Sat 28-Jul-12 03:42 PM | edited Sat 28-Jul-12 04:57 PM by PeterBeckett
Hi Guys, Due to my attention having to be focussed elsewhere for a while, I've done very little FoCal testing for the last few days. However, I did some yesterday - for a Pro photographer buddy who brought his D800 and several lenses to me for fine-tuning with his D800. In the hour before he arrived, I ran some Q&D tests to check that everything was working satisfactorily. A couple of those quick tests confirmed some suspicions that I had.
I believe I'm coming to the conclusion that the absolute numbers which FoCal calculates for each test may NOT be able to be used in an absolute sense. The numbers are affected by just too many factors. Actual illumination level of the target and how well the camera's WB matches the target illumination are just two of many.
Nevertheless, the QoF values CAN DEFINITELY be used to compare two lenses at identical settings and evaluated in reasonably quick succession using the same body. Likewise for the same lens on different bodies (as Larry has been doing). Also, I think I can say that FoCal produces very consistent optimal AFFT values for the same lens when the target is illuminated differently - even tho' the QoF values can be at substantially different levels.
When I get back to "serious" FoCal testing next week, I plan to focus on the effects of different target illumination on the absolute QoF numbers. Yesterday, I had the target in bright sunshine, and made comparative runs with/without an umbrella shading the direct sun from the target.
Please note, I am not criticising FoCal in any way by doubting its use for absolute sharpness evaluation. That's not a claim the developer makes. It's merely a hope that I had/have, and will continue to evaluate a little more. Nevertheless, it is looking as if it may be impractical to control all the test variables well enough for FoCal to be used in that role.
*** EDITED to provide a clarification *** In para 3 above, when I talked about comparing two lenses, I should have made it clear that my experience relates to two different versions of the same lens. i.e., two different "vintages". Specifically, My own 70-200 is the earliest model that had VR. My buddy's version is absolutely up-to-date. I believe there was at least one other version released in-between these two. Comparative measurements reveal what appears to be a major optical improvement. VR was NOT enabled in either case. I hope I'm not shooting myself in the foot by sharing this extra comment, 'cos I shall be viciously attacking my piggy bank to fund an upgrade "soon". Therefore, my existing 70-200 will find itself on the IWTS forum in the not too distant future!