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10-05-2009, 02:39 PM   #1
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K20d + Sigma 50mm 1.4: frontfocus only at large apertures

Hi all,

I have just bought my first Pentax, a K20d.
As my intention is to use it to make photos of my newborn daughter, without flash, I have paired it with a Sigma 50mm 1.4 (with good reviews including on dpreview).

When the lens focuses right, the ensemble is good. But, there is a huge problem: there is strong front-focus at high apertures, from 1.4 to 2.4. The AF correction needed is -10 (giving somehow the impression that at 1.4 a -12 would be better).

The problem is that with this correction, at low apertures (from 2.8 on) it backfocuses badly. The correct "correction" for aperture 8 is 0 (no correction).

I have tried both focus charts, moire effects, real objects, made a few hundred test shots, I am sure about the behavior.

Do you have any idea of how could one identify the culprit (in other words, is it the camera or is it the lens)? I have no other lens, not even the kit lens, as I bought it body only.

I have still a couple of days allowance to send back the gear. I don't want to send back (to amazon) a camera that's good if the lens is in fact producing the problem. The same for the opposite situation, where the lens is good but the camera is flawed. The most I would hate to send them both, not knowing which was wrong (and then also being afraid to buy an identical setup, that I still like when it focuses).

Now, you are the experts in Pentax. What test could I do to clarify the issue?

Thank you

10-05-2009, 03:04 PM   #2
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What you have described about the Sigma 50mm f/1.4's focusing is unfortunately not specific to Pentax. I know of two friends who encountered the same focusing issues with this lens with their Canon 5D Mk IIs. Both had to send their lenses for calibration with the local agent. Agent could reduce but not completely lick the issue. The lens gives nice images but I'm not sure why Sigma still has this issue, which also reared its head on the earlier Sigma 30mm f/1.4 too.
10-05-2009, 03:13 PM   #3
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QuoteOriginally posted by chrisg Quote
When the lens focuses right, the ensemble is good. But, there is a huge problem: there is strong front-focus at high apertures, from 1.4 to 2.4. The AF correction needed is -10 (giving somehow the impression that at 1.4 a -12 would be better).
Try your focus tests in natural light (sunlight), you'll probably find no problem. AFAIK most camera systems (including Pentax and Canon) have a front focus problem under Tungsten light with very large apertures.
10-05-2009, 06:33 PM   #4
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QuoteOriginally posted by audiobomber Quote
Try your focus tests in natural light (sunlight), you'll probably find no problem. AFAIK most camera systems (including Pentax and Canon) have a front focus problem under Tungsten light with very large apertures.
Well I can say for certain the focusing issues on my friends' Sigma/Canon combo was present irrespective of whether shots were taken in daylight or artificial illumination. Also confirmed by the Sigma service centre.

10-06-2009, 12:47 AM   #5
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Thank you very much to both of you who replied. I had read about the yellow light effect on auto-focus, so I had made the test at daylight.
It is too bad that the camera system is "closed". It should have been easy to implement (program) an aperture-dependent correction. Storage for the parameters could have been the SD card in the worst case, if the eeprom was too small.
10-29-2009, 04:55 AM   #6
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Hi chrisg,
I have the same problem with my Sigma on K200D rig.
I made some tests on close and far distances.

If I shot at f/1.4 - f/2.5 I get sharp and crisp pictures,
however when changing the aperture on the camera to 2.8 or higher (value) makes the camera refocus the lens differently.

In case you dont understand what I mean...Put the camera on tripod, point it towards a wall or whatever's contrasty enought for af system. Then halfpress the button on f/1.4 - you'll get the lens to focus somewhere and lock the focus(no matter if its going to be a sharp picture or FF, or BF). Then change the aperture to f/1.7 or f/2 - halfpress the button - you should just get a bip without any refocusing - cause the object didt move. However...
If I change aperture to 2.8 (or higher f/3.5;f/4 etc.)on the camera and then halfpress the shutter realise button - it refocuses the lens like the object had moved !


Last edited by J.Scott; 10-29-2009 at 07:01 AM. Reason: placing wide tags on image
10-29-2009, 03:46 PM   #7
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Sigma 50mm f1.4 does autofocus terribly wrong with accuracy on canon mount cameras while it is performing fantasticly on nikon bodies.

There is a recent admission by sigma to imrpove its autofocus on canon mount sigma 50mm f1.4 production but continuous reports fo sigma 50mm f1.4 is still posted in forums.

Buying sigma is like buying lottery ticket. You need a lucky charm to hit the jackpot.
10-29-2009, 04:29 PM   #8
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Usually all it takes is a trip to Sigma in NY or even CRIS in AZ to address any FF/BB/decentering issues. Might take a couple weeks but I have always had excellent results getting Sigma lenses adjusted under warranty. In fact on the SigmaUSA site the entire process of sending your lens in for service is automated. Just fill out a form, include the needed documents or a request for a repair quote if out of warranty and it gets handled. Pretty slick really...

But if you decide to return the 50/1.4 consider the Sigma 30mm instead...everyone I know with one is thrilled...then again I have my 35ltd, so phhhffftttt on them...

EDIT: I am really sorry I just noted you are not in the US...you are in Berlin...so I imagine the return for service is a whole different situation. So, very sorry for missing that before posting!


Last edited by brecklundin; 10-29-2009 at 04:35 PM.
10-29-2009, 04:34 PM   #9
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I thought I would add if you made a global AF correction (not the lens correction but the line above that under the AF Correction menu item) the camera with the lens mounted to see if that fixes the issue. Or maybe you have made the correction to that setting rather than the lens specific correction?

Also, is the lens reporting the proper focal length to the camera? It could be an issue with the lens firmware reporting the wrong info or the K20D not having that lens in it's look-up table? I am sorta new to Pentax myself so I am not sure how they AF adjustments and lens correction functions. But if the camera thinks the lens is a different lens it possible that could be the cause.

Worth a look just in case.
10-30-2009, 02:26 AM   #10
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My lens is reporting a proper focal lenght. It is however reported by some software as "Sigma 70-200 2.8".
My lens has already been calibrated by the Sigma service. So either they are bold enough to send back a bad lens to me or ignorant not to know that the lens is inacceptable for normal use.

I see chrisg was having the same problem as me. However I still have another problem with the lens.
Not only it changes focus different way with different apertures (there's a shift between f/2.5 and f/2.8 - which I also dont know if it can be attributed to camera or lens), but also is can have front focus on close distances and be ok on far away shots (apertures below 2.8) or... the other way around - back focus at close distances at f/2.8 (or higher values) and front focus on far away shots.

What a piece of crap I am giving it back to Sigma as soon as I can.
10-30-2009, 03:20 AM   #11
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QuoteOriginally posted by elgreen Quote
Hi chrisg,
I have the same problem with my Sigma on K200D rig.
I made some tests on close and far distances.

If I shot at f/1.4 - f/2.5 I get sharp and crisp pictures,
however when changing the aperture on the camera to 2.8 or higher (value) makes the camera refocus the lens differently.

In case you dont understand what I mean...Put the camera on tripod, point it towards a wall or whatever's contrasty enought for af system. Then halfpress the button on f/1.4 - you'll get the lens to focus somewhere and lock the focus(no matter if its going to be a sharp picture or FF, or BF). Then change the aperture to f/1.7 or f/2 - halfpress the button - you should just get a bip without any refocusing - cause the object didt move. However...
If I change aperture to 2.8 (or higher f/3.5;f/4 etc.)on the camera and then halfpress the shutter realise button - it refocuses the lens like the object had moved !

That sounds just like the AF should work. And you realize that by pressing the shutter button it's very possible to cause the camera to move enough to need a refocus. For your tests I would at minimum decouple the AF from the shutter button and simply use the AF button on the back? Better yet use the Pentax IR remote shutter release. Or even a wired shutter release. Also enable mirror lock-up.

I mention the above because your posted images appear, to me anyway, as if they were not at all from the same perspective. Even a mm can matter depending on the DOF. So I am not certain what you are showing is all that meaningful in tracking down your problem.

Trust me I have been where you are with a new lens, it's VERY frustrating trying to figure out if it's me, the body or the lens or any combination of the three.

But you seem pretty unhappy so why let it be a problem? Just return it and move to something that makes you happier. Life it far to short to get so amp'd up over a camera lens. And yes, it is possible for a lens to come back still not right...from ANY lens maker, Canon, Pentax, Sigma, Nikon...etc...think about it, if it was possible to get every lens right at the time of manufacture, none would ever need service in that area. There are so many variables in calibrating, cleaning and reassembling a lens that if any one goes wrong then it's still bad. It could also be a problem with one of the elements being not perfect enough...hard to trace down from what I have read.

Or you could do more testing and see if you induced any issues into the process like even pressing the shutter button with a razor thin DOF. Have you used a DOF chart or calculator to know ahead of time the approximate DOF you should be able to expect at any given FL & aperture? If not here is a nice DOF calculator to work from:

Online Depth of Field Calculator

I would add that it takes a lot of practice and attention to detail if you need to check this sort of thing out...good luck...if you keep testing to figure out the true source of the problem.
10-30-2009, 03:32 AM   #12
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I would add that in your post with the image you show shots at f2 & f2.8 (and yeah I realize you are using the 50mm not the 70-200mm...but at least the actual focal length is right.

But assuming you are about 36" (or about 1m away) your DOF at f2 is about 1" (or about 3cm @ 1m) at f/2.8 and about 1.4" at 36" away (4.3cm @ 1m)...and at f1.4 you have about 1/3" (1.1cm) from the same distance.

I am only bringing this up because it's a common misconception people have when using these super fast lenses...and if ya already know this, it's not intended as trying to point out the obvious, only that I have made the same sort of oops! hehehehehe....
10-30-2009, 04:35 AM   #13
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Hey all,

I've got a Pentax FA 50mm f1.4. Paired with a k20d, there is the same front focusing problem at large apertures that's been described with the Sigma. Stopping the lens down to f3.5 all but cures the problem, with no front or back focusing at all. Perhaps this front focusing problem is inherent in such lenses at large apertures? Just a thought.
10-30-2009, 06:24 AM   #14
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brecklundin,

Its not so easy to make the lens refocus - sometimes I point into object which are 0,5 - 1m distant from each other and the camera is not refocusing.
But you're right - seems that camera moved on those two. But this was just a clear example - I have the same consistent results no matter if I use tripod, ir pilot and 2s mirror lockup.
I can give you another set of pictures:


Tripod used, a wire to shot remotely and probably a 2s mirror lockup.

Both are f/3.5 - however the first one was shot with the focus locked after shoting @f/1.4 - I put f/1.4 on camera - made the shot - and then switched focus into manual and then changed f/1.4 into f/3.5.

The second was shot normally - I turned AF on and made the foto with a wire pilot. You can easily see that something is wrong.

It looks now that it is programmed to change focusing plane over f/2.8 towards back in close distance focusing, and to shift focus plane toward the front when focusing on further objects.

Last edited by elgreen; 10-30-2009 at 06:34 AM.
10-30-2009, 07:39 AM   #15
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that's interesting...you know what it looks like to me? That is way too much difference to be a FF/BF issue, it almost looks as if the camera is using a different focus point. Are you using the center focus point or all of them?

I say this because, well, that looks like a 20-30ft difference in what is being focused on. To my eye in the 2nd shot the bushes just past the grass are in focus and in the second, again to my very tired eyes right now, the home across the road is the focus target.

FF/BF is usually not that prounced is it? Then again the lens does of a bit of a roll of the dice to get a good copy reputation. But there sure seems something is just not right. Nice example of what is happening, and on a large scale. As I see it either the camera is choosing a different focus point in each mode or the lens is just evil!! Have you tried the shot in full manual mode rather than AV (not that this should make a rats arse of a difference...)...other than ensuring you are using selecting the focus point and not the camera, I am guessing a return for more service might be the only solution? Sigma does reverse engineer their firmware and such so maybe you could ask the tech folks at Sigma if the lens needs to be rechipped/firmware updated?
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