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10-15-2012, 05:48 AM   #1
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Back- and front focus issues

Hi, I own a white K-x with a Sigma 17-70, a Sigma 50-150 and the 18-55 kit lens and I'm experiencing some back- and front focus. I did some testing and got the following results:

White K-x:
Sigma 17-70 backfocus becomes less when zooming in
Sigma 50-150 backfocus becomes less when zooming in, perfect at 150mm
Pentax 18-55 (kit) backfocus becomes frontfocus when zooming in

Pink K-x (my sisters):
Sigma 17-70 frontfocus at every length
Sigma 50-150 backfocus becomes frontfocus when zooming in
Pentax 18-55 (kit) frontfocus at every length

Sigma checked their lenses and indicated they are okay (within their margins). At this point I have no idea if it's me, the lenses or the camera(s), but I would really like to get the "problem" fixed. Do any of you have advice? Thanks!

Sorry for any misspellings or bad grammar, English is not my native language.

10-16-2012, 10:18 AM   #2
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I adjusted my K-x. Now both lenses have backfocus zoomed out and have frontfocus when zoomed in. Somewhere in the middle they focus perfectly. But shouldnt a zoom lens focus the same at every length?
10-17-2012, 03:56 AM   #3
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All this is considered 'Normal' with Pentax
10-17-2012, 04:18 AM   #4
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QuoteOriginally posted by 123jippo Quote
All this is considered 'Normal' with Pentax
I think it is considered normal with any camera or you wouldn't see cameras with focus adjustment in their menu...

Honestly? I think people jump to conclusions very quickly and start "adjusting" while in 90% or more of the cases things were fine before. Where people say "I have front/backfocus" our first question should be: how did you measure and under which circumstances?

I'm not an optics expert but I can imagine that on zoomlenses, exact focus simply cannot be the same throughout the zoomrange due to the movement of internal optic components. Normally, the standard variance in DOF and the uncertainty of the focus sensors should mask that. Start measuring it however and you will find front- and backfocus is every zoomlens, always, with every camera.

Second issue I have is that with the mentioned zoomranges (17-70, 50-150 and 18-55) any target that you frame at the widest setting becomes too close to handle on the extreme zoom end. Vice versa logic should tell us that a target framed at the extreme zoom setting becomes impossibly small to judge at the wide end. Also, when zooming, exposure changes as more or less of the area around the target is weighed by the exposure automation. Lastly, on most zooms, maximum aperture changes as you zoom in or out.The only other solution would be to move the target itself but unless that is mounted on some kind of rail system connected to the camera the movement itself introduces set-up changes that affect the measurement to the point of becoming invalid to begin with.

I'm not a prime user (only have the F-50mm and a Sigma 50 macro) but have experienced no issues on my K-5 on my zooms (Sigma 17-70 and 8-16, Tammy 70-300 and Pentax 50-135).

Honesty requires to state that on the 8-16 even at its widest, DOF is so large that any front or backfocus would be impossible to determine anyway, the Tammy is so soft that the exercise becomes a bit useless and the 17-70 would probably benefit only from a measurement at the tele-end because at its widest DOF again becomes bigger than any focus adjustment could cover anyway. Remains my 50-135 and it seems dead on at all settings at least I haven't yet noticed any focus issues in real-life images.

Are we allowing us to be influenced so that we should doubt our own judgement because the common agreement is that if you do not adjust, something MUST be wrong with you?

My thought has always been that the thinner the DOF, the more important the exact focus but the more moving/zooming parts the less benefit any adjustment can have unless the lens is really, really off at all settings. Were I to have a 200mm or 300mm f2.8 prime I suppose focus micro adjustment would make an incredible amount of common sense - as things stand, I'm not touching it.

10-17-2012, 10:36 AM   #5
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QuoteOriginally posted by Hein V Quote
shouldnt a zoom lens focus the same at every length?
Are you changing the focal length before or after your camera auto focuses?

In the bad old days of manual focus only, the photographer assumed that the focus point would change slightly whenever you changed the focal length, unless the lens manufacturer specifically said that focus would remain constant across the zoom range. And as you might guess, those were typically pricey lens. The greatest use of a constant focus zoom lens was to zoom out to the longest focal length, focus, then zoom back to your working focal length.

So, back to my question... you should be making the camera re-focus following each change of focal length. Are you?
10-17-2012, 10:52 AM   #6
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This is very much a Pentax thing lol. This sounds like another case of user not knowing the size of the autofucus points. The focus points are huge ! The centre focus point in the K5 reaches out to the ( ) spot meter markings of your viewfinder. Forget the little red square, as that is not the focus sensor ! Now start to think differently ! Stop trying to be to precise on your focus target. IE... If you try to focus on an eye for example..... the eye is very close to the edge of the head. This means, because the sensor is so large, it can easily focus on the ear or nose, or miss alltogether if there is some thing in the background with lots of contrast. So instead of the eye, just pick the face ! Remember always, keep it in mind that its the ( ) markings in your viewfinder that best represents you focus point. Hopefully that should sort it. If you have messed with the fine focus options I would set them back to factory settings.
10-17-2012, 11:05 AM   #7
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QuoteOriginally posted by westmill Quote
If you have messed with the fine focus options I would set them back to factory settings.
Aha! The voice of reason, finally! I thought I was alone on this.
10-17-2012, 11:25 AM   #8
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QuoteOriginally posted by newmikey Quote
Aha! The voice of reason, finally! I thought I was alone on this.
Its one of those things that should be a sticky or something. Ive never understood it. It causes more frustration to owners than anything I know.
Its a shame really. Shame on Pentax for not being more up front with info too. Another one that causes a lot of focus problems is not so much the camera as individual lenses that have field curvature issues. Like the tamron 17-50. It suffers really bad field curvature at the 17mm setting. Im not sure which is the biggest problem... the size of the focus points or the owners simply being unaware

10-18-2012, 02:28 AM   #9
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I tested with a simple setup: big ruler at 45 degree angle, camera on small tripod, single focus point (center), wide open, refocusing when changing focal length, selftimer, av setting.

But reading all your reactions (thank you!) I guess I should quit trying to get my camera and lenses to work perfectly. The pictures coming out of the camera (after some adjustments) are nice, especially when focal length is not at an extreme end. And maybe I should take pictures with smaller apertures: more forgiving and at 4.0 (instead of 2.8) still giving great DOF.

Thanks again for your replies!
10-18-2012, 04:40 AM   #10
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QuoteOriginally posted by Hein V Quote
I tested with a simple setup: big ruler at 45 degree angle, camera on small tripod, single focus point (center), wide open, refocusing when changing focal length, selftimer, av setting.

But reading all your reactions (thank you!) I guess I should quit trying to get my camera and lenses to work perfectly. The pictures coming out of the camera (after some adjustments) are nice, especially when focal length is not at an extreme end. And maybe I should take pictures with smaller apertures: more forgiving and at 4.0 (instead of 2.8) still giving great DOF.

Thanks again for your replies!
Just some thing for you to think about ! Now you know the size of the sensor ! Where was it focussed ? Your lenses are more than likely spot on. If you really need to shoot very acurately then use manual focus. I bet if you take a pic of a pic or watever on a flat wall it is spot on. One other thing.... turn your SR off
10-21-2012, 01:05 AM   #11
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My two cents, if you really need to apply focus correction (depending the degree of misfocus) you are probably better off doing it at the big focal ranges (zoomed), since then the depth of field is smaller and the focus really counts. Slight back/front focus at the wide end wouldn't hurt since the depth of field is already huge and you are most likely shooting at high F stop anyway.
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