Forgot Password
Pentax Camera Forums Home
 

Reply
Show Printable Version Search this Thread
01-28-2019, 10:23 AM   #1
New Member




Join Date: Jan 2019
Location: New York
Posts: 13
Pentax FA 645 lenses with hard stop at infinity?

Hello, I'm a newbie here and I have a specific need for lenses that have a hard stop at infinity, meaning lenses that are properly focused at infinity when the infinity symbol is aligned with the focus distance mark at the end of focus ring rotation. This was always the case with my Leica M lenses and older Pentax SLR manual focus lenses. It also appears to be true for manual focus Pentax 645 lenses. Do any of the 645 FA lenses hard stop correctly at infinity? If you've worked with one that does or does not, I would greatly appreciate your observations.

If I've missed something in the database that has this information, or if someone knows of an external resource about this, please let me know. Thanks for your help!

01-29-2019, 03:16 AM   #2
Site Supporter
Site Supporter




Join Date: Mar 2016
Location: La Vienne (86), France
Posts: 204
I've just checked a few of mine and they do go slightly beyond the infinity mark....which seems to be the norm for AF lenses. Based on my greater knowledge of Canon's AF lenses then this allows for thermal expansion and continuous AF which requires movement marginally beyond the target distance.

Bob
01-29-2019, 09:25 AM   #3
New Member




Join Date: Jan 2019
Location: New York
Posts: 13
Original Poster
Thanks, Bob. I'm beginning to suspect that most if not all of the FA lenses move beyond infinity. As you mentioned, this is probably to compensate for the thermal expansion, and in my understanding this is done when lower quality materials are used in the lens (many manufacturers do this). However, I hadn't thought about the continuous AF factor. In any case, I might need to use manual focus lenses only for my application, which is fine because I appreciate the high build quality of Pentax's older 645 lenses. Still, if anyone finds a FA lens that does hard stop at infinity, please let me know. Thanks again.
01-29-2019, 06:56 PM   #4
Loyal Site Supporter
Loyal Site Supporter




Join Date: Dec 2017
Photos: Gallery
Posts: 381
QuoteOriginally posted by JonVdG Quote
... As you mentioned, this is probably to compensate for the thermal expansion, and in my understanding this is done when lower quality materials are used in the lens (many manufacturers do this). ...
It is not the quality of the materials, it is the need to maintain the optical properties over temperature by using lens spacers that have just the correct thermal expansion properties. This requires iterative analysis over temperature of all the focal plane properties being controlled against various candidate materials (usually metals) in all the spacer gaps, along with any effects of the overall barrel, with the added complication of doing this with a zoom lens in many cases. And then there is the flange to sensor/film plane thermal variation to consider. For fast lenses, flat focal planes, low CA, etc., this is a non-trivial process. Some of this may be automated today with the more advanced optical design programs, but optical design programs were relatively primitive when the A and FA lenses were formulated.

The practical way out is to dynamically measure focus while moving the focus mechanism -- whether by hand or motor. And yes, servo systems hate running into mechanical limits when they need to measure on each side of a setting to ensure that the setting is optimal.

01-30-2019, 07:19 PM   #5
New Member




Join Date: Jan 2019
Location: New York
Posts: 13
Original Poster
Thanks for your comment. What I was referring to was the quality of the materials used for compensation. Sorry, should have been more clear about that. I have used dozens of fixed focal length lenses and zoom lenses, that hard stop at infinity, in temperatures from -40 to +110 degrees F, and not one has ever missed focus (other than for user error). My experience has been with lenses from Leica, Nikon, Tokina, Minolta, and Sony. Temperature compensating materials have been in use for hundreds of years in various types of instruments and it seems like photographic lenses have no problems with extreme temperatures so I assume manufacturers have taken advantage of this, except when trying to keep their costs low. However, I could imagine that fast autofocus mechanisms with a lot of torque that are trying to keep up with today's continuous AF demands might benefit from not constantly hitting their physical limit. One of my Tokina zoom lenses was constantly slapping its hard stop and it just seemed like that might not be a good thing over a long period of time.

It appears the FA 75mm has a hard stop at infinity, but I'm just guessing based on photos of the lens that never show its focus mark reaching beyond the center of the infinity symbol. Can anyone confirm this?
01-31-2019, 03:16 PM - 1 Like   #6
Loyal Site Supporter
Loyal Site Supporter




Join Date: Dec 2017
Photos: Gallery
Posts: 381
OK, I pulled out an SMC Pentax FA 645 1:2.8 75mm lens. I'm not trying to focus it with a camera attached, merely examining the focus mechanism. Temperature is approximately 74F. Sighting down the alignment mark with the focus in its hard stop, the center of the infinity symbol is minutely past (greater than infinity) a projection of the alignment line. This offset is not greater than 0.2mm at the focus ring. Moving the focus ring to align the center of the infinity symbol requires barely any movement. To the casual viewer, they are aligned. This offset is within the estimated location of the r.h. f/2.8 depth of field mark, if one were provided.

I believe a camera experiment will be needed to be sure, because I think the existing alignment may be within the likely mechanical tolerances of building the lens assembly.
01-31-2019, 05:03 PM   #7
New Member




Join Date: Jan 2019
Location: New York
Posts: 13
Original Poster
Thanks, this helps a lot. I've seen lenses from other manufacturers behave the same - ever so slightly past the center of the infinity mark, and I really had to do a close examination to notice. All of these had correct infinity focus at the hard stop so I suspect the Pentax does as well. Leica lenses seem to be consistently perfect at the center of the infinity mark, and most of my old Nikkor MF lenses were as well. Again, thanks, I think I'll buy the FA model and see if I can confirm it.
01-31-2019, 06:27 PM   #8
Pentaxian




Join Date: Nov 2015
Location: California
Posts: 586
I'm not sure if this is what you are looking for but here is what I get with the fa150 and the fa75.



01-31-2019, 08:44 PM   #9
New Member




Join Date: Jan 2019
Location: New York
Posts: 13
Original Poster
Thanks, that definitely helps! It appears the 75mm does do a hard stop at infinity but the 150mm might drift a little beyond it.
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!
645 fa lenses, 645d, 645z, camera, fa, fa 645 lenses, focus, hard stop, infinity, lenses, medium format, pentax, pentax fa
Thread Tools Search this Thread
Search this Thread:

Advanced Search


Similar Threads
Thread Thread Starter Forum Replies Last Post
does your 300mm ed 67 lens have a hard stop infinity focus? destroya Pentax Medium Format 6 10-06-2017 09:26 AM
DA 12-24mm infinity focus hard stop/slap Lititz Pentax SLR Lens Discussion 3 06-25-2014 06:23 AM
D FA 100mm f/2.8 Macro - Hard Infinity Stop? tjwoon Pentax SLR Lens Discussion 7 02-15-2013 01:29 AM
Hard Stop at infinity cali92rs Pentax SLR Lens Discussion 7 04-27-2012 03:44 PM
Why don't all lenses have infinity exactly on the infinity stop? peterh337 Pentax SLR Lens Discussion 19 01-15-2011 08:53 PM



All times are GMT -7. The time now is 09:49 PM. | See also: NikonForums.com, CanonForums.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