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02-15-2014, 08:48 AM   #1
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Square plastic on the back of the lens?

I tried to google-it but I couldn't find anything about it.
I saw that some of the lenses on their rear end have square shape of plastic and some don't. I was wondering, is there any difference with image quality?
Square shape of plastic and usual shape.

Thank you all!

02-15-2014, 09:03 AM   #2
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Your sensor is technically a rectangle, not a circle. Therefore, projecting a rectangle from the lens is actually better than projecting a circle. This works especially well if the lens is intended for APS-C use only, as it cuts down in internal reflections within the mirror box.
02-15-2014, 09:33 AM   #3
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So its like a secondary hood on the inside? This should protect from ghosting, sensor reflections, and loss of contrast due to light bouncing around the mirror box
02-15-2014, 09:39 AM   #4
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QuoteOriginally posted by malenisjaj Quote
I tried to google-it but I couldn't find anything about it.
I saw that some of the lenses on their rear end have square shape of plastic and some don't. I was wondering, is there any difference with image quality?
Square shape of plastic and usual shape.

Thank you all!
What lens is that with the rectangular opening? It might be worth adding such a mask to old lenses for use on APS-C. Digital sensors are more shiny than film was, and the reflection can bounce off the back surface of the lens, which in old ones was not well coated. Gluing on a bit of black paper might improve contrast. An experiment is needed.

02-15-2014, 10:42 AM   #5
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Sigma DSLRs seem to have an element in front of the mirror. I wonder if someone could fashion something like that to a backup body to compare shots....

02-15-2014, 10:47 AM   #6
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QuoteOriginally posted by clicksworth Quote
What lens is that with the rectangular opening? It might be worth adding such a mask to old lenses for use on APS-C. Digital sensors are more shiny than film was, and the reflection can bounce off the back surface of the lens, which in old ones was not well coated. Gluing on a bit of black paper might improve contrast. An experiment is needed.
It's smc 135mm f2.5.

QuoteOriginally posted by manacho2005 Quote
Sigma DSLRs seem to have an element in front of the mirror. I wonder if someone could fashion something like that to a backup body to compare shots....
That would be really interesting to know.

Last edited by malenisjaj; 02-15-2014 at 10:52 AM.
02-15-2014, 11:17 AM   #7
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Thanks. I have both types of 135 F2.5 with M42 mount but they don't have the mask. Pentax must have thought it worthwhile. It might even be made a bit smaller for a crop sensor.
02-15-2014, 12:44 PM   #8
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The old Pentax 100mm f4 Macro has the rectangle opening also.

02-15-2014, 12:49 PM   #9
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QuoteOriginally posted by manacho2005 Quote
Sigma DSLRs seem to have an element in front of the mirror. I wonder if someone could fashion something like that to a backup body to compare shots....
On Sigma DSLRs, that's the removable dust-protection/IR cut filter. I think they can be bought as spare parts from Sigma, but I'm not gonna touch the one on my camera!
02-15-2014, 01:04 PM   #10
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QuoteOriginally posted by panoguy Quote
On Sigma DSLRs, that's the removable dust-protection/IR cut filter. I think they can be bought as spare parts from Sigma, but I'm not gonna touch the one on my camera!
Replacement units for the SD1 cost $60 in the U.S.. The ones the SD14 and SD15 use are only $30. I've taken mine off a couple of times for infrared photography. It just pops out, no tools required. It's fun, you should try it.
02-15-2014, 08:53 PM   #11
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That would be neat if Pentax implemented that dust protector to their weather-sealed bodies. But improved in a way to protect the body from the elements regardless of what lens was used (sealed mirror box).
02-16-2014, 04:08 PM   #12
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One fell out of my M 135 and it didn't seem to make any difference in image quality

Randy
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