Sunday, June 24, 2007

Exposure variations

Short story: please go to this Picasa album, view the slide show with captions on, and come back here and tell me which exposures with Flash:off and which exposures with some value of Flash Exposure Compensation you liked. Refer to the number, e.g., "I330" at the beginning of each caption.

Longer story:

Y'day, a number of what would have been good photographs I blew because I didn't get the exposure right. The photographic situation is: person in the shadow, bright background. Hence this set of two series.

The constants are:
Canon 5D on tripod
24-105 mm f/4 lens at 105.0 mm
580 EX II flash.

All files are jpgs from the camera, dragged into the Picasa upload tool.

There were some light clouds, occasionally obscuring the sun. The series was taken between 12:10 and 12:30.

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My conclusions:

If you don't have flash handy, use spot or partial metering to get the face correctly exposed. You will likely sacrifice the background, but you don't have much choice.

Things greatly improve if you have a flash. Use evaluative metering, and (on the 5D) set exposure compensation to -1/3 to -1 - this will preserve highlights in the background. Set flash exposure compensation to -1 or more, otherwise you will overexpose the face.

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Yes, I know post-camera RAW file processing opens a whole new large parameter space. But for the most part, I don't think I want to spend a lot of time in front of the computer, I want the exposure to require minimum to zero touch up.

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Criticism, better technique, suggestions for another experiment with other settings, I'd really welcome.

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Postscript: Remember, this is for unposed photographs of people who are not going to stay still. I don't think I'd have the time to meter on middle gray, exposure lock and then reframe and take the picture.

2 comments:

Rajan P. Parrikar said...

Although you haven't explicitly said it, I shall assume that you metered all these by pointing the central focus point at the mask. In this instance it appears that the mask skin is for the most part close to middle gray, so spot metering would yield a correct exposure of the face but would blow out the background. So best to stick to Eval with Flash.

The best exposures, in my opinion, are I331 and I338. You didn't try an exposure compensation value of -2/3.

Also - why did you change the f-stop for this experiment (unless you wanted to fiddle with the depth of field)? You could have put the mode dial on M(anual), fixed an aperture size (say, f/5.6) and done a sequence of shutter speeds. This could have yielded the necessary information since exposure compensation simply provides a finer gradation for change in shutter speed. In quick situations, it is true that you may not have time to set up the M mode. So Av, with -2/3 to -1 exp comp and -2/3 to -1 Flash comp seems to be the way to go.

My question - does Flash compensation change the shutter speed (like exposure compensation) or does it change the Flash power output?

Arun said...

To answer your last question first - flash exposure compensation seems to change the flash power - to the point where only the EXIF data assures me that the flash did indeed fire.

My guess is that a sensor on the camera integrates the light received from the foreground and tells the flash to shut off within microseconds when enough light has been received.

I changed the f-stop sometimes inadvertently when twiddling with dials, but mostly whenever the speed exceeded the Xsync speed (whatever that is) of 1/200 seconds. i.e., stop down when 200 is flashing in the view finder).

Yes, the central focus point was squarely in the mask.

Yes, I need to try exposure compensation of -2/3 also.

Only later I realized that I need to do this experiment with a dummy wearing white clothes - because that is a common scenario, especially with desis. That may require going down 1/3 stop or more in flash exposure compensation to avoid making them look like models in a detergent advertisement.

I should also probably get a diffuser and see if it changes things much. Though I don't think a diffuser is necessary in bright conditions.

Also, one of the problems with the Halloween mask is that it is not as reflective as skin.