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Thread: Flash, low light & long DoF question

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    whited3's Avatar
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    Flash, low light & long DoF question

    I was given a used (but basically new) Canon 420EX speedlite which I've hardly used. But of course Xmas day came around and all of a sudden I'm under enormous pressure to take some good, indoor pics

    Our table set up was ~ 7m long and I wanted to capture everyone, shooting down the length of the table. So standing 1.5m back from one end I wanted a DoF of say 7m+ using a 500D @ 18mm.

    P mode locked the shutter speed to 60 and the aperture to f4, thus killing the DoF and blurring everyone at the end of the table.

    Av mode at f11 for the DoF drove the shutter speed down too low and all I got was camera shake.

    I did manage to get some half decent shots but it was luck and the camera's IS system, not skill. Practising later I found that I could take the perfect shot in M mode at ISO Auto (400), 1/60, f11 and let the flash sort it's levels out. Bounce off the ceiling seemed best.

    So in this situation, is M mode the way to go?
    I assume (??) that the flash will give me the equivalent of 2 stops more light max, which means I can compensate down 2 stops in aperture to gain the DoF I may want?
    Should ISO be on auto?

    Cheers.

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    Re: Flash, low light & long DoF question

    Quote Originally Posted by whited3 View Post
    I was given a used (but basically new) Canon 420EX speedlite which I've hardly used. But of course Xmas day came around and all of a sudden I'm under enormous pressure to take some good, indoor pics

    Our table set up was ~ 7m long and I wanted to capture everyone, shooting down the length of the table. So standing 1.5m back from one end I wanted a DoF of say 7m+ using a 500D @ 18mm.

    P mode locked the shutter speed to 60 and the aperture to f4, thus killing the DoF and blurring everyone at the end of the table.

    Av mode at f11 for the DoF drove the shutter speed down too low and all I got was camera shake.

    I did manage to get some half decent shots but it was luck and the camera's IS system, not skill. Practising later I found that I could take the perfect shot in M mode at ISO Auto (400), 1/60, f11 and let the flash sort it's levels out. Bounce off the ceiling seemed best.

    So in this situation, is M mode the way to go?
    I assume (??) that the flash will give me the equivalent of 2 stops more light max, which means I can compensate down 2 stops in aperture to gain the DoF I may want?
    Should ISO be on auto?

    Cheers.
    Hi Mark,

    You've got a lot going on with your setup for an indoor photo shoot. Please add information regarding the type of lighting used (incandescent, flourescent, etc.), this will help assist you with any color cast issues and possibly any ISO noise effects you may have endured. A good starting point is AUTO for white balance. You can correct for color cast after a few shots.

    ISO-any setting in low lighting will increase the shutter speed and create out of focus images due to camera shake. You need a tripod for long exposures.

    Any flash, from a speedlight or on camera has its desired and undesired effects, red-eye, blaring color and exposure, you need to look at your manual for flash output compensation (flash sync) and see what occurred in your photos that you would want to eliminate and what you want to emulate.

    The undesired effect that I try to avoid is red-eye and it can be controlled either with your flash setting or post processing, it looks like in-focus images is your main dilemma and this can be controlled through camera stabilization, aperture setting, and flash control.

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    Re: Flash, low light & long DoF question

    Hi Mark,

    I think you've come across the classic "something has to give" "trilogy" of photography. "P" mode won't let the shutterspeed drop below 1/60th (it's an arbitary "camera shake" speed) - so - to get enough light in you open the aperture, and "bang" goes your depth of field.

    Some things that might help ...

    1. Max the ISO out or set it very high (say 1600 or 3200). Yes, this will increase noise, but noise is the lesser of 3 evils (better than camera shake or insufficient depth of field).

    2. Stick the camera in Av mode and choose a "sensible" aperture (I'd probably start at about F11 and see how it went)

    3. Back away from the subjects. The further away you are, the more your DoF will increase in this situation, and focus about 1/3 into the scene.

    If the shutterspeed is too low then going manual might work better; it'll let in less ambient light (if you stick with same iso / aperture, but faster shutterspeed), and you'll be relying more on the flash output to balance the exposure.

    Basically what happens when you use Av mode with flash is that the camera will try to secure a correct exposure in what it perceives to by the foreground zone using the flash, and use ambient light to balance out the "background". Unfortunately, if this means selecting a shutterspeed that results in camera shake then "so be it" - so in low light situations a tripod is your best friend.

    Unfortunately, there are a couple of other "gotchas" - (a) the 420EX is pretty limited for bounce shots (and that's probably your best option from what you've described) and (b) you may run into issues with colour casts due to mixed temperature lighting.

    Getting a professional quality shot in those circumstances is a LOT easier if you have the gear ... if I were doing it I'd probably use about 4x gelled 580EX II flashes on stands into shoot through umbrellas, and a tripod, or even a couple of studio strobes, but without the gear I think it's going to be a compromise at best.

    Hope this helps!

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    Re: Flash, low light & long DoF question

    Mark,

    Your 420EX uses TTL metering. This means that it will do a pre-shot flash to measure the expose using your camera settings and then automatically adjust its output as needed. You'll have to check your camera manual to find out the fasted flash sync speed that it's capable of. This is the fastest speed that the curtain is fully open during the exposure. At faster speeds the curtain will open as a narrow slot that passes over the frame. The maximum normal sync speed on my Nikon D300 is 1/250 but I have to use M mode to get that. The speed is automatically set to 1/60 in A and P mode. Your Canon is probably the same. I would try setting everything to manual including the ISO and white balance to flash and do some test shots and see what works best for you. You can use a bounce flash against the ceiling if it's white otherwise you'll introduce a color cast that will be hard to correct for. The next step up would be to get some color correcting gels to put over the flash to correct it for the ambient light. This will give the flash a tint to match either florescent or incandescent lights. You're white balance should be set accordingly.

    -Brad

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    Re: Flash, low light & long DoF question

    Thanks Folks. As said, I should have worked it our prior to the shot, not after No matter, PP was my Friend in the end and I'm now somewhat wiser re the use of the flash.

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    Re: Flash, low light & long DoF question

    Quote Originally Posted by bpdean View Post
    Your 420EX uses TTL metering.
    * ETTL II (reflected light doesn't bounce off sensors the way it did off film )

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    Re: Flash, low light & long DoF question

    i dont know much at all about flash, i would have thought that by putting the camera on manual mode and using ttl the camera would work out what power of flash to use for correct exposure ?
    is there a serious pitfall by doing it this way ?
    its something i need to understand too, i dont like flash because i dont understand it, cheers martyn

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    Re: Flash, low light & long DoF question

    Quote Originally Posted by nomadr View Post
    i dont know much at all about flash, i would have thought that by putting the camera on manual mode and using ttl the camera would work out what power of flash to use for correct exposure ?
    is there a serious pitfall by doing it this way ?
    its something i need to understand too, i dont like flash because i dont understand it, cheers martyn
    Hi Martyn,

    Short answer is "yes" and "no".

    The problem is that the intensity of light falls off with the square of the distance from the source, so although the camera / flash will do it's best to work out the correct exposure, this will only be for what it perceives to be the foreground zone.

    If for example you have a person standing 3 metres in front of a background then that's an easy one ... but if it's a group of folks ranging in distance from near to far then that presents a problem because (a) the light required to correctly illuminate the person in the foreground is vastly different to the amount required to illuminate the person in the background, and (b) the camera has no way of knowing what's important and what's not anyway (other than focal distance information which biases the exposure with ETTL II) (assuming Canon here). About all you can do is increase the distance from the flash to the first person so that the amount the light falls off is less pronounced between the first and last person (which already decreases the effectiveness of an already small flash) (but would be partly compensated for by the use of a high ISO).

    If you want to learn more, then the definitive guide is Mastering Canon EOS Flash by N K GUy (he was kind enough to send me a free autographed copy) - the next best (and free) resource on the topic is his online "book" (more a collection of notes) to be found at photonotes.org.

    Does this help?
    Last edited by Colin Southern; 29th December 2010 at 06:39 PM.

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    Re: Flash, low light & long DoF question

    thanks colin, i am a nikon user, i understand that shooting a group near and far is going to result in some people being blown out and others left in the dark when using a direct flash,
    a wide open aperture will let in more ambient light but thats not so good for dof, good tip about standing further back, cheers martyn

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    Re: Flash, low light & long DoF question

    Quote Originally Posted by nomadr View Post
    thanks colin, i am a nikon user, i understand that shooting a group near and far is going to result in some people being blown out and others left in the dark when using a direct flash,
    a wide open aperture will let in more ambient light but thats not so good for dof, good tip about standing further back, cheers martyn
    No worries Martyn,

    Unfortunately it's often one of those "how long is a piece of string" type things, with many variables. With minimal equipment I think the best solution is high ISO - reasonable aperture - tripod - and "brief the subjects to be as still as possible". A flash might add something, but may just as easily cause additional issues.

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