[kevinwolves]

I have an old Centon 500mm Mirror Lens. I would like to know if anyone out there has the same and if they use the rear UV filter on the lens. Somewhere at the back of my mind I have a feeling this should be fitted at all times but can find no info about it on the Interweb. I will be using it mainly for bird photography on my K-X, so any advise which will get me better pictures with the lens would be greatfully received.

[SpecialK]

UV does not affect digital sensors.

[Marc Sabatella]

It doesn't need to be UV, but most (all?) mirror lenses require some piece of glass at the rear of the lens. If not one of the colored filters they are often inexplicably sold with, then a clear one. Whether or not it has UV properties is immaterial; the glass is needed for its refractive properties as part of the optical design. I think most mirror lenses are sold with a simple clear glass "filter" to be used for this purpose.

[kh1234567890]

It doesn't need to be UV, but most (all?) mirror lenses require some piece of glass at the rear of the lens. If not one of the colored filters they are often inexplicably sold with, then a clear one. Whether or not it has UV properties is immaterial; the glass is needed for its refractive properties as part of the optical design. I think most mirror lenses are sold with a simple clear glass "filter" to be used for this purpose.

Here we go again. Have you any actual evidence of this being the case - theoretical or experimental - or is it just something you've read on the Internet ?

[Marc Sabatella]

Here we go again? I wasn't aware there were any credible sources that challenged this claim. It's practically universally agreed upon. On the Internet, yes, also instruction manuals that come with mirror lenses. Why else would they bother including the clear glass element?

[RKKS08]

That's from the manual of my Tokina 8/500
.

[kh1234567890]

All the UV or Skylight rear filter (or its absence) does is to change the flange distance slightly. Normally mirror lenses are set up to focus well beyond infinity anyway so this is of little practical consequence. But if the Nikon and Tokina manuals say so, it must be a good thing to have another two reflective surfaces with dodgy coating and a bit of flat glass stuck in the optical path .

I'm yet to see any difference myself or find any example shots that clearly show one.

[Marc Sabatella]

??? How cold inserting a filter change the flame disance? How could it *not* affect the optical path? What you're saying doesn't make sense, and files in the face of virtually everything published on the topic. I can't tell if you actually have an advanced optics degree and know something you aren'tt sharing, but if so, now would be a good time to explain why you think you know more about this than the lens manufacturers do.

[kh1234567890]

??? How cold inserting a filter change the flame disance? How could it *not* affect the optical path? What you're saying doesn't make sense, and files in the face of virtually everything published on the topic. I can't tell if you actually have an advanced optics degree and know something you aren'tt sharing, but if so, now would be a good time to explain why you think you know more about this than the lens manufacturers do.

It has been discussed here and elsewhere many times before without any definitive conclusion reached and no clear practical proof presented that the filter makes any real difference. Eg. https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/44979-mirror-l...tml#post436062

[RKKS08]

I think I can offer an explanation for the problem. If some of it sounds a bit too simple, keep in mind I don't have a degree in Physics.

Any ray of light going through a material with a different diffraction than air will get a slight parallel shift, except if the ray enters and leaves the material square to the surface. This happens even with window glass, but you will not see it as you usually cannot compare to light not shifted. The amount and direction depends on the diffraction factor and the thickness of the material.

As mirror lenses use filters behind the rear of the lens, the only way not to have this effect would be to design a lens where all light rays would leave the rear lens exactly parallel to the optical axes. Real life lenses are not designed this way.

As mirror lens manufacturers have to offer the possibility to use filters, the easiest way to circumnavigate possible problems is to integrate a piece of plain glass to the calculations of the lens. So a very slight, but not necessarily linear zoom effect of every part of a picture (outside of the centre) would be cared for in the optical design of the lens itself, and then corrected by the filter.

Probably also sharpness and bokeh outside the center of the picture could be affected, but I am not sure about that.

[Marc Sabatella]

Right, that's pretty much the obvious common sense explanation that should satisfy anyone with a basic high school / college level understanding of physics. It is certainly possible that some sort of advanced specialization in optics wold give one insight into why this might not apply in this case the way basic physics suggests it should. But absent such an explanation, I see no reason to doubt the common sense one, especially given that, again, manufacturers are also pretty explicit about it.

Of course, whether the magnitude of the effect is enough to worry about is another matter. Problem is that is tough to do really good controlled tests given the shallow DOF and imprecision of focus with most mirror lenses. I did actually try to measure this effect once, but quickly realized my own measurement error was great enough that I had little hope of seeing it.

[Marc Sabatella]

It has been discussed here and elsewhere many times before without any definitive conclusion reached and no clear practical proof presented that the filter makes any real difference. Eg. https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/44979-mirror-l...tml#post436062

Not sure which specific post in that that thread you think constitutes reason to doubt the conventional wisdom and the technical explanations given in that same thead. One guy saying he was having a had time doing a conclusive test is hardly reason to doubt the scienc. But yes, I would agree that the magnitude of the effect is not "night and day".

[kh1234567890]

So really there is no answer because there is no real effect. And you'd better not put a planar UV filter on the front of any lens (telephotos might just be OK though) because the non-parallel rays away from the optical axis will really screw up sharpness, bokeh and whatever. Unless of course the designers of the lens took all that into account

[RKKS08]

And you'd better not put a planar UV filter on the front of any lens (telephotos might just be OK though) because the non-parallel rays away from the optical axis will really screw up sharpness, bokeh and whatever. Unless of course the designers of the lens took all that into accoun.

Now it's getting absurd, and I think you know it. If parallel shifting of light rays in front or back of a lens had a comparable effect, shift lenses wouldn't make sense.

One should not respond to trolls, but the OP wanted his question answered.

[kh1234567890]

Now it's getting absurd, and I think you know it. If parallel shifting of light rays in front or back of a lens had a comparable effect, shift lenses wouldn't make sense.

One should not respond to trolls, but the OP wanted his question answered.

So would I, but as you must agree, a plausible explanation is still lacking.

In practice I could never see the filter or its absence making any real difference to sharpness or aberrations. The only difference it makes is that the focus is shifted slightly. If the planar bit of glass at the back of the lens affected the image in a way that is suggested then the usual low pass filter in front of the sensor would also have a similar detrimental effect on pre-digital era lenses.

I'm quite prepared to be proven wrong by experimental or ray-tracing evidence. But I've so far not found that anywhere, the answer always being 'it says so in the Nikon/Tokina instruction manual and on the Internet'.