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Infinity (and beyond)

flossie
Posted 10/09/2010 - 08:45 Link
Just a quick question - why do modern AF lenses let you go "beyond" Infinity when you manually focus them, but old MF lenses stop at infinity?
Still shooting in the dark (literally and metaphorically)...
Edited by flossie: 10/09/2010 - 08:46
bforbes
Posted 10/09/2010 - 09:12 Link
I don't understand, infinity is infinity how do you go infinity+1?
thoughton
Posted 10/09/2010 - 09:38 Link
Do old MF lenses stop at infinity? The only MF lenses I regularly use at infinity are both long mirror lenses, and both focus considerably past infinity (annoyingly so). I guess that may be a special case because they are mirror lenses though.
Tim
AF - Pentax K5, Sigma 10-20/4-5.6, Tamron 17-50/2.8, Sigma 30/1.4, Sigma 70-200/2.8, Tamron 70-300/4-5.6
MF - Vivitar CF 28/2.8, Tamron AD2 90/2.5, MTO 1000/11
Stuff - Metz 58 AF1, Cactus v4, Nikon SB24, Raynox 150, Sigma 1.4x TC, Sigma 2x TC, Kenko 2x macro TC, Redsnapper 283 tripod, iMac 27, Macbook Pro 17, iPad, iPhone 3G
Flickr Fluidr PPG Street Portfolio site
Feel free to edit any of my posted photos! If I post a photo for critique, I want brutal honesty. If you don't like it, please say so and tell me why!
flossie
Posted 10/09/2010 - 09:38 Link
Just nipped outside and did these very unscientific demonstrations with the DA* 50-135 @50mm, looking at a house about 50m away - a DoF calculator has the near point at 23.5m which is about the end of my garden.

- Only cropped for 1:1, no sharpening or any PP

Infinity :

Comment Image


Beyond Infinity :

Comment Image
Still shooting in the dark (literally and metaphorically)...
pentaxian450
Posted 10/09/2010 - 10:05 - Helpful Comment Link
Most lenses now use plastic material in their construction, and they go "beyond" infinity to allow for plastic components thermal expansion. Even long lenses made of metal will suffer thermal expansion, too.
Yves (another one of those crazy Canucks)
Algernon
Posted 10/09/2010 - 10:11 Link
Won't it be because of the AF adjustment on the camera. They probably set infinity on a camera that was on the limit for being out of adjustment.

Some Vivitar lenses such as the Series 1 28-105mm let you adjust infinity by rotating the front element.
Half Man... Half Pentax ... Half Cucumber

Pentax K-1 + K-5 and some other stuff

Algi
DoctorJeff
Posted 10/09/2010 - 10:27 Link
Tim wrote
Quote:
The only MF lenses I regularly use at infinity are both long mirror lenses, and both focus considerably past infinity (annoyingly so). I guess that may be a special case because they are mirror lenses though.

And so does my mirror lens. But I suspect that what Tim and I (and others) mean is that one can turn the focus ring so that the infinity mark has gone beyond the index mark.

I think that answers bforbes
Quote:
I don't understand, infinity is infinity how do you go infinity+1?

It's a bit like the definition of parallel lines:
"Two lines that meet at infinity"
But how far away is that?
Geoff
Water can wear away a stone - but it can't cook lunch
X-5
istDS
K2000
P50.
Lenses Digital: 50-200, 18-55 KAF: 28-80.
Lenses KA & K: SMC-KA f2.0, SMC-K f1.4, SMC-K f1.7 Tokina KA 28-70 , SMC Pentax 70-210 F4, Sigma KA 75-300 , Hanimex 500mm Mirror, and the Tamron Adaptall-2 stuff.
and then there's all the M42 kit, and the accessories ...
Dangermouse
Posted 10/09/2010 - 11:05 Link
I've never been entirely sure how far away infinity should be anyway - I recently spent some time trying to get an old K 28-50mm adjusted by the simple approach of attaching it to my Super A (which has a split focus screen), putting it on a tripod, then lining up on a ridge line which I would guess to be about five or six miles distant. Hopefully that was far enough!
Matt

Shooting the Welsh Wilderness with K-m, KX, MX, ME Super and assorted lenses.
Algernon
Posted 10/09/2010 - 11:52 Link
Dangermouse wrote:
I've never been entirely sure how far away infinity should be anyway - I recently spent some time trying to get an old K 28-50mm adjusted by the simple approach of attaching it to my Super A (which has a split focus screen), putting it on a tripod, then lining up on a ridge line which I would guess to be about five or six miles distant. Hopefully that was far enough!

Should be. The moon would probably be better

There is a method where another camera with a lens known to focus on infinity accurately can be used as a collimator to set up a 2nd camera and lens to infinity. See here
Half Man... Half Pentax ... Half Cucumber

Pentax K-1 + K-5 and some other stuff

Algi
johnriley
Posted 10/09/2010 - 14:07 Link
It is all about the focus point changing as the lens expands and contracts. This becomes relevant with big telephoto lenses in particular.
Best regards, John
rparmar
Posted 10/09/2010 - 14:53 - Helpful Comment Link
I hope I am not the only one to catch the film reference in the title of this thread.
Listen to my albums free on BandCamp. Or visit my main website for links to photography, etc.
thoughton
Posted 10/09/2010 - 14:58 Link
Who could resist a little buzz?
Tim
AF - Pentax K5, Sigma 10-20/4-5.6, Tamron 17-50/2.8, Sigma 30/1.4, Sigma 70-200/2.8, Tamron 70-300/4-5.6
MF - Vivitar CF 28/2.8, Tamron AD2 90/2.5, MTO 1000/11
Stuff - Metz 58 AF1, Cactus v4, Nikon SB24, Raynox 150, Sigma 1.4x TC, Sigma 2x TC, Kenko 2x macro TC, Redsnapper 283 tripod, iMac 27, Macbook Pro 17, iPad, iPhone 3G
Flickr Fluidr PPG Street Portfolio site
Feel free to edit any of my posted photos! If I post a photo for critique, I want brutal honesty. If you don't like it, please say so and tell me why!
Edited by thoughton: 10/09/2010 - 14:58

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