Forgot Password
Pentax Camera Forums Home
 

Reply
Show Printable Version Search this Thread
11-16-2008, 03:12 PM   #1
Senior Member




Join Date: Mar 2008
Location: Brisbane, Australia
Photos: Gallery
Posts: 141
Sigma 70-200 focus issues, worth sending to Sigma?

Hi all, on the weekend I shot some portraits of my girlfriend. After reviewing the shots I found that the majority of the shots had focus issues, 60% were usable. I later tested the focus and found the lens to have pretty severe back-focusing up until 160mm and severe front-focusing up until 200mm.

I've set the AF adjustment at -10 and it's pretty much fixed the back-focusing but obviously the front-focusing at the long-end is worse.

Have any of you guys had any experiance with sending lenses to Sigma for adjustment (especially in Australia)?

11-17-2008, 06:23 AM   #2
Senior Member




Join Date: Mar 2008
Location: Brisbane, Australia
Photos: Gallery
Posts: 141
Original Poster
Nobody???

I should mention that after I got the impression the lens was miss-focusing I used the focus charts to get some solid confirmation. Stopping down to around f4-5.6 negates the problem but I'd love to get reasonably consistent results wide open.

Could using the focus then recompose method be a contributing factor?
11-17-2008, 09:28 AM   #3
Veteran Member
Ben_Edict's Avatar

Join Date: Jul 2007
Location: SouthWest "Regio"
Photos: Albums
Posts: 3,303
First of all: I have an old version of the Sigma 70-200/2.8. It works flawlessly and focuses right on the spot. Especially for portraits, focusing is critical. Usually one would aim at the eye of the subject. In my experience this is rarely achieved at the first try, because the AF spot is hard to place at the eye. So the result is a focused nose or focus on the hairs, but a slight unsharpness of the face - looks like front or backfocus.

I have a very strong conviction, that focus charts are very, very overrated in their value. First of all, they are usually used at near distances. Then, and this is really important, they simply do not take into account, that the actual AF sensor may be not exactly placed where the viewfinder's little blinking mark is. It is well-known, that the AF sensors can be offset from the viewfinder marks by a millimeter or so (and that is not only the case for Pentax). This will lead to misfocusing.

Personally I only trust flat test charts with varying distances from the camera. This is the only way to distinguish real front or backfocus from simple misalignment between sensor and marks.

Also, the results of a focus test will be more meaningful at typical shooting distances. At near distances, abberrations like spherical abberation or a curved field will lead to results similar to front or backfocus. These abberations won't be as severe at further distances. Only macro lenses should be tested on a table top target, because this is representative of their use.

Ben
11-17-2008, 02:50 PM   #4
Senior Member




Join Date: Mar 2008
Location: Brisbane, Australia
Photos: Gallery
Posts: 141
Original Poster
Hi Ben, I completely agree with everything you're saying. I might do some flat wall test later as they mimic real world shooting better than the focus charts do. I must admit that it's a bit odd that this is the first time I've noticed these issues with the lens in nearly 6 months of use.

Which is best to achieve critical focus, I was trying to focus on the eyes and the out-of-focus shots were exactly as you described. I've read that focusing on the eye-brows can provide more consistent results.

11-17-2008, 03:06 PM   #5
Veteran Member
Ben_Edict's Avatar

Join Date: Jul 2007
Location: SouthWest "Regio"
Photos: Albums
Posts: 3,303
QuoteOriginally posted by nobbsie Quote
Hi Ben, I completely agree with everything you're saying. I might do some flat wall test later as they mimic real world shooting better than the focus charts do. I must admit that it's a bit odd that this is the first time I've noticed these issues with the lens in nearly 6 months of use.

Which is best to achieve critical focus, I was trying to focus on the eyes and the out-of-focus shots were exactly as you described. I've read that focusing on the eye-brows can provide more consistent results.
I haven't read the eye-brow tip. But sounds quite good. Whether it works better, than the eye, is mostly a question of light, I think. And as the eye-brows are not recessed, like the eye-ball, they should be easier to target. I will try that myself, sound rely interesting. If it works, it makes life easier... But not with f/1.4 at 4 feet distance, the depth of field would be too shallow, I think (but didn't calculate).

Ben
Reply

Bookmarks
  • Submit Thread to Facebook Facebook
  • Submit Thread to Twitter Twitter
  • Submit Thread to Digg Digg
Tags - Make this thread easier to find by adding keywords to it!
adjustment, back-focusing, focus, front-focusing, issues, k-mount, pentax lens, shots, sigma, slr lens
Thread Tools Search this Thread
Search this Thread:

Advanced Search


Similar Threads
Thread Thread Starter Forum Replies Last Post
K10d Focus Issues w/ Sigma 120-400mm HSM weldingblues Pentax DSLR Discussion 1 07-10-2010 03:13 AM
Sigma 70-200 HSM and front-back-focus alexulo Pentax SLR Lens Discussion 0 04-08-2010 05:34 AM
Sigma 70-200 HSM Slow Focus Issue joelovotti Pentax SLR Lens Discussion 2 09-14-2009 06:12 PM
Why wil my sigma 18-200 not auto focus? zak Pentax SLR Lens Discussion 7 05-13-2009 01:50 AM



All times are GMT -7. The time now is 01:29 PM. | See also: NikonForums.com, CanonForums.com part of our network of photo forums!
  • Red (Default)
  • Green
  • Gray
  • Dark
  • Dark Yellow
  • Dark Blue
  • Old Red
  • Old Green
  • Old Gray
  • Dial-Up Style
Hello! It's great to see you back on the forum! Have you considered joining the community?
register
Creating a FREE ACCOUNT takes under a minute, removes ads, and lets you post! [Dismiss]
Top