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Thread: Exposure and metering in Av mode with popup flash

  1. #1

    Exposure and metering in Av mode with popup flash

    Hello all
    Long-time reader, first-time questioner.

    I wonder if anyone can help me understand the following phenomenon please.
    When I use the popup flash on my Canon 550D in P (program auto-exposure mode), it uses a (fixed) shutter speed of 1/60 and it varies the ISO and aperture for the correct exposure. This is fine.
    But when I go to Av mode (aperture-priority), set the same ISO and aperture as was previously selected, the camera seems to ignore the fact that the flash is up and sets exposure with very slow shutter speeds. If I flip down the flash and snap the same scene, the camera will set the same shutter speed as when the flash was up. It's as if it does not care that the flash is there when in Av.
    Why's that please? Is it because it's assuming I want only auxiliary fill-in flash rather than using it as the primary light source? If so, am I more-or-less limited to P mode when I need flash?

    Many thanks

    Daniel

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    Melkus's Avatar
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    Re: Exposure and metering in Av mode with popup flash

    Think I'm reading your question right so I will give it a try. That is how Canon EOS cameras are designed to work. P, Av, Tv and M modes all meter for flash in different ways. Keep in mind that the camera meters for ambient (existing) light conditions and flash illumination independently.

    P (program) mode keeps the shutter speed between 1/60 sec and the maximum flash sync speed your camera can handle. It does this so that you shouldn’t need a tripod, even if light levels are low. It then tries to illuminate the foreground using flash.

    Av (aperture priority) and Tv (shutter speed priority) modes set the shutter speed or aperture to expose for the existing light conditions correctly. They then fill in the foreground using flash. If light levels are low you will need a tripod to avoid blur.

    M (manual exposure) mode lets you set both aperture and shutter speed to be whatever you want. The camera then automatically controls the illumination of the foreground subject using flash.

    Not sure this help much, maybe someone who uses a Canon more than I have will chime in.

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    Re: Exposure and metering in Av mode with popup flash

    Page 82 of the English 550D manual explains the behavior of Av mode with flash.

    Page 79 gives a somewhat limited explanation of Tv mode with flash.

    Page 64/65 goes into flash use with P mode, including information that is useful for other modes.

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    Re: Exposure and metering in Av mode with popup flash

    In basic terms
    The default in AV mode is to set the shutter speed for the background and use the flash for fill in such as often used in portraits. However in poor lighting to expose the background correctly will involve long shutter speeds. Under the speedlite option in menu is the option to change this default to either a fixed shutter speed or a range that stops at normally 30th second, so camera remains hand holdable for shooting.

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    Melkus's Avatar
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    Re: Exposure and metering in Av mode with popup flash

    Here something I dug up.

    P mode: Shutter speed is automatically set from 1/60 sec to the camera’s maximum X-sync speed. The lens aperture is automatically set according to the camera’s built-in program.

    Tv mode: You can set any shutter speed between 30 seconds and the camera’s maximum X-sync speed. The lens aperture is automatically set to match the shutter speed you have set.

    Av mode: Shutter speed is automatically set between 30 seconds and the camera’s maximum X-sync speed to match the lens aperture you have set. You can set any lens aperture you like.

    M mode: You can set any shutter speed between 30 seconds and the camera’s maximum X-sync speed. You can set any lens aperture you like.
    Last edited by Melkus; 4th January 2013 at 07:48 PM.

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    Re: Exposure and metering in Av mode with popup flash

    There are potential dangers in all these settings, Daniel.

    P mode, makes all the decisions so you risk having too slow a shutter speed for the scene (subject movement etc) and a wide open aperture which gives a very shallow depth of focus.

    Tv allows you to control the shutter speed up to the max allowable (usually 1/250) but there is a risk of a wide open aperture and loss of focus depth. But can be useful for moving subjects.

    Av allows you to control the aperture but at the expense of shutter speed. So you can end up with very slow shutter speeds which are too slow for handheld shots, or subject movement. But can be useful for static subjects when using a tripod.

    M allows you to be in full control of shutter speed and aperture. Also Iso, unless you are set to auto Iso. But note the max shutter speed warning. This is the method I use. The danger is that you need to be fully aware of what the scene requires in the way of optimum settings.

    Some experimentation and use of flash output compensation may be required. These settings may need variation as you move between different scenes. But once you master the technique, shooting with manual camera settings and using the flash TTL control can give the best, and most controllable results. Particularly when shooting directly towards the subject.

    Bouncing the flash etc is another situation but this only applies to tiltable external flash units.

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    William W's Avatar
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    Re: Exposure and metering in Av mode with popup flash

    Quote Originally Posted by dc197 View Post
    I wonder if anyone can help me understand the following phenomenon please.
    When I use the popup flash on my Canon 550D in P (program auto-exposure mode), it uses a (fixed) shutter speed of 1/60
    No it doesn’t. In P Mode the 550D will choose a Shutter Speed between 1/60th second and 1/200th second.
    1/200th second is the camera’s MAXIMUM FLASH SYNC Shutter Speed.

    You only have experienced the camera using 1/60th second, because you have been shooting in lighting situation where the camera selected that particular Shutter Speed.

    ***

    Quote Originally Posted by dc197 View Post
    . . . and it [the camera] varies the ISO and aperture for the correct exposure.
    Again: No it doesn’t.

    If the ISO is being automatically adjusted by the camera, then you have selected AUTO ISO and that is separate to and NOT a function of you selecting P MODE.

    ***

    Quote Originally Posted by dc197 View Post
    But when I go to Av mode (aperture-priority), set the same ISO and aperture as was previously selected, the camera seems to ignore the fact that the flash is up and sets exposure with very slow shutter speeds. If I flip down the flash and snap the same scene, the camera will set the same shutter speed as when the flash was up. It's as if it does not care that the flash is there when in Av.
    Yes. That describes generally, the Pop up Flash’s functionality and the Camera’s functionality, when the camera is in Av Mode. . . and it is a relatively dark general scene.

    In Av Mode (with any dedicated Flash set in an auto mode) the Camera’s TTL Meter will set the Shutter Speed for the General Scene and the Flash’s Output will fill the FOREGROUND SUBJECTS to the ‘correct’ exposure.

    As you have already found out, if the general scene is a low light level and the camera is in Av Mode, the camera might select a relatively long Shutter Speed which will suit the Aperture and the ISO ypu have sleceted: this is so the GENERAL SCENE is 'correctly' exposed by the AMBIENT LIGHT.

    Also note that, the shutter speed which the camera selects is dependent upon the METERING MODE you have selected.

    ***

    Quote Originally Posted by dc197 View Post
    Why's that please? Is it because it's assuming I want only auxiliary fill-in flash rather than using it as the primary light source?
    Something like that, yes.

    Certainly in BOTH Av Mode and Tv Mode the PuF (Pop up Flash) and any Dedicated Flash in Automatic Mode will act as ‘Flash as Fill’ for the FOREGROUND SUBJECT.

    In P Mode the scenarios get a little more complex as P Mode will act in TWO (actually three) DIFFERENT functionalities - depending upon the Ambient EV (Light Level).

    In Low Level Light (around EV ≤10) in P Mode the Flash will act as the KEY LIGHT attempting to illuminate the Foreground Subject(s) (and sometimes the Middle-ground Subject(s) at the expense of overexposing the Foreground) with the Flash – the Flash is used at an high power and the recycling time can be relatively long.

    In brighter lighting (around EV ≥ 13), the automated system assumes that Flash will be Fill and the Flash is set to a low power and will be metered as ‘Fill’ for the FOREGROUND Subject(s).

    When we get a scene between EV 10 and EV 13 is where some fun can begin: and although this is NOT technically a third functionality of P Mode with PuF – it is can be a tricky area to negotiate as (for one simple example) the Automatic Program may change its mind depending upon the COLOUR of the MAIN foreground Subject – so a WHITE dress and a BLACK suit swapping as the foreground Subject will likely confuse the Automatic Functions of the Camera and the Flash in this Shooting Scenario.

    In P Mode, with the PuF activated you do NOT have access to EXPOSURE SHIFT: however you can use FEC (Flash Exposure Compensation) and also EC (Exposure Compensation) and if you choose to do that, it is wise in most shooting situations, to use BOTH.

    ***

    Quote Originally Posted by dc197 View Post
    If so, am I more-or-less limited to P mode when I need flash?
    However your question asks if you are limited to using only P Mode, with the PuF – and the answer to that question is:
    No. You can use mostly any Mode you wish.

    I assume you are asking about when: 'I really need flash because it is very dark’ – in these cases it would be best to use either P Mode or M Mode.

    M Mode gives you more flexibility more quickly, once you understand it.

    But P mode will give quick results without much thinking and ‘good’ results most of the time.

    ***

    I cannot stress how important the METERING MODE is, to getting the (ambient light) results you require.

    You should take time to understand how the various METERING MODES work and how differently each might meter a dark scene with people in the foreground.

    ***

    In any case, the PuF is arguably a MORE DIFFICULT tool to master than a Dedicated Hot Shoe Flash.

    My general advice would be to seriously consider buying a Dedicated Hot Shoe Flash with Tilt and Swivel like the 580MkII or the 430MkII – as either of these tools will be much easier to use and more powerful and capable of much easier bounce, diffusion and other Light Modifications for very good and rewarding Flash Photography.

    WW

    Addendum:

    I note that your question specifically asks about the Pop up Flash (PuF).

    The PuF (unlike an Hot Shoe Flash) - has no ‘separate controls’ as such.

    This is mentioned as an addendum footnote, because various Flash Controls which are applicable to dedicated Hot Shoe Flash Units, are not applicable to the PuF.
    Last edited by William W; 7th January 2013 at 06:55 AM.

  8. #8

    Re: Exposure and metering in Av mode with popup flash

    Thanks, all, for the replies. There is some excellent, if a little pedantic, information in the responses, for which I am grateful.

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    Re: Exposure and metering in Av mode with popup flash

    Quote Originally Posted by dc197 View Post
    Thanks, all, for the replies. There is some excellent, if a little pedantic, information in the responses, for which I am grateful.
    There isn't a easy way to answer your Q and something like flash can be daunting when first using flash. I hear a lot of people say they won't use flash because they would rather have a natural light image. All well and good when you have light that you can take a image without flash, but to me all they are saying is,"I don't understand how flash works and so I won't use it". When you think that when you are using auto, P, Tv, SV the manufacture whether it be Nikon, Canon or one of the other top brands they put into the camera thousands of variables that you may come across when you select one of the scenerios to take your image.

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