[rowdyjhawk]

I would like to protect my 600mm lens with a UV filter. What diameter of filter fits the front threads on a 600mm/f4 lens for the Pentax 67? The threads appear to be coarser than the threads on my other Pentax lenses. Any recommendations on filters to consider?

[surfotog]

I looked at my Pentax literature, and there is no mention of the front filter size. The maximum diameter is listed as 170mm. A 600/4 lens has a front element at least 150mm in diameter so the filter would be somewhere between 150mm and 170mm. Pentax made clear protective filters of 112mm and 128mm for the later 400mm M* and 800mm M* lenses, but to my knowledge, not for the earlier super-teles.
My suggestion would be to measure the inside diameter with a metric ruler and start looking. It won't be easy to find a filter that size. Check KEH, they sometimes have oddball size Russian filters.
Perhaps Steve (Desertscape) can be of some help as I believe he owns the 600.

[Ben_Edict]

I would like to protect my 600mm lens with a UV filter. What diameter of filter fits the front threads on a 600mm/f4 lens for the Pentax 67? The threads appear to be coarser than the threads on my other Pentax lenses. Any recommendations on filters to consider?

Why do you need a front filter for that lens at all? I guess, you won't just throw it in you camera bag or use it in a dessert storm. It is so big and massive and requires a long lens hood anyway, that I can hardly imagine any need for a protective filter at. I would, in all seriousness, make a matching lens hood instead.

Ben

[desertscape]

I measured mine and it is about 156mm although it is hard to say that the filter will be exactly that size. Be aware that when placing any filter in front of a big telephoto, that the filter must be perfectly flat, otherwise it will affect your image! I have been through this, trust me. Any second rate filter will easily be seen as softness on slides or prints. I do not use any front filters on mine, only rear 77mm ones. If you are going to use the rear filter bay, use multi coated filters if possible. Uncoated ones cause relections that can show up on film.

[ghelary]

I own this lens as well and there are no front filter thread. There's never had been any point in it as this lens takes 77mm filters at the back the lens. (Yes the mount is that big)

Given the fact that this lens is very much chromatic aberation prone, putting even a high quality filter on front would cause nightmare on the quality side.

While I see the point of making a custom lens cap from glass fiber or carbon (the stock one is leather), like Ben, i don't see the point of your request. What such a filter would be able to protect ?


My 2 cent
Guillaume

[Pål Jensen]

You cannot use an UV filter on such a lens; it will reduce optical quality significantly. . The filter nees to be part of the optical system. Many newer Pentax super telephotos are provided with a dedicated protection filter (PF filter) that comes with the lens (Eg. FA* 600/4). .

[desertscape]

Just to clarify things: An optical flat in front of a high magnification lens like the 600 will not degrade the image unless it is not perfectly flat. When an optical flat is placed in the converging light beam (at the rear of the lens) it (1.) changes the spherical aberration correction just a bit, resulting in a slightly softer image and (2.) moves the focus further back in the optical system. An optical flat up front will not add to the lateral or longitudinal chromatic aberration nor change the focus point (ie infinity will still be infinity).

[Pål Jensen]

Just to clarify things: An optical flat in front of a high magnification lens like the 600 will not degrade the image unless it is not perfectly flat. ).

I suspect nothing will be in focus if you put a filter in front of the 600mm lens. Eg. graduated filter (even the best ones) cannot generally be used on lenses longer than approximately 200mm if you want the images in focus. I don't think generic filters are totally flat.....

[desertscape]

Pal, my 400mm Takumar has a rear filter bay which I have used over the years. It has caused some reflections when using a polarizer, so, I decided to put a 105mm B+W polarizer up front instead. It does not degrade the images but a cheaper one did. The more the magnification of the lens, the flatter the filter up front has to be.

[Ben_Edict]

I suspect nothing will be in focus if you put a filter in front of the 600mm lens. Eg. graduated filter (even the best ones) cannot generally be used on lenses longer than approximately 200mm if you want the images in focus. I don't think generic filters are totally flat.....

Generic filters ARE totally flat - or at least they should be. Front filters are usually not part of the lens design, except for special application lenses (industrial, telecentric etc.).

In the past one or two manufacturers experimented with spherical front filters (but with even thickness, so without refracting propertes. But it is obvious, that you get different optical path lengthes through the filter glas for incoming parallel rays, with such a filter, which in effect may lead to undesired effects (colour aberrations mostly). It is indicative of te problems with a non-flat filter, that these filters have been on the market for only a short time.

Desertscape is quite right. Only a rear filter needs to be part of the lens design, because of the reasons Desertscape gave (namely spherical aberration correction and location of focal plane).

Ben

[ghelary]

Generic filters ARE totally flat - or at least they should be. Front filters are usually not part of the lens design, except for special application lenses (industrial, telecentric etc.).

In the past one or two manufacturers experimented with spherical front filters (but with even thickness, so without refracting propertes. But it is obvious, that you get different optical path lengthes through the filter glas for incoming parallel rays, with such a filter, which in effect may lead to undesired effects (colour aberrations mostly). It is indicative of te problems with a non-flat filter, that these filters have been on the market for only a short time.

Desertscape is quite right. Only a rear filter needs to be part of the lens design, because of the reasons Desertscape gave (namely spherical aberration correction and location of focal plane).

Ben

Ok, lets explain my point of view. If we ever make a front filter for this lens, the surface of it would be around 5 times bigger than of a 77mm filter.

A filter is made of ultrathin plastic or glass to avoid things like adding chromatic aberations or influenting the optical patch. At such a size, it would be exotic to produce perfectly flat surface, but the mechanical contraints on it (mount, gravity) would also make it tricky to keep it flat while mounted on the lens (even glass can be deformed at some point)

Therefore a "wise" technical solution would be to use thicker glass, potentially introducing chromatic aberations.

And we not mentionning also the added potential for flare, as the coating on the front element of the lens was likely not optimised to reduce reflections (nothing was supposed to be put on front of it)

[Ben_Edict]

Ok, lets explain my point of view. If we ever make a front filter for this lens, the surface of it would be around 5 times bigger than of a 77mm filter.

A filter is made of ultrathin plastic or glass to avoid things like adding chromatic aberations or influenting the optical patch. At such a size, it would be exotic to produce perfectly flat surface, but the mechanical contraints on it (mount, gravity) would also make it tricky to keep it flat while mounted on the lens (even glass can be deformed at some point)

Therefore a "wise" technical solution would be to use thicker glass, potentially introducing chromatic aberations.

We all know about the problems filters introduce. What you wrote now is exactly why I answered in my first post in this thread, that I would not recommend using a filter.

Also, quite as you wrote, producing an optical flat is basically the most complicated thing to do - agreed on that, but ofcourse you can get optical flats up to a considerable higher diameter, if money is of no importance.

But I was not questioning these things, I was just questioning, what you wrote about front filters being a part of the optical design of a lens. That is not necessary and is not common at all for standard photographic lenses.

And we not mentionning also the added potential for flare, as the coating on the front element of the lens was likely not optimised to reduce reflections (nothing was supposed to be put on front of it)

The coating on that lens is the same SMC, that is used on all lenses of that generation. And ofcourse SMC was/is Pentax highly successful) mc to reduce reflections. I mean, after all, reducing reflections is the exact purpose of multi-coatings by any vendor. Why that would work less effectively than on smaller lenses, I do not really understand.

Nevertheless I agree, that not using a filter in front of that lens is the best way to go.

Ben

[Pål Jensen]

Generic filters ARE totally flat - or at least they should be. Front filters are usually not part of the lens design, except for special application lenses (industrial, telecentric etc.).

Thats not what the Pentax lens manual says. The FA* super telephotos have front PF (protection filters) that are part of the optical design. Hence, they should not be removed. The same is true for the rear filters: a filter must always be used cause they too are part of the design.

[ghelary]

We all know about the problems filters introduce. What you wrote now is exactly why I answered in my first post in this thread, that I would not recommend using a filter.

Also, quite as you wrote, producing an optical flat is basically the most complicated thing to do - agreed on that, but ofcourse you can get optical flats up to a considerable higher diameter, if money is of no importance.

But I was not questioning these things, I was just questioning, what you wrote about front filters being a part of the optical design of a lens. That is not necessary and is not common at all for standard photographic lenses.



The coating on that lens is the same SMC, that is used on all lenses of that generation. And ofcourse SMC was/is Pentax highly successful) mc to reduce reflections. I mean, after all, reducing reflections is the exact purpose of multi-coatings by any vendor. Why that would work less effectively than on smaller lenses, I do not really understand.

Nevertheless I agree, that not using a filter in front of that lens is the best way to go.

Ben

Well, technically the coating on the front element is different from the inside as it has been hardened to be scratch resistant.

But true we are nippicking here about a solution that was is a dead end from the start.

[Ben_Edict]

Thats not what the Pentax lens manual says. The FA* super telephotos have front PF (protection filters) that are part of the optical design. Hence, they should not be removed. The same is true for the rear filters: a filter must always be used cause they too are part of the design.

Rear filters is clear. They need to be part of the optical design. For front filters they don't need to be. I have seen so far only one example of a lens, that profits to some degree from using a front filter and that's the old Tarmon 60B 300/2.8. Here the front filter will increase sharpness and contrast, at fully open aperture, because this lens seem to transmit more unwanted UV than usual and shows increased softness without the filter.

But if the manufacturer explicitly states, a front filter should be used, than be it so, I just have not seen such a recommendation by Pentax for the lens here in question, in fact, Pentax does not even give any info on the front filter diameter, only about the rear one.

Ben