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Photo Forum / Digital Photography / DSLR Cameras / July 2006Stopped down focus
Can any camera auto focus with the lens stopped down? I know it would be dimmer, but suppose there is more than enough light. Suppose the lens stopped down is sharper than wide open, would that make auto focus more accurate and faster? > Can any camera auto focus with the lens stopped down? I know it would > be dimmer, but suppose there is more than enough light. Suppose the > lens stopped down is sharper than wide open, would that make auto focus > more accurate and faster? It really doesn't matter how sharp the lens is, as the AF only looks for the maximum sharpness. AF searching often happens because it is trying to focus in an area where there is very little edge contrast. And having more depth of field with a stopped down lens would, I imagine, make finding maximum edge contrast more difficult, not to mention that AF works best with maximum light. Toby >> Can any camera auto focus with the lens stopped down? I know it would >> be dimmer, but suppose there is more than enough light. Suppose the [quoted text clipped - 9 lines] > > Toby When you stop the lens down you increase depth of field. So, the fact that more of your photo is in focus at a smaller aperture would mean that an auto focus mechanism would see more in focus, therefore making it less accurate. Also, as the poster says, there is less light to focus with. When the photo is actually taken the lens will (should) automatically go to the proper aperture, or if you are using aperture priority it will go to whatever you have it set at while the shutter speed adjusts for the proper exposure. According to Sheldon <sheldon@XXXXXXXXsopris.net>: [ ... ] > > It really doesn't matter how sharp the lens is, as the AF only looks for > > the maximum sharpness. AF searching often happens because it is trying to > > focus in an area where there is very little edge contrast. And having more > > depth of field with a stopped down lens would, I imagine, make finding > > maximum edge contrast more difficult, not to mention that AF works best > > with maximum light. [ ... ] > When you stop the lens down you increase depth of field. So, the fact that > more of your photo is in focus at a smaller aperture would mean that an auto [quoted text clipped - 3 lines] > aperture, or if you are using aperture priority it will go to whatever you > have it set at while the shutter speed adjusts for the proper exposure. The problem may be with a lens on extension tubes or a bellows. Note that the focusing aids in viewfinders often consist of tiny prisms to route to the eye information from extreme angles (out near the edge of the diaphragm), to get a maximum baseline for detecting the difference between two adjacent image parts. When you stop down to produce an aperture below that angle, the prisms black out. And when close to the limit, your pupil position became quite critical, with one half or the other blacking out unless you were precisely centered. I'm not sure exactly how the autofocus mechanism is designed in these cameras, but I could certainly imagine it utilizing a similar mechanism with pattern recognition algorithms to detect breaks in lines. Back in the days of the Nikon F, focusing screens with prisms (both the split-image in the center, and the ones with a grid of tiny prisms surrounding the center) had several versions -- typically four, to allow you to select ones appropriate for the maximum aperture of the lens which you were currently using. The only ones which would work with all apertures were the plain ground glass, and the clear center spot with a fine crosshair. On that one, you needed the camera on a stable mount (tripod or equivalent), and you moved your eye from side to side, looking for relative motion between the crosshair and the part of the subject on which you were attempting to focus (parallax focusing). That one worked very well, if you had time to use it and a stationary subject and camera. So -- it *may* be that the autofocus will not work below a certain minimum aperture even with plenty of light. I believe that I remember reading a warning about this in my manual. Enjoy, DoN.  SignatureEmail: <dnichols@d-and-d.com> | Voice (all times): (703) 938-4564 (too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html --- Black Holes are where God is dividing by zero --- There are several ways to do the autofocus in a SLR. 1. Using different light path's in the lens. One light path goes through the left of the lens on goes through the right of the lens. Stopping down would not influence the focusing up to the point the the light path's are blocked making focussing impossible. (There are camera's with 2 set's of light path's one for large apparatures one for small apparatures. The large is better because of the large angle between the two light path's so stopping down would result in slower less accurate focus. 2. Some focussing systems work with sensors places at different depth's and maximise contrast differences in the sensor. Using a smaller apperature would make less light reaching the sensor and making is less fast and making the contrast difference less fast changing and therefore less accurate. 3. Some systems hunt for the focus (probably not used in SLR systems). This hunting would be hindered if less light was let into the system. The only advantage of stopped down AF would be that shifting of the focus by using different apparatures would be counteracted. But shifting of focus is far less than depth of field so this shifting is hardly a problem. So stopped down AF systems are not preferable. ben > Can any camera auto focus with the lens stopped down? I know it would > be dimmer, but suppose there is more than enough light. Suppose the > lens stopped down is sharper than wide open, would that make auto focus > more accurate and faster? > Can any camera auto focus with the lens stopped down? I know it would > be dimmer, but suppose there is more than enough light. Suppose the > lens stopped down is sharper than wide open, would that make auto focus > more accurate and faster? Were it possible then you would have less control over the placement of the beginning and end of the apparent in-focus area (the DOF) as the contrast would be inside the hysteresis of the focus phase sensor over a greater depth of focus. Put a little more simply, from shot to shot of the same subject, the plane of sharpest focus would not be at the same distance. Don't forget that AF is always less accurate than MF in a MF lens due to the measurement system tolerances in the AF lens. Further, as others have said, the AF would have additional trouble due to the reduced light level that the AF depends on. Cheers, Alan. Signature-- r.p.e.35mm user resource: http://www.aliasimages.com/rpe35mmur.htm -- r.p.d.slr-systems: http://www.aliasimages.com/rpdslrsysur.htm -- [SI] gallery & rulz: http://www.pbase.com/shootin -- e-meil: Remove FreeLunch. > Can any camera auto focus with the lens stopped down? I know it would > be dimmer, but suppose there is more than enough light. Suppose the > lens stopped down is sharper than wide open, would that make auto focus > more accurate and faster? Some cameras use ultrasound or IR for focusing - rengefinding would be more accurate since they work like radar, time out and back equates to distance. Cameras which utilize semiconductors behind the lens require a reasonably wide aperture to work. Light levels aren't the problem, it's to do with the f/stop. Canon, and I think Nikon, need a minimum f/5.6 for focusing to work. Some lenses when used with a 2x extender won't focus because the effective maximum stop is smaller than f/5.6, typically f/8 or so. All Canon lenses have a minimum aperture of f5.6 at any point in the zoom range; but some third-party lenses have only f6.3 at the long end, and they won't focus properly. Colin D. SignaturePosted via a free Usenet account from http://www.teranews.com >>Can any camera auto focus with the lens stopped down? I know it would >>be dimmer, but suppose there is more than enough light. Suppose the [quoted text clipped - 4 lines] > more accurate since they work like radar, time out and back equates to > distance. Ultrasound yes. IR no. You would need sub nanosecond timing for accurate enough focus using IR. (speed of light is about 1 foot per nanosecond). IR assist shines a pattern on the subject to get contrast lines for the AF to focus on. AF assist is sometimes body mounted and more often accessory flash mounted. Cheers, Alan Signature-- r.p.e.35mm user resource: http://www.aliasimages.com/rpe35mmur.htm -- r.p.d.slr-systems: http://www.aliasimages.com/rpdslrsysur.htm -- [SI] gallery & rulz: http://www.pbase.com/shootin -- e-meil: Remove FreeLunch. > >>Can any camera auto focus with the lens stopped down? I know it would > >>be dimmer, but suppose there is more than enough light. Suppose the [quoted text clipped - 21 lines] > -- [SI] gallery & rulz: http://www.pbase.com/shootin > -- e-meil: Remove FreeLunch. Yep, thanks. Colin D. SignaturePosted via a free Usenet account from http://www.teranews.com > Ultrasound yes. IR no. You would need sub nanosecond timing for accurate > enough focus using IR. (speed of light is about 1 foot per nanosecond). IR has been used very extensively in Point and Shoot camera's to determine the distance. This was done with an IR beam and a sensor. System 1. (Oldest) The IR beam made a sweep and the sensor picked it up. Only a primitive sensor is needed for this. The sweep of the IR beam was linked to the focusing mechanism. System 2. A fixed IR beam. With a multiple sensor setup. The beam and the sensor were places some distance apart on the camera. Most used positions were left and right above the lens. I think the first Pentax SLR system with AF worked this way. The system was integrated in the lens and not in the camera. ben >>Ultrasound yes. IR no. You would need sub nanosecond timing for accurate >>enough focus using IR. (speed of light is about 1 foot per nanosecond). > > IR has been used very extensively in Point and Shoot camera's Ben, Why did you snip the part where I said: "IR assist shines a pattern on the subject to get contrast lines for the AF to focus on. AF assist is sometimes body mounted and more often accessory flash mounted. " That is what IR AF assist does. It does NOT do ranging as Colin first implied (accidently or otherwise). That's what I was clearing up. Signature-- r.p.e.35mm user resource: http://www.aliasimages.com/rpe35mmur.htm -- r.p.d.slr-systems: http://www.aliasimages.com/rpdslrsysur.htm -- [SI] gallery & rulz: http://www.pbase.com/shootin -- e-meil: Remove FreeLunch. > >>Ultrasound yes. IR no. You would need sub nanosecond timing for accurate > >>enough focus using IR. (speed of light is about 1 foot per nanosecond). [quoted text clipped - 9 lines] > That is what IR AF assist does. It does NOT do ranging as Colin first > implied (accidently or otherwise). That's what I was clearing up. Actually, you could also measure distance with IR easily: 1) two sensors, one emits, the other detects; given the reception angle, you know the distance 2) make the incoming reflected beam interfere with one directly from the IR source; you could then deduce the elapsed time, hence the distance (however, only modulo one wavelength, ie of the order of 600nm; that is, 10m and 10m plus any integer multiple of 600nm (eg 6m) would be the same to this system; so please ignore this method...). I have no clue if method 1 is actually used, though I suspect it's what was used in 35mm compacts; never having owned a film compact, however, I don't know. achilleaslazari...@yahoo.co.uk wrote: > Actually, you could also measure distance with IR easily: Ben Brugman has also written above that triangulation was indeed used with old compacts. >> >>Ultrasound yes. IR no. You would need sub nanosecond timing for >> >>accurate [quoted text clipped - 16 lines] > 1) two sensors, one emits, the other detects; given the reception > angle, you know the distance It's not easily. Most sensors are not capable of detecting an angle. Although triangulation is done, this is not done by sensors which detect an angle, but with a row of sensors, where the detection is done by registring on which of the sensors the centre of the beam falls and working out the angle. Or on older camera's the beam of the camera sweeps, the sweeping mechanism is connected to the focus mechanism. The sweep is stopped when the sensor detects the beam. > 2) make the incoming reflected beam interfere with one directly from > the IR source; you could then deduce the elapsed time, hence the > distance (however, only modulo one wavelength, ie of the order of > 600nm; that is, 10m and 10m plus any integer multiple of 600nm (eg 6m) > would be the same to this system; so please ignore this method...). 600 nm is not 6m. A nanometer 10-9 meter, or 0.000000001 meter Detection of this scale is difficult, and not usefull for AF. Ben > I have no clue if method 1 is actually used, though I suspect it's what > was used in 35mm compacts; never having owned a film compact, however, > I don't know. > I wrote: > > 2) make the incoming reflected beam interfere with one directly from [quoted text clipped - 6 lines] > > Detection of this scale is difficult, and not usefull for AF. Well, this is what I said. Also, what I wrote is "any integer multiple of 600nm (eg 6m)". Well, 6m is an integer multiple of 6nm (as is 6km, for that matter). And the reason I give for not being able to use interference between two beams for distance measuring at this scale is valid. You are actually the third person in this thread to "explain" to me that 1nm=0.000000001m (when reading what I wrote would indicate that I do know this, I would think); I imagine it will make continuation of any discussion rather difficult if we all start doubting if the others have any clue what they are talking about. Anyway, obviously nobody here reads carefully what the others say before answering, so the whole thing is a waste of time for all of us. >> I wrote: >> > 2) make the incoming reflected beam interfere with one directly from [quoted text clipped - 12 lines] > interference between two beams for distance measuring at this scale is > valid. If I was the third person to make the same "explaination" than you probably were not very clear on the matter. You also wrote "1) two sensors, one emits, the other detects; given the reception angle, you know the distance" Sensors doe not emit, that is not there function, combined with the fact that this is an English language newsgroup, I assumed (wrongly) that you didn't understand the nanometer, because of being part of the metric system. (If Nasa can make that mistake, most Americans can make that mistake). I did read carefully, but because not information was supplied I made some wrong assumtions. So sorry for that. ben > You are actually the third person in this thread to "explain" to me > that 1nm=0.000000001m (when reading what I wrote would indicate that I [quoted text clipped - 4 lines] > Anyway, obviously nobody here reads carefully what the others say > before answering, so the whole thing is a waste of time for all of us. > If I was the third person to make the same "explaination" than you > probably were not very clear on the matter. Maybe I was and maybe not. I don't really see how I can "unclearly" say eg "seven nanometres", but what do I know? Anyway, all sorts of miscommunications may occur with a text-only medium. >> IR has been used very extensively in Point and Shoot camera's > [quoted text clipped - 3 lines] > AF to focus on. AF assist is sometimes body mounted and more often > accessory flash mounted. " Because in the 'IR assist' mode it is only used as a light source with a pattern build in. Although light is neccesary for the focus system the 'IR assist' is not an essential part of the workings of the focus system. The method I described uses IR as an integral part of the focussing. The IR was triangulated, killing the IR beam would disable the focussing system of those camera's. > That is what IR AF assist does. It does NOT do ranging as Colin first > implied (accidently or otherwise). That's what I was clearing up. > Ultrasound yes. IR no. You would need sub nanosecond timing for > accurate enough focus using IR. (speed of light is about 1 foot per > nanosecond). I don't see the problem. Computers do a couple of GHz nowadays. So you can get a stable tact rate in these frequencies without trouble. 1 Hz => cycle 1s 1 kHz => cycle 1 millisecond 1 MHz => cycle 1 microsecond 1 GHz => cycle 1 nanosecond No problem even for consumer electronics. Even waaay back (1990 or earlier) my Dad had a video camera that would measure distances by IR light. No, no patterns there, and it didn't focus correctly when filming through a glass window. (OK, focussing requirements are lower for moving images on small chips with low resolution.) -Wolfgang > 1 Hz => cycle 1s > 1 kHz => cycle 1 millisecond > 1 MHz => cycle 1 microsecond > 1 GHz => cycle 1 nanosecond > > No problem even for consumer electronics. But the return trip for an object 1m away would be less than 7ns, ie 7 cycles. I suppose it is possible to emit a signal and measure the number of cycles until something is received, but I imagine that 15 years ago this was not feasible. Also, I suppose you'd need to do some processing to work out if the thing you received really is the signal you wanted, and I don't know if this is possible in such short times (since you must be ready to restart if it's the wrong signal). And did compact cameras in 1990 have 1GHz oscillators in them? Again, I don't know; it seems hard to believe, but somehow they autofocused. Well, as I said, I have no clue, so please correct me if I am wrong. >> 1 Hz => cycle 1s >> 1 kHz => cycle 1 millisecond >> 1 MHz => cycle 1 microsecond >> 1 GHz => cycle 1 nanosecond >> No problem even for consumer electronics. > But the return trip for an object 1m away would be less than 7ns, ie 7 > cycles. I suppose it is possible to emit a signal and measure the > number of cycles until something is received, but I imagine that 15 > years ago this was not feasible. Sorry, having seen that with consumer VHS video cameras back then, I disagree. > Also, I suppose you'd need to do some > processing to work out if the thing you received really is the signal > you wanted, Send many signals and average over time (the focus motor has a finite speed, thus doing averaging itself), and use either a specific frequency or a specific pattern (patterns are no problems either, how do you think IR remote controls work? And they have been around a _long_ time. > and I don't know if this is possible in such short times > (since you must be ready to restart if it's the wrong signal). No problem. Throw away signals detected a spurious, continue sending pulses as before. > And did compact cameras in 1990 have 1GHz oscillators in them? Oscillators are not _that_ new an invention. > Again, I don't > know; it seems hard to believe, but somehow they autofocused. Hey, the Mercury capsule worked. Without any computers. -Wolfgang > >> No problem even for consumer electronics. > [quoted text clipped - 5 lines] > Sorry, having seen that with consumer VHS video cameras back > then, I disagree. But did they measure distance by measuring the time between emission and reception? I don't know, I am asking. > > Also, I suppose you'd need to do some > > processing to work out if the thing you received really is the signal [quoted text clipped - 5 lines] > either, how do you think IR remote controls work? And they have > been around a _long_ time. What do you mean? If your distance measuring is done by detecting the time elapsed between emission and reception, then you have to emit, wait until you receive the signal, and then divide that time by the speed of light to get the distance, and only then activate the focus motor to focus properly. Furthermore, changing the focus has absolutely no effect on the signal received by the IR sensor; so I fail to see how the speed of the motor enters into the discussion. Basically, I don't understand what you're saying. As for IR remotes, I don't know how quickly the TV (say) reacts, but I suppose there is some time lapse between reception and reaction, since you have to react differently according to the signal so some processing is needed. > > and I don't know if this is possible in such short times > > (since you must be ready to restart if it's the wrong signal). > > No problem. Throw away signals detected a spurious, continue > sending pulses as before. What I was trying to say is this: You emit a signal, start a stopwatch and wait; when its reflection is detected, you stop the stopwatch, and may calculate the distance. However, to do that, you have to make sure that the received signal is the reflection of whatever you emitted, so you need to do some processing to check. If you process and find that it is, then all is fine; but if it's not, then you have to wait some more. The problem is that while you're processing the spurious signal, the real one might have arrived. What do you do? I don't know if you can process a signal while managing a queue of incoming candidates so quickly (of the order of nanoseconds). > > And did compact cameras in 1990 have 1GHz oscillators in them? > > Oscillators are not _that_ new an invention. No, but I was questioning whether 1GHz oscillators were built into VHS cameras. As I said, I don't know, it's a question (although it sounds too high-frequency, to be honest) I think I am misunderstanding you. Could you please specify whether you are talking about a system that measures the elapsed time between emission and reception (as opposed to the angle of reception, a completely different method)? If so, could you explain how the speed of the focus motor has anything to do with the feasibility of doing this? Thanks. >> >> No problem even for consumer electronics. >> > But the return trip for an object 1m away would be less than 7ns, ie 7 >> > cycles. I suppose it is possible to emit a signal and measure the >> > number of cycles until something is received, but I imagine that 15 >> > years ago this was not feasible. >> Sorry, having seen that with consumer VHS video cameras back >> then, I disagree. > But did they measure distance by measuring the time between emission > and reception? I don't know, I am asking. Yes. To the best of my knowledge and memory. They would be mis-focussing on windows, they were not hunting, the light was not visible. >> > Also, I suppose you'd need to do some >> > processing to work out if the thing you received really is the signal >> > you wanted, >> Send many signals and average over time (the focus motor has >> a finite speed, thus doing averaging itself), and use either a >> specific frequency or a specific pattern (patterns are no problems >> either, how do you think IR remote controls work? And they have >> been around a _long_ time. > What do you mean? If your distance measuring is done by detecting the > time elapsed between emission and reception, then you have to emit, > wait until you receive the signal, and then divide that time by the > speed of light to get the distance, and only then activate the focus > motor to focus properly. The maximum stated range was 10 meters. So the maximum time to wait would be 20m / c ~= 6.7 * 10^-8 seconds. If you allowed just 1/1000s before activating the focus, you'd be able to do more thas 10.000 measurements. (Not that you'd want to, it'd be draining the NiCd battery (yep, that old), but you can cram in a few ...) > Furthermore, changing the focus has absolutely > no effect on the signal received by the IR sensor; so I fail to see how > the speed of the motor enters into the discussion. The focus motor was not _very_ fast. So what would happen if you had the following pattern of distance results (probably a few dozen per seconds): 4m focus to 4m 4m still focussing 4m still focussing ... ... arrived at focus ... 4m 4m still at focus 0.3m start focussing motor, now at 4.0m 0.3m still focussing (now at 3.95m) 4m reverse focus motor (now at 3.85m) (inertial mass!) 4m still reversing (now at 3.9m) 4m arrived at focus ... The fact that the focus motor is slow DOES mean you average out spurious signals. Think about this sequence. The whole sequence probably takes half a second or less: 4.0m at 4.0 4.1m at 4.0 starting for 4.1 3.9m at 4.05 starting for 3.9 4.0m at 4.0 stop motor 3.9m at 4.0 starting for 3.9 4.1m at 3.95 starting for 4.1 4.1m at 4.0 4.0m at 4.1 starting for 4.0 You see, even with a fairly fast motor and slow updates, the speed of the motor evens out a lot. > As for IR remotes, I don't know how quickly the TV (say) reacts, but I > suppose there is some time lapse between reception and reaction, since > you have to react differently according to the signal so some > processing is needed. First patent for remotes: 1893, Nicola Tesla, US Patent 613809. First remote controlled model airplane: 1932. First remote controlled SAM missile "Wasserfall", WWII. First wireless TV remote control "Flashmatic" 1955 (visible light) First ultrasound TV remote control "Zenith Space Command", 1956. (4 buttons, 4 frequencies, no batteries needed!, and 6 extra tubes in the TV) Many-Button remotes, prototypes at 1977-78. IR-remote controls from the early 1980s. Learing remote controls from mid 1980s. Remember that the whole unmanned space exploration (and a good part of the manned one as well) are using remote Of course some lapse occurs because you have to decode the whole signal, and probably wait for more pulses to come, but the pauses are not that long. >> > and I don't know if this is possible in such short times >> > (since you must be ready to restart if it's the wrong signal). >> No problem. Throw away signals detected a spurious, continue >> sending pulses as before. > What I was trying to say is this: You emit a signal, start a stopwatch > and wait; when its reflection is detected, you stop the stopwatch, and > may calculate the distance. Exactly. All you need is a 'counter' which can cope with a GHz pulse and some start, stop and readout electronics. If you want to do that digitally. You can probably get by by discharging a capacitor over a resistor while the light is going there and back again, and measuring the rest voltage. And since the voltage is dropping fast at first, you get increased accuracy with close targets. (Accumulate by discharging over multiple bounce cycles.) > However, to do that, you have to make sure > that the received signal is the reflection of whatever you emitted, so > you need to do some processing to check. Nope. It would be nice if you had. You can average over 10 or 100 or so measurements. You can use a well-defined frequency. And you can say 'I don't care, it's consumer electronics anyway'. > If you process and find that > it is, then all is fine; but if it's not, then you have to wait some > more. We are talking about times like .000000067 seconds per measurement for the light to travel. Ok, take 1.000 times that time to handle the stuff. We still talk about .00067 seconds. Negible. About 1/2000s lagtime, even with generous handling time. > The problem is that while you're processing the spurious signal, > the real one might have arrived. What do you do? You switch off or ignore the receiver once you get the first signal. After all, you _will_ often get scatter from the background, after the target has reflected. > I don't know if you > can process a signal while managing a queue of incoming candidates so > quickly (of the order of nanoseconds). You are thinking digital. Try thinking analog. > I think I am misunderstanding you. Could you please specify whether you > are talking about a system that measures the elapsed time between > emission and reception (as opposed to the angle of reception, a > completely different method)? Elapsed time. > If so, could you explain how the speed of > the focus motor has anything to do with the feasibility of doing this? As I said, if you get a few spurious data (we are talking about that VIDEOcamera), the focus speed can average them out. For a photo camera you'd probably average measurements electrically. -Wolfgang > Yes. To the best of my knowledge and memory. They would be > mis-focussing on windows, they were not hunting, the light > was not visible. That proves it's IR radiation; it could be by triangulation, not by measuring the time for the signal to come back. > >> Send many signals and average over time (the focus motor has > >> a finite speed, thus doing averaging itself), and use either a [quoted text clipped - 13 lines] > 10.000 measurements. (Not that you'd want to, it'd be draining > the NiCd battery (yep, that old), but you can cram in a few ...) But my point wasn't that the time interval is too long; it's that it's too short. > > Furthermore, changing the focus has absolutely > > no effect on the signal received by the IR sensor; so I fail to see how [quoted text clipped - 3 lines] > you had the following pattern of distance results (probably > a few dozen per seconds): > You see, even with a fairly fast motor and slow updates, the > speed of the motor evens out a lot. OK I see what you mean: you measure many times, the errors have little effect. Well I don't think this is what cameras did/do. Feel free to disagree. > > As for IR remotes, I don't know how quickly the TV (say) reacts, but I > > suppose there is some time lapse between reception and reaction, since [quoted text clipped - 18 lines] > whole signal, and probably wait for more pulses to come, but > the pauses are not that long. What do these dates have to do with anything? And "not that long"? We're talking about reactions that must occur in nanoseconds. > > What I was trying to say is this: You emit a signal, start a stopwatch > > and wait; when its reflection is detected, you stop the stopwatch, and [quoted text clipped - 3 lines] > pulse and some start, stop and readout electronics. If you want > to do that digitally. All you need is a gigahertz oscillator? OK, I never paid any attention to circuits etc, but it doesn't sound trivial to me. OK, maybe I am wrong, but I'd be rather surprised if it's easy to build an accurate gigahertz oscillator with cheap components. As I said, I may be wrong. > You can probably get by by discharging a capacitor over a resistor > while the light is going there and back again, and measuring the > rest voltage. And since the voltage is dropping fast at first, > you get increased accuracy with close targets. (Accumulate > by discharging over multiple bounce cycles.) > > However, to do that, you have to make sure > > that the received signal is the reflection of whatever you emitted, so [quoted text clipped - 12 lines] > handle the stuff. We still talk about .00067 seconds. Negible. > About 1/2000s lagtime, even with generous handling time. Did you bother to read what I wrote? I am arguing exactly the opposite of what you're answering to. > > The problem is that while you're processing the spurious signal, > > the real one might have arrived. What do you do? > > You switch off or ignore the receiver once you get the first > signal. After all, you _will_ often get scatter from the > background, after the target has reflected. And what if the signal is spurious? You measure many times, would probably be your answer. OK. I don't think anything works this way. > > I don't know if you > > can process a signal while managing a queue of incoming candidates so [quoted text clipped - 14 lines] > As I said, if you get a few spurious data (we are talking about > that VIDEOcamera), the focus speed can average them out. OK, I can see this will become long and tedious. I give up. Frankly, I don't know how it works, so maybe you're right. I don't think so, but maybe you are. >> Yes. To the best of my knowledge and memory. They would be >> mis-focussing on windows, they were not hunting, the light >> was not visible. > > That proves it's IR radiation; it could be by triangulation, not by > measuring the time for the signal to come back. It was done by triangulation. There are no cheap commercial detectors available which receive enough light to detect a signal with an accuracy of only nanoseconds. The sensors are just not sensitive enough to detect light in such a short time. At the time there where no cheap commercial electronics to work fast enough to do the timing. So the method described of timing light was (and still is) not feasable for consumer products. Look at the price of a laser gun ten years ago to get an estimate of how much that technologie would cost. (The lasergun emits so much laserlight (not just ir) that this technologie would not be good for the health of the persons being taken pictures of, if this technologie would be used). ben >> >> Send many signals and average over time (the focus motor has >> >> a finite speed, thus doing averaging itself), and use either a [quoted text clipped - 129 lines] > don't know how it works, so maybe you're right. I don't think so, but > maybe you are. > >> Yes. To the best of my knowledge and memory. They would be > >> mis-focussing on windows, they were not hunting, the light [quoted text clipped - 10 lines] > fast enough to do the timing. So the method described of timing > light was (and still is) not feasable for consumer products. Ben, Thanks. I thought so. > Look at the price of a laser gun ten years ago to get an estimate > of how much that technologie would cost. > (The lasergun emits so much laserlight (not just ir) that this > technologie would not be good for the health of the persons > being taken pictures of, if this technologie would be used). But does a police laser gun work like this? I'd have thought it would use the doppler shift to measure speed (actually I'm sure). On the other hand, I know that there are laser guns that measure distance (used eg by the artillery), so these presumably do work by measuring the elapsed time. Do you know if this is so? > But does a police laser gun work like this? I'd have thought it would > use the doppler shift to measure speed (actually I'm sure). On the I stand corrected, I suppose that the doppler shift is the method used. ben > other hand, I know that there are laser guns that measure distance > (used eg by the artillery), so these presumably do work by measuring > the elapsed time. Do you know if this is so? >>But does a police laser gun work like this? I'd have thought it would >>use the doppler shift to measure speed (actually I'm sure). On the [quoted text clipped - 5 lines] >>(used eg by the artillery), so these presumably do work by measuring >>the elapsed time. Do you know if this is so? No, it seems that, for laser guns, it is not doppler (it's doppler for radar guns). I remember the BBC ran a program where they demonstrated how such a gun can result in a nonzero reading off a wall (the user pulled the triger and scanned along the wall). Had it worked using the doppler effect, it would have returned zero; it didn't because it works by measuring the distance many times. So you were correct. >>> 1 Hz => cycle 1s >>> 1 kHz => cycle 1 millisecond [quoted text clipped - 30 lines] > > Oscillators are not _that_ new an invention. Oscillators aren't, but inexpensive electronic devices that can operate at gigahertz frequencies are. If you know of any cameras that use the timing of a reflected light signal for distance measurement please name them. >> Again, I don't >> know; it seems hard to believe, but somehow they autofocused. > > Hey, the Mercury capsule worked. Without any computers. > > -Wolfgang Signature--John to email, dial "usenet" and validate (was jclarke at eye bee em dot net) > If you know of any cameras that use the timing of a reflected light signal > for distance measurement please name them. Speedometercamera's with lasergun technologie use that technologie. >> If you know of any cameras that use the timing of a reflected light >> signal for distance measurement please name them. >> > Speedometercamera's with lasergun technologie use that technologie. I think you'll find that they use doppler, not interval--they don't care how far away the subject is, just how fast it is going. Signature--John to email, dial "usenet" and validate (was jclarke at eye bee em dot net) >>>If you know of any cameras that use the timing of a reflected light >>>signal for distance measurement please name them. [quoted text clipped - 3 lines] > I think you'll find that they use doppler, not interval--they don't care how > far away the subject is, just how fast it is going. I thought so too, but it seems that the laser guns (as opposed to radar guns) simply measure the distance (by measuring the elapsed time) many times. >>> If you know of any cameras that use the timing of a reflected light >>> signal for distance measurement please name them. [quoted text clipped - 4 lines] > how > far away the subject is, just how fast it is going. I think you are correct and that I was wrong. Ben |
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