Green-yellow 'Oil mark' visible with telescope on Nikon

tonyowen · Aug 27, 2021

When my Nikon is attached to a telescope, there appears to be an oil spot in the 'viewfinder'.
Examination of the mirror shows no oil spot.
When a Nikon lens is attached no oil spot is evident in the 'viewfinder'
No oil spot is evident on the single front lens of the telescope.
The telescope is circa 75mm diameter and circa 1200mm fl and f16.
Any suggestions welcome

tonyowen · Aug 27, 2021

tonyowen said:
When my Nikon is attached to a telescope, there appears to be an oil spot in the 'viewfinder'.
Examination of the mirror shows no oil spot.
When a Nikon lens is attached no oil spot is evident in the 'viewfinder'
No oil spot is evident on the single front lens of the telescope.
The telescope is circa 75mm diameter and circa 1200mm fl and f16.
Any suggestions welcome

Update
Tried with another Nikon camera, same problem
Correction of effect - it is more greeny- yellow like an old bruise that is 'coming out'

AgX · Aug 27, 2021

Maybe you can explain what you mean by "oil spot". Also that you have it about the mirror in the headline, but exclude such in your first post is puzzling.

tonyowen · Aug 27, 2021

AgX said:
Maybe you can explain what you mean by "oil spot". Also that you have it about the mirror in the headline, but exclude such in your first post is puzzling.

When I look though the viewfinder I am looking at the image via the mirror.
I've corrected 'oil spot' to a greeny- yellow colour in my update.
At all times I'm looking through the viewfinder onto the mirror hence my use of the word 'mirror.
The source/place of the 'discolouration' is 'unknown BUT seems to be on the mirror - hence the title.
regards

AgX · Aug 27, 2021

Now I am even more puzzled, as you say it seems to be on the mirror, after having the inspected the mirror and found no such. But your viewfinder contains more than that mirror...

You refer to inspecting the front lens. With a (astro?) telescope the front lens element is far out of focus, so you coulds not see such a spot in the voefindr of your camera anyway.

I leave it to others to unveil this enigma.

dynachrome · Aug 27, 2021

This sounds like a focusing screen issue. Defects in the focusing screen become more apparent when the lens is closed down or is very slow to begin with. I don't know which Nikon you are using. With a lens that long and that slow, a grid type or plain matte screen would be a good starting point.

tonyowen · Aug 27, 2021

dynachrome said:
This sounds like a focusing screen issue. Defects in the focusing screen become more apparent when the lens is closed down or is very slow to begin with. I don't know which Nikon you are using. With a lens that long and that slow, a grid type or plain matte screen would be a good starting point.

Agreed - maybe!! BUT why the same effect/illusion on two different Nikon camera???

Huss · Aug 27, 2021

tonyowen said:
Agreed - maybe!! BUT why the same effect/illusion on two different Nikon camera???

It's obviously something with the lens, not the cameras.

MattKing · Aug 27, 2021

It may be the result of the combination of the telescope - a really high magnification and relatively slow optic - and the viewfinder imaging system employed by the Nikons.
Have you examined photographs taken with the system?
Perhaps the telescope produces chromatic aberrations that the camera's viewing system is sensitive to.
Is the greeny-yellow area in the same area as a focusing aid on the screen?

tonyowen · Aug 27, 2021

MattKing said:
It may be the result of the combination of the telescope - a really high magnification and relatively slow optic - and the viewfinder imaging system employed by the Nikons.
Have you examined photographs taken with the system?
Perhaps the telescope produces chromatic aberrations that the camera's viewing system is sensitive to.
Is the greeny-yellow area in the same area as a focusing aid on the screen?

The results are unblemished [on a dslr albeit the 'illusion' is visible in both slr and dslr cameras] - the 'greeny-yellow area is at the 'bottom' of the 'viewed image' -
I assume it is something to do with the telescope -- BUT WHAT??
It is only a 'problem' when viewing an object and a small, but irritating one.

MattKing · Aug 27, 2021

Thread title updated to include colour and telescope.
Do you have the ability to change viewing screens on any of the cameras to see if that makes a difference?

tonyowen · Aug 27, 2021

reddesert · Aug 27, 2021

Tony, with respect, your description of this issue is so vague that it's impossible to say anything about it. Can you post a picture of the blemish? Can you post a picture of the optical setup? I'm not even sure what kind of telescope this is - a refractor with an objective lens and no eyepiece? What models of SLRs are you using? Do they have an illuminated viewfinder display?

The mirror of an SLR is out of focus when you look through the finder, so a spot on the mirror doesn't show up as an in-focus spot on the focusing screen. I might guess some kind of scattered light, but it's not possible to say with any certainty.

tonyowen · Aug 28, 2021

image show setup and a 'capture of 4-5 miles away.
Checked 'oil marks' - best description is blue star surround by 'damaged' yellow square, surrounded by area of fuzz

i

AgX · Aug 28, 2021

MattKing said:
It may be the result of the combination of the telescope - a really high magnification and relatively slow optic - and the viewfinder imaging system employed by the Nikons.

All more modern SLR-viewfinders contain next to the groundglass a converging lens ("field lens") under the prism. The idea is to sent as much rays dispersed at the ground glass to the eye, to yield an even lighted image. This lens even may be divided into two lenses: a massive static one, and fresnel one, making part of the groundglass.

This field lens makes part of the optical system and thus must be apt for the taking lens chosen. Or more specific: apt for the angle of ncidence of the light cone hitting the groundglas and the grid of the groundglass.
A mismatch may lead to a hot-spot.

The OP however reports an "oily spot", that from his description reminds me of Newton rings, or just those rainbow reflections from thin layers. However I do see such originate from a mismatch as such between taking lens and viewfinder arrangement.

Kino · Aug 28, 2021

I would guess that your telescope does not have enough covering ability for 35mm and you are seeing optical aberrations from the periphery of the imaging system.

Telescopes like this are designed for the human eye, not for film.

AgX · Aug 28, 2021

Yes, but then the color fringes/artefacts would be at the periphery. And not in center, as I understand the OP.

AgX · Aug 28, 2021

AgX said:
All more modern SLR-viewfinders contain next to the groundglass a converging lens ("field lens") under the prism. The idea is to sent as much rays dispersed at the ground glass to the eye, to yield an even lighted image. This lens even may be divided into two lenses: a massive static one, and fresnel one, making part of the groundglass.

This field lens makes part of the optical system and thus must be apt for the taking lens chosen. Or more specific: apt for the angle of ncidence of the light cone hitting the groundglas and the grid of the groundglass.
A mismatch may lead to a hot-spot.

The OP however reports an "oily spot", that from his description reminds me of Newton rings, or just those rainbow reflections from thin layers. However I do see such originate from a mismatch as such between taking lens and viewfinder arrangement.

I now remember having perceived iridescent colours at AF-SLR screens, though not at manual-focus SLR screens, when viewing through the eyepiece with no lens attached.
Maybe with a lens/viewfinder mismatch here may be the cause.
This is a wild guess, by no means substantiated by any contemplating on it.

grain elevator · Aug 28, 2021

tonyowen said:
image show setup and a 'capture of 4-5 miles away.
Checked 'oil marks' - best description is blue star surround by 'damaged' yellow square, surrounded by area of fuzz

iView attachment 283771 View attachment 283772

Where in the picture is the problem? I don't see it.
Or do I understand you correctly in your previous post that the issue is only the the viewfinder image, not in pictures taken with the setup? But then why would you show us a picture that doesn't shown the problem rather than a pic through the viewfinder? I'm confused.

tonyowen · Aug 28, 2021

How on earth do I take a picture of what can be seen through the viewfinder???
I've no microscope lens or a lens capable of mm focal range!!!

MattKing · Aug 28, 2021

tonyowen said:
How on earth do I take a picture of what can be seen through the viewfinder???

Try using a cel phone.

tonyowen · Aug 28, 2021

Mobile phone not very successful
Attached image from camera

tonyowen · Aug 29, 2021

another shot using a mobile phone this time.
I note that the 'illusion' is brighter on bright days unfortunately we are overcast for the next few days

grain elevator · Aug 29, 2021

Most interesting! My wild guess is that it has something to do with the field lens Agx mentioned, or the fresnel, being designed for shorter focal lengths and collecting a strong image of non-image-forming light or internal reflections.

MattKing · Aug 29, 2021

Do you see any change if something moves across the field of view closer to the telescope?
I'm thinking it has something to do with an interference pattern caused by the interaction of the focusing screen, the rest of the viewing system in the camera and the optics of the telescope.

Green-yellow 'Oil mark' visible with telescope on Nikon

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