Does backfocus/frontfocus change?

Question

I have adjusted the back-/frontfocus setting in my camera for different lenses.

Now I am wondering if I have to readjust the lenses regularly? Does the back-/frontfocus for a lens change much over time?

Answer

No. Well, I've never seen it happen.

If it does, I would suspect an imminent mechanical failure.

What I did see once is front or back focusing change between focus-distances. In that case the lens focuses well at one distance but is off at another. Since I've only see it once, it may a defect rather than tolerance error.

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What do “Frontfocus” and “Backfocus” mean?

Question

I've read those terms in a couple of lens reviews, and not even Wikipedia has an article about it.

So my questions are:

• What is back/frontfocus?
• What causes back/frontfocus?
• Is there any preventive measure to avoid getting back/frontfocus?

Answer

Front focus is when the lens and camera focus in front of your intended focus point. Your subject will look slightly out of focus and something in front of them will be razor sharp in focus.

Back focus is correspondingly when something behind them is in focus, instead of your intended subject.

As to why.. it could be mis-aligned, mis-calibrated equipment. It could be you or it could be your subject. You or your subject could have moved slightly after focusing and this would also cause front or back focus.

In terms of why it would be this way for the equipment:

• Things may be out of spec from the factory, due to poor QA.
• Things could be in an acceptable QA range, but your combination of camera and lenses could be at opposite ends of the acceptable range.
• Your autofocus could be not properly functioning due to a software issue (this seems to be the case for the Pentax K-x camera with tungsten light sources...go figure).

There's a host of why, but most of them boil down to something being not quite calibrated, aligned, or working 'properly'.

Modern choices to fix it usually include one of three options that I'm aware of:

1. Send your camera and lens to the manufacturer to be 'fixed' or calibrated.

2. Your camera may have a global adjustment in which you can tell it to always adjust its autofocus front or backwards by a small adjustable amount.

3. Your camera may have a per lens adjustment in which the camera will remember that this lens always needs a certain small adjustment that you provide in order to function properly. (Usually up to some reasonable number of lenses.)

Which of these is an option, typically depends on the 'level' of camera you have. More entry levels (like my Nikon D3100) have no adjustment options. The Pentax K-x (mid level), for example, offers a global adjustment and the Pentax K-5 (mid-upper level) offer a per lens adjustment.

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What is the best way to micro-adjust a lens?

Question

I have a Canon 7D with a 50mm f/1.4 lens, and I think the auto-focus of the lens is off. How can I test and adjust this reliably?

Will this approach work with all of my lenses? If I had a different camera body, would I have other/different options?

Answer

Moire Fringe Method

Use Bart van der Wolf's moire fringe method (also explained here and here, and archived here):

It works by exploiting the interference patterns or moiré between the R/G/B LCD elements and the camera's LCD elements when directly viewed with Life View [sic]. With good optics and perfect focus, the moiré is maximized.

Compared to focus charts

Pros:

• Much more precise.
• Unaffected by tungsten / incandescent lighting, which causes front focus. (I'm not positive if extreme monitor color temperatures affect it.)
• Easier to line up 100% perpendicular, yet less affected by it.
• Doesn't require taking a picture: liveview is sufficient with magnification.

Cons:

• Without liveview, I'd imagine it'd be tedious.
• Can't calibrate for tungsten lighting. (Though you can use a focus chart to supplement, and estimate the offset you'd need to give it for tungsten)

The Target Pattern

Load this file (or from this alternate location). It's a black-and-white image of concentric rings which get increasingly small and close as the they get further from the center circle.

There's nothing particularly magic about this image: anything which produces a moire pattern on an LCD screen should work, but this one is designed to give good results in many situations. Bart van der Wolf also produced an earlier moire target design which some people apparently find works better.

Steps

Setup and familiarization:

1. Load the target pattern at 1:1 / 100% view in any image viewer — your web browser will do, but make sure it's showing the image unscaled.
2. Set up your camera on a tripod perpendicular and at the appropriate distance away from the screen
   • Camera-to-subject distance should ideally be no less than 50 times the focal length of the lens. For a 50mm lens, that would be at least 2.5 meters (25m for a 500mm).
3. Turn on liveview and magnify until the image is close to filling the screen.
4. In manual focus mode, adjust the focus distance and become familiar with the maximum interference pattern

Method 1:

1. Go to the point of maximum interference. You do this by focusing manually (contrast detect may not be as precise, but you can try).
2. Switch to phase detect and push the AF button.
3. If the focus changes, dial in microadjustment in the correct direction and repeat.

Method 2 (more accurate, in my opinion):

1. Set focus to infinity or closest focus.
2. Autofocus using phase detection. Some cameras let you do this while in liveview.
3. In liveview, manually adjust focus to see if it was front or back focused.
4. If so, adjust and repeat.

Troubleshooting

If you can't see a moire effect, see these tips, which are, in summary:

• If the focus is too far off, it won't work.
• You could be too close for the focal length.
• You could be too far away for the focal length.
• The lens has poor resolution.
• The lens's manual focus control is too coarse to nail the spot.

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