Front/Back focusing distance related?

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shufflemoomin

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Hi all,

I'm using a Canon 6D with a Canon 50mm 1.4 at some gigs I do. I recently did a MFA on it using a chart and the dot-tune method. I usually use the centre point and recompose for bands that aren't overly energetic. I've noticed that while some shots are pin-sharp with focus where I'd set it, other shots are clearly front focusing by quite a bit. Can anyone explain if distance from the subject and have a bearing on whether front/back focusing can happen and any suggestions on what to do about it?
 
Distance does make a difference especially if you are up close and personal. Still I think it is more likely to be a problem with the focus recompose if your shooting at or near 1.4.
 
With some body/lens combinations I think there is a problem with variable focus accuracy over distance (and light wavelength). IME the 50/1.4 and 5D2 is a perfect example of such a problem child. I guess 6D and 50/1.4 might not be dissimilar.

As Canon would say - "The best results are obtained by calibrating at the shooting location.", or words to that effect, which I take to mean conditions of light temperature, air temperature, distance and possibly even light intensity. So if you calibrate at 2.5m in bright daylight and then shoot at 8m (for example) under geled tungsten lighting you may indeed run into difficulty, not helped any by the less than stellar focus precision of the 50/1.4.
 
Ive also experienced this but with a couple Sigmas, fine up close but more than +20 at a distance.
 
daugirdas said:
The 50/1.4 should have no trouble focusing accurately over the whole focal distance range. You are either experiencing faulty lens, poor AF or using bad focusing technique
Click to expand...

Then put it down to a faulty lens, because it is not technique that is the problem. When attempting to accurately set AF microadjustment the consistency of focus is far, far wore than with a lens such as my 85/1.8. I'm not in the least surprised as the AF mechanism is crude by comparison and at f/1.4 the dangers of a focus miss even higher when used wide open.

AFMA plot from 50/1.4, showing very random focus precision....

20111224_110001_000.png


AFMA adjustment plot from 85/1.8, showing a perfect bell curve of sharpness vs AFMA value....

20111224_111713_000.png
 
tdodd said:
Then put it down to a faulty lens, because it is not technique that is the problem. When attempting to accurately set AF microadjustment the consistency of focus is far, far wore than with a lens such as my 85/1.8. I'm not in the least surprised as the AF mechanism is crude by comparison and at f/1.4 the dangers of a focus miss even higher when used wide open.

AFMA plot from 50/1.4, showing very random focus precision....


AFMA adjustment plot from 85/1.8, showing a perfect bell curve of sharpness vs AFMA value....
Click to expand...

85mm is a lot better lens due to proper USM motor, but before writing off 50mm it needs to be tested with something more advanced than 5DII. I have seen lenses that struggled with 40D (same as 5DII AF) and worked well with 1D.
 
I get similar results with my d800 and 85 especially. Shooting wide open at 4.5m(50x FL) and under it's pin sharp at 9+ MA using focal and real shooting.

Shooting at infinity/10m it's totally out, you can see the ground In front about a foot ahead is where the dof lands.

A tad annoying as I mainly shoot closer for head shoulder shots but occasionally shoot longer.

I've not used any other focal parts bar the MA tune so may test it on that.
 
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