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DR testing and color tracking (was: Re: Further thoughts on the Camera Evaluations)

Adam Wilt

 I don’t think you can ever look at DR on a chart though.

If you don’t have access to a Xyla (they’re available with up to 26 stops these days), a Stouffer transmission wedge can be used. But the ones I have top out at 13.3 stops and it’s very difficult to get the lighting perfectly even. The 13-stop limitation can be worked around by shooting two different exposures, but by the same token you can do the same thing with a multiple differing exposures of a reflective grayscale chart.

Of course these things have to be used in pitch-black environments to keep ambient light from filling the shadows and inflating the perceived DR. And you have to shoot at a stop where the lens’s illumination is flat edge-to-edge, and watch out for flare (more of a problem with Stouffer wedges than Xylas; I frame Stouffers entirely in the top half of the frame so flares from patches on the bright end fall below the patches on the dim end).

So what I do is try to create an clipping exposure ramp.  Using a leko say slashed at a very steep angle on a white wall. 

Also a useful test. If you slash it along a textured wall (such as one with “orange-peel” surface treatment) you can see very clearly what happens with detail rendering at exposure extremes.

Now this doesn’t tell me how colour tracks into near clipping

I find it very useful to shoot a tungsten ramp with daylight balance, and vice versa (using a wall slash, a Xyla, or a Stouffer wedge. Just bias the camera’s color balance about 150 mireds in either direction from the light source if you can’t gel or change the source). You’ll see quite clearly where colors start to clip, distort, or smoothly desaturate. Doesn’t handle a broad range of colors the way overexposing Geoff’s stress-test chart will, but it’s a graphic indication, contained in a single frame, of how the camera handles colored highlights in general.

Adam Wilt
technical services: consulting / coding / camerawork
Vancouver WA USA (no, not that Vancouver, the other one)