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Is it possible to DIY fix front focus?

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Thread started 23 Sep 2008 (Tuesday) 04:35   
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KarlosDaJackal
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I thought my 2nd EF 50mm 1.8 II might be front focusing so I've loaned it out to a couple of people and they seem to think the same. Its not enough to make the lens unusable if your aware of it.

So my question is, what causes a lens to front/rear focus. How do you fix this (assuming you can't send it to someone to do it for you). As this lens cost so little its not worth repairing it, I'm tempted to open it up and try fix it myself but I've no idea what to look for inside it.

Assuming the 5 cameras this lens has been on don't all front focus the same amount. Is it a case of something being too loose or to tight in the lens? Has a gear skipped a tooth when it was manufactured. What are the variables that feed into lens focus issues?

Post #1, Sep 23, 2008 04:35:21


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mercury-666
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what is front focusing?

Post #2, Sep 23, 2008 05:03:14


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KarlosDaJackal
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mercury-666 wrote in post #6363342external link
what is front focusing?

When in Autofocus, you focus on something take the shot, but something closer to the camera is in focus and the thing you aimed at is not. Like these 2 pictures I took of an xbox controller below.

* The top is my sigma 24-70 at 50mm f/2.8 nailed focus on the green cross button in the middle.
* The bottom is my EF 50mm at f/2.8 same camera, same focus point same framing, same everything, but it appears to have front focused so the controller stick on the left appears to be more in focus.
* The lens still works perfectly if you manual focus it.

IMAGE NOT FOUND IMAGE IS A REDIRECT OR MISSING!
http://farm4.static.fl​ickr.com ...60128066_c7cc857c62​_b.jpgexternal link
HTTP response: NOT FOUND | MIME changed to 'image/gif' | Redirected to error image by FLICKR

Post #3, Sep 23, 2008 05:09:21


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TeamSpeed
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I expect it is not that easy, I suppose Sigma fixes parameters on the chip to move relative 0 back or forward. However, the 50D, 5DII and 1D(s)III all have the capability to compensate focus via the menu. Very easy and works well!

Also maybe try a different test. Put something of high contrast on the carpet and sit back a ways, then focus on that object with both lenses, same settings. You can use the carpet fibers to tell where focus ends up.

Post #4, Sep 23, 2008 05:54:40


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KarlosDaJackal
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I've more tests done, and have similar results from the real world, similar results from other togs, and similar results with other bodies.

Even if my lens was fine, i'd still like to know what mechanically would be adjusted.

Post #5, Sep 23, 2008 06:03:56 as a reply to TeamSpeed's post 9 minutes earlier.


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mercury-666
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ok thanks :) i thought it had something to do with the front piece turning or something. i dotn actually have and slr...

Post #6, Sep 23, 2008 06:35:40 as a reply to KarlosDaJackal's post 1 hour earlier.


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cctsm
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Well, it does depend on the lens. However, front-focus is due to the focusing elements of the lens moving too much/little, and is compensated for in software. I really don't think there are any screws to turn or parts to move. You just slap the lens on a calibration machine, and let it twiddle itself; I'm not sure if the protocols for the lens allow tuning, so the lens might have to be opened to access a debug port.

Post #7, Sep 23, 2008 06:41:57


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KarlosDaJackal
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I get what you are saying, but.....

All EF50mm 1.8 II are built to the same mechanical standards and tolerances. All should be given the same setting for focusing during manufacture I believe, or is every lens given a specific calibration before it leaves the factory?


If my assumptions are correct...

The fact that some front/back focus either means that a different calibration went on for that one lens (not likely) or the mechanical components fall out of spec (more likely as different pieces of factory tooling get worn/replaced at different times)

I think the lens mount looks to sink in a bit more than it should, maybe that is moving the lens a millimetre away from the sensor and causing the focus error. If I run my finger around the edge with the metal contacts their is definitely a ridge between that and the back of the lens element. I can see this back piece is held on with 3 or 4 very small screws. I don't know though, i'm only speculating.

Post #8, Sep 23, 2008 07:17:50 as a reply to cctsm's post 35 minutes earlier.


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fxk
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I don't know for fact - but I believe all Canon lenses are calibrated via changes to the "chip" in the lens - not by mechanical means.

Post #9, Sep 23, 2008 12:04:05




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EoSD30fReAk
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most canon lenses are calibrated with a computer, but the 50 f1.8II is one of a few Canon lenses that are calibrated with soldering or desoldering contacts on the main board.

so if you are handy with an soldering iron you could do it yourself.

if you want, i can post some pictures here with a step by step guide on how to do it.

Post #10, Sep 23, 2008 12:15:50 as a reply to fxk's post 11 minutes earlier.


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KarlosDaJackal
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I can solder with the best of them, so post away :cool:

Post #11, Sep 23, 2008 12:19:24 as a reply to EoSD30fReAk's post 3 minutes earlier.


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EoSD30fReAk
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ok,

first you have to remove the part where the serial number is stamped.

i use a pair of tweasers to open it, starting from the contacts

IMAGE: http://img.photobucket.com/albums/v59/EoSD30fReAk/50mm%20focus%20repair/50mm01.jpg

IMAGE: http://img.photobucket.com/albums/v59/EoSD30fReAk/50mm%20focus%20repair/50mm02.jpg

then turn the AF ring out so that the rear lens lowers and the main board becomes more accessible.

now you'll see these 3 contacts:
IMAGE: http://img.photobucket.com/albums/v59/EoSD30fReAk/50mm%20focus%20repair/50mm03.jpg

if they are open, fill them with solder and if they are closed open them.
try one by one until you find the right combination.

example 1 open 2 open 3 open
1 closed 2 open 3 open
1 open 2 closed 3 open

so there are a few combinations but in my experience it's not so much work.
and you'll have a perfectly sharp 50mm :D

Post #12, Sep 23, 2008 12:46:54 as a reply to KarlosDaJackal's post 27 minutes earlier.


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KarlosDaJackal
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Cool :cool:

I hereby award you my post of the month award for most useful information in a single post.

Post #13, Sep 23, 2008 12:50:56 as a reply to EoSD30fReAk's post 4 minutes earlier.


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EoSD30fReAk
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you're welcome,

and thanks for the award :D

Post #14, Sep 23, 2008 13:12:46 as a reply to KarlosDaJackal's post 21 minutes earlier.


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number ­ six
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Always good to see people ready to jump in there with a soldering iron!

Awaiting report...

-js

Post #15, Sep 23, 2008 14:10:08


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