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I have searched for this but because I don't know what I'm looking for I cannot be sure how to use these. (I can search for "why I'd want to")

A mirror lens I purchased has come with 3 filter elements, they are small and fit to the rear of the lens (where the lens joins the camera) not the front. Why? Cheaper to manufacture? Mirror lenses are immune to whatever damage UV can do to lens-lenses?

My question is why are there 3 and how do they differer, they are:

  • KM-500 (phi) 30.5 SKYLIGHT 1A - visually it appears clear
  • KM-500 (phi) 30.5 ND2X - slightly grey filter
  • KM-500 (phi) 30.5 ND4X - quite a dark filter

I am sorry this might be a bit of a silly question but searching for "what to do with 3 filters that came with lens", I'm not even sure they're UV filters TBH.

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The second two filters you have listed are not UV filters but neutral density filters. They are used to reduce the amount of light reaching the sensor by a set number of stops, to allow for use of slower shutter speeds than would otherwise be possible. The names ND2X and ND4X stand for 'Neutral Density 2x' and 'Neutral Density 4x' - indicating that they will halve and quarter the light respectively (i.e. darken by 1 and 2 stops respectively).

The Skylight filter will reduce the amount of UV light hitting the sensor, however it is of more use with film photography where the blue-sensitive part of the film was also sensitive to some wavelengths of UV light. Modern digital cameras are calibrated not to respond to UV so it won't have an effect on your images. A skylight filter is also slightly pink tinted to remove the colour cast caused by shooting under a blue sky; this is because film has a fixed white balance which can result in a slight blue cast under these conditions. Software white balance correction means this is not an issue for digital cameras.

The reason the filters fix to the rear of the lens is because your mirror lens probably (you don't state which model) has a very large diameter front, which would make filters that attach to it bulky and expensive. Filters that attach to the rear could also feasibly be used on more than one lens, whereas if you had different lenses with different front element diameters, you'd need one filter of each type per lens.

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You need to have at least one filter (there's usually a plain glass "filter" in the kit; the 1A is probably playing that role in your lens's kit), since that element is part of the lens design. Because you can't adjust the aperture of a mirror (catadioptric) lens, the only means you have of changing the amount of light coming through the lens is to use a neutral density filter. Mirror lenses tend to have a rather large front element, which would normally mean a very large and expensive ND filter -- that's rather at cross purposes to the simple and relatively inexpensive lens design. So they use a much smaller rear filter instead. Because the filter lives in the lens's light path, when there's no filter in place something that bends light in the same way that the filter does needs to be put in its place, so a clear (or minimal filter, like a UV or a 1A) fliter has to be installed.

The good news is that you will probably never need to use either of the ND filters. They made a lot of sense when cameras had maximum shutter speeds of around 1/1000; modern cameras tend to go to either 1/4000 or 1/8000. Your lens probably has an aperture of f/8 (I'm only aware of one reflex lens that was faster, the Minolta RF-Rokkor 250mm f/5.6 MC) and a T-stop (actual light transmission) of about T10 (so it lets light through as if it were f/10 because of the rearward-facing mirror in the centre of the front of the lens, but has a depth of field of f/8), so it would be a very rare thing that you'd run out of shutter speed. You can likely just install the 1A and leave it there permanently.

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Just to add - UV light doesn't damage a lens (as suggested by the OP), whether it's a mirror lens or not. UV light just has the potential to affect the image. A UV filter attempts to filter out UV light, so it doesn't matter where the filter is positioned, as long as the light going through the lens also goes through the filter.

There are many different kinds of photographic filters, for many different purposes/effects. You can read more about filters here: http://en.wikipedia.org/wiki/Photographic_filter

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  • I always thought that was weird but assumed it may damage the coatings and stuff. You often see "Lens has had UV filter entire life" and stuff
    – Alec Teal
    Mar 20, 2015 at 0:06
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    People often keep a UV filter permanently attached to their lens for the simple reason that it protects the lens from dust, dirt, sand, scratches, etc, and otherwise has no adverse effects. Also, if a lens suffers an impact, it's cheaper to replace a broken UV filter than to replace a broken front element.
    – osullic
    Mar 20, 2015 at 0:36

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