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02-04-2010, 03:58 PM   #1
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What is this called & what is it for?

Curious if anyone can tell me what the black masking part on the back of some lenses is called:


and explain to me what it is for. (The thread I found the pic above in, for the SMC K 135mm f2.5, shows one with and one without it.) I'm just wondering what it's function is and if it would have a corresponding performance reason-for-being if you used a lens on a APS-C DSLR (only sized down correspondingly for the sensor size).

I have a (currently hair-brained) theory that the purple fringing you see (caused not by the lens, but by the microlenses on the sensor) might be cut down a bit if this opening was optimized for the APS-C sensor size.

Any optical geniuses out there care to educate me? Thanks in advance!

02-04-2010, 04:04 PM   #2
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There's something like it on my Super Tak 300mm f4, which is honestly my WORST lens when it comes to PF'ing

My guess its that the mask is o stop internal reflections.
02-04-2010, 04:58 PM   #3
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I would agree with little laker

reduce the strey light hitting the sides of th emirror box.

most telephoto pentax K lenses have them
02-04-2010, 05:00 PM   #4
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I am curious to find out too because the Tamron 70-200/2.8 has that thing too. Lens works just fine though.
If that causes any problems on Little Laker's lens, I wonder if it could be removed/reinstalled without breaking anything and find out what it does.
JP

02-04-2010, 05:26 PM   #5
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QuoteOriginally posted by jpzk Quote
If that causes any problems on Little Laker's lens, I wonder if it could be removed/reinstalled without breaking anything and find out what it does.
JP
I could try removing it, however I might not have the time until early next week.
There's too much low cloud for a proper test, and I have to work straight through the weekend.
02-04-2010, 06:01 PM   #6
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OK... exercising my Google Fu, it appears that the wording most often used to describe this part is "baffle" (as in "light baffle" although the word "light" seems to be often omitted.)

Still digging to see if I can find the actual optical purpose. (Certainly cutting down on internal reflections would seem obvious. What is less obvious, at least to me, is why they would later do away with it [as coatings improved???] - why would one still not have been a good idea? Similar to lens hoods??? You HAD to have a lens hood with earlier coatings - or no coatings - but don't you still get better contrast with a hood + multicoatings?)

Another possible explanation (I just thought of) is that it had to do with obtaining accurate METERING inside the camera and later advances in ways of metering did away with the need for them.

Last edited by cheekygeek; 02-04-2010 at 06:07 PM.
02-04-2010, 10:21 PM   #7
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No need to over-think this. It is as Lowell says.

The intent is to reduce internal reflections off the internal surfaces of the mirror box. Think of it as a lens hood for the rear element of the lens.

Steve
02-05-2010, 06:32 AM   #8
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I look at it as "understanding", not "over-thinking". That explanation does not account for why it went away (on the same lens/optical design) at some point.

This thread seems to answer my questions, and show that it (or rather its absence) can relate to the incorrect exposure problem that some lenses have on modern DSLRs. Note the russian translation page link in the discussion: http://forum.mflenses.com/someone-could-explain-accessory-for-crop-bodies-t1...size+mask.html.

02-05-2010, 06:54 AM   #9
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The baffle is simply there to reduce stray light and improve contrast. As antireflection coatings and light absorbing materials used to coat the inside of the lens body have improved the need for these baffles has been removed. If your lens has one of these baffles then it may work better on a cropped sensor if the opening in the baffle was reduced. I the exact size it should be redued to will depend on the lens but reducing the opening by the crop factor would propably be a good place to start. Try sticking a black cardboard cutout on with tape and see what happens. This will however have no effect on purple fringing which is caused but chromatic aberations in the lens. The microlenses on the sensor cannot cause purple fringing as each microlens covers a single pixel .
02-05-2010, 07:49 AM   #10
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It is indeed a baffle (still used widely in telescopes). It plays mostly the same role as a lens hood, but positionned differently so it blocks light at different angles. It was probably dropped in lens designs because it increased costs, manufacturing complexity, and didn't really improve performances on well designed lenses with good coatings.

Also, it is entirely likely that if you opened a modern lens, you would find its equivalent positionned elsewhere in the light path. In optical design terms, it is usually refered to as an "aperture stop". Sometimes the iris is the aperture stop, sometimes for various reasons you want another area to act as the aperture stop.

Remember that some Limited lens hoods have exactly the same shape
02-05-2010, 11:24 AM   #11
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QuoteOriginally posted by cheekygeek Quote
I look at it as "understanding", not "over-thinking". That explanation does not account for why it went away (on the same lens/optical design) at some point.

This thread seems to answer my questions, and show that it (or rather its absence) can relate to the incorrect exposure problem that some lenses have on modern DSLRs. Note the russian translation page link in the discussion: Manual Focus Lenses :: View topic - Someone could explain...accessory for crop bodies.
Blaming the lack of a baffle for the exposure issues is a bit of a stretch in my mind. The vast majority of lenses manufactured in the metered SLR era were made without baffles with no metering issues. It never really went away because it really never was a universal feature. Some lenses (mostly teles) have a baffle, most do not. The exposure issues with Pentax and other brand dSLRs is a fairly recent phenomenon and appears to be related to current screen technology.

Steve
02-06-2010, 02:31 PM   #12
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In that case, no use to test-remove it because it obviously has a purpose.
I wouldn't remove the one present on my Tamron 70-200/2.8.
Thanks for the answers to all.
JP
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