how do you fine tune the lens so that it Is not front or back focus

[desh4106]

how do you fine tune the lens so that it Is not front or back focus

[Skirball]

The only way to fix a lens is to send it back to the manufacturer.   You make the adjustments in the camera, not the lens.  However, only certain cameras have micro-adjustment (AFMA).  Which model are you using?

There are DIY ways to do it, but I'd suggest plunking down a little money for a program like Reikan's FoCal and have it do it for you.

http://www.reikan.co.uk/focalweb/

[hsbn]

It's called Micro Focus Adjustment. It's not available on all camera. Here is an article from Canon : http://www.learn.usa.canon.com/resources/articles/2011/af_microadjustment_article.shtml

Like Skirball said, there are free method of doing it but it is not as reliable. I've heard good thing about Reikan Focal.

[7D5D]

If your body supports AFMA, then I would highly recommend Reikan FoCal.  I had a very bad experience with LensAlign and I would not recommend it.

If your body does not support AFMA, then you will need to send your body and lens back to Canon.  They will not do it for free unless there is a issue and your gear is under warranty.

I have not personally tested it but I would expect f/4 or smaller lens should not have any issues with front/back focus unless there is an out-of-spec defect with the lens.

[diverhank]

Hi Desh4106...I'm a "little late to the party" but I'm gonna provide you with my 2 cents...

For us users, we technically don't  fine tune the lens on the lens, rather we compensate using the camera.  Only a certain high end cameras have this feature.  For modern Canon cameras, the 50D (not the 60D), 7D, 6D, 5D's, 1D's have this feature called AF Micro-adjustment (MA).  This feature allows you to enter in the amount of adjustment the camera will apply to the focus point to compensate for front or back focus.

In order for you to apply this amount, you need to be able to determine this.  As mentioned in the replies herein, you can buy a software program to help you do this.  Or you can do it yourself.  I find that it's not that hard to do it.  A quick google search will afford you all kinds of instructions and graduated charts designed to help you.

One thing to keep in mind is that the amount of front or back focus depends not only on the lens but also on the camera.  Regarding the camera, depending on what kind of light and how much light there is, the amount of back/front focus will vary.  It also depends what focus mode (single point, multi-points, etc).  Page 109 of the 5DMk3 manual mentions this and I find it to be absolutely true.  This means that you need to pick a certain shooting conditions you will adjust for.

To adjust, the idea is to aim at one point, take a picture and verify whether that point is the sharpest or some other points in front or back are.  If the point in front is sharper, then it front focuses and vice versa.   To help with this, there are graduated charts (for the MA value)  you can download for free to help you determine this.  Plug that in the camera, take the picture again and verify that it now focuses on the point that you aim at.  Of course, your camera and target cannot move at all during all of this.

Personally, the only meaningful micro-adjustment (that really helped me) was on the 100mm f/2.8L Macro IS lens that I use to take closeup pictures of bugs and flowers where depth of field is razor thin.  I was frustrated in not getting the sharpness I would expect.  After adjusting, I was able to get the picture razor sharp at my point of aim and I'm a believer in MA.

Canon has a nice article on this that you should read.

http://www.learn.usa.canon.com/resources/articles/2011/af_microadjustment_article.shtml