Why Does The Bottom Crop Sample Vary in Brightness?
Variances of brightness in the bottom crop sample can be attributed to two things. First, at wide apertures, the difference is usually light fall-off/vignetting. This difference is very helpful to see and compare. At narrow apertures, where light fall-off is no longer an issue, the power of the flashes must be turned up dramatically. It is very hard to illuminate the chart perfectly evenly at this flash power level and the corners may be slightly brighter due to light fall-off across the chart from the flashes. Also, even though the lights are close to the test target plane, reflections are still a problem when testing super-wide angle lenses. I’ve implemented a newly-available larger test chart in 2008 to help with this issue. Are The Crop Samples Accurate Image Quality Indicators? For the most part, I find the results to be very indicative of the image quality of the lens being tested and the comparisons work very well – the results are even better than I had hoped for. However, I’m
