Monday, August 30, 2010

Taming E-TTL


Honeybee Studio Shot II
Originally uploaded by Dalantech.
It’s been a while since I talked about lighting in general, and one of the things I’ve struggled with the most is E-TTL flash metering. Before I dive into an explanation of E-TTL I’ll answer the one question that’s sure to come up in the comments: “Why not just use manual flash mode”. The answer is simple: I can only adjust the MT-24EX in full stop increments and it’s just not enough control. I typically end up with a shot that’s too under exposed (and under exposing leads to more image noise and less detail). Also minor changes in angle can make significant changes in the amount of reflected flash light that comes back into the lens, and I just don’t have time to make a lot of adjustments when shooting active subjects. So I’d rather use E-TTL but to make the most of it I had to figure out how it works…

When I press the shutter release on my camera the flash sends out a short burst of light that the camera’s metering system uses to determine how long the main flash pulse is going to fire. If the subject fills the frame, or there is something close behind the subject for the light to bounce off of, then odds are the light meter in the camera will be able to correctly determine how long to turn on the flash to give me a properly exposed image. But if the subject doesn’t fill the frame, and there isn’t anything close in the background for the flash to bounce off of, then the light meter is going to turn on the flash so long that I’ll probably get an over exposed image. That’s how Evaluative–Through The Lens (E-TTL) flash metering works. To get the most consistent results with E-TTL I need to get as much of the metering flash burst back into the lens as possible, and that’s what I’ve done with this field studio:

Honeybee Studio

The trick is to initially bate the flowers that the bees are feeding on. Once the girls get into the habit of looking for the corn syrup (believe it or not it will take them a while) simply cut a piece of the flower petal, place it in a convenient spot close to where the bees are feeding, and set up a back drop (in this case a leaf). You’ll get better E-TTL metering since there’s a backdrop to reflect the metering pulse back into the lens, and you’ll have more time to concentrate on framing and composition because the camera is taking care of the exposure.

Honeybee Studio Shot

You can do a similar trick with dormant subjects –just cut the perch that they are using and hold it up in front of a leaf (or anything that will reflect light back into the camera and give you a pleasing background):

Gymnast

Footnote: The light meter in most cameras samples the frame at the same areas as the auto focus system. So a camera with more auto focus points will typically have a more accurate light meter.

Until next time, happy shooting :)

John

8 comments:

Kenny said...

Good explanation, John. Now I understand even better why, more often than not, my subjects tend to get 'over flashed' when there's no nearby background. How ya been? Still hot as Hector here in TX but it's getting better.

Kenny

Dalantech said...

Thanks Kenny -was hoping that I explained that well :)

The weather here is starting to turn, feels more like fall every day.

Chris said...

Thanks for that explanation on how the flash metering works with E-TTL.

It finally explains to me why it messes up when further away from subjects too. Same problem the background is not there.

Dalantech said...

Very true Chris -the light meter in the camera is exposing for the subject, so the background ends up dark.

Elise said...

Thanks for the explanation... I need to carry around a few big leaves with me wherever I go now ;) I wondered if you're using high-fructose corn syrup as bee bait? I'm guessing not, but I just wanted to check, as I was a little concerned given studies that show a potentially harmful substance may form in high-fructose corn syrup that may contribute to honey bee deaths and colony collapse disorder (I use local, raw, unprocessed honey myself when baiting bees). For more info on HFCS and HMF see http://www.sciencedaily.com/releases/2009/08/090826110118.htm (this article links to the original scientific article, but the latter requires payment to view).

Dalantech said...

@ Elise: Actually I do use corn syrup as bait because some beekeepers say not to use honey -seems like I can't win!

I'll go back to using honey as well, since the quantities that I use shouldn't be harmful. Thanks for the article link!

Eliza Divine said...

This may be a silly comment, but I just wanted to say that the photo of the little bee hanging onto the twig (gymnast) is adorable in ways that boggle my mind.

Larry said...

I just got myself a flashgun for my 30D so this post was quite useful for me!! I used to used flash years ago before we had TTL flash and everything was done manually, with tape measure and patience-these days it is so much eaier!