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Photo Forum / Digital Photography / DSLR Cameras / April 2007Lighting questions
Okay guys, here is what I have: Canon Rebel XT, 430EX Flash, two Smith-Victor 110i monolights with a white reflective umbrella, a silver on black reflective umbrella, and a shoot through umbrella, a "camera flip" stroboframe, and a Interfit Flash meter . I need to trigger strobes with either on camera flash or 430EX. I will be taking photos of from 1 person to a small group. Is it possible to set up lights in one place and not move them? Any suggestions on camera settings? Any input would be greatly appreciated, this will be my first time using strobes. Tx blah > Okay guys, here is what I have: Canon Rebel XT, 430EX Flash, two > Smith-Victor 110i monolights with a white reflective umbrella, a silver on [quoted text clipped - 4 lines] > them? Any suggestions on camera settings? Any input would be greatly > appreciated, this will be my first time using strobes. Tx blah You will have to use manual settings on the camera to avoid the pre-flash that Canon (and Nikon) dslrs fire to measure the exposure a fraction of a second before the main flash. Slaves on monolites or other discrete units will cause the slave flashes to fire on the pre-flash. Alternatively, I believe there are trigger sensors for firing remote flashes that ignore the first flash and fire on the main flash. You will have to research where those can be bought. Colin D.  SignaturePosted via a free Usenet account from http://www.teranews.com > You will have to use manual settings on the camera to avoid the > pre-flash that Canon (and Nikon) dslrs fire to measure the exposure a > fraction of a second before the main flash. Nope: He will have to use manual settings on the _flash_ to prevent the pre-flash. Manual setting on the camera is to set the aperture as needed and shut out ambient light (short exposure). -Wolfgang >> You will have to use manual settings on the camera to avoid the >> pre-flash that Canon (and Nikon) dslrs fire to measure the exposure a [quoted text clipped - 7 lines] > > -Wolfgang I'd like to add a little side note: With a Speedlite 550EX or 580EX the camera will always emit a pre-flash if set to master mode (it uses this to control speedlite slaves). I appreciate that this doesn't apply to the OP's situation, but thought it might help others. >> You will have to use manual settings on the camera to avoid the >> pre-flash that Canon (and Nikon) dslrs fire to measure the exposure a [quoted text clipped - 7 lines] > > -Wolfgang Nope again. The eos cameras fire a pre-flash from the both the pop-up flash and the hot-shoe flash, whichever is in use, for all settings except manual on the camera. My 420EX flash does not have a way of turning off the pre-flash; it just does what the camera tells it to do. Setting the manual exposure to the flash meter readout for aperture, and a fast enough speed to cut out ambient light is the right way to do it if you are using non Canon flash lights. If you are using all Canon flashes for the slaves, then I understand the slaves emit a preflash, enabling the camera to calculate the exposure, as for a single flash. Colin D. SignaturePosted via a free Usenet account from http://www.teranews.com >>> You will have to use manual settings on the camera to avoid the >>> pre-flash that Canon (and Nikon) dslrs fire to measure the exposure a >>> fraction of a second before the main flash. >> Nope: He will have to use manual settings on the _flash_ to >> prevent the pre-flash. >> Manual setting on the camera is to set the aperture as needed >> and shut out ambient light (short exposure). > Nope again. The eos cameras fire a pre-flash from the both the pop-up > flash and the hot-shoe flash, whichever is in use, for all settings > except manual on the camera. Interestingly, this is not the case. The DSLR EOS-cameras fire a pre-flash not only on P, Av and Tv, but also on M.[1] > My 420EX flash does not have a way of > turning off the pre-flash; it just does what the camera tells it to do. And to be able to tell the flash anything, a pre-flash is needed ... > Setting the manual exposure to the flash meter readout for aperture, and > a fast enough speed to cut out ambient light is the right way to do it > if you are using non Canon flash lights. You mean "manual flash units". You can set e.g. the 550EX to manual mode and select the flash power from 1/1 to 1/128 in full stops. But I start repeating myself. -Wolfgang [1] It's trivial to check: if you see a flash through the view finder, then the mirror must be down, and thus it's a pre-flash. You may also want to bookmark http://photonotes.org/articles/eos-flash/ : >>> You will have to use manual settings on the camera to avoid the : >>> pre-flash that Canon (and Nikon) dslrs fire to measure the exposure a [quoted text clipped - 19 lines] : And to be able to tell the flash anything, a pre-flash is : needed ... If I'm not mistaken, preflashes are a rather recent invention. In earlier times, you dialed the film speed into the flash unit, which monitored the light reflected off the subject during the actual flash and decided when to shut down. (At least that's what the literature advertised, and I had a flash unit for my Nikon film cameras that claimed to work that way.) It may be that modern flashes are too fast for that; in olden days, flashes couldn't be faster than around 1/1500th of a second or the film wouldn't respond. : > Setting the manual exposure to the flash meter readout for aperture, and : > a fast enough speed to cut out ambient light is the right way to do it [quoted text clipped - 9 lines] : view finder, then the mirror must be down, and thus it's : a pre-flash. But the converse isn't true. The preflash doesn't have to fire with the mirror down; it just has to fire before the shutter opens. There's at least an order of magnitude difference between the time it takes to get the mirror out of the way and the time it takes to fire a preflash. : You may also want to bookmark : http://photonotes.org/articles/eos-flash/ > But the converse isn't true. The preflash doesn't have to fire with the > mirror [quoted text clipped - 3 lines] > the > way and the time it takes to fire a preflash. A retracted mirror blocks the metering sensor - so pre-flash has to fire before mirror retraction. > On Fri, 23 Mar 2007 13:58:25 +0100, Wolfgang Weisselberg > If I'm not mistaken, preflashes are a rather recent invention. A recent neccessity, since there is no other way to do TTL flash metering. Film reflects diffusely (think whitecard), so a sensor can meter off the film, but CCDs and CMOS reflect directly (think mirror), and thus you get spurious readings from reflections. > In earlier > times, you dialed the film speed into the flash unit, which monitored the > light reflected off the subject during the actual flash and decided when to > shut down. Ah, an automatic flash. Yes, you can use that even today to good effect, but it'll only meter averages, won't think about distances if the lens reports them and you have to dial in the aperture, as well. (OK, the data can be transmitted via the extra contacts in the flash shoe, _if_ you know the protocol.) And even earlier you could only set the flash manually, or not at all. > It may be that > modern flashes are too fast for that; in olden days, flashes couldn't be > faster than around 1/1500th of a second or the film wouldn't respond. That is a problem of chemical films (they don't react 'normally' to very short or very long exposure times) >: [1] It's trivial to check: if you see a flash through the >: view finder, then the mirror must be down, and thus it's >: a pre-flash. > But the converse isn't true. Irrelevant: the claim was "set the camera to 'M' to avoid preflashes", and this disproves the claim. > The preflash doesn't have to fire with the mirror down; Except for the fact that's the only way to meter the returning light properly in DSLRs, yes. I seem to remember there was another Canon flash, some ages ago, which pre-flashed to be able to give "out of range" warnings. > it just has to fire before the shutter opens. There's at least an order > of magnitude difference between the time it takes to get the mirror out of the > way and the time it takes to fire a preflash. True, but irrelevant, unless you are talking about "tell the slave flashes how much to flash, after we collected their pre-flash results". -Wolfgang > If I'm not mistaken, preflashes are a rather recent invention. In earlier SLR Pre-flash metering began as a means to improve the flash exposure even before digital cameras required it. I'm probably off on the dates but IIRC it began in the late 1990's. > times, you dialed the film speed into the flash unit, which monitored the > light reflected off the subject during the actual flash and decided when to > shut down. (At least that's what the literature advertised, and I had a flash > unit for my Nikon film cameras that claimed to work that way.) That's what TTL flash is about: Stopping the flash when enough light has bounced off of the film to indicate a nominal exposure. This is done via a transistor called a thyristor which opens the circuit to the flash tube very quickly. It may be that > modern flashes are too fast for that; in olden days, flashes couldn't be > faster than around 1/1500th of a second or the film wouldn't respond. Not sure what you mean by "olden days" but films since WW II should respond to 1/50,000 or slower flash periods if enough photons struck. After all, they did. Typically, however, a high end attached flash in usual conditions (f/4 - f/8, ISO 400 - 100, range of 8 - 20 feet) will output 1/8 to all of their energy and this can take 2ms or more (1/500 or slower). > The preflash doesn't have to fire with the mirror > down; it just has to fire before the shutter opens. There's at least an order > of magnitude difference between the time it takes to get the mirror out of the > way and the time it takes to fire a preflash. What really matters is total shutter lag. The order of events is otherwise irrelevant. The major contributor to lag is the mirror. Sans mirror and a decent SLR can do about 6 ms or better. With the mirror, the fasted SLR's get about 40 - 45msec. However, the main reason the pre-flash fires "mirror down" in most (all?) DSLR's is to use the same exposure sensors as available light for the measurement. These sensors are up in the prism area or behind the mirror (in a semi-silvered area). The SLR OTF TTL-flash sensors could be used with the mirror up and shutter closed by perhaps adding white or grey to the lens side of the shutter curtain. (Similar to the white circle on the shutter curtain of the Leica M6 for available light metering). OTOH, by not adding OTF sensors and doing both available light and flash pre-metering with the prism area (or behind the mirror) sensors the cost of the system is reduced. All of the SLR makers are working hard to absolutely minimize cost. 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. > That's what TTL flash is about: Stopping the flash when enough light has > bounced off of the film to indicate a nominal exposure. This is done > via a transistor called a thyristor which opens the circuit to the flash > tube very quickly. My understanding is that a thyristor, due to the avalance breakdown inside, _can_ be triggered into conducting by an appropriate gate signal, but _cannot_ be triggered into non-conducting (unlike a transistor) by any gate signal --- it stays conducting as long as the electric current flows strong enough. (Ok, you could use a thyristor to discharge the capacitator powering the flash or similar ...) Or am I misunderstanding things? -Wolfgang > My understanding is that a thyristor, due to the avalance breakdown > inside, _can_ be triggered into conducting by an appropriate gate > signal, but _cannot_ be triggered into non-conducting (unlike a > transistor) by any gate signal --- it stays conducting as long > as the electric current flows strong enough. Exactly, analogously to the gas tube thyratron. Which my ancient (1962) Dictionary of Electronics says are "very widely used as an electronic switch in control circuits". Those were the days! Mike. SignatureIf reply address = connectfee, add an r because it is free not fee. >> This is done >> via a transistor called a thyristor which opens the circuit to the >> flash tube very quickly. A thyristor is _not_ a transistor, they're different things. > My understanding is that a thyristor, due to the avalance breakdown > inside, _can_ be triggered into conducting by an appropriate gate [quoted text clipped - 6 lines] > > Or am I misunderstanding things? True for a "normal" thyristor but there are a few special types like the GTO Thyristor, IGCT and MCT that can be turned off. http://en.wikipedia.org/wiki/Gate_turn-off_thyristor http://en.wikipedia.org/wiki/IGCT Sander > True for a "normal" thyristor but there are a few special types like the > GTO Thyristor, IGCT and MCT that can be turned off. > http://en.wikipedia.org/wiki/Gate_turn-off_thyristor I cannot see that being used in a flash --- we are talking about 1/1000s to 1/30.000s flash time, and with a TTL-flash you have no advance warning of shut-off-time. With a turn off time of 150 micro-seconds, we are talking about 15% for a full power 1/1000s flash, and 450% of a short low-power flash --- i.e. you'd be off 2 full stops in flash power, if you used a GTO Thyristor as a switch. > http://en.wikipedia.org/wiki/IGCT While they could do 1/30.000s flashes (40kHz) according to this article (and have high losses!), they are not used in Canon flashes supportig 'FP' mode: the flash cycles at 70kHz then, I understand. http://www.allaboutcircuits.com/vol_3/chpt_7/10.html doesn't metion switching times, and I am lazy. -Wolfgang [1] X-Sync time of a Canon 20D is 1/250s --- i.e. this is the shortest time where both curtains are in a full-open position during the shot. So the flash must be faster than that --- and 1/1000s sounds like a sensible number. A 550EX can be set from 1/1 to 1/128th power (manually). Since flashes regulate their power solely via the burn length of the flash bulb, a 1/128th power shot wold be 1/32.000s evn for a 1/250s full power flash ... and 1/128.000s, if the full power flash is 1/1000s. > Okay guys, here is what I have: Canon Rebel XT, 430EX Flash, two > Smith-Victor 110i monolights with a white reflective umbrella, a silver on [quoted text clipped - 4 lines] > them? Any suggestions on camera settings? Any input would be greatly > appreciated, this will be my first time using strobes. Tx blah I don't know the Canon equipment but I suspect that if you mount the flash on the camera, set the camera to Manual (exposure) and the flash to manual and at a lowish power (say 1/8 or 1/16) and point that at the ceiling, it will be enough to trip the slaves (monolights). This assumes, of course, that the monolights have a slave sync. This is all, of course, to avoid the "pre-flash" which will trigger the slaves. The flash power and aperture is set for where the subject is v. the lights. The camera can be anywhere as long as the subject/light position does not change much. Unless you like continuously re-setting your lights and aperture. It gets very tedious. Assuming you set a fill light at 1 - 2 stops lower than aperture and a key light at aperture, you should get some nice shots. With a group you are better having your key light at less of an oblique than with a single person. To meter, simply turn on one light, meter it (say the fill light) for f/4 (dim) and seperately the key for f/5.6 (a bit brighter) and you'll have a simple 1:2 lighting ratio which is fairly low contrast. The key could be with the shoot through or reflected. If you reduce the fill further (f/2.8) then you'll be at 1:4 ratio and get more contrast. To meter the lights, go to the subject position and point the dome at the light being set. Measure them independantly. To verify overall setting (with both lights on) point the dome (from the camera position) at the camera lens position. For the above, that should yield f/5.6. I shot a stage show using several lights. I set the strobes and then measured with my meter. During the shooting I varies aperture according to subjects on the stage. I had very few missed shots. Have fun. At least with digital you can experiment your way to effective exposures in a short session. 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. Thanks Guys for your help! It seems F8 is the best I am getting. I practiced a little today. Should I move the light closer and try again? Is there some other trick? Tx blah > Okay guys, here is what I have: Canon Rebel XT, 430EX Flash, two > Smith-Victor 110i monolights with a white reflective umbrella, a silver on [quoted text clipped - 4 lines] > move them? Any suggestions on camera settings? Any input would be greatly > appreciated, this will be my first time using strobes. Tx blah > Thanks Guys for your help! It seems F8 is the best I am getting. I > practiced a little today. Should I move the light closer and try again? Is > there some other trick? Tx blah Assuming the flash is in manual, you should be able to set its power in whole stop increments (1/1, 1/2, 1/4, 1/8...) and thus get to shallower apertures. 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. Thanks Alan >> Thanks Guys for your help! It seems F8 is the best I am getting. I >> practiced a little today. Should I move the light closer and try again? [quoted text clipped - 6 lines] > Cheers, > Alan |
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