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Relex-Nikkor C 500mm f/8 - prelimiary sharpness test


Cadmium

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These are reflex lenses (mirror lenses). Just preliminary test to compare sharpness. Full spectrum no filters yet so far.

I had the Lentar for many years, collecting dust, got the Nikon version to compare differences.

Sharpness was the best I could do. Distance is about 20-25 feet.

 

Reflex-Nikkor C 500mm f/8

post-87-0-21766200-1543050004.jpg

 

Lentar 500mm f/8

post-87-0-14345000-1543050074.jpg

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You have that 1000mm F/11, right? With the filter turret, right?

No support. Both of those shots were hand held, high ISO, poor light, same settings, few times. The Lentar is consistently very un-sharp in comparison.

The Lentar has a filter turret. I will see what I can do to fix the sharpness on it, but it doesn't look good.

Will compare for UV transmission next. Lentar already tested UV, transmission is good.

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Filter glass size for rear filters for Reflex-Nikkor 500mm are 35mm-35.4mmm (acceptable range) x 2mm-2.1mm thick.

Uses compression ring type retention ring, not threaded type, filter thickness is not negotiable, using ring, unless glued, etc..

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I have a Soligor 300mm reflex which pales in comparison to the Reflex-Nikkor-C 500/8 (not the latest) in sharpness. I can tell immediately just using a scope converter..

 

I was using the reflex 500 as a spotting scope but if I can get a UV-pass filter for the rear filter replacement, I will explore the UV possibilities with the D800M.

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Unfortunately, this Reflex-Nikkor C 500mm f/8 does not transmit UV as well as one might anticipate. It changes pink UV light to blue, and that is with no rear filter installed.

The design of that this lens has rear elements, and those are probably what are cutting the depth of UV transmission.

5 elements in 3 groups.

Diagram:

https://www.mir.com....500mmreflex.pdf

 

Interestingly enough, the Lentar does not cut UV color at all, even though it has a rear element, and it seemingly transmits the same as my Kuri 35mm lens, although I have not tested it with the Sparticle yet.

I will check the Lentar again to see if something is amiss with it before I give up on it.

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I am not giving up on the Nikkor 500mm exactly.

All the Nikon lenses shift the UV color toward blue. 18/4, 35E, 105Q and 200Q... any of them, but all those I just listed still work for UV,

not that I would pick one of them for UV these days unless I had to.

Unfortunate I can't make the Lentar focus any sharper, but it doesn't have acceptable focus sharpness, it is no where near as sharp as the Nikkor 500mm.

Both lenses have very clean glass.

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Found some filter glass for UV-only to stack and fit behind the Reflex-Nikkor C 500mm.

U-340 2mm + BG40 2mm (to be exact), see stack graph at bottom.

Not the best stack, given it only has just under OD3, but didn't have anything else to fit at the moment.

Here is a shot of a black light, UV only (cuts below 400nm), high ISO because this was hand held (focus was the same in live view).

Pardon the high ISO grain/noise, I didn't feel like setting up my tripod.

Just a quick test. I have not yet tested it with the Sparticle.

 

This shot is about 25 feet away, according to the lens focus ring, which seems about right. Same exact distance as the top shot of the tape reel.

Oh, and this shot was cropped to make it more square and cut off some of the larger black area, and also this is using a a DX (D7000, same as above shots), so more crop from that than from an FX.

Lower reflection is off of white enameled light shade interior.

post-87-0-40902100-1543208274.jpg

 

post-87-0-48871500-1543208430.jpg

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Lentar was a reseller--I wonder who actually made that one lens..perhaps it was a Tokyo Koki product. Good bandpass; too bad about the terrible image quality.
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It could be missing something perhaps...

I have noticed that other copies I have seen have a rear clear filter (1A filter?) between the turret and the mount. Perhaps just to keep the dust out, but perhaps it is coated and somehow limits the wavelength enough to keep the image sharper?

I don't know, but my copy didn't come with the rear clear filter.

That rear filter may or may not make the lenses sharper, but it probably also cuts UV transmission to the same as the Nikkor C.

I may get another copy of the Lentar just to double check if the sharpness is really as bad as this copy I have is. This time one with the rear filter included, to include that in the tests also.

I certainly can't ignore the UV transmission depth of the Lentar, and the image quality seems almost unbelievably bad, and I have a hard time believing they are all that bad.

So I may try to figure that out a little bit more.

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Can it might be that the rear " filter " has been a very slightly curved lens instead?

 

Otherwise in some optical designs the presence of a flat window might affect the sharpness.

If the sharpness of your current lens improves with a rear "filter" borrowed from another copy of the lens might help possibly you could make a filter of similar thickness in a suitable UV-passing glass type.

 

It might also be that the mirrors or other optical components has lost their alignment.

I would guess that lenses with long focal length could be more sensitive to angular misalignment.

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That certainly looks improved in the second copy. Perhaps it is the misalignment mentioned above causing the blur? It still has some blurring but the top one has a bias not present in the other copy.
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Well, I looked for information about aligning the lens, but I found nothing.

The lens is extremely simple, and has no alignment adjustment inside or out.

At this point I know the Lentar 'like the back of my hand', inside and out, and there is no way to align it.

Some telescopes have something called 'collimation', I don't know if Birna means something like that by alignment,

but there is no adjustment for this lens.

Example:

https://garyseronik....to-collimation/

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Alignment by recollimating the mirror lens, yes. Several of the mirror lenses I own or have used over the years provide such means.
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Cadmium,

I found the manual here:

https://www.google.com/url?sa=t&source=web&rct=j&url=http://jameskbeard.com/Photography/Legacy_Canon_Manuals/Donated%2520Manuals/Instructions_500mm_T-Mount_Mirror_Lens.pdf&ved=2ahUKEwiDnpeM-4bfAhXywVkKHZWrB9Y4MhAWMAB6BAgDEAE&usg=AOvVaw3eyKECnoNh7IlV9RQH7DZb

 

Looks like the sky filter is optional and added to the t mount. The front can take 77mm filters.

Your uv performance is interesting. I have found some conflicting diagrams for the lens. Not sure if this one is real.

post-188-0-64872500-1543953908.gif

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Birna, Thanks.

The rear mirror in the Lentar sits on a rubber ring, like an O-ring, only square with 4 flat sides in cross section. The mirror is held in place by a large threaded retaining ring, like that used in a filter ring.

There is no possible adjustment, even from side to side because the walls of the lens barrel are too close to the edges of the mirror, only the tightness of the retaining ring is adjustable.

 

In the front, the front mirror is held against the inside of the front glass element with a center screw which goes through a center hole in the front glass element,

unlike some mirror lenses which I think have the front mirror glued onto the inside of the front glass element.

 

Dabateman, Thanks for that manual! I didn't have a copy, and I had not looked for one. Nice to see and have.

Your diagrams differ in the focus point. The diagram on the left is front focus (the entire front glass element and attached front mirror move back and forth in relation to the back mirror and rear elements).

That is the design of both the Lentar and the Nikkor C I have.

The right hand diagram has a rear focus mechanism (bellows style, essentially moving the entire lens back and forth, I think), one of the earlier Reflex-Nikkor lenses is designed like that I think.

 

I have to remove the rear clear Sky filters to install the rear UV-only stack I am using right now.

I have not tried a visual focus sharpness test comparison with/without the 1A installed on the Lentar yet.

The clear filter that comes with the Nikon C is a L37c with 39mm thread.

The clear filter that comes with the Lentar is a 1A filter with 35.5mm threads.

Here is a quick UV transmission test comparison of the the Nikon L37c and the 1A filter. The L37c cuts slightly more UV, but they both cut a lot of UV.

 

post-87-0-28220300-1544053506.jpg

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Cadmium,

This may be a dumb question. But is the back primary mirror on your first Lentar clean?

You said it was gathering dust. It may have gathered slight humidity and caused a faint film to form which may be the cause of the blur.

Try cleaning it and see if that improves the sharpness.

Otherwise that rubber o-ring may not be sitting flush, offering a slight tilt that may need adjustment.

Thats all I can think off quickly that maybe causing the slight blur you see.

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Yes, it is very clean, actually cleaner than the second copy.

I have cleaned the first one, and the mirror sits evenly all the way around, and is determined by the retaining ring which is tight all the way around.

Interesting idea though, to possibly replace the rubber ring with a new one, however it doesn't seem like there is any issue with that now.

The second copy I have not cleaned, except just the front glass outside with a microfiber cloth.

Thanks for your ideas.

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Update/Correction.

I posted previously that the Nikon and Lentar both had the same rear filter threads, but that was not correct. I have edited the previous post.

The clear filter that comes with the Nikon C is a L37c and has 39mm thread.

The clear filter that comes with the Lentar is a 1A filter and has 35.5mm threads.

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