Scoop! 50L Pic IN Focus

Tried it out at work last night, with the dreaded centre focus point/f1.2-2.8/low light combo that seems problematic with some examples.
ISO 1600, f2.5, 1/200th, Looks fine to me...

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And the obligatory crop...

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Looking good! Looking forward to getting my hand on it!

What body are you useing?

sootyvrs wrote in post #3620009
Looking good! Looking forward to getting my hand on it!

What body are you useing?

400D. Forgot to include +2/3 exposure comp info.

bildeb0rg wrote in post #3620095
400D. Forgot to include +2/3 exposure comp info.

And the most important question of all, of course: what was the focusing distance?

Kind regards, Wim

These are near useless, as the center part of the images have little or no contrast, making it very difficult to determine back/front focusing issues.

Try again, but place the center FP on a higher contrast object so that we can better determine if the lens is right on, or backfocuses.

My own 50L has a very bad BF issue, and if I used it to take the shots you did, this issue would not really show....

Right....not the best target selection for evaluating any moderate focus calibration drifts.
Suggest a simpler contrasty target inclined about 45 degrees wrt the sensor plane, and shoot at f/2.8 preferrably, 2-3 ft distance.

PetKal wrote in post #3622237
Right....not the best target selection for evaluating any moderate focus calibration drifts.
Suggest a simpler contrasty target inclined about 45 degrees wrt the sensor plane, and shoot at f/2.8 preferrably, 2-3 ft distance.

I'm not in favor of the 45-degree angled target due to the fact that the AF sensors on the non-1-series cameras are considerably larger than the indicators in the viewfinder. The AF sensor can easily lock focus anywhere on the area it covers.

This 10D AF picture superimposed over a ruler illustrates this point:

Tom W wrote in post #3622390
I'm not in favor of the 45-degree angled target due to the fact that the AF sensors on the non-1-series cameras are considerably larger than the indicators in the viewfinder. The AF sensor can easily lock focus anywhere on the area it covers.

This 10D AF picture superimposed over a ruler illustrates this point:

In this case, if the left side of the AF sensor detects focus, it will appear to front focus, whereas if the right side of the AF sensor detects, it will appear that the lens is backfocusing. I prefer a flat target, with an angled scale or other array to show depth.

Sorry about the slide-rule. Apparently, it was the only ruler I had available at the time.

Tommy, your argument has certain force despite its source.
There are traps in whatever one does.....one has to be careful while testing.
However, the key is that one has to be able to see a focus drift if present. The target geometry/contrast has to allow that.

Assuming it functions as advertised, will the micro adjustment available on the Mark III (not saying the OP owns one) correct a back focusing lens, or front focusing one for that matter?

MDJAK wrote in post #3622468
Assuming it functions as advertised, will the micro adjustment available on the Mark III (not saying the OP owns one) correct a back focusing lens, or front focusing one for that matter?

I don't think it would assuming the 50L defocusing amount is a function of aperture.
I believe that MKIII microadjustment does a linear focus biasing....that's not good enough for the non-linear focus calibration error some of us have observed with that lens.

MDJAK wrote in post #3622468
Assuming it functions as advertised, will the micro adjustment available on the Mark III (not saying the OP owns one) correct a back focusing lens, or front focusing one for that matter?

If it consistently back- or front-focuses at all apertures, then yes. But if the problem is focus shift at particular distances and apertures, then no.

Yohan Pamudji wrote in post #3622506
If it consistently back- or front-focuses at all apertures, then yes. But if the problem is focus shift at particular distances and apertures, then no.

Correct, and I believe that the focus problem on the 50/1.2L is one that is related to focus distance and aperture. At least that's what I've gathered from various discussions around the net. Never tried one myself.

PetKal wrote in post #3622458
Tommy, your argument has certain force despite its source.
There are traps in whatever one does.....one has to be careful while testing.
However, the key is that one has to be able to see a focus drift if present. The target geometry/contrast has to allow that.

I recall some anchient test I performed back in the 10D days, where I focused on an inverted golf tee, with the angled scale next to it to provide a visual indication of AF performance. The Tee was the target, the scale was, well, the scale....

Anyway, something like that with a nice, high-contrast target that is larger than the AF sensor and a measuring tool would work well, I think.

Tom W wrote in post #3622577
I recall some anchient test I performed back in the 10D days, where I focused on an inverted golf tee, with the angled scale next to it to provide a visual indication of AF performance. The Tee was the target, the scale was, well, the scale....

Anyway, something like that with a nice, high-contrast target that is larger than the AF sensor and a measuring tool would work well, I think.

Another simple test is to make the target -camera relative geometry rigid...i.e., nothing moves. Then, take shots at incresing apertures, from f/1.2.....to f/2.8. Observe the change in focus drift. Normalize the results to the drift at f/1.2.

In such case, the AF sensor size relative to target detail is taken out of the equation.

Incidentally, on the 50 f/1.2's I checked, the focus drift at f/1.2 was negligible.