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06-26-2009, 01:03 PM   #1
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Front-back focusing issue

Hi i've bought a new lens - Tamron 28-75mm f/2.8 and.. i doesn't focus correctly..

what a dissapointment.. but luckily I have a k20d.. so this issue can be solved.. but I also have a K-m.. and on it - it's useless..

can any one explain something to me.. how it is called, is it back focusing or front focusing if:

in the viewfinder i focus on some spot, and the real focus spoton, zone is closer to the camera than the spot, that I was focusing in the viewfinder..

So on K20d i adjusted focus to negative side.. -3. is it common or it's very bad?

how long the callibration will take? how long it usually takes?

I think I should recallibrate it even though k20d and later k-7 and next cameras will have focus adjustment feature, i still will want to use it on km or whatever other camera.. right?

06-27-2009, 12:36 AM   #2
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Well...I suppose they (whoever they are) put this facility on the K20d and the K7 as it is a common problem. I would think that if you want to use it on the Km it will have to be calibrated. The time varies for service but I reckon you should allow a couple of weeks at least. Best of luck...
06-27-2009, 01:17 AM   #3
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Do not bother with fine tuning of k20d. It is just a gimmicky feature with my experience. Just learn the way it mal-focuses with various camera to lens combo
06-28-2009, 06:22 AM   #4
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QuoteOriginally posted by Mystic Quote
in the viewfinder i focus on some spot, and the real focus spoton, zone is closer to the camera than the spot, that I was focusing in the viewfinder..
Are you using AF for focsing?
If so, then your camera/lens combination is front-focusing.

Have a look at some focus adjustment tips.

AFAIK, it is next to impossible to get a lens that will show no BF/FF on every camera.

QuoteOriginally posted by roentarre Quote
Do not bother with fine tuning of k20d. It is just a gimmicky feature with my experience.
What makes you say that?
Because AF adjustment is dependent on too many variables, like FL on a zoom, distance to subject, ...?

QuoteOriginally posted by roentarre Quote
Just learn the way it mal-focuses with various camera to lens combo
And then? How do you compensate, i.e., obtain optimal focus?

06-28-2009, 06:45 AM   #5
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Here's some information I posted in another thread. I hope it is helpful.

1. The focusing screen does not affect auto focus at all. It only affets manul focus.

2. Some after market focusing screens, like KatsEye screen, are slightly thicker than the sandard screen. It causes the focus to shift slightly forward.

3. If the camera with standard screen has slight back focus, then chances are KatzEye will improve it, or make it perfect.

4. If the camara with standard screen has perfect manual focus, the KatzEye will cause front focus. So the shim located on top of the screen (between the screen and its seat in the camera body) has to be replaced with a thinner shim. The problem is that some of the bodies do not have a shim. So there is nothing to replace with a thinner shim.

5. If you feel your manual focus is slightly off to the back, I think it should be okay without having to worry about shim. The KatzEye will shift the focus to the front and improve the manual focus.

6. If your manual focus is already perfect, then open the focusing screen to check and see if there is a "removeable" shim. The shim is located on top of the screen between the screen and the prizm. If there is a shim, then adjustments can be made.

7. If the MF is way too far to the back, the Katz Eye with increased thickness may be able to bring the focus to the front far enough to make it perfect. In that case a shim has to be put in there to bring the focus forward even farther.

8. I have played with KatzEye and three Pentax bodies. On the K10D KatzEye worked perfectly. I guess it must have had backfocus before changing. On a K20D KatzEye caused front focus and there was no shim to remove. So it did not work. I put the original screen back in and got perfect MF. On a K100D I removed the copper shim and I made a shim from glossy photo paper. The paper was slightly thinner than the copper shim and the focus moved back to being perfect.
06-28-2009, 09:49 AM   #6
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Hi Startaio,

Not wanting to hijack this thread would you please have a look at my post here
https://www.pentaxforums.com/forums/pentax-dslr-discussion/64311-focus-screen-thickness.html
and explain why the thickness of the screen affects focusing.

Regards
Greg

Last edited by gregmoll; 06-28-2009 at 10:09 AM.
06-28-2009, 11:29 AM   #7
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Original Poster
thanks everybody for your replies..

I do not use MF i use AF and when I look at the pictures on my computer screen, all flaws appear. ant the flaws are - perfect focus is in front of the spot, that I was focusing while took the picture.

so as Class A advised it's front focusing..

The AF adjustment actually solved the issue, and during my last photoshoot, when adjusted AF showed pretty ok ratio of perfect focus shots.

however I will take this lens to service, because since i paid the money, I want it to be good.

thanks again for everyone.
08-26-2009, 12:34 PM   #8
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K20D backfocus problem solved

Had my K20D camera for about 11 months now and always had a focusing problem but blamed my cheap (really cheap) poor lenses. Ponied up some ducets and bought a couple of better lenses. Now I could really see a severe back focus problem. Testing only agrivated me more. I was consistantly 1 to 3 inches back of my focus point. (I had been compensating by stopping down).

I shoot only manual focus. Usually with a tripod, and don't notice much difference with shake reduction on or off.

I called Pentax support. They referred me to the adjustment procedure for auto-focus lenses. They they asked about my lenses. Opps, we can't help you with your problem because some of them are not Pentax branded, though some were.

I kept telling them the problem was in the body and most likely the focusing screen. Not a bit of willingness to hear me out.

While I was stewing we ordered a new focusing screen from the Pentax store and the tool to replace it. (Don't order the tool, it comes with the screen).

I installed the LI-80 screen as opposed to the standard screen and tested it out on my sharp old trusty Pentax A f2.0 and it looked good.

Next I installed my 85mm f1.4 portrait lens which had about 3" of back focus. Now we focused on some real human subjects and checked the results and it was perfectly focused on the point I chose. YES. YES. YES.

I figure Pentax owes me the cost of a new focusing screen and tool but am not holding my breath. I am this .... close to chucking 30 years of shooting Pentax for another brand.

Check out the cats eye focusing screen website. Am considering a purchase there as they are better designed for low light and manual focus which is primarily where all my shooting is done.

08-26-2009, 06:11 PM   #9
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This whole FF/BF thing is pet hate of mine!

Why bother buying a lens faster than 2.8 if it is useless at this F-stop? I also think that this where a lot 'soft wide open' comments come from. FF or BF not an inherent softness in the lenses.

The fact that the bodies now have the AF adjustment as part of the standard menus is like an admission of very lose tolerances in pentax QA. Either the body or all fast lenses should come with a test chart and instructions on how to use it.

I'm sure too many people stop down their fast lenses to compensate. I used to and i used to question what it was that i had paid for.

Fortunately, I now have a K7 so a can adjust the AF individually by lense. So far so good. AF (at the open end) is yeilding great results. At the moment i would not say that it is a gimmick.

BTW my experiences are based on the DA LTDs.... how the hell do people with FA LTDs or the FA 1.4 get along? Are these lenses even viable to use wide open? Or do you keep sending them back until you get a good one (assuming you have a body pre k20d)?

mike
08-26-2009, 06:57 PM   #10
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My FA50 f/1.4 had a front-focus problem on the K100D. I sent the K100D back for adjustment - all the pre-K20D bodies had individual lens adjustable AF as well, but only Pentax service personel could access it with their service software. Pentax do it for free during either the body or lens warranty period, but I had to pay for the mail. After that it focused well, but still hit and miss at large apertures (a different issue).

On the K-7, it needed a front-focus adjustment as well (-3), all is well now...the K-7's AF is a very different level of performance

'Soft wide open' is a separate lens resolution issue, though BF/FF problems often get mistaken for that. The act of stopping down to a smaller aperture also increases the DOF , so it helps cover a BF/FF problem.

The FA50mm, for example, really is "Soft" at f/1.4, even with after the focus adjustment is "perfect". It simply cannot resolve the same level of detail you get at f/4, or even f/2.8 for example.

Someone mentioned chucking Pentax for another brand - BTW, other Camera brands all have such issues with both lenses and body. ....they may likely run into the same problem with their shiny new Canon or Nikon....

Last edited by kittykat46; 08-26-2009 at 07:08 PM.
08-26-2009, 08:15 PM   #11
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If you're focusing manually, then what you are describing isn't front focus or back focus at all. It's most likely your failure to udnerstand that the viewfinder is lying to you and showing too much DOF at wide apertures, causing things to appear in focus in the viewfinder that aren't actually in focus. That is unfortunately normal; it takes practice to get a sense of what subset of what appears in focus will actually be. Although if you're find that things are in focus in the picture that *weren't* in focus in the viewfinder (as opposed to simply too much looking in focus), then you have a misaligned focus screen.
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