Sunday, July 19, 2009


The reversing ring showed up in the mail Saturday. This ring screws into the lens's filter threads and mounts it backwards--something that according to the books should improve the image quality. Since I am pushing the resolution of the 50mm garage sale lens I shall have to do some experiments at lower magnification to test that out.

At 5X magnification, five millimeters of the iris adjustment mechanism is focused down on to ~3800 pixels. Since it takes two pixels minimum to resolve a feature this works out to a theoretical resolving power of 380 line elements per millimeter. And since the numbers I've seen posted in the newsgroups and forums for camera lens resolutions range from 40 to 100 lines elements per millimeter, the slight blur in the image I worked to get rid of wasn't caused by my less than perfect focusing. At 5X magnification the 10 megapixel sensor has far more resolution than the lens.

One test for this problem is how much sharpening is needed. On this image I needed a 2.8 pixels radius and a strength of 400 using Adobe Camera RAW. Such grossly overshapening would have totally destroyed a normal image.

Another challenge was illumination. For maximum DOF I shot a f22, the smallest iris opening on the lens. On top of this you add in an exposure factor of 36--the (magnification pus one) squared. I had to clamp my flash 6 inches away from the mechanism and run it a full power to have the needed light. With a 150 watt halogen lamp almost as close--my first attempt--I was so light starved I couldn't see anything in the viewfinder at f22.

The vitals for this image are:

50 mm lens with an extension of 125mm--the maximum the bellows will allow
5 mm by 3.3 mm image area
4 mm from out-of-focus bottom plate to the plate stretching the spring.
ISO 200 at 1/160 sec.
f22 with an exposure factor of 36.

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