What exactly is Front/Back focus problems?

Hey Guys!

I need some clarification on what exactly is front and back focus issue and what cause it. Does is happens due to a defect on lenses or it is the cameras fault.

Why I need to know? I thinking about getting the Sigma 50mm for my 5DII and I seem a few photographers complaining that they had to send it back to Sigma so they can calibrate the lens to their 5DII.

I sent an email to Sigma asking then and they confirmed the issue. They mentioned the mirror box but I have no idea what that it

Email trail below:
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Contact Us Submission from Erik Figueroa

Erik



You would only know when you put the lens onto your body. If your mirror box is off then a calibration will be needed. We have 2 5D mark 2’s here, one forward focuses and the other back focuses.



Regards,



Marc Farb

mfarb@sigmaphoto.com

Technical Representative/Custome​r Service

Sigma USA

15 Fleetwood Court

Ronkonkoma, NY 11779

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First Name: Erik
Last Name: Figueroa
Email: Deleted for privacy
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Contact Question: Technical

Question: I use a Canon 5D Mark II. If I buy the Sigma 50mm 1.4, Do I need to have it calibrated for my camera for sharpness? I have seems several photographers complaining about focus issues. Thanks

Submitted: September-05-2012 02:32:27 PM

Assuming the setting would be wide open for the lens when taking the test shot picture.

- Front focus = When you focus to the spot and the spot is not sharp. While the area in front of the spot is sharp.

- Back focus = Not sharp at focus spot, but sharp behind the focus spot.


To fix this, the lens needs to be calibrated to the camera for spot on focus. But you have a 5D2, you can MA it. Or you increase the f-stop to widen the DOF, so the subject will be in focus at the focus spot. If I am paying for f1.4, I would expect the lens to be sharp on my camera at f1.4 without MA.

Both cameras and lenses have tolerances from the factory, so the specific matching of your camera with any lens could be different to the next person, therefore, unless the lens is particularly bad, it isn't necessarily clear cut as to where the problem lies.

As mentioned above, you have the ability to micro focus adjust on the 5d2, so if it is front of back focusing just slightly (on your body) then you should be able to compensate for it... there are numerous testing methods out there you can use to evaluate whether it's focus needs adjusting!

Can the lens just be 'not sharp' but there's actually no ff/bf issue? Reason I ask is I have the sigma and I just find it way too soft.

I have tested the lens with various focus charts and seems to be focusing at where it should be.

So should I send my lens in? What can Sigma do to make it sharper? Or am I simply expecting too much?

see the article in the link:

http://www.lensrentals​.com …s-is-soft-and-other-myths

kin2son wrote in post #14954715
Can the lens just be 'not sharp' but there's actually no ff/bf issue? Reason I ask is I have the sigma and I just find it way too soft.

I have tested the lens with various focus charts and seems to be focusing at where it should be.

So should I send my lens in? What can Sigma do to make it sharper? Or am I simply expecting too much?

This is definitely possible yes... the lens could be focusing correctly but just isn't capable of producing the lovely sharp image you're wanting. My Sigma 70-200 went for calibration a few months back and it ended up going to Japan for optical correction because it was too soft based on their tests...

PLLphotography wrote in post #14954723
see the article in the link:
http://www.lensrentals​.com …s-is-soft-and-other-myths

+1

So lens microcalibration is what Sigma does to the lenses when someone send it with the body?

That's a great read! Thanks for the link.

PLLphotography wrote in post #14954723
see the article in the link:

http://www.lensrentals​.com …s-is-soft-and-other-myths