Did I get an expected result from Reikan FoCal?

[jonathan7007]

All,
Please comment. I just bought a used 1DsMk3 and saw a consistent "back focus" -- that is, with a few different lenses images appeared to be focused on a point a little behind the intended point of focus... best I could remember, of course, where I asked for focus. Not 100% but a lot of images showed this.

I own Reikan FoCal and made some time today to test as the sun went toward setting (we rarely have low winds here...) and got through the following lenses: 24-70 v2 (at 70); 70-200 v2 (at 200); and the Sigma 35mm f1.4. The last test was asked to soldier on through what I know was too dim a light. Yes, I know it's best to do this with sunlight.

The results were all positive numbers like 4, 7, and 5, respectively.

So are positive numbers in FoCal consistent with my description/observation of back focus?

I do plan to redo this under better conditions. Hard to get all the factors -- including time -- together. I did like having the test run automatically, as my 5DMk3 has the annoying SDK failings that Reikan tells us are NOT in the 6D camera software.

Thoughts?

jonathan7007

 

[Hydrogen]

If you suspect and are certain (from reviewing your general usage exposures) that your camera is back focusing and FoCal is revealing all positive numbers, this would be indicative of either:

A) Most likely: Did you have enough light on the target? Check your test result reports if you saved them from FoCal. Look for the Exposure Value (EV) of the shots of your target. FoCal recommends at least 10EV or more for accurate results. My target typically has 11 to 12EV's.

B) Possible?: Your camera is actually FRONT focusing but you are referring to it as back? Positive AFMA numbers such as those you received from FoCal shift the focusing of a lens or body toward the telephoto (BACK) side of the focal range; or

C) Least likely: All of your lenses are front-focusing and in need of back (+) adjustment.

-Darius

 

[jonathan7007]

Darius,
My focus points observed in files are BEHIND the point I believe I chose. But the intended focus is a bit of a guess, of course, as these are not exposures made in a test. I take it you think the "logical" FoCal result would have been a negative number?

jonathan7007

 

[Hydrogen]

Jonathan,

Thank you for clarifying that the focal point is behind your intended focus point. Yes, I would expect as I have personally experiencing a need for a negative number coming out of FoCal, with one exception...

I am starting to believe from purely anecdotal evidence (I've tested eight lenses on two bodies numerous times in FoCal and added more actuations to my new 5D3 than I'd cared to) that positive and negative AFMA settings, especially within +/- 5 are more of a 'secret sauce' that translates into a formula the camera uses when giving instructions to the USM focus motors within the lenses.

In other words, just because you select a positive AFMA adjustment for a lens (which should technically focus it FURTHER BEHIND your focal point), does not necessarily mean that it will and could actually bring the focus FORWARD/CLOSER to the camera. Here's why I believe this...

Using the AF Consistency test within FoCal, you can see that certain AFMA values deliver far more consistent AF results than other. There will almost always be at least one ore more "outlier" results at certain AFMA settings where the autofocus missed significantly compared to the exposures.

I always take the AFMA result given to me by FoCal from the Fully Automated test, and go into the AF Consistency test. From there, I select the "AFMA Range" option and enter for the minimum, a value 2 or 3 points FORWARD (subtract from the AFMA result given to you by the Full test) up to 2 or 3 points BEHIND (add to the AFMA result), in steps of 1, with 10 exposures at each AFMA increment.

You may be surprised at the results. The POSITIVE numbers you are getting from FoCal may actually yield more CONSISTENT focus results than negative numbers (which we would expect).

Give it a try at least for one of your lenses and see what you get and set the AFMA setting for that lens in your camera to the AFMA setting which gives you the *most consistent* result with a high QoF (Quality of Focus) score.

If your images continue to miss the mark and focus behind your subject, you may seriously want to consider sending your camera body into Canon for them to calibrate it and ensure it is working to spec.

-Darius