ETTL Autoexposure

It took me some time to realize what was going on when I first started using my 580EX flash. I would consistently get what seems like underexposure. I almost never shoot without bouncing the flash off something, but even in direct flash result don't seem consistent.

This is a sample shot with no flash exposure compensation. I had the main dial set on manual - 1/80s at f/5.6. No ambient light is coming through, this is all bounce flash. Note the histogram has nothing in the top 1/4, where the most information is stored (in terms of digital 'bits').

At first I thought it was because when the flash is bounced, there isn't enough light, but when I set the flash to manual on full power (1/1), the scene ends up almost totally white. This tells me I have plenty of power, even when bouncing.

Here is a second shot with the exposure compensation for flash turned up to +1 1/3. This completely changes the histogram to look 'right'. Data all the way, and more what you would expect in a scene like this. Note that I'm not claiming this is exposed properly, but nothing is blown out, and you can pull back a bit on exposure after the fact. In the previous case, if I pull the exposure up, I get a lot of noise in the shadows with my 20D.

I find this to be the case with my Canon flash when used in ETTL. I almost always need overexposure on the flash to get a properly exposed shot. I typically run with +2/3 when shooting in auto.

The bottom line is that you need to go manual if you want to nail the exposure every time, as ETTL shifts around a bit depending on the brightness and reflectance of the subject.

Getting Started

The purpose of this blog is to discuss and document the things I learn about EOS photography. I have used Canon cameras since 1985, when I got my first Canon AE-1. I didn't know for about 3 years that I could set my lens to A and get aperture priority! (I was 12 at the time and bought the camera in Hong Kong, didn't get a manual I could read, and my third party Tokina lens didn't actually have the 'A', but a circle.)

Nowadays, I am using a digital camera, the Canon 20D. I won't go into detail about my other equipment, except to say that I have been experimenting heavily with off camera flash within the Canon wireless system. I've learned lots from the ubiquitous Strobist website, as well as many others.

I intend to post tips and tricks that I have learned, as well as before and after shots for stuff I learn along the way. Not really intending this to become something many read, but if I can save someone from having to learn the hard way, it will be worth doing. At the same time, it is a place to put all the things I figure out for myself to refer back to. I suspect I will be surprised to look back in a few years and see what I was doing.

Sometimes writing something down is another way to cement the lesson learned.