Friday, January 18, 2008

ETTL II sucks?

In a comment to yesterday's post Mike correctly points out that the ETTL II pre-flash should allow the camera to set the proper flash power regardless of whether the improvised bounce card was in place. I did a quick experiment where I took two pictures with the bounce card, one at 17mm and the other zoomed at 61mm. Pictured above are crops of the bear, resized to be the same size with the bounce card in place. Its pretty clear that the picture taken at 61mm is underexposed, for some reason the camera didn't set the flash power high enough. I tried changing the metering mode and it had no effect, so I'm puzzled. Is there some other factor at work? I didn't think the on-board flash of the Rebel XT had a zooming head but even if it did with the preflash the camera should be able to compute the proper flash power level. Maybe ETTL just sucks. Maybe this is one of those instances that all those folks who speak about the prowess of the Nikon flash system are referring to. I'm welcome to suggestions.

Pictured above is the uncropped versions, again the 17mm one is on the right.


Michael said...

Hmmm, very interesting results. I can't come up with any explanation off the top of my head. I'm rather mystified as to why the camera chose a higher flash setting for the zoomed out version.

I'm 99% positive the onboard flash doesn't have a zoom head, and I'm pretty sure it doesn't use focal length information from the lens either.

If both photos were shot with center weight metering, I can sort of understand why the 61mm shot would be underexposed, because it would be trying to make the bear come out at 18% grey, and might be thrown off by the brightly coloured clothing. Meanwhile, the zoomed out shot has more of the dark sofa material in the center of the frame, so it would be trying to make that 18% grey.

However, my theory goes out the window if you get the same results using evaluative metering.

Aravind Krishnaswamy said...

These were both taken using evaluative metering. I tried center weighted metering on the 61mm shot and it didn't make much of a difference.

Cyber Samurai said...

GREAT TIP - I have the same camera and lens you have and I was exited about the results. YOu are truly the real MacGyver!!!