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Auto Focus - Lens and Camera Calibrations

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Auto Focus - Lens and Camera Calibrations

Postby Paz_Pazzaz » Mon Dec 19, 2016 1:00 am

I've been doing some lens testing lately and I'm not particularly happy with my results. It appears that not only can a lens either back focus or forward focus, the camera body itself can affect auto focus. From an article I've just skimmed over, apparently it is possible to calibrate camera body / lens - combinations in some DSLRs but from one of the comments, perhaps NOT from a mirrorless camera.

I bought a Canon 100-400 L 4.5 - 5.6 and LOVE it. It focuses quickly and accurately with the GH4, using a Kipon (brand) "Canon lens to micro 4/3" adapter, the vast majority of the time. Most of the time I use it in the "Ex. Tele Conv" mode, which, if I have my numbers right, at 400mm gives me a reach of about 960. (400 x 2.4) (What is happening is that the camera is giving a 1080 crop from the center, sharpest portion of the lens. This feature is not available in 4k. It is sort of like a camera created crop in the center of 4k.

ftp://ftp.panasonic.com/camera/om/dmc-gh4_en_adv_om.pdf

The magnification is so high I cannot use it without a tripod, especially since it is so heavy, even if I didn't usually use it in the most zoomed in mode, so I wanted something as good that I could hand hold. I tried a Canon 70-200 L f/2.8 IS. The image quality was fabulous. Even a little better than the 100-400. BUT it was also heavy I couldn't use it hand holding and since the range pretty much overlapped, I returned it.

Then I read reviews saying the Canon 70-200 L F/4 is one of the sharpest lenses Canon has ever made - at almost half the weight of the F/2.8 version. I tried it. When I focused on a flower, the rocks about 3 inches behind the flower were in perfect focus. Returned it and they sent me another. This one isn't focusing perfectly either. And sometimes seems really sharp and other times seems worse than my Tamron lenses. So I tested both lenses, the 100-400 and the F/4 70-200 with two camera bodies, the Canon 7D and the Panasonic GH4, at the same ISO 400 and 800, and the same F/8, and same focal length of 200mm. I was really surprised that the Canon 7D images were not nearly as sharp as the GH4 images.

Then I tried using auto focus with both cameras, all other things being the same, and manually tweaking. I could not get the Canon 7D body to produce images as sharp as the GH4. Oh - forgot to mention that the GH4's firmware upgrade now allows auto focus WITH peaking, while in "manual focus" mode while actually filming!!! It has made all the difference for me. LOVE it.

Anyway, here is a link to the body/lens article - and I'd like to hear anyone's opinion related to calibrations of lenses and/or cameras. I'm torn between returning this latest lens - but is it the lens - or is it me??? Or my equipment? Should I keep this lens, which is pretty good, but not fabulous, and send it to Canon for calibration? How could they let two out that are not calibrated correctly in the L line? And how could I get them both???

I DO understand that 4k focus is critical. If you're taking 4k footage with the idea of later zooming in for good quality at lower resolution, you need to be sure the 4k is perfectly focused in the first place, or zooming in does not give a good image.

the article:
https://www.slrlounge.com/calibrate-len ... ry-images/

Comment relating to mirrorless cameras:

Greg Thomas

What about mirrorless cameras like the Sony A7ii? How do you fine tune the focus for them?
============================================================
Dave Haynie

This is only an issue for DSLRs. In a DSLR, the PDAF sensor is a separate sensor, not part of the image sensor. The lens is calibrated to match PDAF and image sensor focus, but it’s not always perfect.

In a mirrorless, all focus sensors, whether CDAF or OSPDAF, are on-sensor. So you’re always focusing at the exactly sensor plane. Most of these use hybrid AF, where the OSPDAF sensor is used for fast focusing (a single PDAF measurement can accurately tell, not just whether the image is out of focus, but in which direction and by how much), the CDAF sensor used to accurately dial in that last little bit. So no calibration needed.


Assuming Dave Haynie knows what he's talking about, does that mean a mirrorless camera is easier to auto focus? If so, then my latest lens really might not be up to snuff and I should ask to try yet another? I know my GH4 is a contrast system and the phase detection systems are supposed to be better at tracking, but now that I can re-focus continuously when needed, I dont' see that as as much of a problem as I did at first - that is - if the body/lens combination are focusing correctly when I press for auto focus.

Anyone here ever done any of this?

thanks,

Paz
Whatever you do, don't set your coffee cup adjacent to your turps cup. :CS:
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