I am shooting with a Sony A100. I just got the F56 flash gun and am working to understand how to use it properly. Please excuse these photos as I'm just using them for practice. From what I've read about TTL flash, the unit/camera determines how long to flash the strobe based on the through the lens reading. When I look at these two pictures the second one is so much darker. Why is this? The only thing that changed from the first to the second photo was the focal length. Why didn't the flash compensate for the change? Am I missing something?

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Is your metering point linked to your focus point or is it a set area of metering metering? If it is linked to your focus point what area(s) of the image were you focusing/metering on? What could happen is if you have your flash metering linked to your focus point it could have focused on a different area in the second image and that could explain why the second image is a little darker.

A common problem with TTL is that sometimes people do not notice that even though they are taking the same image in the same situation the metering area is over a different part of the image that has different reflectivity which can greatly effect the image results.

Also, TTL re evaluates the situation every time. Just like auto white balance or AV, or P modes on your camera. So, when it re evaluated the second time, it made a different decision on how much flash to put out than the first time. If you want 100% the SAME results, you need to shoot your camera and flash on Manual everything.

Rosemary and Kari,

Thanks so much for ther responses. It makes sense. When I played with the second image I was able to make it match the first with a curves adjustment. Working with a flash is very interesting!