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Photo Forum / General Photo Topics / General Topics / February 2008Auto-focus on digital SLRs.
I am new to the auto-focus world, both my film cameras are manual focus. I just got my hands on my sister's Digital Rebel XT (DRXT) to just learn some basics of the camera and see what features I really like. One thing I am developing a disliking for is auto-focus. On the DRXT it sometimes goes into fits zooming in and out then refuses to snap a picture. I find the auto-focus slow and a pain in the a.s. I understand that the speed of auto-focus can be a function of the lens and the effectiveness on the amount of light (if I understand this correctly) Other than things like numbers of points and speed of lenses I don't see much on the effectiveness of auto-focus. How do the hi/low end cameras differ for canon? Is it better/worse with other brands? If this is representative of the other cameras as well I think I may add weight to the quality of manual-focus for my future purchase and largely ignore auto-focus. Thanks, Steve > I am new to the auto-focus world, both my film cameras are manual focus. > I just got my hands on my sister's Digital Rebel XT (DRXT) to just learn > some basics of the camera and see what features I really like. Being an old school old fart, I didn't think I'd like autofocus when I switched to digital. But I love it. It takes a bit of getting used to (and turning off all sensors except one), but it can focus a lot quicker than these old eyeballs. >> I am new to the auto-focus world, both my film cameras are manual focus. >> I just got my hands on my sister's Digital Rebel XT (DRXT) to just learn [quoted text clipped - 4 lines] > (and turning off all sensors except one), but it can focus a lot > quicker than these old eyeballs. And, when you only have about 2% of "normal" vision, it becomes indeispensible... > And, when you only have about 2% of "normal" vision, it becomes > indeispensible... My use of soft-focus lenses in a semi-darkened studio for many years may have something to do with it too. >> And, when you only have about 2% of "normal" vision, it becomes >> indeispensible... > > My use of soft-focus lenses in a semi-darkened studio for many years > may have something to do with it too. Yes, eye-strain is definitely a hazard of the profession... > > I am new to the auto-focus world, both my film cameras are manual focus. > > I just got my hands on my sister's Digital Rebel XT (DRXT) to just learn [quoted text clipped - 4 lines] > (and turning off all sensors except one), but it can focus a lot > quicker than these old eyeballs. In the old days- one knew how to pre-focus; yet oddly enough it works with AF cameras-if one knows what the hell one is doing.  SignatureReality is a picture perfected and never looking back. : > > I am new to the auto-focus world, both my film cameras are manual focus. : > > I just got my hands on my sister's Digital Rebel XT (DRXT) to just learn [quoted text clipped - 7 lines] : In the old days- one knew how to pre-focus; yet oddly enough it works : with AF cameras-if one knows what the hell one is doing. You are so right. And your observation reminds me of one of my pet peeves: Why don't modern automobiles have a place to insert a starting crank? Autos should be crank-started. It's what Henry Ford intended, and our latter-day reliance on electric starters is a stark reminder of how modern society has gone soft. Bob > On Fri, 22 Feb 2008 15:42:07 -0500, ____ > <internetphobic@deletedmail.com> [quoted text clipped - 27 lines] > > Bob Sounds like a crank to me! >: > > I am new to the auto-focus world, both my film cameras are manual focus. >: > > I just got my hands on my sister's Digital Rebel XT (DRXT) to just learn [quoted text clipped - 12 lines] >be crank-started. It's what Henry Ford intended, and our latter-day reliance >on electric starters is a stark reminder of how modern society has gone soft. A crank starter sure would be nice if you accidentally leave your lights on. And if the engine is too big to crank start, one of those clockwork wind-up crank starters would work great. Just like manual focus can work great if the autofocus isn't doing what you want. Steve > A crank starter sure would be nice if you accidentally leave your > lights on. And if the engine is too big to crank start, one of those > clockwork wind-up crank starters would work great. Just like manual > focus can work great if the autofocus isn't doing what you want. > > Steve Good point, to place back in context any suggestions for speed focusing rings for manual control? SignatureReality is a picture perfected and never looking back. >> A crank starter sure would be nice if you accidentally leave your >> lights on. And if the engine is too big to crank start, one of those [quoted text clipped - 5 lines] >Good point, to place back in context any suggestions for speed focusing >rings for manual control? Well, I have an old one-touch Nikon 80-200 f/2.8 zoom that I got on ebay. The huge combo ring (twist to focus, push-pull to zoom) really works great for manual focusing. Fast and easy to handle because you don't have to move your hands anywhere to switch from focus to zoom. Switch it to auto-focus and twisting doesn't do anything, only push-pull to zoom. Steve >> A crank starter sure would be nice if you accidentally leave your >> lights on. And if the engine is too big to crank start, one of those [quoted text clipped - 5 lines] > Good point, to place back in context any suggestions for speed focusing > rings for manual control? If you really want to go that route, you could always place an object at various distances in a well lit setting, and then autofocus on it. Scribing a reference line at some point of the housing and a small mark on the focusing ring could help. But, after playing around with a Rebel XT and trying to get it to exhibit similar behaviour, I have to say it actually is quite good. The only time it would hunt around was when I was pointing it at a white wall, or some other nondescript object with no texture or detail. If there is something to work with, the AF unit seemed to find it. Also, if the AF unit couldn't find anything to focus on, it only went through its range twice -- once rotating out and once coming back in. It took about a second, and it wouldn't try again if it wasn't initially successful. Does the problem get worse as you zoom out? If so, it could be the result of the variable aperture of the zoom lens. How old is the camera? Could the sensor be dirty? Lastly, it is important to keep in mind that red-eye reduction features tend to slow down camera operation in low light situations. The camera might hunt a bit for focusing, and then sit there for another second with the red-eye reduction light eluminated even if you don't need it. This might be mistaken for slow AF performance. I don't know if any of this applies, but it doesn't hurt to mention it. Good Luck, Dudley > >> A crank starter sure would be nice if you accidentally leave your > >> lights on. And if the engine is too big to crank start, one of those [quoted text clipped - 37 lines] > Good Luck, > Dudley Good advise however does not apply directly - because mine is a speed issue, doing at a wedding etc. I need better focus quickly and see the down side of AF compared to manual focusing. SignatureReality is a picture perfected and never looking back. [AF pumping and failing] > Does the problem get worse as you zoom out? If so, it could be the result > of the variable aperture of the zoom lens. If it's not, it could also be camera shake. An in-lens IS helps. -Wolfgang > How do the hi/low end cameras differ for canon? > Is it better/worse with other brands? I can't say much for Canon cameras being a Nikonian, but there is definitely a difference in autofocus speed and precision depending on the lens/camera. For example, my D50 (yeah, baby Nikon I know) is dead in the water when I had the opportunity to try out a D200. The D200 focuses with far more areas and works far better in low light. Also, my Tamron 70-300mm lens uses the camera's drive system for focus, it takes roughly 2 seconds to get from one end of the screw to the other. My Nikkor lens does that in less than half of a second with its internal motor. I've entirely missed the film era--I've used digital all my life, getting my first dSLR just over 2 years ago. Strangely enough, I still prefer my dad's 50mm Micro Nikkor when I have the opportunity to do some macro shots. - -- Brendan Gillatt | GPG Key: 0xBF6A0D94 brendan {a} brendangillatt (dot) co (dot) uk http://www.brendangillatt.co.uk "Brendan Gillatt" <brendanREMOVETHIS@brendanREMOVETHISgillatt.co.uk> wrote in message > I can't say much for Canon cameras being a Nikonian, but there is > definitely a difference in autofocus speed and precision depending on the [quoted text clipped - 3 lines] > had the opportunity to try out a D200. The D200 focuses with far more > areas and works far better in low light. Wait till you tyr the D300. > Also, my Tamron 70-300mm lens uses the camera's drive system for focus, > it takes roughly 2 seconds to get from one end of the screw to the other. > My Nikkor lens does that in less than half of a second with its internal > motor. > I've entirely missed the film era--I've used digital all my life, getting > my first dSLR just over 2 years ago. Strangely enough, I still prefer my > dad's 50mm Micro Nikkor when I have the opportunity to do some macro > shots. I use my old 200 Nikkor macro lens for close ups and macro work. Sometimes I use it with a 2x extender and extension tubes. I don't think autofocus is at all useful for those kind of shots. but play around, and try to extract the abstract color shape and form. While few may recognize it as a flower, it doesn't really matter. SignaturePeter When it hunts (zooms in and out), what are you photographing at the time? An image with very little contrast? A low light shot? Is the subject moving? Also, what focusing mode are you using? AI Servo, AI Focus or One Shot? >I am new to the auto-focus world, both my film cameras are manual focus. > I just got my hands on my sister's Digital Rebel XT (DRXT) to just learn [quoted text clipped - 21 lines] > > Steve > When it hunts (zooms in and out), what are you photographing at the > time? An image with very little contrast? A low light shot? Is the > subject moving? > > Also, what focusing mode are you using? AI Servo, AI Focus or One Shot? I was on One shot at the time. In this case it was a rather low light shot of my dog laying on a sofa. Indoors with lamp lighting. I don't think he was moving at the time. I think I tried the flash at the time and it did the strobe thing and couldn't make up its mind. He is a rather brown dog with a dark face. I think he has rather good contrast including the sofa. It's strange that the 'strobe thing' you experienced (which is AF assist) didn't get a focus lock. I would be interested to know what lens you were using and how far away you were from the subject. >> When it hunts (zooms in and out), what are you photographing at the >> time? An image with very little contrast? A low light shot? Is the >> subject moving? >> >> Also, what focusing mode are you using? AI Servo, AI Focus or One Shot? > I was on One shot at the time. > [quoted text clipped - 5 lines] > He is a rather brown dog with a dark face. I think he has rather good > contrast including the sofa. > It's strange that the 'strobe thing' you experienced (which is AF assist) > didn't get a focus lock. > > I would be interested to know what lens you were using and how far away you > were from the subject. Why? It happens all the time in low light situations. That's why the lenses have a switch to shut off the AF. Tokina even makes lenses that you pull the focus ring towards you and overide the AF (which is nice). If you haven't experienced the joy of AF that can't find a focus point, you haven't used your camera enough. Sometimes you need to throw on a flash just for the IR focusing. > >> When it hunts (zooms in and out), what are you photographing at the > >> time? An image with very little contrast? A low light shot? Is the [quoted text clipped - 12 lines] > > - Show quoted text - Have to disagree. Just as an extreme experiment, I just switched off all lights and took a photo in total darkness (AV, with flash up). It still focused. The reason I asked what lens is because a small aperture lens will not be as effective in low light. The reason I asked how far was because maybe he was too far away from the subject for the flash to effectively illuminate the subject for AF assist. I agree that another option is to use IR, such as a Speedlight or an SE-E2. "Pat" <groups@artisticphotography.us> wrote in message news:36fbc671-1ce1-49ad-ad96- > It's strange that the 'strobe thing' you experienced (which is AF assist) > didn't get a focus lock. > > I would be interested to know what lens you were using and how far away > you > were from the subject. Why? It happens all the time in low light situations. That's why the lenses have a switch to shut off the AF. Tokina even makes lenses that you pull the focus ring towards you and overide the AF (which is nice). If you haven't experienced the joy of AF that can't find a focus point, you haven't used your camera enough. Sometimes you need to throw on a flash just for the IR focusing. >I am new to the auto-focus world, both my film cameras are manual focus. > I just got my hands on my sister's Digital Rebel XT (DRXT) to just learn [quoted text clipped - 21 lines] > > Steve Remember auto-focus is a contrast detection system, if there's little or no obvious 'edges' under the focus points, its will fail. Often the trick can be to turn off the multi-point focus system and just use the central sensor, point that at an obvious edge at the distance you want to focus on; half press the shutter to let the camera focus then reframe. Like everything, it takes a bit of getting used to. > Remember auto-focus is a contrast detection system, on a dslr, it's a phase detection system (unless in live view mode on some cameras). point&shoots use contrast detection. >> Remember auto-focus is a contrast detection system, > > on a dslr, it's a phase detection system (unless in live view mode on > some cameras). point&shoots use contrast detection. True, but whichever system is used they all need to be able to detect some difference in contrast. Try getting a dslr to focus on a white card or a blank wall when there's nothing else in view... > Try getting a dslr to focus on a white card or a blank wall when there's > nothing else in view... kinda hard to manually focus on that. >> Try getting a dslr to focus on a white card or a blank wall when there's >> nothing else in view... > > kinda hard to manually focus on that. Not with an old manual focus, mechanical camera. You just set the focal scale to the distance the wall is from the focal plane and, presto, it's in focus. Takes less than a second. Well, maybe a second and a half if you're clumsy. Smile, Dudley >>> Try getting a dslr to focus on a white card or a blank wall when there's >>> nothing else in view... [quoted text clipped - 5 lines] > in focus. Takes less than a second. Well, maybe a second and a half if > you're clumsy. And for those who can't judge distances accurately, a US dollar bill is six inches long. No, a five dollar bill is not two and a half feet! Back in the days of sync cords and manual flashes and guide numbers, I had a long sync cord (15'?) that I applied a piece of narrow tape around it every foot. Made it easy to measure distance to the subject. >> Try getting a dslr to focus on a white card or a blank wall when there's >> nothing else in view... > kinda hard to manually focus on that. Ever heard of microprisms? As in "microprism ring"? Try Google on that term, or ask your local dealer, preferably one that has a proper "old" manual focus SLR on hand for demonstration ... -Wolfgang >>>Try getting a dslr to focus on a white card or a blank wall when there's >>>nothing else in view... [quoted text clipped - 7 lines] > > -Wolfgang Also there is the likelyhood that unless your blank wall or white card has no surface texture (ie perfectly smooth) and/or is illuminated completely evenly (no shadows from any surface texture), there is probably more than enough contrast for a DSLR camera to AF on the surface and also it's enough for manual focus without the aid of a split prism and microprism ring on the viewfinder screen anyway. In article <47c0c0bc$0$27300$5a62ac22@per-qv1-newsreader-01.iinet.net.au>, > Also there is the likelyhood that unless your blank wall or white card > has no surface texture (ie perfectly smooth) and/or is illuminated > completely evenly (no shadows from any surface texture), there is > probably more than enough contrast for a DSLR camera to AF on the > surface and also it's enough for manual focus without the aid of a split > prism and microprism ring on the viewfinder screen anyway. By real life experience: There are more than enough walls with inadequate contrast and even lighting to fool AF. SignatureReality is a picture perfected and never looking back. > By real life experience: > There are more than enough walls with inadequate contrast and even > lighting to fool AF. Since all AFs are created equal, it's completely unimportant to state the AF system and age used. -Wolfgang > Remember auto-focus is a contrast detection system, if there's little or no > obvious 'edges' under the focus points, its will fail. Often the trick can > be to turn off the multi-point focus system and just use the central sensor, > point that at an obvious edge at the distance you want to focus on; half > press the shutter to let the camera focus then reframe. You are so correct! > Like everything, it takes a bit of getting used to. A lot of- at first. SignatureReality is a picture perfected and never looking back. >Remember auto-focus is a contrast detection system, No, it is not. On dSLRs as the OP is talking about it's phase detection. jue >I am new to the auto-focus world, both my film cameras are manual focus. > I just got my hands on my sister's Digital Rebel XT (DRXT) to just learn [quoted text clipped - 21 lines] > > Steve Obviously, the more you pay, the more you get. But, when it comes to autofocus, the digital Rebel is one of the better cameras on the market -- especially in its price range. Given that the Rebel XT is a Canon, you can use older EF lenses on it which have an autofocus switch; it's located near the camera body on the left side of the lens. (I'm not sure if the newer FS lenses have that switch or not.) You can just turn off the autofocus mechanism and focus manually. But, there won't be a focusing scale that is of much use to you. My wife uses that camera a lot, and her only complaint is that the viewfinder is a bit dimmer than our old A2's viewfinder, but that is usually only a problem in very low light conditions when manually focusing. Also, you might want to check if the AF assist light is turned on. The AF assist light really improves the cameras performance in low light conditions. Hope this helps, Dudley >I am new to the auto-focus world, both my film cameras are manual focus. >I just got my hands on my sister's Digital Rebel XT (DRXT) to just learn [quoted text clipped - 3 lines] >On the DRXT it sometimes goes into fits zooming in and out then refuses >to snap a picture. This can happen if there isn't enough light or the lens isn't fast enough. Most manufactures explicitely limit AF to lenses faster than approx. f/5.6. > I find the auto-focus slow and a pain in the a.s. Normally autofocus on a dSLR is almost instantanious, at the order of a fraction of a second. And normally they don't hunt either because of the phase detetction system used, which allows a direct, non-incremental adjustment. >I understand that the speed of auto-focus can be a function of the lens >and the effectiveness on the amount of light (if I understand this >correctly) Quite so. Unfortunately you don't tell us anything about what lenses you are using. There is quite a difference between a 20+ year old screw-driver type and a modern ultra-sonic AF. Or are you by any chance using auto-focus with a closed apperture? That would explain your problems, too. Of course, you can always turn off AF easily enough with the flip of a switch. jue > This can happen if there isn't enough light or the lens isn't fast enough. > Most manufactures explicitely limit AF to lenses faster than approx. f/5.6. [quoted text clipped - 4 lines] > phase detetction system used, which allows a direct, non-incremental > adjustment. The seeking attribute of AF even with a slower lens, is greatly improved by a TTL flash, I have a blast painting people with the ir grid using the off camera cord. SignatureReality is a picture perfected and never looking back. > Quite so. Unfortunately you don't tell us anything about what lenses you > are using. There is quite a difference between a 20+ year old > screw-driver type and a modern ultra-sonic AF. > Or are you by any chance using auto-focus with a closed apperture? That > would explain your problems, too. Sorry didn't mean to skimp on the details. I believe this is a kit lens, EF-S 18-55mm 3.5-5.6 II. I was loaned another lens but for what I am doing (reading through a magic lantern guide and testing all the features) I have not had cause to use it; a 70-200 IS lens. It is a better lens but for indoor dog shots I thought it was overkill. --Steve > Quite so. Unfortunately you don't tell us anything about what lenses you are > using. There is quite a difference between a 20+ year old screw-driver type > and a modern ultra-sonic AF. Canon EOS doesn't have screw-driver (AF motor in the body). Nikon has such lenses. > Or are you by any chance using auto-focus with a closed apperture? That > would explain your problems, too. Again, that would be ... somewhat hard with a Digital Rebel. > Of course, you can always turn off AF easily enough with the flip of a > switch. Or move the AF from half-press-shuttor to another button and use FTM lenses. -Wolfgang Manual focus is a wonderful tool. Unfortunately the viewing systems in most autofocus SLRs, film and camera, are not made for critical manual focusing. Most have at best a ground glass surface that tends to make thinks look focused that may not be. Also because many autofocus zooms have a maximum aperture of f4 and go down to f5.6 at the long end the viewing image may be a bit dim. The manual focusing cams on many autofocus lenses do not have the firmness of manual focus lenses either but there are not many complaints about slipping out of a manually adjusted position. Some dSLRs, like the otherwise great D70, are severely hampered by the dim small view they give the user. Modern SLR autofocus under most circumstances however is probably more accurate than manual focus overall under most conditions. Adding facial recognition algorithms may also be an advance, although I have not experimented much yet with my camera that has this feature. > Modern SLR autofocus under most circumstances however is probably more > accurate than manual focus overall under most conditions. Adding facial > recognition algorithms may also be an advance, although I have not > experimented much yet with my camera that has this feature. That depends. I have a split prism screen in my D200 and find that manually focusing is more accurate if I am not tired, provided I use a lens with a reasonably long focus throw. For example, with a manual focus 28mm lens I have I get good accuracy and repeatability, while with a tamron 17-50mm which I also have it's practically impossible to focus reliably because of the short throw of the focus ring (and the fact that it has some play). Of course autofocus doesn't get tired and works at lower light levels than I can focus in. It's also much faster than me. So both systems have advantages. And certainly autofocus is good enough for most things, and the only way for some. |
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