Tuesday, September 9, 2008

Micro adjustments with Canon MKIII cameras

MicroAdjustment on the Canon 14mm II

I just got back from a shoot on Little Cayman where I was using my new Canon 14mm II to shoot a model as an over/under, with starfish being the underwater component. The focus was on the starfish, and I expected the model would go to soft focus given the refractive difference between water and air. But, still, intuitively I thought the model in the background should have been relatively sharper.

(Top shot)

That thought nagged at me today as I did the 20-hour crossing from Ensenada to Guadalupe Island aboard Nautilus Explorer. After all, when I wake up tomorrow I’ll be hoping to photograph great white sharks, and this is no time to have a lens performing less than optimally. After all, if a lens does not focus properly topside, there is no way it will work satisfactorily underwater.

I have had problems with lenses focusing improperly in the past. Actually, only once, and that was with a 15mm lens that should have been very easy to focus because of its extreme depth of field. The photos would look sharp through the viewfinder, and auto focus indicated they were spot on. But, a critical look at the files at 100% magnification revealed they are soft where they should be ultra-sharp. That lens, plus the camera body, had to be sent back to Canon for calibration.

But now, with the MKIII Canons (I have a 1DsMKIII and a 1DMKIII) the cameras themselves have micro adjustment as a custom function, which means I can dial in the auto focus accuracy of my lens on each of my cameras, and anytime a 14mm II is attached to the camera, it detects it and dials in any specific auto focus adjustment I might apply. It can’t tell which specific 14mm II lens of course, but any 14mm II will be recognized as such by the camera and the micro adjustment applied.

To improve my focus functionality I had to first determine exactly what setting would give better results than the factory setting of “zero” adjustment. The camera allows +/- 20 points of correction, so I did a test in 5-point increments, beginning at –20 and running through zero to +20. Obviously, I had to improvise (I was on a boat, after all) and didn’t have a tripod. I chose aperture priority at F-2.8 for the test, to assure the very least depth of field and worst optical performance the lens could deliver. I assumed things could only get better from there. I set the center AF zone on the numbers “1434” on my targeted book cover.

Second shot shows the whole scene, from which the 100% screengrabs were taken.

(Third shot is at minus 20,
Fourth shot is at plus 20)

As it turned out, minus 5 was best for my Canon 1DsMKIII and plus 5 was best for my 1DMKIII with my personal 14mm II lens. Your mileage may vary, and probably will.

Now, having dialed in the micro adjustment correct for this lens, I wanted to find out what working apertures would give me the depth of field sufficient to hold both foreground and background reasonably well, for a situation like my starfish + model. I used the two magazines in the foreground as reference, and again focused on the type on the novel, “1434”. The Outdoor Photographer in the foreground would have to be way out of focus at wide apertures since the focus was so much farther back, but I wanted to see what aperture might bring it reasonably into focus. Again, these are screengrabs, this time from Camera Raw at 100%.

In this admittedly extremely difficult situation to hold focus both at the center of the frame and the subject in the near foreground, F-11 was where I began to see satisfactory results. For two subjects on the same plane, F-5.6 might have been acceptable, but this tells me for subjects of the kind of dimension I might find underwater, F-8 to F-11 is the maximum aperture I am comfortable with. So, instead of working at ISO 100 with this lens, I’ll standardize on ISO 200. After all, the difference between 100 and 200 ISO on a MKIII is virtually indiscernible, but the difference between F-5.6 and F-8 on this lens is significant.

(Fifth shot is at F-5.6,
(Sixth shot is at F-22)

Bring on the white sharks, because my 14mm II and I are now ready for them! It is nice to know this level of precise control is available on my MKIII cameras, especially when problems are diagnosed impossibly far from the nearest Canon repair facility.

Oh, by the way, I forgot to show the target when it was properly dialed in. I added that one at the end, a minus 5 on the 1DsMKIII. Double click on the shots of the targets to get a sense of how significant the difference is throughout the range of adjustment. No sharpening or processing has been applied to any of the shots. Just straight screengrabs from either Photo Mechanic's browser or CS3's Camera RAW.


obakesan said...

great stuff! Thanks for that information. Now, if only my EOS bodies had this option

oh well ... spend spend spend ;-)

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