I’m thinking about doing a report comparing several 100ish mm macro lenses, and I need a protocol to do modulation transfer function (MTF) and longitudinal chromatic aberration (LoCA) testing. I find it much more fun to devise new protocols than perform the old ones, so I jumped at the chance.
I can’t use any of my printed targets for this kind of testing, because I can’t print the edges sharply enough. I’d read that a razor blade is the gold standard for a slanted edge, so I came up with the following precision razor blade mount:
The blade is stuck into an AF tuning tool, which is mounted to a tripod with a gear head that I can use for alignment. Behind the razor blade is a white piece of paper illuminated by two Westcott 5000K LED panels. When you turn the room lights off and the panels on, here’s what you see:
For on-axis pictures, you put the blade edge in the center of the image:
After that, it was just a matter of modifying my old motorized-rail protocol. Here’s the first experiment I tried:
- Sony a7RII
- ISO 100
- Manual exposure
- Manual focus, set at one position
- Sony 90mm f/2.8 G Macro lens
- Cognisys computer-driven focusing rail
- 130 exposures 250 um apart
- Lens distance set at 1:2 magnification mark
- Document mode decoding with DCRAW
- MTF50s for all the raw color planes plus withe balanced calculated using MTF Mapper
- Graphing in Excel
Here’s a plot at f/2.8:
Distance is the horizontal axis, with the left hand size having the subject farther from the camera than the right hand side (The camera moves closer to the subject by 250 um after each exposure).
The first thing I noticed was that it looks like I missed the peak of all three raw planes because the steps were too big. Imagine that! 250 um steps are too big! Next time I’ll use 100 um or maybe even 50 um. I may have to make more exposures to make sure I get the peaks in the middle.
The next thing was the absolute value of the green and blue peaks. They are quite high for this lens, considering that we’re looking at raw channels with absolutely no sharpening and we probably missed the actual peaks. It looks like a razor blade may be a better slanted edge target than my printed ones, even at much longer subject distances.
The third thing of note was the LoCA, with the red channel achieving best focus with the camera farther away than the best-focus distance for the other two channels. The difference is about 500 um; I’ll be able to measure it more accurately when I redo the test with smaller steps.
Same idea, but the lens is getting sharper. I probably missed the peak in the red and blue channels.
Not quite as sharp, and the LoCA is about the same.
Sharpness is coming down, but the lens is still quite sharp.
Now we’re really losing sharpness. The LoCA hasn’t actually gotten better, but you won’t see it in real images because it will be masked by the depth of field.
There is some, but it’s not bad. I’ll check again with smaller steps.
I think this protocol will work fine.