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Old May 11, 2015, 8:20 AM   #1
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Default micro adjustment for 50mm 1.2 lens

This past weekend a local camera store was offering to do microadjustments on the spot, so I gave them a try.

When I came home, I did my own test.

I used a 36 inch yard stick resting on its edge and set it at a 45 degree angle to the camera about 5 feet away.
I center focus was aimed at the 6 numeral in the numeral 26.

When looking at the photo, the 27 was sharper than the 26. So I made a few micro adjustments of my own and got the 26 to be sharp, with the 25 and 27 being slighlty less sharp.

They gentleman who did the micro adjustment was using the black and white grid. I was using a 45 degree ruler.

Should I trust my own test or go back to the setting that he made with the grid? I'm not sure which is more reliable. In my opinion, his setting caused this camera and lens to front focus.

How do you know which micro is right?

Thanks!
FP
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Old May 11, 2015, 9:19 AM   #2
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One of the tings that happens a lot with very large aperture lenses like the Canon 50/1.2 is that the focus will shift as you stop down.

See http://diglloyd.com/articles/Focus/FocusShift.html and http://www.lensrentals.com/blog/2010...perture-lenses

You may have set the microadjustment to give you accurate focus at f/1.2, but if you use it at f/2.0 the focus will be off. If you never stop it down, what you did will be fine. It's possible that what the guy in the store did was to set it at a happy medium.

As long as the subject is within the depth of field, everything should be fine, but with an f/1.2 lens that depth of field can be pretty small. What matters is how you use it.
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Old May 11, 2015, 10:02 AM   #3
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Thanks TCav, you're the best!

I have read that lenses often perform their best about 2 stops down from their lowest fstop. So the 1.2 sweet spot is about a 2.0.

So if I shoot that lens mostly at 2.0, I'd be best off by doing a micro adjustment at with the camera set at 2.0 and stop worrying about it. Is that about right?

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Old May 11, 2015, 10:57 AM   #4
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That lens is pretty sharp in the center at any aperture, but it seems to be sharpest from f2.8 to f/5.6. If you set it at f/2.8, that should cover your work at f/2.0.

But whatever you do, the camera will always pick it's focus with the lens wide open, so after you set the microadjustment at whatever aperture you chose, make sure it's still close at f/1.2. Otherwise the focus will always be off (though probably inside the DoF.)
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Old May 11, 2015, 10:58 AM   #5
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You had no idea you were opening such a big can of worms, did you?
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Old May 11, 2015, 11:53 AM   #6
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Try repeating your test with the ruler angled at 45 degrees in two different directions and compare them. Be sure to use the same aperture setting, as focus falls off sooner, closer to the camera. If you obtain the same results, use your correction.
Cameras normally use maximum apertures when focusing, so whatever your aperture setting, the focus is done wide open unless you manually focus with the lens stopped down.
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Old May 11, 2015, 5:13 PM   #7
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Therein lies the problem. Very large aperture lenses experience focus shift, where the camera sets focus when the aperture is wide open, but if an aperture other than wide open is used for the exposure, the subject that was in focus at f/1.2 is no longer in focus at f/2.8.
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Old May 11, 2015, 9:28 PM   #8
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I'm having a bit of trouble with that concept. It doesn't seem to fit with how lenses are designed. Will have to look into it some.
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Old May 12, 2015, 4:36 AM   #9
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See http://diglloyd.com/articles/Focus/FocusShift.html and http://www.lensrentals.com/blog/2010...perture-lenses
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Old May 15, 2015, 6:36 PM   #10
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The mirrorless brigade chuckle....
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