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03-15-2011, 03:24 AM   #1
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another focus thread but with small twist

sigma 17-50mm f2.8, all over the place, could not focus correctly under daylight but worked ok indoors even under very low light (hows that for a change?)
no microadjustment would make it perfect in all conditions, fantastic lens via LV focus, best sigma zoom lens i ever seen but donkey with normal focus.
back to adorama with this thing but sad to do so as it worked without problems on k20d and i really though it will be my favorite lens for years to come

pentax * 16-50 f2.8 two copies same issue, back focus under daylight.
-6 adjustment fixed daylight problem but what is really odd and i wish someone could explain this to me why indoors it focus fine even without microadjustment ?!?! first copy was returned to adorama as it had some mark or dust on the inside, but focus wise both performed idetically and both worked like sigma just fine on k20d so no buts there is something off with K5 AF.

was pulling my hair out thinking that if not all of my lenses i would be thinking about leaving pentax camp but I don't give up easly and i been with pentax for too long to just walk away (mostly i would just hate to use canon or nikon like every Tom, Harry and theirs dogs out there) so then i figured lets try some other lenses, to my surprise they ALL worked fine, even FA50 1.4 wide open would be focus spot on, i'm yet to test outdoors under clear sky as i run out of time today for the final test but it is looking good.

so if not for 17-50 and 16-50 lenses I would never even know there was a problem with K5 focusing system and there is no question for me that there is a problem.
i'm sort of happy now since microadjustment fixed my 16-50 and it is WR so i guess i'm ok with keeping it but if there was no problem with K5 i would have sigma for less money and a fair bit lighter.

point of my post? a freindly reminder that maybe before you knock people for whining too much or before you claim it must be "user error" please consider that K5 can act very differenlly with some lenses and just because you don't see the problem doesn't mean there is no problem.
this also works both ways, just because someone claims to have no problems doesn't mean they lieing and are blinded pentax fanboys, they might have combination of lenses that just don't demonstrates the problem so don't lock horns over things like this guys.

03-15-2011, 05:25 AM   #2
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QuoteOriginally posted by Croc Quote
both worked like sigma just fine on k20d so no buts there is something off with K5 AF.
There is a thread somewhere that explains why this happens...The thing is, to put it bluntly and as i remember from my reading some days ago, that manufacturers have some quality standarts that accept some amount of deviation. In terms of focus units lets range it from -4 to 4 being 0 perfect focus...
Your body may have a -3 factory calibration and will work perfectly with the lenses that have a +3 factory calibration...but if when you change bodies the new one has got a +3 factory calibration all the lenses that worked fine before will not now...
You should really look for that thread it was very enlightning, well written and explained.
Does the K5 have the microcalibration feature the K20D has? if not it's a huge mistake from pentax to have removed it from a pro body...
03-15-2011, 05:38 AM   #3
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QuoteQuote:
Does the K5 have the microcalibration feature the K20D has?
Yes, both K7 & K5 have AF microadjustments. You set adjustments for up to 20 different lenses as long as these lens have a specific code set in their identification chip. Some Sigma use the same lens-id code, eg 70-200 HSM II & 120-400 HSM, so the camera can not distinguish which lens is mounted. If you have both lenses you have to compromise if their AF is not the same.
03-15-2011, 08:28 AM   #4
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QuoteOriginally posted by Coeurdechene Quote
You should really look for that thread it was very enlightning, well written and explained.
I think this is what you are referring to:
LensRentals.com - “This Lens Is Soft” and Other Facts

03-16-2011, 02:45 AM   #5
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QuoteOriginally posted by Coeurdechene Quote
There is a thread somewhere that explains why this happens...The thing is, to put it bluntly and as i remember from my reading some days ago, that manufacturers have some quality standarts that accept some amount of deviation. In terms of focus units lets range it from -4 to 4 being 0 perfect focus...
Your body may have a -3 factory calibration and will work perfectly with the lenses that have a +3 factory calibration...but if when you change bodies the new one has got a +3 factory calibration all the lenses that worked fine before will not now...
You should really look for that thread it was very enlightning, well written and explained.
Does the K5 have the microcalibration feature the K20D has? if not it's a huge mistake from pentax to have removed it from a pro body...
i understand that very well but this is not what is happening with K5
same lenses work fine on K20D so if that was the case there would be shift with all of them on K5 one or other way but it is not, some lenses focus spot on while some clearly show problem and ONLY under specific conditions, both copies of 16-50mm focus just fine under indoor light on K5. there is something odd there but I got tunned in to my satisfaction and since I don't plan buying other lenses I should be happy camper now
03-16-2011, 03:25 AM   #6
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QuoteOriginally posted by Coeurdechene Quote
There is a thread somewhere that explains why this happens...
That is a good piece, but then it would not really seem expain the "why" beyond "manufacturing tolerances" (of what exactly ?). It seems that the actual cause of the amount of BF/FF varying with the copy of a lens has not been explained anywhere I know of. That it varries with body one can explain by the AF sensor / image sensor mismatch in physical placement. That it varies by a copy of a lens is a mystery (to me) as the AF system should be about adjusting until sharp. The mechanics of the focus adjustment play a part, but with them and the nature of the AF system ("adjust until focused") one would expect their contribution to be an inconsistency corresponing with play in the mechanics and such, that is, not a consistent, repeatable shift.

One thing I would like to know is what is done when lenses are adjusted to eliminate BF/FF which is reportedly done with good results.
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