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by garik

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My question is, if you drop a camera, and you think you might have an AF issue, what are the mechanics involved that might cause a mis-calibration? I can imagine that the lens has movable parts -- but what about the body? In the body's case, it's mostly electronic and one would expect an 'all-or-nothing' breakage? Am I wrong?

My new 6D dropped onto carpet with a 35mm 1.4L attached to it, from a height of 18 in (45 cm). It's hard to tell whether focusing is spot-on because the camera is new, so I have no basis of comparison. I'm trying to set up more pixel-peeping-type focus tests to see if it's front or back focused. Right now I'm suspecting the lens might have been affected slightly. It doesn't seem as sharp...

This is less a 'what do I do here' question than a 'how does autofocus work, exactly, inside' type of question. I'm curious how a bump can affect AF in the body itself.

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I would be really surprised if a 6D and a lens built like the 35L would suffer from that short of a drop - but I am still sorry to hear it happened. I would just send the lens to Canon for calibration, and potentially even the body to match them up perfectly - the piece of mind alone will be worth the small cost. I know I'm not addressing your "how does AF work" question, that's why this is a comment and not an answer. –  dpollitt Jan 26 '13 at 19:25

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Wow, there are so many things that can be affected... Assuming there is damage (an 18" drop onto carpet isn't likely to have damaged anything, but it's possible) the most likely situation might be a warping of the lens mount so that the mount isn't parallel with the sensor (either the focus sensor or the main imaging sensor). The focus sensor would focus on one point, but another point at the same distance elsewhere on the sensor might be out of focus. Here and here are nice writeups with interactive graphics illustrating the two most common types of SLR autofocus systems.

Although somewhat broad, here is a comprehensive set of slides covering autofocussing/focus mechanisms that I find incredibly interesting...

To check your focus you can do a 'brick wall' test to see if focus is consistent across a parallel field. You can also do an 'autofocus' test to check front/back focus.

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You should also test with another lens, so if you do think focus is not consistent you can deduce if it's a body or lens problem. –  MikeW Jan 26 '13 at 23:04
    
Thanks for informational links. That's really useful. One test I found useful - but not ideal on a FF camera - was to take a picture of a sheet of music at an angle. I didn't have a test chart handy or a printer to print one out, but by focusing on certain parts of the music sheet, I could use it to see if I'm front or back focusing. It's hard on a 35mm FF due to needing to pick a point on the sheet of music to be honed in on by the AF point, but it's doable (I'll try the same on my cropped camera, so it'll look bigger!) –  Emmel Feb 1 '13 at 18:50
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Sure you can use a sheet of music. The only thing to be aware of is that you need to have a well defined focus 'point' (or line, as in the autofocus test page) so the camera doesn't inadvertently focus somewhere else. –  BobT Feb 1 '13 at 19:39

What SHOULD it do

A 35 mm f/1.4 anything stopped down to say f/4 or above and used to take well lit carefully focused closeup of eg a head of hair should produce an image so sharp that you'd cut yourself on the individual hairs.

Put the system in manual focus, put on a tripod and focus on a test target with fine detail. Take photos while moving test target through focal point in small steps (Start at say 3mm (1/8th inch) steps. You may wish to refine this to 1 mm or less in due course). There will be a best focus point. How sharp is it? Is it where the focusing view indicated it should be.

Put system in AF, focus, return to manual. (Some cameras allow you to use AF while in MF by holding a button down to focus. I don't know if your camera has that ability). Repeat tests as above. How do they compare. Is focus point as before. Is focus point where AF sets it?

Will it blend?

Dropping a piece of fine precision machinery that relies on precision positioning for its operation is capable of causing significant damage with even relatively mild shocks. Parts that move relative to each other during system calibration and which are then clamped in place, can shake loose the fetters of their clamping system when subject to forces 100+ times greater than their weight.

If a falling camera gives you the chance, put your foot under it. Even kicking a camera across the ground is probably less damaging than a direct impact - see below.

Your experience will vary with circumstance, luck, impact surface and angle that the camera hits. If there is any "give" at the time of impact it will help greatly.

Deceleration when dropped onto carpet will vary greatly but the g's experienced by the body are Fall_height / Stopping_distance. For a fall of 450mm a
1 mm stop = 450g (very thing matting),
3 mm stop = 150g still fairly thin.
5 mm = 90g (somewhat springy)
10 mm = 45g (shag pile or very spongy)

Forces exerted are usual_weight x g factor.
With bad luck a fall onto shagpile at that height can kill something.


YMWV

Your experiences will vary - mine did. I try to not make a habit of dropping cameras, but I've managed it 3 times in the last 15 or so years. In each case I dropped a Sony DSLR with inbuilt anti-shake (7D, 5D, A700) - the antishake has a reasonably sensitive mechanical mechanism that slews a "stage" in two dimensions at high speed.

1st drop (7D) was off a car sill (when I pulled a rolled up mains power cord out of a door). The camera fell perhaps 300 mm onto concrete and killed the anti-shake mechanism (11pm, wedding next day, Whoops!).

2nd drop (5D) was at a wedding reception - onto a wooden dance floor from ~= 1 metre - everyone gasped at the sound - and it worked perfectly afterwards. (Two cameras around neck, swapping cameras for different shot of dancers. Ah - it wasn't around my neck! Doh!).

3rd drop (3 is 3 too many) was onto a lift floor - probably somewhat more resilient than some surfaces. The camera struck on the lens hood, damaging the hood but leaving the camera working perfectly.

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