Forgot Password
Pentax Camera Forums Home
 

Reply
Show Printable Version Search this Thread
05-14-2009, 07:23 AM   #1
Senior Member




Join Date: Apr 2009
Posts: 167
Why focus past infinity

I was wondering why lenses have the ability to focus past infinity. Does this actually have any practicle value? I'm trying to figure out why this would be possible.

05-14-2009, 07:37 AM   #2
Pentaxian
Lowell Goudge's Avatar

Join Date: Jan 2007
Location: Toronto
Photos: Gallery | Albums
Posts: 15,314
note that if you consider thermal expansion and contraction, and the temperature range that lenses are expected to work, focusing a little past infinity may insure that you can actually focus to infinity at all temperatures.
05-14-2009, 09:59 AM   #3
Pentaxian
Just1MoreDave's Avatar

Join Date: Feb 2007
Location: Aurora, CO
Photos: Gallery | Albums
Posts: 4,815
Time travel - but that's only fully implemented on the K-8. I know because I have traveled back in time to post this.

Zooms can have slightly different infinity focus points at both ends of the zoom. I have also read that autofocus logic works better if the lens can go beyond infinity and then back.
05-14-2009, 11:21 PM   #4
Veteran Member
Sean Nelson's Avatar

Join Date: Sep 2008
Location: Vancouver BC Canada
Posts: 353
Most of my old manual focus lenses are designed with a hard stop at exactly infinity, and in my experience that stop is accurate - objects at infinity are in focus when you crank the lens all the way to the stop.

Thus, I'm skeptical about the "thermal expansion" argument.

What I have noticed is that the zoom and autofocus lenses that I've come across (and admitedly I haven't come across all that many of them) have a bit of space for going beyond infinity. So I tend to attribute it to one of two things:

1) Tolerances for the more complex assembly of zoom lenses, or

2) Allowance for the autofocus mechanism to slightly overshoot infinity without stressing the mechanism.

05-15-2009, 07:56 AM   #5
Loyal Site Supporter
pacerr's Avatar

Join Date: Jan 2008
Location: Henry, TN
Photos: Gallery | Albums
Posts: 2,850
(With tonge in cheek) how do you know it focuses at infinity? I've never been able to focus at infinity myself - I can't see that far!

H2

[Thermal expansion and manufacturing tolerances are the accepted, and logical, answers. Hyperfoacl distance covers a lot of slop too.]
05-15-2009, 11:15 AM   #6
Pentaxian
Just1MoreDave's Avatar

Join Date: Feb 2007
Location: Aurora, CO
Photos: Gallery | Albums
Posts: 4,815
QuoteOriginally posted by pacerr Quote
(With tonge in cheek) how do you know it focuses at infinity? I've never been able to focus at infinity myself - I can't see that far!

H2

[Thermal expansion and manufacturing tolerances are the accepted, and logical, answers. Hyperfoacl distance covers a lot of slop too.]
When I lived in Hawai'i and had to reset the focus on a lens, I used the line between ocean and sky for infinity. I'm not sure how far away that is exactly. Under super clear conditions I could see the high parts of O'ahu, maybe 60-70 miles. Now I use Long's Peak, 55 miles away.

That's way farther than necessary. Fool around with a depth of field calculator and you'll find that infinity is pretty easy to hit. A 50mm lens at f8 and focused at a subject 52 feet away will produce an image where everything beyond the subject is in acceptable focus.
05-15-2009, 11:33 AM   #7
Veteran Member
Sean Nelson's Avatar

Join Date: Sep 2008
Location: Vancouver BC Canada
Posts: 353
QuoteOriginally posted by pacerr Quote
(With tonge in cheek) how do you know it focuses at infinity? I've never been able to focus at infinity myself - I can't see that far!

[Thermal expansion and manufacturing tolerances are the accepted, and logical, answers. Hyperfoacl distance covers a lot of slop too.]
I've done a lot of astrophotography using manual lenses, both of the moon and stars. There's nothing farther that you can focus on, so for all intents and purposes I think it's reasonable to consider that these objects are "at infinity".

My star photography is done with the lens wide open in order to minimize exposure times. If it's in focus at the hard infinity stop wide open when focused on a star, I think we can safely discount "depth of field" as being relevant.

I haven't seen any difference in sharpness between summer and winter shots taken over a range of temperatures of about 15 to 20 degrees Celsius. So I conclude that for my manual lenses "thermal expansion" is not producing any adverse effects, at least not in this temperature range.

That's why I'm skeptical that "thermal expansion" is the reason that modern lenses focus past infinity - in my experience it wasn't a problem for older lenses and my understanding is that modern lenses with plastic barrels are LESS affected by thermal expansion issues than my old metal-bodied lenses.

For auto-focus lenses it seems logical to me to allow a small beyond-infinity range to accommodate the focusing system slightly "overshooting" the infinity position while zeroing in on the exact focus point. This would prevent it "slamming" into the stop and stressing the mechanism.
05-15-2009, 01:59 PM   #8
Senior Member




Join Date: Mar 2009
Location: Massachusetts
Photos: Gallery
Posts: 186
To take pictures of Buzz Lightyear?


;-)

05-15-2009, 02:41 PM   #9
Senior Moderator
Loyal Site Supporter
Parallax's Avatar

Join Date: Apr 2008
Location: South Dakota
Photos: Gallery
Posts: 15,107
QuoteOriginally posted by Just1MoreDave Quote
Time travel - but that's only fully implemented on the K-8. ....................
Oh, I don't believe that's true. If it were RiceHigh would have told us about it!

Last edited by Parallax; 05-16-2009 at 07:28 AM.
Reply

Bookmarks
  • Submit Thread to Facebook Facebook
  • Submit Thread to Twitter Twitter
  • Submit Thread to Digg Digg
Tags - Make this thread easier to find by adding keywords to it!
focus, infinity, k-mount, pentax lens, slr lens
Thread Tools Search this Thread
Search this Thread:

Advanced Search


Similar Threads
Thread Thread Starter Forum Replies Last Post
Pentax 85mm f/2 -- does yours focus past infinity? deadwolfbones Pentax SLR Lens Discussion 10 04-30-2010 10:32 PM
Infinity Focus DblArrow Pentax SLR Lens Discussion 2 01-24-2010 01:03 AM
SMC P-A 35-105/3.5 Focusing Past Infinity arpaagent Pentax SLR Lens Discussion 1 01-11-2009 02:48 AM
Why would you ever want to focus past infinity? Isaac314 Pentax SLR Lens Discussion 18 10-22-2008 02:02 PM
Sigma zooms focusing past infinity?? druitt Pentax SLR Lens Discussion 3 02-08-2008 03:36 AM



All times are GMT -7. The time now is 05:28 PM. | See also: NikonForums.com, part of our network of photo forums!
  • Red (Default)
  • Green
  • Gray
  • Dark
  • Dark Yellow
  • Dark Blue
  • Old Red
  • Old Green
  • Old Gray
  • Dial-Up Style
Hello! It's great to see you back on the forum! Have you considered joining the community?
register
Creating a FREE ACCOUNT takes under a minute, removes ads, and lets you post! [Dismiss]
Top