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Tokina RMC 500mm/8.0 mirror lens rear filters
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PostPosted: Fri Mar 03, 2017 9:15 am    Post subject: Tokina RMC 500mm/8.0 mirror lens rear filters Reply with quote

I just received a really nice looking Tokina 500mm f/8 mirror lens. It seems to be in very nice condition, very clean and all, but one problem. The set of three rear filters that is supposed to come with the lens is missing and also there is no rear filter on the lens itself. I googled and found different opinions. Some say the rear filter is not strictly necessary, others say it is. My lens is not able to create a truly focused sharp image. I think it might be because of the missing rear filter because otherwise the lens is in very good condition. My questions for you:

1. Does anyone know with certainty whether a rear filter is necessary or not? If someone has this lens, can you test with and without rear filter?

2. If a filter is absolutely necessary, does it have to be an original Tokina filter that was specifically made for this lens or can it be any filter that fits? The diameter is 35.5mm which I don't have so can't test.

3. If a 35.5mm filter is really what I need, does anyone have a spare? Perhaps salvaged from a broken 500mm Tokina?


PostPosted: Sat Mar 04, 2017 2:59 pm    Post subject: Reply with quote

I have tried to see if there is a difference with/without filter on my tamron mirror but I could not detect any difference.


PostPosted: Sat Mar 04, 2017 4:42 pm    Post subject: Reply with quote

I can only judge by my own lens: the Minolta 250mm. I do have the ND filter, but it's missing the normal UV filter. The lens performs perfectly well without any filter mounted, although they claim that the normal UV filter is a part of the optical path. Honetsly: i've always wondered how it can be that a simple flat piece of glass is "part of the optical path", but of course i am not an optical expert.

All of the following pictures in the links were taken without a filter mounted.
http://forum.mflenses.com/minolta-rf-rokkor-250mm-f-5-6-mirror-lens-t66821,highlight,%2Bminolta+%2Brf+%2Brokkor.html

Good enough i think....


PostPosted: Sat Mar 04, 2017 5:32 pm    Post subject: Reply with quote

Thanks. After some more googling it looks like the Tokina manual also states a filter must be mounted, and not only that, but the glass must be of the exactly correct thickness, so only original Tokina filters are supposed to be good. But I too can't understand why this would be important. I didn't find any direct comparisons with/without filter.


PostPosted: Tue Mar 07, 2017 9:10 am    Post subject: Reply with quote

I had tokina 500/8 years ago, without filter. It was worst tele lens I ever used. no sharp image at all. Now I read your find, same case then.
could it be because it must be using filter? LOL


PostPosted: Tue Mar 07, 2017 12:04 pm    Post subject: Reply with quote

I just went on US eBay and did a search on "tokina 35.5" and got several hits, including filters meant specifically for your lens. Not cheap, but necessary.

The best deal, by far, is this one:f Click here to see on Ebay

Hurry!


PostPosted: Tue Mar 07, 2017 12:54 pm    Post subject: Reply with quote

Thanks. I already found that. And he even lowered the price since when I saw it first. But shipping to Europe is prohibitively expensive (more than the item itself). I also found a guy more locally. Well, Germany. He's selling those filters individually, not as a set, 14 euros a piece plus postage, the bastard. Anyway I ordered the plain one (skylight or whatever, I don't need the NDs). What can you do... I'll report when it arrives.


PostPosted: Tue Mar 07, 2017 1:20 pm    Post subject: Reply with quote

A flat filter between lens and film/sensor changes focus about 1/3 of its thickness. THis is quite much for the 16mm Zenitar fisheye, but not much for the 500mm mirror lens. With a bit of luck you could reach infinity with those mirror lenses even without filter.

It would be a bit more complicated with a fast lens lkike f/2 and faster, there are additional effects. This is the reason why high aperture microscope lenses need to be used with correct cover glass thicknes above the object. Some of those lenses have a dial to choose the correct cover glass thickness, other are made for no cover glass use.
Furthermore this could be a problem with fast lenses which are designed for film and now used on digital cameras with IR-cut and Antialiasing filter.


PostPosted: Tue Mar 07, 2017 2:41 pm    Post subject: Reply with quote

ZoneV wrote:
A flat filter between lens and film/sensor changes focus about 1/3 of its thickness. THis is quite much for the 16mm Zenitar fisheye, but not much for the 500mm mirror lens. With a bit of luck you could reach infinity with those mirror lenses even without filter.


It is not at all unusual for long lenses to have an optical flat as part of the optical path, but with many, removing this flat has little, if any, effect. I own two refractors that have optical flats before and after the elements that are doing all the work. I've played around with them some without the flats, and didn't see much difference. However, this Tokina is a departure from the norm. Its rear filter obviously has some power to it for focus to be affected so drastically. So the Tokina must have that filter in place in order for it to function properly.


PostPosted: Tue Mar 07, 2017 4:31 pm    Post subject: Reply with quote

[quote="cooltouch"]
ZoneV wrote:
It is not at all unusual for long lenses to have an optical flat as part of the optical path, but with many, removing this flat has little, if any, effect. I own two refractors that have optical flats before and after the elements that are doing all the work. I've played around with them some without the flats, and didn't see much difference. However, this Tokina is a departure from the norm. Its rear filter obviously has some power to it for focus to be affected so drastically. So the Tokina must have that filter in place in order for it to function properly.

This obviously must be true because I doubt any non-defective lens could be this bad. It's not that it can't focus. It can, but the focused image is not at all sharp. It looks like if it was given a dose of the motion blur filter with some other blur filters mixed in. I'll post with/without filter side by side comparisons in due time.


PostPosted: Tue Mar 07, 2017 6:04 pm    Post subject: Reply with quote

You know, that reminds me. A couple years ago -- right before I bought my Tamron mirror in fact, I bought a -- I dunno, a Rokinon, maybe? It came without filters. I bought it used from an eBay seller. Made by Samyang, it was an 800mm mirror and I returned it because it was not sharp. I bought it, however, because the 800mm Samyang actually had/has a good reputation. Now, I wonder if maybe it was missing a rear filter? Huh.


PostPosted: Wed Mar 08, 2017 8:57 am    Post subject: Reply with quote

The filter arrived and there's bad news. It has little effect on sharpness if any at all. Here's a direct comparison as promised:



Yes, that's the sharpest focus I could get. It's also very very difficult to get a photo not affected by motion blur because at 500mm (plus 1.5x aps-c crop!) the whole thing is extremely sensitive to any movement or vibration even on a fairly strong tripod. But these photos really are as sharp as this Tokina gets.

I can't see any difference between the photos with and without the filter other than a change in colour (the skylight filter has a slightly pinkish hue).

For comparison I included the Tokina 400mm/5.6 which is the next longest lens I have. Probably not the sharpest 400mm in existence but it's at least decent and obviously better than the 500mm.

Now I wonder why this might be. The lens as stated looks very nice, no signs it was ever dropped or otherwise damaged or even just tinkered with. No marks on the glass, minimal on the housing and minimal amount of dast particles inside. And it carries that familiar gold "quality passed" sticker proudly, so at one point it must have been a good lens. Or at least "good enough".

Since the lens manual states the presence of a rear filter is necessary and that even the thickness of that filter is important, is it possible that the digital sensor filter stack has such a devastating effect on image quality? I know many shorter lenses, especially wideangles designed for rangefinders (but also some others) have huge problems on many digital cameras. Is it possible that also some mirror lenses are affected?

I'll do a test on film if I can, just out of curiosity.


PostPosted: Wed Mar 08, 2017 9:39 am    Post subject: Reply with quote

It's not the first time that i read about the Tokina 500 mirror performing worse than expected. I don't think you'll get better results on film.

There is an article somewhere on the net (lost the link) about old mirror lenses. It's about the tension the rear mirror gets in over the years. The author shows samples of unsharp pictures and pics taken after he slackened the screws of the rear mirror, releasing the tension and as such giving sharp pictures again.

I tried to find it again, but no luck. There are however some other links that popped up while searching, like this one: https://www.flickr.com/groups/10494981@N00/discuss/72157629758572451/

So there seems to be some truth in that story, it seems the smallest amount of pressure can deform the mirror in such a way that it's performance is disturbed significantly. It's worth a try i think!


PostPosted: Wed Mar 08, 2017 4:13 pm    Post subject: Reply with quote

Thanks. Makes sense. But I can't get to any screws holding the rear mirror. There's a ring that goes off easily and under it are some set screws but when take them out, nothing else moves. Can't find any instructions either. It's a simple construction but I can't get over the last step. I guess this will have to be a project for some other time...


PostPosted: Wed Mar 08, 2017 8:21 pm    Post subject: Reply with quote

miran wrote:
... difficult to get a photo not affected by motion blur because at 500mm (plus 1.5x aps-c crop!) the whole thing is extremely sensitive to any movement or vibration even on a fairly strong tripod. But these photos really are as sharp as this Tokina gets. ...


Faster shutter speed can freeze (minimize effects of) that motion blur. Increase iso...

Near subject can be sharper than far subject sometimes, although accurate focus difficulty increases due to narrow dof combined with short focus ring throw.

Low contrast mrror-lens images, apparent sharpness sometimes benefits from increase in contrast during pp.