DIY Resolution Target
February 2, 2010
I know you’re all pacing a groove in the carpet, waiting for hot news about this month’s upcoming micro Four Thirds cameras. I am too.
But to give you something to occupy yourself in the meantime, here’s a handy little test target. It’s useful for checking how closely a camera’s resolution approaches the spacing of its sensor pixels:
Download the PDF file here, and print it out at full size. It’s a 600 ppi bitmap, and each tile should measure one inch square.
The numbers give the divisions per inch—that includes both the black and white stripes.
Now, photographed at a certain magnification, one spacing of stripes will exactly match the pixel pitch of the camera. Ideally then, zooming all the way into the image should show one row of black pixels, then a row of white pixels, etc.
Let’s say you have an 8 megapixel camera, which creates 3280 x 2460 pixel images. To match up the 40-lines/inch sample to your pixel spacing, divide 3280 by 40; you get 82 inches.
On a nearby wall, put up two marks 82″ apart (using tape, post-its, etc.). Set up the camera and zoom/move until the picture width exactly matches the marks. (Note that many viewfinders show slightly less than the true image size).
Tape up the test target near the center of the frame, where lens aberrations ought to be lowest. The setup looks something like this:
To make your test more reliable, be sure to:
- use the highest JPEG quality setting
- put the camera on a tripod
- repeat the shot a few times, and select the sharpest
- use the lowest available ISO setting
- check that other settings (sharpening, etc.) are typical of your use
When you take a magnified look at the resulting image, you may see something resembling this:
The 30-line sample looks a little lumpy; but it is actually resolved (you can count the correct number of lines). Not too surprisingly, the 50-line sample is not resolved—although you do get a hint of the line orientations.
But most interesting is the 40-line sample. Ideally, you’d see black & white lines exactly matching the pixel spacing. But because of the Bayer demosaicing, some decidedly funky things have happened. (Read a good explanation here about how demosaicing works.)
The target lines aren’t resolved—and they’ve actually created some false colors and textures.
Now, more natural, irregular subjects won’t show such alarming artifacts as this. And notice that the hairline around the number squares remains faintly visible, even though it’s much narrower than one pixel.
But down at the individual-pixel level, sensor resolution can become a bit shaky. Check for yourself and you might be surprised.