70-200mm sigma front and back focus K10D

S

steve_t

Messages
112
Name
steve
Edit My Images
No
Hi,

I've just picked copy of sigma 70-200 apo dg ex f2.8 lens but I'm experiencing front focus at 200mm wide (a little)and back focus at 70 wide (quite alot) :(

I've used the k10d debug menu to resolve the BF issue with my fa 50 1.4 but how can you resolve front and back focus issues within the same lens?

Is it more likely a camera or lens issue? both are out of warranty but if the
lens is faulty I should be able to return it.

Anyone come across having front and back focus issues on the same lens? if so what did you do to fix it?

Any pointers would be most gratefully received

Cheers
Steve
 
If you've had to resolve an issue already with another lens, then I would be inclined to suspect body AF caliberation not the lens.

This isn't something you can fix.
 
hi,

thanks for the reply, what i'm confused by could a miscalibration cause front to back focus issues along it's focal length? Most of what I can find on the issue is lenses are front focusing or back focusing but not both....:shrug:

cheers
 
most (or many) zoom lenses when zoomed need to be re-focused (they are not par-focal) so I can understand (theoretically) how this can happen.

But this isn't something you as an end user can fix. Even if you had AF fine tune per lens on your camera, it wouldn't work for you as you can only adjust for BF or FF at one particular focus length.

In the first instance I would suspect the body. If you want to send the lens to Sigma they will ask to see your body too.
 
Ok, cheers for that...

Looks like I may have to send it in, the camera was bought in march last year but I've only now been able to test the AF with a fast zoom - It may have always been faulty.....:(
 
How are you testing your lenses for focus accuracy?

If it is using that sheet of A4 paper test shot at 45 degrees I would say that not only is it prone to errors, both from a technical point of view and also user error, but it is also extremely critical to the point where it shows up perceived errors that are actually within accesptable tolerance, ie there's nothing actually wrong!

Can you post any pictures showing the focus errors you mention?
 
Hi,

I post some images at http://www.flickr.com/photos/41529211@N03/

They are 70mm f2.8, tripod, you will see some with the correct focus point and
some with it backfocusing ( and a whole lot more backfocusing images on the drive)

The front focus at 200mm has no real impact overall and overall from 100mm to 200m I quite pleased (135 to 200 very good)

Any suggestions?

Cheers
 
What, specifically, is wrong with those test pics? Only #2 of the railings is not sharp where you have placed the letter F (and that looks none too sharp anywhere - shutter speed?).

How are you focusing? Centre point, lock and reframe, or some other method?

Basically, I'm not getting a handle on your alleged problem yet... :thinking:
 
I am no expert but you don't say about your camera settings. Have you tried different metering or are you just using say spot metering, what about DOF etc. May I suggest you take the same picture several times with different settings then you can determine better what the problem may be. It is all too common to make a judgement on one picture with one camera setting, we have all made that error.

That particular lens has a minimum focus distance of100cm(39.4 inches), have you physically measured the distance you took the picture at?

Realspeed
 
hi

I use a centre focus point, take a look at the one with the blinds and the focus point on the corner, from 70-100 the focus point is way behind -100-200 is ok i've done 100's of pic tests like this but around 20% dont back focus so i want to show that it can get it right

I'll put up more bf pics when i get home

Cheers for taking the time to look
 
thanks for the replys all,

I was passing a retail park today and currys are selling the k20d with lens for £449:eek: so I picked one up (as I need a second body and was going to get another k10d for around £250-£299).

Went to the same place and sat the k20d on the tripod, f2.8 and centre fp set, tried the full range 70-200 and it focused fine (quite a few tests). Swap over to the k10d and it back focused from 70-100 then got better.

Kind of answers my question really,

Cheers all
 
Excellent news, glad that answers your question :)

Also, a spot on call from Puddleduck in post #2 (y)
 
You must log in or register to reply here.
Back
Top