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Thread: Manual Flash Exposure with Zoom lenses ??

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    Manual Flash Exposure with Zoom lenses ??

    ok this might sound like silly question for the more experienced flash gurus among us but here it is anyway ..

    Lets say Im using off camera strobes and a zoom lense, my lens is a 18-70 and I am at a focal length of 35mm and my shutter speed is set at 1/125th. I take an incidental meter reading with a handheld meter and the right exposure is for eg. f11 ..

    Now my question is if I start shooting pictures at the correct exposure and start zooming my lens and changing focal lengths, will this affect the exposure ??
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    No, focal length should not effect exposure

    On TTL in Nikon land, it will effect flash output though
    Darren
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    This would depnd on the lens that your using. If the lens is a constant f stop then no it won't but if your lens is a say a lower quality with a variable f stop compared to zoom length say f4-5.6 for instance then yes it must.

    Then again if your using CLS you wouldn't have to worry at all would you.

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    Quote Originally Posted by kiwi View Post
    No, focal length should not effect exposure

    On TTL in Nikon land, it will effect flash output though
    thats kinda what I was getting at Darren .. my example was of using an off camera strobe in full manual mode and using triggers .. No TTL readings at all .. if my camera is in manual mode and set to nominated SS (1/125th) and nominated aperture (determined by incidental meter reading) .. These values will remain constant regardless of whether I zoom the lens or not. Will my exposure change despite my varying focal length ?? I wouldnt think so, but im not sure, thats why im asking.

    and no Mark in this example the lens is not a constant 2.8 ... so using this same lens in semi manual mode, A or S, or Tv or whatever, as you zoom it the max aperture at any given focal length would vary .. im wondering if this same thing occurs, and whether it has an effect on your exposure youve determined and set from your incidental meter reading ??

    am i making sense here ??
    Last edited by bigdazzler; 08-07-2009 at 8:01pm.

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    AS you zoom you are changing the lens's ability to deal with light so something must change by at least one stop which is the variation in the lens.

    Maybe I'm wrong - I keep getting a pain in the head thinking about this issue as I'm going round and round in circles thinking about equivalents.

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    You're using your flash to light a subject, nothing you do to your camera will effect that light, you can only effect your camera's ability to deal with that light. As mentioned before if you don't have a constant aperture zoom lens you may not be able to obtain the fstop you had previously set at a wider angle. But essentially the light is the same, therefore the exposure is the same.

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    Mark, you are thinking along the right lines, but winding up with the wrong answer. The key here is that the f ratio is indeed a ratio.

    To keep things simple, let's start by thinking about a constant aperture zoom - f/4, let's say. The absolute aperture - the actual size of the hole the light passes through, measured in mm - does indeed change as you zoom. However, we are not concerned about the absolute aperture, we are only concerned about the amount of light falling on each square mm of the sensor, and that is determined by the ratio between the absolute aperture and the focal length. This the key figure we need to pay attention to: the relative aperture, better known as the f/ number. Notice that "/" after the "f" - it is written that way to remind us that isn't a measurement in itself, it is the focal length divided by the absolute aperture.

    In other words, yes, the lens' ability to transmit light may or may not change, depending on the design of the lens, but if it does change, the changed f/ratio reflects that.

    The upshot is that, provided we are shooting at the same f/ratio (f/4, let's say), focal length has no effect at all on exposure.

    Of course, distance does have an effect if you are shooting with flash, but the only distance of interest is the distance between the flash gun and the subject. The distance to the camera is not relevant. If we shoot a given subject at 1/125th and f/11 with a 50mm lens and the exposure is correct, provided we don't move the flash head, we can also shoot the same subject at 1/125th and f/11 using a 500mm lens.
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    Quote Originally Posted by Dalzine View Post
    You're using your flash to light a subject, nothing you do to your camera will effect that light, you can only effect your camera's ability to deal with that light. As mentioned before if you don't have a constant aperture zoom lens you may not be able to obtain the fstop you had previously set at a wider angle. But essentially the light is the same, therefore the exposure is the same.

    Dale
    so in a nutshell .. the answer is NO. I can fire the flash at my meter, take a reading, set my camera to manual with a shutter speed no higher than the flash sync speed and aperture already determined from the meter reading, and then fire away at any focal length within my zoom range provided I dont alter the distance of the flashhead to subject, and it will not alter my exposure ??

    Real life scenario .. I shoot a correct exposure at 35mmm 1/125th and f11,and it looks spot on on the histogram, then zoom in to 70mm, take the same shot and the histogram should be identical ??

    yea ??

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    yea, as long as your lens has not changed focal length because it's variable, is it ? what lens ? Is it f/11 at 35mm and at 70mm ? Check the EXIF.

    I assume you are getting differing exposures for some reason ?

    Exposure is determined by only 4 factors

    Light
    ISO
    S/S
    Apperture

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    Quote Originally Posted by bigdazzler View Post
    so in a nutshell .. I can fire the flash at my meter, take a reading, set my camera to manual with a shutter speed no higher than the flash sync speed and aperture already determined from the meter reading, and then fire away at any focal length within my zoom range provided I dont alter the distance of the flashhead to subject, and it will not alter my exposure ??
    Yes.

    On the Canon cameras - I can't imagine that Pentax, Nikon, etc do this any different - you can zoom as much as you like in either direction and the aperture doesn't change unless you have selected an aperture that the lens can't do at all focal lengths, in which case it selects the closest apeture it can do.

    So at f/11, it will never change at any focal length.

    With, say, an f-3.5-5.6 lens, if you set f/8 it will never change.

    If you set f/5.6 when zoomed in, the camera takes that as a hint that you want the lens to be wide open, so as you zoom out, it will go to f/4 and eventually f/3.5.

    If you set f/5.6 when not zoomed in (i.e., at any focal length where you can have less than f/5.6) the camera takes that as a hint that you want f/5.6 and does not change it no matter what you zoom to.

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    Quote Originally Posted by kiwi View Post
    yea, as long as your lens has not changed focal length because it's variable, is it ? what lens ? Is it f/11 at 35mm and at 70mm ? Check the EXIF.

    I assume you are getting differing exposures for some reason ?

    no Darren, quite the opposite .. during a quick play today with my new studio flash and using my Sony 18-70 3.5-5.6 kit lens I found that after I set my exposure at one focal length and took the shot, I could then zoom my lens, change the focal length and take the same shot and nothing changes .. the histogram and exposure was identical.

    Im gonna have a good play tomorrow and test this out again, ill try and post some shots with exif info included.

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    Thanks Tony

    I think I have the concept now.

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    Quote Originally Posted by Tannin View Post
    Yes.

    On the Canon cameras - I can't imagine that Pentax, Nikon, etc do this any different - you can zoom as much as you like in either direction and the aperture doesn't change unless you have selected an aperture that the lens can't do at all focal lengths, in which case it selects the closest apeture it can do.

    So at f/11, it will never change at any focal length.

    With, say, an f-3.5-5.6 lens, if you set f/8 it will never change.

    If you set f/5.6 when zoomed in, the camera takes that as a hint that you want the lens to be wide open, so as you zoom out, it will go to f/4 and eventually f/3.5.

    If you set f/5.6 when not zoomed in (i.e., at any focal length where you can have less than f/5.6) the camera takes that as a hint that you want f/5.6 and does not change it no matter what you zoom to.

    Thanks Tony .. great explanantion. Much appreciated.

    So basically if I set an aperture beyond 5.6 in initial exposure, this will never change despite my chosen focal length. Got it.

    ok another question more a long the lines of my OP .. if I were to have my lens at 70mm and aperture at 5.6 (wide open) for correct exposure, say i zoomed back to a wider angle, say 28mm, and the lenses max aperture is now f4 , this will affect my exposure ?? Theoretically, if exposure was correct at 5.6 @ 70mm, zooming back to 28mm @ f4, the shot would be overexposed by 1 stop yea ??
    Last edited by bigdazzler; 09-07-2009 at 7:47am.

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