Slide holding for Scanning - Ohnar and Other Options

Halina3000 wrote:

I tried the Razor test and MTF Mapper using the 80mm Rodagon lens this evening. I don't have any fancy ways to do fine focus adjustments.

I first focussed manually using my usual steps of setting the lens to F4 and using tethered live view to focus on a slide image which includes some wire mesh detail. Then did a series of shots at progressively narrower F stops (with the exposure time increased to suit). I did not find a huge variation - 0.06 to 0.09 with the higher values generally in the middle although there were some ups and downs.

Then I set the lens to the middle mark between F8 and F11 (which is where I usually use it). I wound the focus forwards by three ridges on the bellows focus wheel grip (that's about as accurate a measurement as I can do) and then took photos (8 in total), moving the bellows back by one ridge each time. I don't think I went forward far enough but the result was quite a wide plateau reading 0.09 or 0.10 which fell off to 0.05 by the last photo.

so you managed to install mtf_mapper and did get results. I assume these numbers were read directly from the mtf_mapper output.

I pre-process the raw file and crop to my chosen region of interest, either center or further out portion of edge. I call the resulting file pix.ppm

linux options (in version 5.15) to produce full MTF output file: edge_sfr_values.txt

mtf_mapper pix.ppm . -q -b --bayer green

I tried to save the results but just got empty directories.

I've no idea what 0.09 or 0.10 means. Higher seems to be better. I do think this has confirmed that I'm using the lens in the best range and that there is some tolerance for focus when I stop down to the middle mark.

I may test the EF 40mm on the 21mm extension at some point but it's a bit of a pain to swap the rig between a bellows mounted and a camera mounted lens.