Flash/ETTL/Iso question.

If I'm using a flash in ettl mode, I understand it's prefiring a flash to evaluate the required flash power at the current camera setting, right?

My question then, is why does iso affect exposure so much? If the flash is adjusting itself to create a decent exposure, why does changing iso change exposure so dramatically. Changing shutter speed doesn't seem to work the same way. (it's not as dramatic)

Thanks!

Keep in mind that a flash exposure has 2 different exposure components to consider. The baseline or ambient exposure, and the flash exposure. If you change the ISO you brighten the parts of the image not lit by the flash (the background). When you raise the ambient the contrast between the flash and the background is lessened. We can get into some pretty arcane convultutions about the role of changing ISO vs shutter speed but your hypothetical isnt clear about the role of aperture so its best to stop here for now.

It is due to the short duration of a flash burst.

Your shutter is set at 1/60th, your aperture at f/8 and your ISO at 400 When you fire a flash the burst lasts between 1/200th and 1/1000th of a second. This means that the shutter is open for far longer than the duration of the flash burst.

Now close your aperture (or reduce your iso) by one stop. Half as much ambient light and also half as much light from the flash will now reach the sensor - this is because the aperture/iso settings will be active for the entire duration of the exposure.

Now reduce the shutter speed by one stop to 1/125th of a second. Half as much ambient light will now reach the sensor during the exposure but the flash burst is still much faster than the shutter speed. This means you still get the entire (or almost the entire) light from the flash before the shutter closes.

Thanks for the info. Gonzogolf, I see what you're getting at.

I guess I would assume that when I bump up the iso, the flash would lower itself to compensate.
But then I'll suspect that the ambient/subject lighting ratio would change.

Dan Kearley wrote in post #16322347
Thanks for the info. Gonzogolf, I see what you're getting at.

I guess I would assume that when I bump up the iso, the flash would lower itself to compensate.
But then I'll suspect that the ambient/subject lighting ratio would change.

In ETTL the flash does lower itself, but you've raised everything else up. Keep in mind that if ETTL is overexposing, or underexposing the way to adjust isnt to change ISO but to change flash exposure compensation instead.

Listen to this ^^^^ person. Here is a great example of how changing shutter speed does not effect flash exposure.

The flash was on manual and did not change. Also notice that the ISO and aperture do not change in each image, only shutter speed.

http://neilvn.com …ues/dragging-the-shutter/

Dan Kearley wrote in post #16322185
My question then, is why does iso affect exposure so much? If the flash is adjusting itself to create a decent exposure, why does changing iso change exposure so dramatically. Changing shutter speed doesn't seem to work the same way. (it's not as dramatic)

I think you are seeing something OTHER than ISO affecting exposure density!

Here is a test to show ISO 100 vs. ISO 1600...

No one has made the distinction between "fill" flash and "full" flash.... Which are dependent upon the exposure mode you have set on your camera. You can use either one with ETTL flash, but the camera and flash's behavior will be different.

FILL: If your camera is set to any of the auto exposure modes (Av, Tv, P)... It will set exposure per the ambient light, then treat the flash as "fill", as a secondary light source that's dialed down about 1.333 to 1.666 stops from rendering "full" flash.

FULL: If your camera is set to a manual mode (M, or B in the case of really long exposures), then the flash is treated as the primary light source. Ambient light will be ignored, but depending upon your settings can still have major effect on the image. If your settings are below what ambient light alone would require, the flash will fire fully, as the sole light source (as far as the camera and flash are concerned).

Note, even though the camera's exposure mode is set to M, when the flash is the primary light source and is set to ETTL, you still will get auto exposure.

In either fill or full, when using ETTL you can modify the automatic flash exposure by dialing in + or - Flash Exposure Compensation (FEC... which is the same as Exposure Compensation when using an auto exposure mode without flash, except FEC pertains only to the flash).

amfoto1 wrote in post #16331521
No one has made the distinction between "fill" flash and "full" flash.... Which are dependent upon the exposure mode you have set on your camera. You can use either one with ETTL flash, but the camera and flash's behavior will be different.

FILL: If your camera is set to any of the auto exposure modes (Av, Tv, P)... It will set exposure per the ambient light, then treat the flash as "fill", as a secondary light source that's dialed down about 1.333 to 1.666 stops from rendering "full" flash.

FULL: If your camera is set to a manual mode (M, or B in the case of really long exposures), then the flash is treated as the primary light source. Ambient light will be ignored, but depending upon your settings can still have major effect on the image. If your settings are below what ambient light alone would require, the flash will fire fully, as the sole light source (as far as the camera and flash are concerned).

Note, even though the camera's exposure mode is set to M, when the flash is the primary light source and is set to ETTL, you still will get auto exposure.

In either fill or full, when using ETTL you can modify the automatic flash exposure by dialing in + or - Flash Exposure Compensation (FEC... which is the same as Exposure Compensation when using an auto exposure mode without flash, except FEC pertains only to the flash).

There is a great amount of misinformation about 'fill' vs. 'primary light source' in terms of amount of light output by the flash. In fact, whether you are in P or Av or Tv or M, when the comination of shutter speed and f/stop are the same in all four modes, the amount of flash intensity is IDENTICAL for all four situations. That is, the flash metering is not somehow different for Av than it is in any other mode!

What determines 'fill' is merely the relative intensity of fill vs. ambient light contribution, and that is not affected by flash metering...ever (unless you dial in FEC to deliberately tell the flash metering to alter the relative contribution...but FEC is not mode dependent upon its behavior)

Wilt wrote in post #16331865
There is a great amount of misinformation about 'fill' vs. 'primary light source' in terms of amount of light output by the flash. In fact, whether you are in P or Av or Tv or M, when the comination of shutter speed and f/stop are the same in all four modes, the amount of flash intensity is IDENTICAL for all four situations. That is, the flash metering is not somehow different for Av than it is in any other mode!

What determines 'fill' is merely the relative intensity of fill vs. ambient light contribution, and that is not affected by flash metering...ever (unless you dial in FEC to deliberately tell the flash metering to alter the relative contribution...but FEC is not mode dependent upon its behavior)

This is misinformed. There is a difference between how Canon ETTL handles flash in Av/Tv and M/P modes. This information is easy to find. It is even easier to corroborate for yourself if you own a Canon EOS body and a Canon EX flash.

Mr. Myers is correct. Wilt is misinformed.

EDIT: I left the original as is for reference. On second reading, I think Wilt and Mr. Myers are both correct. The way flash is metered does not change with mode selection. The combination of ambient and flash is what is altered by using different modes. Mr. Myers and Wilt are both correct. My response was well-intentioned, even if hasty. I tried to flesh out the differences in the following post.

In P mode the camera will default to fill-flash if ambient is sufficient for decent exposure. The camera will allow the BG to go dark and expose for flash on the subject if light is lower. In Av/Tv the camera does not do this. It will set BG exposure to obtain a properly exposed BG without the flash. If that results in 1/2 shutter speed, so be it. In M mode, it is up to the shooter to control ambient with shutter speed and ISO; flash metering only concerns itself with the subject matter. That makes at least 4 different ways of combining flash and ambient, depending on the mode the camera is in.

Well, it's just a question about how you look at it.
P, Av and Tv all strive to expose the ambient correctly, just a tad under, to allow for adding the flash without creating accumulated overexposure.

But since P mode doesn't extend the shutter speed more than to 1/60 s, it will change from fill to full when it gets darker.
In Av mode, you can achieve exactly the same thing if you limit the shutter speed range to 1/60 to 1/250 s. The only difference is that you select the aperture.
Tv mode tries to do the same as Av normally does, but runs more often out of apertures than Av runs out of long exposure times. But if you enable safety shift, then you get the same effect in Tv as in unrestricted Av.
Then finally M becomes what you set it to, as stated above.

I think it is easiest to shoot flash in Manual mode.

I agree. The only time it's not is if you use the flash to fill in shadows outdoors, and the ambient is varying too much to keep the same setting in M all the time. Then there's sometimes too much to fuzz about. In such a case Tv or Av may be better.

Wilt wrote in post #16331865
There is a great amount of misinformation about 'fill' vs. 'primary light source' in terms of amount of light output by the flash. In fact, whether you are in P or Av or Tv or M, when the comination of shutter speed and f/stop are the same in all four modes, the amount of flash intensity is IDENTICAL for all four situations. That is, the flash metering is not somehow different for Av than it is in any other mode!

What determines 'fill' is merely the relative intensity of fill vs. ambient light contribution, and that is not affected by flash metering...ever (unless you dial in FEC to deliberately tell the flash metering to alter the relative contribution...but FEC is not mode dependent upon its behavior)

I was wondering where this was going to go. I followed a different thread where you explained this. I tested it and when I shot in AV and then I matched all the settings in M I got the same flash exposure.