What sorts of variations are you seeing? We really try to control all the variables as best we can, given the limitations of a 3-D target, some necessary variation in lenses, and differences between cameras.
To make sure that the results are directly comparable, we adjust exposure so the shots turn out with exactly the same brightness value (123 +/- 1-2 units) for the middle grey on the mini-MacBeth chart at the bottom. Depending on the camera's tone curve, there may be minor variations in shutter speed needed to achieve that. We also make small adjustments (a couple of inches one way or the other) in the distance between the lights and the target, to make exposure adjustments of less than the 1/3 EV that most cameras can manage. For Canon, most Nikon, Pentax, and Sony, we use the same model lens, a Sigma 70mm f/2.8 macro, the sharpest lens we've ever tested for SLRgear.com. For other platforms, we use the sharpest lens we've been able to find, but they are different lenses, which means different T-stop values, which would also lead to different shutter speeds. (The actual amount of light transmitted by the lens at a given aperture, here's the Wikipedia description of this:
F-number - Wikipedia, the free encyclopedia.) Different lenses also mean different shooting distances, as is also the case with cameras with different sensor sizes. We control the shooting angle, but adjust the distance so the subject always fills the frame to the same extent.
We've tried to standardize on an aperture that's as small as we can manage (for depth of field) without diffraction-limiting. Somewhat arbitrarily, we've settled on f/5.6 for cameras with larger sensors, and f/4 for ones with smaller. Recently, we've seen some cases with intermediate-sized sensors where f/5.6 is diffraction limiting, so we've gone to f/4 for some of those as well.
As to focus, we *try* to always focus the same, pointing the camera at the neck of the vinegar bottle, as it has good contrast and is roughly in the middle of the depth of the target itself. We also shoot at a slight downward angle, so the plane of best focus aligns better with the arrangement of objects in the scene.
Of course, though, we're dealing with a 3-dimensional subject, so variations in depth of field with different sensors will come into play. Full-frame cameras have shallower depth of field, so some parts of the subject in front of or behind the plane of focus may be rendered less sharply than by some smaller-sensor cameras.
There have also been some minor changes over time, as we've gradually tightened our control over the variables. In particular more recent shots have much better-controlled exposure levels (less variation in the shade of the gray swatch), and at some point, we switched from shooting at f/8 for better depth of field to f/4 or f/5.6, to avoid diffraction limiting in the lens. We've also significantly improved our control over framing and shooting angle in recent years, as compared to some shots you might have seen back when we first began using the current target.
Let me know what you're seeing, if it doesn't fall into one of the categories I've just discussed. We try to control for everything we can, and there are unavoidable differences between cameras, but if we're missing something, I surely want to know about it.
- Dave Etchells
Founder & Publisher
Imaging-Resource.com