[MSL]

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.

[jt_cph_dk]

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

[reeftool]

My K300/4 has it. My M135/3.5 does not.

[blackcloudbrew]

I have several 645 lenses that have that window as well.

[MSL]

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?

[johnyates]

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.

[jopenmx]

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.

[Just1MoreDave]

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

[Sagitta]

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.

[Sagitta]

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.

[bdery]

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.

[pacerr]

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

[Sagitta]

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.

[MSL]

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.

[Lowell Goudge]

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.