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FORUMS Canon Cameras, Lenses & Accessories Canon EF and EF-S Lenses 
Thread started 11 Nov 2014 (Tuesday) 05:45
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Canon 500mm reflex filter question

 
Theo43
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Nov 11, 2014 05:45 |  #1

I have recently got a Canon 500mm mirror lens, and I have a question about the drop-in filters. I know that the lens originally came with a set of ND filters, but mine has only the clear filter. It appears to be flat (without any curvature) so I am wondering if it is necessary at all. Can the lens be used without this filter in place, and, would doing so have any positive effect on image quality? Thanks,




  
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macroimage
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Nov 11, 2014 18:16 |  #2

With my Sigma 600mm mirror taking out the filter doesn't improve image quality in any noticeable way. The focus shifts a little with it out. I would think that it would be similar with your Canon 500mm.


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DreDaze
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Nov 11, 2014 21:46 |  #3

you have the lens, take some pics with it in place, and not...i'd guess that it is a UV filter, in which case you wouldn't really need it on a DSLR...that is assuming you've somehow adapted it to EF mount


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Nov 12, 2014 00:38 as a reply to  @ DreDaze's post |  #4

DUST!
It never crossed my mind with mine but if you would remove the filter I should at least tape off the opening to prevent dust from contaminating the inside fo the lens/mirror.




  
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amfoto1
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Nov 12, 2014 13:14 |  #5

Most lenses that use drop in filters require there at least be a clear filter in there all the time. I know all the Canon super teles and other similar lenses from other manufacturers do. I don't know about the Canon mirror 500mm specifically, but various other mirror lenses I've used (Tamron, Sun, Sigma, Vivitar) have always had at least a rear-mounting Sky or UV filter.


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Theo43
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Nov 12, 2014 21:41 as a reply to  @ amfoto1's post |  #6

Thanks for all of your thoughts. It occurs to me there is a simple test, namely, set it on a tripod and focus on something, then remove the filter and see if it is still in focus. This should be easier and surer than trying to eyeball a subtle IQ difference. I am reluctant to take my stuff outdoors in freezing weather, but as soon as there is a suitable day I shall try this.




  
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Nov 13, 2014 01:06 |  #7

Theo43 wrote in post #17268692 (external link)
Thanks for all of your thoughts. It occurs to me there is a simple test, namely, set it on a tripod and focus on something, then remove the filter and see if it is still in focus...

This will reulst in unsharp images. The filter has an influence on back focal distance!

From my thinking those flat filters inside old SLR lenses are better removed for DSLR camera use - from image quality theory. This requires sometimes a back focal lenght / infinity setting adjustmend on the lens, which is not always possible to perform correct, especially for lenses with floating lenses or internal focussing.

Why I think it is better?
All the old SLR lenses are constructed to be direct in front of the film - no glass between lens and film. With a DSLR there is in most cameras the Anti Aliasing filter, in nearly all the IR-Cut filter, and in all the sensor cover glass. With this in mind the additional filters in the lens could be removed to compensate for all the camera filter.

But now the more practical side: It seems that f/2.8 and slower lenses show nearly no effect. The ray angle is too small to cause much aberration. So for the f/8 lens the filter has only some more reflections in case it has proper optical quality.'

It is the same effect that Roger Cicala learnt the hard way with his MTF measurments (sensor stack thickness influence). Fast lenses need to be designed with the camera filter stack in mind (I have some basic optical design knowledge).

For example the Canon EF 50/1.0 lens is most likely designed only with film use in mind. Likely it would be better on a DSLR / mirrorless camera without all those thick filters inside, or with less optical thickness.


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Theo43
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Jul 04, 2015 10:13 |  #8

I have finally got around to testing my original question regarding the drop in filter for the Canon 500mm reflex. All I did was take images with and without the filter and compare them at 200% magnification. I found it necessary to refocus from one condition to the other. My conclusion is that, if there is any difference at all, the images without the filter are sharper.




  
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Canon 500mm reflex filter question
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