Infinity focus with telephoto lens

studio17 · 07-29-2013, 07:08 AM

I have a Pentax K100D camera - having upgraded from film camera - and use my Pentax-FA 28-90 & 80-200 lenses I used to use with my 35mm Pentax.
But of course, the camera will not focus at infinity over approx 100mm.
Do I have to buy another lens, or is there an adaptor available of some sort to enable me to use my lens to max potential?

yusuf · 07-29-2013, 07:38 AM

If you are unable to focus at infinity with both the lenses, problem could be in camera. Have you tried to focus manually?

Not a Number · 07-29-2013, 08:10 AM

All Pentax K-mount cameras have the same film/image sensor register distance. Lenses that focus to infinity on one body should do the same on another body. As mentioned above if all your lenses have a problem on the K100D body then the problem is in the body.

What happens if you set the focus manually?

You should only need adapters with screwmount or other non-K-mount. The flangeless, glassless screwmount adapters should allow infinity focus.

studio17 · 07-29-2013, 08:47 AM

It's only with my 90-200 lens when I zoom beyond around 100.
Makes no difference whether I manually focus or not, but OK on my 35mm camera.
Speaking with a friend that uses another camera, he said its a 'well known fact' that lenses designed for film cameras don't focus to infinity on zoom!? & he had an adaptor for his.

Na Horuk · 07-29-2013, 09:22 AM

Originally posted by studio17

Speaking with a friend that uses another camera, he said its a 'well known fact' that lenses designed for film cameras don't focus to infinity on zoom!? & he had an adaptor for his.

I dont know what he is talking about. Usually its adapters that ruin infinity focus. And besides, you dont need adapters on K-mount lenses. Adapters are for other mounts, like m42 mount.
Pentax FA lenses should work on Pentax DSLRs exactly as they did on SLRs.
The only thing I can imagine is that the photo is not OoF, but just blurry due to:
a) hand-shake and slow shutter speed
b) lens quality is okay for film, but it doesnt have enough resolution to be "sharp" on digital sensors. (this would explain why it only happens at the long end of the zoom)

Feel free to check out the lens reviews for your lenses here (there are also sample photos):
Pentax FA Zoom Lenses - Reviews and Specifications - SLR and Interchangeable Lenses - Pentax Lens Reviews & Lens Database

Edit: It would be great if you could upload a photo or two, with intact exif data

studio17 · 07-29-2013, 09:36 AM

Hi all & thanks for all your advice...
Just looked out my MZ6 and popped the lens on it - & it is now the same on that too. (i.e. won't focus to infinity beyond around 100)
Looks like I'll either have to get it serviced or replace it

Sorry to have wasted your time.

07-29-2013, 07:08 AM	#1
studio17 New Member Join Date: Jul 2013 Location: Rojales, Alicante, Spain Posts: 4	Infinity focus with telephoto lens I have a Pentax K100D camera - having upgraded from film camera - and use my Pentax-FA 28-90 & 80-200 lenses I used to use with my 35mm Pentax. But of course, the camera will not focus at infinity over approx 100mm. Do I have to buy another lens, or is there an adaptor available of some sort to enable me to use my lens to max potential?

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07-29-2013, 07:38 AM	#2
yusuf Pentaxian Join Date: Nov 2008 Location: Palo Alto, CA Photos: Gallery Posts: 2,497	If you are unable to focus at infinity with both the lenses, problem could be in camera. Have you tried to focus manually?

07-29-2013, 08:10 AM	#3
Not a Number Moderator Join Date: Mar 2012 Location: Venice, CA Posts: 10,526	All Pentax K-mount cameras have the same film/image sensor register distance. Lenses that focus to infinity on one body should do the same on another body. As mentioned above if all your lenses have a problem on the K100D body then the problem is in the body. What happens if you set the focus manually? You should only need adapters with screwmount or other non-K-mount. The flangeless, glassless screwmount adapters should allow infinity focus.

07-29-2013, 08:47 AM	#4
studio17 New Member Join Date: Jul 2013 Location: Rojales, Alicante, Spain Posts: 4 Original Poster	It's only with my 90-200 lens when I zoom beyond around 100. Makes no difference whether I manually focus or not, but OK on my 35mm camera. Speaking with a friend that uses another camera, he said its a 'well known fact' that lenses designed for film cameras don't focus to infinity on zoom!? & he had an adaptor for his.

07-29-2013, 09:22 AM	#5
Na Horuk Veteran Member Join Date: Feb 2012 Location: Slovenia, probably Photos: Gallery \| Albums Posts: 11,186	Originally posted by studio17 Speaking with a friend that uses another camera, he said its a 'well known fact' that lenses designed for film cameras don't focus to infinity on zoom!? & he had an adaptor for his. I dont know what he is talking about. Usually its adapters that ruin infinity focus. And besides, you dont need adapters on K-mount lenses. Adapters are for other mounts, like m42 mount. Pentax FA lenses should work on Pentax DSLRs exactly as they did on SLRs. The only thing I can imagine is that the photo is not OoF, but just blurry due to: a) hand-shake and slow shutter speed b) lens quality is okay for film, but it doesnt have enough resolution to be "sharp" on digital sensors. (this would explain why it only happens at the long end of the zoom) Feel free to check out the lens reviews for your lenses here (there are also sample photos): Pentax FA Zoom Lenses - Reviews and Specifications - SLR and Interchangeable Lenses - Pentax Lens Reviews & Lens Database Edit: It would be great if you could upload a photo or two, with intact exif data

07-29-2013, 09:36 AM	#6
studio17 New Member Join Date: Jul 2013 Location: Rojales, Alicante, Spain Posts: 4 Original Poster	Hi all & thanks for all your advice... Just looked out my MZ6 and popped the lens on it - & it is now the same on that too. (i.e. won't focus to infinity beyond around 100) Looks like I'll either have to get it serviced or replace it Sorry to have wasted your time.