This is a continuation in a discussion of spatial frequency response (SFR) and modulation transfer function (MTF) testing reproduciblity. The series starts here:
A reader suggested that I reconfigure my testing protocol to move the razor blade target rather than the camera. That approach has several advantages:
- The target is in general lighter than the camera, so less wear on the rail
- I could accommodate heavy camera/lens combinations which exceed the capacity of the rail.
- For magnifications of less than 1:1, the effect of rail vibrations on the test capture is less if the target moves than if the camera moves. This means I can program faster settling times.
There is one disadvantage. I now need cables to connect the rail controller to gear mounted on two tripods (one cable to trip the shutter, and one to drive the rail). If the target is a couple of meters from the camera, then the controller should ge on the tripod with the camera. If the camera and the target are much farther apart, then the controller should go on the tripod with the target to keep the length of the cable that drives the rail down.
However, all the little stuff above is trumped by one huge advantage: moving the target gives me the geometry I need to do off-axis testing.
On-axis geometry is undemanding. Aim the camera at the target, center it, and it’s on the lens axis. If you move the camera towards the target, the target stays on-axis, and everything works fine. If you move the target towards the camera along the lens axis line, that works too.
But what if you want to measure corner SFR? You line up the camera so that the razor blade is in the corner, and focus to get rid of field curvature effects. But if the camera is on the rail, how do you line the rail up to move the camera along the line traced by the path from the razor blade through the lens to the place where the blade is imaged on the sensor? It makes my head hurt just to think about it.
If you move the target in a direction orthogonal to the plane of the razor blade, you’re golden, assuming the blade was square to the camera when everything was on-axis.
So now it should be easy to check the MTF anywhere in the frame.
At least, it seems like it should be easy. With these things, as with most things in experimental science or engineering, you never know for sure until you try. So I set up a test. I put the Sony 90mm f/2.8 FE macro on a Sony a7RII 2 meters from the target, set the magnification to as far as it would go in the upper right of the image, adjusted the aiming of the camera so that the razor blade was centered, set the rail for 190mm of travel and 101 exposures, and made a series at whole stops from f/2.8 through f/11.
I ran the files through my MTF cruncher, and that’s when I ran into trouble. The razor blade wasn’t in the same location at both the near and far ends of the rail. I suspect two things.
First, after I line up the camera on-axis, I should really rotate the camera/lens assemble around the lens node just as if I were doing stitching. I didn’t think of that.
Second, it’s a lot harder to line the rail up precisely facing the camera than it is to line the rail up facing the target. The camera has a viewfinder, which is a precise and accurate aiming tool, an the rail itself does not. It would probably be a good idea to attach a laser pointer to the rail for aiming.
Because of both these things, I think I’m going to have to revise my procedures so that I’ll get results even with small movements of the target in the frame of the captured images.
There are two pieces to this. First, block off the internal perforations in the razor blade with gaffer tape so I’ll have more freedom to place the region of interest without creating what MTF Mapper will identify as edges. Second, rewrite the MTF Mapper front end program to track changes in the target position. That’s going to take a while, but will yield other benefits, since I’ll make the code more modular while I make the changes.
Hmm, thinking about it, aren’t there much longer razor blades available, like refills for snap-off type craft knives?
That would allow you to test at a much longer distance than ordinary double-edged razors for facial grooming.
Worth exploring. The longer the blade in the image, the more important the alignment, though.
Alignment would be simplified using a laser pointer from the position of the entrance pupil, reflecting it off the flat of the blade. That’s assuming that a perforated blade is flat enough, though.