Metering with different popup flash powers (T3i)

[daviddoria]

When I use the popup flash in Av, it fires a preflash to use for computing the shutter speed. I can then use the flash exposure compensation to change the flash brightness. I'm assuming the metering is always done with 0 EV even when the flash exposure compensation is set to a non-zero value, because the shutter speeds it computes are the same no matter what I have flash exposure compensation set to. Is there a way to get it to tell me the appropriate shutter speed for a selected flash power?

When in M, I don't understand why "flash exposure compensation" is still an option. The idea is to compensate for a misinterpreting of the settings it chooses for what you want to do with the shot. But in M, there is nothing that it chooses. Shouldn't this option just be called "flash power" at this point. (Actually, it's the same for "exposure compenstation" too - why would you ever use this in M?). Again, is there a way to get the meter to tell me what the exposure will be like with my selected flash power?

Thanks,

David

[Crista]

Hi daviddoria!

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Thanks!

[daviddoria]

Sorry Crista! I added the camera (T3i) to the post title.

David

[ebiggs1]

I don't know if I completely understand what you are asking. But in "M" it means manual so the camera will select nothing. Not aperture, not shutter and not flash. It will, however, tell you whether you are under or over on exposure.

[daviddoria]

I guess there are two questions:

1) Can you make the pre-flash metering in Av use the flash power set with "flash exposure compensation" to determine the shutter speed?

2) In M, why is there still "exposure compensation"? Since you have to set everything yourself, what are you doing to "compensate"?

[TCampbell]

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@daviddoria wrote:

I guess there are two questions:

1) Can you make the pre-flash metering in Av use the flash power set with "flash exposure compensation" to determine the shutter speed?

2) In M, why is there still "exposure compensation"? Since you have to set everything yourself, what are you doing to "compensate"?

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The camera has "Exposure Compensation" and "Flash Compensation" (sometimes called "Flash Exposure Compensation").  They are not the same thing.

Exposure compensation causes the camera to alter the exposure it selects after metering a shot.

Flash compensation (or "flash exposure compensation") leaves the metering and exposure settings alone... but changes the amount of power applied via the flash so that the flash will provide either more light or less light than the camera would otherwise have used.

Flash exposure compensation is used for mixed lighting situations and can help create a balance between available light and flash.  I commonly use flash exposure compensation when shooting outdoors during the day... but dialing the flash exposure compensation down to -1 (give or take a little) so that the flash fills in the shadows without overpower and ruining the look of the available light.

[TCampbell · Accepted solution]

This is a common area of confusion.  The camera is actually behaving in an ideal way but if you haven't been shooting flash for years you might not guess why it does this.

When you meter with flash, the camera mostly ignores the flash.  The light meter performs metering for the available light only.

When using E-TTL (really E-TTL II), the computer in the camera calulates the amount of power needed to adequately light the subject based on your exposure settings... but ignoring the shutter speed (so it really only cares about ISO and aperture... well, it needs to make sure the shutter speed is within the "flash sync" range unless the flash has high-speed sync enabled.)

This is because while the available light is more or less "continuous", the flash is a momentary source of light.  The camera needs to use a shutter speed at or below the camera's "flash sync" speed (which varies by camera and is typically in the range of 1/160th to 1/250th sec).  The max "flash sync" speed is based on the amount of time needed for the mechanical focal plane shutter to completely slide open, allow the flash to fire, and then completely slide shut.  The need to operate this way is based on the design of the focal plane shutter.  The shutter uses two doors or "curtains".  One slides open... the other follows later and slides shut.  If there were only one "curtain" then pixels near one edge of the sensor (where it starts to open) would technically get more light then the opposite edge of the sensor ... because the "curtain" would have to reverse.  By using two "curtains" and having them both travel in the same direction, all pixels technically are exposed for the same amount of time.

The amount of time that the flash is illuminated is usually extremely brief.  It varies by flash model and also the power level -- but it could be as brief as 1/4000th sec... but regardless of how brief it lasts, it can fire until the shutter is completely open or part of the sensor will have it's light blocked.

Ultimately this means that if you use an extra long shutter exposure (e.g. suppose you set the exposure for a full 1 second), the flash will fire... (let's say it lasts 1/1000th sec) -- but leaving the shutter open longer will not actually increase the amount of light recevied by the flash, because now the flash has gone dark.  

That extra long exposure time WILL improve the camera's ability to collect more ambient light even though it will not collect more light from the flash.  When you use flash, you start to think of each shot as two different exposures... an exposure for the "available" light and an exposure for the "flash".  Changing aperture or ISO settings affects both equally... but changing the shutter speed only affects the "available light" part of the exposure because the flash is a momentary source of light.

Suppose you are in a room with low light... you take a shot with flash.  If the camera ONLY exposes for flash, then your subject is illuminated, but everything else in the room appears to be nearly black and the shot doesn't look very good.   But if you leave the shutter open longer then the camera can collect more of the available light and the lighting will look better -- you'll have some balance and depth to the shot because you'll be able to see the background.  This technique of leaving the shutter open longer is called "dragging the shutter" and it makes for better flash photography.  

This is one key reason why you'd want your camera to meter for available light even when it's using flash.

The other reason is for daytime shots.  I use flash to shoot outdoors during the daytime (it's actually one of the best times to use a pop-up flash).  This is referred to as "fill flash".  The idea is that the sun is lighting your subject, but the flash is filling in some of the deepest shadows so they don't look so harsh.  You don't want the flash to try to compete with the sun.  So you can set the flash exposure compesnation to -1 and take the shot.    You do similar things if the subject is in shade but the background is in sun -- to balance the lighting between subject and background.

[daviddoria]

Thanks Tim. I think I get the basic idea of thinking of each shot as two exposures. So the camera is good at telling you what the ambient exposure settings should be, and if you think it got it wrong you canuse exposure compensation to adjust. However, how do you find out what it thinks the flash power should be? That is, when would I know to use flash exposure compensation (again, really just "flash power"?) other than just trying one shot with the flash exposure compenstation at 0 and then adjusting to taste? Is there no "flash light meter" as there is an "ambient light meter"? I assume it is doing something as the preflash (actually, there seems to be from 1 to 4 preflashes sometimes??) must be used for something?

David

[ebiggs1]

If one of the autofocus points is not on an area illuminated by the pre-flashes, the camera may have a difficult time finding something to focus on and may flash a few more times while it is trying to focus.

[daviddoria]

ebiggs1 - After looking through the manual again, it seems like those "preflashes" I was talking about are AF-assist, so perhaps they are not used for determining the flash power? That is, if I turn AF-assist off, I do not see any preflashes. So how does the camera decide which flash power to use?

David