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04-19-2014, 11:39 AM   #1
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Focusing on a Pentax-A F3.5 35-105mm

Hi folks,

Late last year, I purchased my first Pentax-A lens for my K5. Reviews seem to indicate that this lens is really sharp, and I can attest that I've managed to get some shots that are indeed sharp, but one thing that has troubled me is the fact that some shots came out really blurry. My first inclination is to blame my lack of skills. So I decided to do some tests - I put it on a tripod, set it to 35mm and try to focus on some houses in the distance. More often that not, I was not able to get a sharp focus. When I zoom to the long end, I am occasionally able to get focus lock (depending on the distance of the house I'm trying to shoot). My question is, is this what's known as the "zoom to infinity" problem? And if so, is this something that is common with this particular lens? I read from some other forum postings about folks with similar issues, but I wasn't sure if it's similar to my problem.

Thanks!

04-19-2014, 11:46 AM   #2
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Check to see if the lens barrel/filter ring is loose.
04-19-2014, 04:54 PM   #3
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Actually, now that you mention it, it seems that there is about a 0.5mm 'rattle' space on the focusing barrel/filter ring. I don't have any other Pentax-A or Pentax-M zoom lenses to compare with, so I thought it was just the age of the lens that makes it 'loose' like that. Is that the likeliest reason for the issue? I guess this begs the next question - how DO you tighten those things?

Thanks for your tip!

Last edited by drewski; 04-19-2014 at 04:54 PM. Reason: typo
04-19-2014, 05:57 PM   #4
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Does the lens barrel/filter ring turn?
Try setting the zoom to 35, the focus to infinity, then hold the zoom and focus ring together with one hand, and with the other hand turn the filter ring counter clockwise (looking at front of lens.
If it moves, keep turning gently and it should tighten up.

04-19-2014, 07:05 PM   #5
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QuoteOriginally posted by Swift1 Quote
Does the lens barrel/filter ring turn?
Try setting the zoom to 35, the focus to infinity, then hold the zoom and focus ring together with one hand, and with the other hand turn the filter ring counter clockwise (looking at front of lens.
If it moves, keep turning gently and it should tighten up.
okay, now I know what you mean by filter ring - I didn't realize that part could 'come off' :-). It was actually pretty tight, and I had to put some rubber bands around it to get a good grip before I could turn it the other way. The part that feels loose/rattly is the focusing ring. I see some screws on the lens itself, but they looked pretty tight as well.
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04-19-2014, 07:20 PM   #6
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QuoteOriginally posted by drewski Quote
okay, now I know what you mean by filter ring - I didn't realize that part could 'come off' :-). It was actually pretty tight, and I had to put some rubber bands around it to get a good grip before I could turn it the other way. The part that feels loose/rattly is the focusing ring. I see some screws on the lens itself, but they looked pretty tight as well.
The focus ring itself is usually kinda loose on these lenses.
If you loosened the filter ring, make sure you carefully tighten it again.
04-19-2014, 07:24 PM   #7
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When I first read your post, I thought your lens would be very similar to my Vivitar 35-105f3.5 lens, but realized that it is quite different. Anyway, the fact that someone find the same lens sharp when you find it to be quite soft usually has something to do with the camera. I would suggest you check if you camera/lens has FF or BF; the simplest method is to take 3 batteries lined up in a row, focus on the middle one from an angle. Use AF fine adjustment to correct any FF or BF.

Btw, my Vivitar 35-105f3.5 is tack sharp wide-open at f3.5 but that lens is so heavy that I don't use it often enough. Good luck...

NOTE: it is perfectly fine for the camera to be accurate for AF-lens while not so for MF-lens. That's why I setup a user mode for MF lens to correct the FF/BF issues.

Last edited by aleonx3; 04-19-2014 at 07:27 PM. Reason: additional information
04-19-2014, 08:05 PM   #8
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QuoteOriginally posted by drewski Quote
Hi folks,

...So I decided to do some tests - I put it on a tripod, set it to 35mm and try to focus on some houses in the distance. More often that not, I was not able to get a sharp focus. When I zoom to the long end, I am occasionally able to get focus lock (depending on the distance of the house I'm trying to shoot). ....
Was this lens opened anytime? In that case it's possible that infinity adjustment was not done after it was serviced. In such case, you may find that one end is going beyond infinity and another end doesn't reach infinity.

btw, If I remember correctly, there are adjustment screws beneath the focus ring rubber.


Last edited by yusuf; 04-19-2014 at 08:37 PM.
04-19-2014, 09:31 PM   #9
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@Swift1 : Many thanks for the tips - learnt something new from you today.

@aleonx3: Yeah I wasn't sure if it was a BF/FF problem. I don't have too many lenses (for now!), the only two AF ones I have is my walkabout DA 18-135 DC AL WR, and a Sigma 28 mm f/1.8 EX DG, both of which seem to focus fine in both AF and MF. The first MF lens I bought after that was the Pentax-M 50mm 1.4, which is Super Sharp (and a few other lesser-known brands which I got to stave off the beginnings of my LBA ), followed by this lens we're talking about here. Being the 3rd most expensive lens I'd bought so far, I wanted to learn how to use it well, and this focusing issue is driving me bonkers as I'm not sure if it's me, the camera or the lens (or a mix thereof). I tried to isolate any BF/FF issues using the method you suggested, but it wasn't very conclusive (all three batteries came out as blurrier, not so blurry, and blurrier!). I might do a proper chart test with a tripod, but as I'm still learning this stuff, I have to go to the chapter on setting up user modes as well. And yes, this IS a heavy lens!

@yusuf : It might have been before I got it. I bought it used off kijiji (I think it's owned by eBay) here in Canada. You may very well be right in that it wasn't serviced properly. I see the screws under the focusing ring, but it looks like it's tight?, and to be honest, I don't have the confidence to actually attempt to try fixing this myself at this point.
04-20-2014, 02:19 AM - 1 Like   #10
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Lots of red herrings here.

The A35-105 is not the easiest lens to focus, the focus ring throw is a bit short, especially at the 105mm end. The lens is parfocal (focus does not change with zoom) at the normal setting but becomes varifocal (focus changes with focal length) at the macro setting.

First of all check that you can actually focus at infinity by using liveview. Don't worry if the infinity focus does not actually coincide with the infinity mark. If it can't, then set it at the infinity stop and take a shot of a road, pavement etc and see where it actually focuses. If it can't reach infinity then it was either taken to bits and not reassembled correctly at some point or the infinity stop needs moving. From what I remember it can be set in three positions simply by moving screwed-in tabs accessible under the rubber grip.

Next, check that focus does not vary with zoom (except when in macro). It is a known problem with this lens that the internal nylon bushes which guide the various bits wear and loosen. The effect is that focus will change with zoom in a strange 'backlash' like fashion. Not easy to fix without major disassembly.

I think that this lens is greatly over-rated.
04-20-2014, 09:04 AM   #11
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Ah... I didn't know this lens was half parfocal half varifocal. I will definitely try out the liveview focus test when I go outside, but I did a quick test in the kitchen here, and my worst fears are confirmed - the focus does vary with zoom.

Was also reading your comments of the lens in the review section, and I concur that it's a heck of a lot sharper stopped down than wide open. And yes, it's a very temperamental lens to shoot with - I was/am willing to put up with the weight if I could get it to work well consistently, but my keeper rate is pretty low with it. Maybe I got a bad/worn down copy here, but I gotta stop learning about these things after I buy the lens. Did the same thing 2 years ago after I got this 'deal' on a Pentax M 50mm f/1.4 - that time I learned what fungus was. *grr*
04-20-2014, 09:40 AM   #12
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QuoteOriginally posted by kh1234567890 Quote
I think that this lens is greatly over-rated.
Don't deny that, however this lens can seriously compete with the best zoom lenses from that era, in fact stands pretty good to modern zooms too.

Although I rarely use mine due to the size, all these shots from a single weekend trip.







04-20-2014, 10:18 AM   #13
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Those are some nice shots - and No, I have not come close to getting that consistent level of sharpness on my copy of this zoom. I have been able to get more consistently good shots on my (2nd, non-fungal) copy of the Pentax M 50mm f/1.4 so that rules out my camera I guess.

Last edited by drewski; 04-20-2014 at 10:24 AM.
04-20-2014, 09:13 PM   #14
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@kh1234567890 and all : well, got a good excuse to go out and take some pictures today! Long story short, the lens won't focus to infinity, more pronouncedly so when in Tele than Wide. None of these have been PPed, just resized/cropped to upload here. First pic is me going all the way to 105mm, with the twigs about 10ft (3.0m) in front of me. The 100% crop shows that it's pretty sharp. Next I turn around and try to zoom in on some ducks in the distance. They may as well be three sea snakes sticking their heads out of the water - I couldn't get the lens to focus at all. Both were taken using a tripod too.

I'm resigned to the fact that there is something misaligned or calibrated wrongly in the lens. Maybe it's better to start saving up for another sharp zoom lens than to try to spend any more money to get this fixed? I spoke to one of the sales people at Henry's (local camera chain) about fixing it, and the guy told me it's not worth the trouble. Although.. I'm not sure if he's saying that because he wants me to buy a new lens from him
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Last edited by drewski; 04-21-2014 at 08:00 PM.
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