Why I can't understand correct EOS flash exposure

[lightwriting by swapan]

...and YES, I have read the excellent photonotes.org article, I have

even downloaded and printed it for multiple reading.

 

Let me present a scenario please..

 

Camera (1v) is in manual mode, f11, 1/30 sec. Shows 5 stops

underexposure to medium tone. I switch on the flash at full power,

indicator says subject is within range. I shoot, get a totally

underexposed result.

I thought camera told the flash how much light it needed to give a

proper exposure for that setting. Is that true ? then why

underexposure ? Is it because it was out of the range of the poor

flash unit even at its full power to provide enough light ? that

seems logical. Am I right ?

 

Next, in AV mode...

 

I have the subject covering 95% of the frame and there is not much

of background. the exposure says 1.6 seconds on f11.

If I read correctly, the camera goes to evaluative metering and

meters the background and provide fill flash to the foreground

subject(depending on EV or becomes the main light source-which will

be here) to balance the exposure. But if the subject and the

foregrounds are in same light, and

I switch on the flash and meter again why doesn't the exposure

change, it still remains the same 1.6 seconds. What is the flash

doing here ? Will it not give extra exposure with the light ?

remenber, basically no background on this shot, my subject covers

the frame!

Why does the exposure not change when I am providing additional

lihgt fron the flash (which is going to be the main light here?)

 

Now I move to "P" mode. Now the camera, for the same frame, sets 3.5

and 1/60, which is not equivalent to f11 and 1.6 seconds which I got

in AV mode!

 

I am sure I am missing things here. Can somebody please explain or

direct me to some more reading, please ?

 

thanks in advance

[greg_tan]

It's very simple. Program mode merely sets a value which enables you to handhold the shot. If that means an underexposed background, so be it. The flash will expose properly for the subject on the active focus. Your problem with the first scenario is why are you using f11 indoors? I'm assuming you're shooting indoors. You'll get little to no ambient light. Unless you've got a few strobes lying out there it will appear very dark.

[ccrevasse]

I don't know about your manual exposure question, except to say it sounds like you were out of the flash's range. Your AV and P mode questions are explained in the photonotes article and, based on your description, your camera is functioning as expected. Going by memory, in the AV mode the camera assumes the active focus point is the main subject on which flash exposure will be metered, so the camera sets ambient exposure based on areas other than the active focus area under the assumption that those areas are the background. Where the frame is filled by the subject, then, exposure will not change when the flash is turned on in AV mode. In P mode, on the other hand, the camera does not try to expose for ambient light, but rather assumes the flash is the intended main source of light. So the exposure settings in P can be quite different from those in AV (or TV).

[kaustubh]

now that chris has answered one part of you question about Av mode, let me take a shot at the other

 

yes, ur photo can get slightly overexposed if it is light toned. You will have to use some flash exposure compensation. here's the deal...even though ur shutter speed is 1.6 sec, the flash duration may not be that much. flash duration, if this is fill flash mode, will be a short one....majority of the time the shutter is open...flash is not exposing the subject. hence, this does not cause gross overexposure.

 

typically, with fill-flash, u will have to dial in negative flash exposure compensation for light toned objects....a little less for middle toned and not at all for dark objects..or perhaps some positive fec for dark ones.

[sheldonnalos]

One additional piece of information...

 

The ambient light meter in Av mode will dial in ambient negative exposure compensation (NEVEC) when shooting fill flash. This is usually 0.5 to 1 stops of ambient underexposure. This occurs especially in brighter conditions where there is an increased chance of overexposing the subject with the flash.

 

You can test this out by metering a scene (try a brightly lit outdoor scene) in Av mode, with and without the flash. I just tried it and my 10D was dialing 1/2 stop underexposure with the flash on.

 

Hope this helps!

 

Sheldon

[bill_goldman]

A little clarification about Program mode with flash: The camera and Speedlite are trying to accomplish two tasks with respect to achieving a good exposure. I.e., in dimly lit scenes, the objective is to properly expose the main subject with the flash while preventing the background from going dark, if possible. It does this by choosing the longest shutter speed possible without risking camera shake. Hence, 1/60th second is the norm along with a relatively large aperture. In bright light, the system will provide automatic flash fill. If you need more depth of field and/or want to control balance between ambient and flash light, manual mode is the way to go.

 

Remember that E-TTL fires a preflash just before the mirror rises and exposure is based on the area around the active focusing point(s). I have found it works best with camera in automatic focusing point selection mode. With the EOS-1v, this usually means that multiple focusing points will be active and this helps to achieve proper exposure since it is unlikely that a single focusing point will be affected by an unusually bright or dark area of the subject.

 

It's also important to remember that with E-TTL, ambient and flash exposure are computed by the camera independently.

 

As has been stated previously, Av (and Tv) modes always meter for the ambient light and flash provides fill only.

[donny_jatisambogo]

At the risk of repeating what everyone has said, here's my two cents: <BR>

Assuming that:<BR>

1. Your flash is a (A-/E-)TTL, Canon-dedicated gun (550EX, perhaps?)<BR>

2. It's a reasonably powerful gun (GN of at least 40.), which includes 420EX, 430EZ, 540EZ, 550EX, and 580EX.<BR>

3. It's set to TTL, A-TTL, or E-TTL.<BR>

<BR><BR>

Camera in Manual mode:<BR>

- The camera exposes for the ambient light (I'd rather not use the terms 'background' or 'foreground' as they confuse me -- and you, too, apparently.) Ambient light is whatever light comes thru the lens without the flash.<BR>

- The flash goes to fill-in mode, and adjusts itself to available light and illumination of subject in focus.<BR>

- It looks as tho your aperture setting was a bit to small (f/11) for the flash to illuminate the subject in focus properly. Remember that the Guide Number provided by the manufacturer is ALWAYS a tad too optimistic, so assuming you have 550EX (GN=55), the maximum distance between you and your subject at f/11 must not exceed 55/11 = 5 meters (that's the 'optimistic' approximation, tho.)<BR>

<BR>

Camera in Av/Tv modes:<BR>

- The camera exposes for the ambient light.<BR>

- The flash goes to fill-in mode, and adjusts itself to available light and illumination of subject in focus.<BR>

- The meter reading of the camera should be the same with or without the flash, unless your shutter speed exceeds your camera's flash sync speed (for 1v it should be 1/250 sec.) In that case, the exposure shifts (it does NOT change!), unless the flash is set to high-speed sync mode.<BR>

<BR>

Camera in Program mode:<BR>

- The camera and the flash work together to get the best illumination for the subject in focus, whether or not it means that the background becomes totally dark.<BR>

- The readings with the flash and without it WILL be different.<BR>

<BR>

Phew! Hope that helps!<BR>

<BR>

Donny

[ccrevasse]

Sheldon, I thought the automatic exposure compensation was for the flash exposure, not the ambient exposure. Could your changed meter reading have been due to the camera altering the metering pattern when the flash is turned on, as I tried to describe in my first response?

[lightwriting by swapan]

Thanks everybody for your time and effort to help me educate.

 

I am going to read all your answers a couple of more times and then will post any further question/comments if I am still unclear.

 

Thanks again

[sheldonnalos]

Chris -

 

You're right about the reduction in flash output in brighter conditions. I neglected to mention that in my previous post. In bright lighting conditions (daylight), the camera body will reduce the flash output by about 1 1/2 stops, assuming that you want fill flash and not a full power discharge. You can turn this off on the 10D via custom function 14.

 

The auto reduction of fill flash is in addition to the negative ambient exposure compensation. The camera actually reduces both ambient and flash exposure in bright conditions when shooting in Av or Tv modes. My guess is that the logic behind this is that the camera is trying to prevent the combined ambient and flash exposures from overexposing the the scene. It's a little frustrating, having the camera doing all the thinking for you, but once you know what it's doing, you can compensate as needed.

 

Hope this helps!

 

Sheldon