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07-04-2011, 06:33 PM   #1
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Filter placement

Ok, this might be an odd question or an obvious one but I can't seem to find anything on it so here goes:

Does the placement of the filter in relation to the lenses make any difference in the images? Most lenses have threads on the very front for addition of a filter, however Ultra-Telephotos often have a slot in the rear of the lens for insertion of a filter (e.g., Sigma Telephoto 500mm f/4.5 EX DG APO). If the light has passed through the filter and then the lens is this potentially different than if it passes through most of the lens, then the filter, and then the rearmost elements?

07-04-2011, 07:24 PM   #2
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I've wondered about this, too. I can't give you an answer myself, but I found a similar discussion here:Use of Filters on REAR Lens thread

Last edited by .a.t.; 07-04-2011 at 07:58 PM.
07-04-2011, 08:22 PM   #3
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can you imagine how much more it would cost to buy filters for the massive front element of an ultra-telephoto lens versus using regular sized filters near the rear of the lens? not to mention how few manufacturers would bother making such large filters, how much less portable they'd be and how much more awkward they'd be to use.
07-04-2011, 08:37 PM   #4
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QuoteOriginally posted by maltfalc Quote
can you imagine how much more it would cost to buy filters for the massive front element of an ultra-telephoto lens versus using regular sized filters near the rear of the lens? not to mention how few manufacturers would bother making such large filters, how much less portable they'd be and how much more awkward they'd be to use.
And that is the only reason why telephoto lenses have filter drawers, the placement of the filter has no effect upon the result obtained from using said filter.Though the use of internal filters can affect the focusing of a lens, which is why with some super telephoto lenses come with a clear filter already in the drawer.

there are wideangle lenses such as the pentax 645 25mm f/4 AL [IF] SDM that uses a filter drawer - this is because with such a wide angle lens the front element is not only huge but it is also has a substantial curvature to it - which makes using flat plate glass filters impossible, even square filter systems have problems with lenses like these.

07-04-2011, 10:38 PM   #5
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I can add that some of the super telephotos use the rear filter as part of the optical equation and if you remove it the focal plan shifts. I discovered this the hard way of course and only advice from this forum got me back on track.
07-05-2011, 04:34 AM   #6
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QuoteOriginally posted by imtheguy Quote
I can add that some of the super telephotos use the rear filter as part of the optical equation and if you remove it the focal plan shifts. I discovered this the hard way of course and only advice from this forum got me back on track.
Correct! The manual to my Tamron SP 350 mm catadioptric mirror lens explicitly warned against removing the rear (UV- or colour or ND) filter due to potential focusing issues. I must confess that I still wonder why and how?

Last edited by Stone G.; 07-05-2011 at 04:35 AM. Reason: spelling error
07-05-2011, 10:04 PM   #7
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QuoteOriginally posted by Stone G. Quote
Correct! The manual to my Tamron SP 350 mm catadioptric mirror lens explicitly warned against removing the rear (UV- or colour or ND) filter due to potential focusing issues. I must confess that I still wonder why and how?
Well, it's all down to refraction. Imagine 2 converging beams of light from a lens which come together at the surface of the camera's sensor. Now imagine a thick glass plate with flat parallel surfaces in the way of the beams: the beams will be deflected within the glass (refraction), and emerge at the same angle as they went into the plate.

Now, it's because they were deflected within the glass that the point at which they converge gets moved somewhat - in other words, the focus changes. Even with a thin filter, the amount may be critical, so that's why you're advised against it. Mind you, I should have thought it would only affect close focusing (and any focus distance markings)...
07-05-2011, 11:22 PM - 1 Like   #8
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Correction: omitting the filter would make infinity focus impossible (assuming the infinty focus stop is set up for operation with the filter).

07-06-2011, 03:11 AM   #9
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QuoteOriginally posted by m42man Quote
Correction: omitting the filter would make infinity focus impossible (assuming the infinty focus stop is set up for operation with the filter).
Ah yes, of course - Thanks!

And since my Tamron 06B focusses a considerably good deal past infinity, that is why I have never been able to verify any real issue ('cause, of course I had try the lens out without filter as soon as I read the instructions!).
07-06-2011, 06:43 AM   #10
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A filter (or any dual plane sheet of glass) will move the focal point slightly. That's the only optical effect (except for resolution decrease and of course the desired effect of the filter). the camera can compensate for that focal point position shift, and it's a small shift anyway.
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