Forgot Password
Pentax Camera Forums Home
 

Reply
Show Printable Version Search this Thread
04-29-2013, 11:36 AM   #1
Pentaxian
MSL's Avatar

Join Date: Feb 2012
Location: Greater Toronto Area
Photos: Albums
Posts: 3,389
Rectangular window on K-135 lens (and maybe others)

I recently acquired a K-135/3.5 lens at auction, despite already owning a Super-Tak 135/3.5 with case and for which I had also bought the original metal hood. When I use m42 lenses I tend to shoot in Av mode and will set the exposure compensation based on my knowledge of the lens, or I'll just bracket my shots, but I tend to avoid the green button as I found I was missing shots when I needed to press it (especially close focus situations).

Anyhow, one thing I noticed is that this lens and its sibling, the K-135/2.5 both have a rectangular window opening behind the rear lens element. I'm guessing that this was done to minimize stray light that would have been outside the 36x24 frame of the negative. Was this construction common for the K series of lenses? Did it make any difference when shooting film, and will it make any difference at all when shooting on a crop sensor?

I find that 135mm is not a focal length I use a lot for normal shots (although I have used the Super-Tak on extension tubes for macro). But before I consign it to the pile of things I really need to sell, I wanted to learn more about this lens.

Thanks.

04-29-2013, 12:21 PM   #2
Site Supporter




Join Date: Apr 2011
Location: Copenhagen
Photos: Gallery | Albums
Posts: 493
Hi. Of the ones that I have: Besides the SMC K 2.5/135, the SMC 2.8/105 have it. The SMC M 2.8/120 does not. I guess you are right in your assumption about the stray light? Regards
04-29-2013, 12:58 PM   #3
Pentaxian
reeftool's Avatar

Join Date: Dec 2007
Location: Upstate New York
Photos: Gallery | Albums
Posts: 8,150
My K300/4 has it. My M135/3.5 does not.
04-29-2013, 01:11 PM   #4
Loyal Site Supporter
blackcloudbrew's Avatar

Join Date: Mar 2009
Location: Cotati, California USA
Photos: Gallery | Albums
Posts: 3,816
I have several 645 lenses that have that window as well.

04-29-2013, 01:27 PM   #5
MSL
Pentaxian
MSL's Avatar

Join Date: Feb 2012
Location: Greater Toronto Area
Photos: Albums
Posts: 3,389
Original Poster
Thanks for the replies so far. So the question is - was this something Pentax experimented with in first developing the K mount lenses? And for you optics folks who think about this, will it make any difference on a crop sensor?
04-29-2013, 03:00 PM - 1 Like   #6
Pentaxian
johnyates's Avatar

Join Date: Aug 2011
Location: Saskatoon, SK
Posts: 1,192
You could think of that mask opening as a lens hood for the rear of the lens--its purpose is to cut down on flare from non-image forming light.

If you wanted to, you could use some black tape to reduce the size of the opening to that of the APS-C sensor. I expect that would reduce flare even more.
04-29-2013, 03:46 PM   #7
Forum Member




Join Date: Oct 2010
Location: vigo
Photos: Gallery
Posts: 65
QuoteOriginally posted by MSL Quote
I recently acquired a K-135/3.5 lens at auction, despite already owning a Super-Tak 135/3.5 with case and for which I had also bought the original metal hood. When I use m42 lenses I tend to shoot in Av mode and will set the exposure compensation based on my knowledge of the lens, or I'll just bracket my shots, but I tend to avoid the green button as I found I was missing shots when I needed to press it (especially close focus situations).

Anyhow, one thing I noticed is that this lens and its sibling, the K-135/2.5 both have a rectangular window opening behind the rear lens element. I'm guessing that this was done to minimize stray light that would have been outside the 36x24 frame of the negative. Was this construction common for the K series of lenses? Did it make any difference when shooting film, and will it make any difference at all when shooting on a crop sensor?

I find that 135mm is not a focal length I use a lot for normal shots (although I have used the Super-Tak on extension tubes for macro). But before I consign it to the pile of things I really need to sell, I wanted to learn more about this lens.

Thanks.
Both my K 135 f/2,5 and also K 200 f/4 have the rectangular window behind the rear lens. Is it perhaps from a common designer ? I guess your idea about the stray light is right.
04-29-2013, 03:47 PM   #8
Pentaxian
Just1MoreDave's Avatar

Join Date: Feb 2007
Location: Aurora, CO
Photos: Gallery | Albums
Posts: 4,858
QuoteOriginally posted by reeftool Quote
My K300/4 has it. My M135/3.5 does not.
I expected to see one on my K300/4 but didn't. I guess I know why it was cheap.

I think of the rectangle as a K telephoto detail. I don't think I've seen it on an M lens

04-29-2013, 03:56 PM   #9
Pentaxian
Sagitta's Avatar

Join Date: Feb 2011
Location: Maine
Photos: Gallery
Posts: 2,938
Knew I had at least one lens with that - my M200 f4 has it, as does my FA28-90. the 28-90 actually has a rectangle with a circle cut in the center, so its sort of a "I don't have it, or DO I?" kind of thing.
05-01-2013, 06:05 PM   #10
Pentaxian
Sagitta's Avatar

Join Date: Feb 2011
Location: Maine
Photos: Gallery
Posts: 2,938
All double posting, but I just notcied my Sigma 10-20mm f/3.5 has that rectangular window as well, so its obviously not relegated to old stuff.
05-02-2013, 05:30 AM   #11
Pentaxian
bdery's Avatar

Join Date: Mar 2008
Location: Quebec city, Canada
Photos: Gallery
Posts: 5,826
The purpose of the window is, indeed, to act as a hood, but with one significant flaw : stray light will still enter the lens and can create unwanted flare or reflections when bouncing on the various optical surfaces. That's why the usefulness of such a window is marginal at best.
05-02-2013, 07:28 AM   #12
Loyal Site Supporter
pacerr's Avatar

Join Date: Jan 2008
Location: Henry, TN
Photos: Gallery | Albums
Posts: 2,909
If you think of reflected light in a lens as the same as noise in an audio system it's obvious that any masking of light rays that don't contribute to a "true" image on the sensor is beneficial. The need for additional 'Di' coatings to defeat interior reflections associated with digital sensors is a good example.

Noise reduction of any kind is relatively expensive from a design and manufacturing stand point and at some point the cost/benefit ratio exceeds a reasonable limit. Especially for lenses designed in the pre-digital era when exhaustive PP wasn't a common pastime.

As mentioned above, you might experiment with masking if you feel it would make a perceived difference - especially when using FF lenses with an image circle that exceeds the needs of the APS-C sensor.

I'd add that, as noted below, this may only be practical on some of the older/longer FL lenses with recessed rear elements. I inadvertently left a round baffle out of an older Vivitar 400mm tele lens after cleaning it - the difference was quite apparent in contrast so the baffle was quickly replaced. Would have been interesting if I'd thought to mask the disk at that time.

H2

Last edited by pacerr; 05-02-2013 at 02:20 PM.
05-02-2013, 10:27 AM   #13
Pentaxian
Sagitta's Avatar

Join Date: Feb 2011
Location: Maine
Photos: Gallery
Posts: 2,938
QuoteOriginally posted by pacerr Quote
As mentioned above, you might experiment with masking if you feel it would make a perceived difference - especially when using FF lenses with an image circle that exceeds the needs of the APS-C sensor.
I wonder if it would be possible to create a rear filter of sorts to mimic this. Trying to think of how you could without running the risk of having your lens hit the thing or even get it in there without screwing up the registration distance.
05-02-2013, 11:35 AM   #14
MSL
Pentaxian
MSL's Avatar

Join Date: Feb 2012
Location: Greater Toronto Area
Photos: Albums
Posts: 3,389
Original Poster
QuoteOriginally posted by Sagitta Quote
Trying to think of how you could without running the risk of having your lens hit the thing or even get it in there without screwing up the registration distance.
Aha! I think you've stumbled on one reason why this feature is limited. I suspect for too many lenses the rear element comes too close to the mount so you couldn't use this feature without having to rethink how the lens focused to inifnity.
05-02-2013, 11:44 AM   #15
Pentaxian
Lowell Goudge's Avatar

Join Date: Jan 2007
Location: Toronto
Photos: Gallery | Albums
Posts: 15,400
fore those keeping count, it is present on the K105/2.8, K135/2.5, K300/4 and M100/4 macro in my collection of lenses

not present on any shorter lenses like K50/1.4, or M35/2

i suspect as others have opined that the rear element is too close to the mirror box and due to the size of the rear element, the shioeld would likely lead to vignetting.

on most other wide angle lenses in my kit, the rear element is so small, there should be no light requiring such a shield any way.
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!
k-mount, lens, lenses, pentax lens, shots, slr lens, super-tak, window
Thread Tools Search this Thread
Search this Thread:

Advanced Search


Similar Threads
Thread Thread Starter Forum Replies Last Post
Maybe Pentax and the K-01 were trend setters? Docrwm Pentax K-01 15 09-26-2012 09:37 AM
For Sale - Sold: Relisting - Sigma 100-300 F4, Pentax DA 50-135, 55mm f1.4, and others mcs Sold Items 5 08-16-2012 02:56 PM
Color fringing on SMC 135 and others touchogrey Pentax SLR Lens Discussion 20 09-29-2010 07:39 AM
For Sale - Sold: Pentax Rectangular Metal Lens hood for 58mm Lens (Worldwide) oneill Sold Items 2 03-19-2010 05:12 PM
For Sale - Sold: Bigma, Takumar 135 2.5 and others. Not Registered Sold Items 21 04-14-2008 10:40 AM



All times are GMT -7. The time now is 03:20 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