Front focus/back focus technique

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Archive 2009 · Front focus/back focus technique

kkroeker
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p.1 #1 · Front focus/back focus technique

I ahve been scouring the net for a technique to test the front/back focus of my lenses. I have tried a few methods and I am getting different results each time. Any advice? I am shooting a Canon 7D.

Is this method accurate?
http://www.focustestchart.com/chart.html

Oct 28, 2009 at 07:31 PM

kylegehmlich
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p.1 #2 · Front focus/back focus technique

That's how I check my focus. Rulers, type on a page, anything flat with some sort of markings, really.

Oct 28, 2009 at 08:01 PM

dcains
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p.1 #3 · Front focus/back focus technique

kkroeker wrote:
I ahve been scouring the net for a technique to test the front/back focus of my lenses. I have tried a few methods and I am getting different results each time. Any advice? I am shooting a Canon 7D.

Is this method accurate?
http://www.focustestchart.com/chart.html

ALL tests where the AF point falls along a diagonal ruler are crap, which you've evidently discovered. Try something like this (good, natural light, distance of ~50X focal length):

This image is copyrighted by the owner

Those are center crops, BTW. Use a tripod, and defocus between shots.

Oct 28, 2009 at 08:11 PM

sapro
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p.1 #4 · Front focus/back focus technique

dcains wrote:
ALL tests where the AF point falls along a diagonal ruler are crap, which you've evidently discovered.

If you have crappy setup, then you will get crappy results. 45 degree ruler method, such as LensAlign products, is the most accurate, and it gives you the real DOF. But you need to know how to set it up, if you can't do it yourself, then buy the LensAlign Pro.

Oct 28, 2009 at 08:22 PM

dcains
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p.1 #5 · Front focus/back focus technique

The LensAlign does NOT have AF points on its diagonal ruler. The AF target is flat and aligned to be perpendicular to the sensor. The ruler simply provides a measure for the amount of front or rear focus error. It's a highly accurate, and more importantly, repeatable method, but as has often been debated here, it's $$$. My 3 soup cans were ~$5, and I had some nice lunches, too.

Oct 28, 2009 at 08:29 PM

RAF09
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p.1 #6 · Front focus/back focus technique

If you set the LensAlign up per instructions the point you are focusing on puts the scale of the ruler in focus so you can tell whether your lens is focusing in front of or behind the the point the CAMERA is saying is in focus. From there you can MicroAdjust the camera to the lens. Canon also has a method for adjusting but it is on a use by use basis. If the soup cans or newsprint on the wall work for you, great. But if you have a larger problem with a particular lens other methods may be of more use.

Oct 29, 2009 at 11:14 PM

kylegehmlich
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p.1 #7 · Front focus/back focus technique

dcains wrote:
ALL tests where the AF point falls along a diagonal ruler are crap

I must've missed the memo. Why are they crap?

Oct 30, 2009 at 02:52 AM

omarlyn
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p.1 #8 · Front focus/back focus technique

kylegehmlich wrote:
dcains wrote:
ALL tests where the AF point falls along a diagonal ruler are crap

I must've missed the memo. Why are they crap?

The best (most accurate) AF tests are done when the objest of focus is at 90 degrees (perpendicular) to the sensor plane. Using the AF test that the OP described introduces a degree of +/- error due to the angle of the focus object. It's best to focus on a single line that is flat & perpendicular to the camera with the ruler/chart below the focus point for reference.

Omar

Oct 30, 2009 at 06:04 AM

digitalbug30d
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p.1 #9 · Front focus/back focus technique

thinks basic photography DOF is confused with a Front focusing/back Focusing problem...plus this issue is as irrating as pixel peeping.

Oct 30, 2009 at 06:16 AM

dcains
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p.1 #10 · Front focus/back focus technique

digitalbug30d wrote:
thinks basic photography DOF is confused with a Front focusing/back Focusing problem...plus this issue is as irrating as pixel peeping.

True. Plus, look at this ridiculous setup:

This image is copyrighted by the owner

Even if one could assume this 45-degree setup works (which it absolutely does not), how many of you guys have lenses this setup would actually work with? I've got one; my Canon 100/2.8 macro, out of my dozen or so lenses. Sure you want to trust this guy? None of the major manufacturers support/endorse this sort of testing, and especially at such a short distance from lens to target. Sorry, but it's crap, and if you don't believe me, run the tests yourself and see if you get consistent results. If the results are not repeatable, every time, they're worthless, and a waste of your time.

If your lens(es) really have a front/back focus issue, you'd see it in actual use a lot more than you'd see with the focus test shown above. I'll admit the only reason I even played with my soup can setup was that I was home from work for a few weeks with a hand injury, and I was bored out of my skull.

Oct 30, 2009 at 12:30 PM

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