One of our blog readers send in the following message.

With regard to your thread on the Tamron 17-50mm over-exposing with TTL/BL, I must disagree with your conclusions. I’m not saying the Tamron doesn’t have issues but in your samples your conclusions don’t make sense. I see one well exposed shot and three looking very grey – perhaps the Nikon lenses you were using are not sending the distance info to the flash so the camera resorts to plain TTL since it can’t use distance info and TTL is under-exposing because of the white.

I’ve just been doing some pretty in-depth tests to work out how TTL/BL works and with direct flash it is much more accurate than TTL because it uses distance rather than reflected light to the meter as its main calculation. I have done these tests with a D90 and Tamron 17-50 and with a D40 and Nikon 18-105 lens with the same results. I have a lot of samples in my photobucket album.

Try setting up a white reflector maybe with some black satin hanging over half of it. TTL flash will over-expose when you point more towards the black and under-expose when you move towards the white. TTL/BL will maintain a constant exposure because it works off distance rather than reflected light. In your example the picture taken with the Tamron is in fact the only correctly exposed image!

Cheers, Desmond.

First of all, I’d like to thank Desmond and other readers for participating in the discussions. The feedback from our readers makes me motivated to contiguously improve this blog.

Secondly, the discussions on Nikon Cafe is very interesting. For anyone who is trying to understand the difference between TTL vs. TTL-BL flash, it is a good brain exercise trying to explain the observations. If you are confused, I’d recommend the excellent Nikon CLS Guide.

Now going back to the particular message I received, all the lenses I tested in my original post are either D or G type so the distance information is indeed involved in the TTL-BL mode captures. The distance information is also recorded in the meta data as I have shown. The photos appeared underexposed (gray), but that is expected because the scene has large areas of white so the meter is tricked to underexpose.

Posted in: Digital photography on May 6th, 2009. Trackback URI
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