How to make a 50mm eye piece from a camera lens

[25585]

Www.skyatnightmagazine.com/feature/how-guide/how-make-50mm-eyepiece

Worth it? 

[Alan White]

Hits me as a tragic waste of a 50mm prime lens, even if its an older legacy one,
these lenses are often very good and easily adapted to live on as a photographic item.

[Stub Mandrel]

I read that anything longer than 32mm focal length will get vignetted to the same filed of view as a 32mm EP by a 1.25" focuser tube.

[25585]

11 minutes ago, Stub Mandrel said:

I read that anything longer than 32mm focal length will get vignetted to the same filed of view as a 32mm EP by a 1.25" focuser tube.

More suited to a 2" tube. Stopping down the lens would make a difference?

[Louis D]

To be honest, I'm surprised it works at all since camera lenses produce a virtual image at the iris location (effectively the eye relief point) and then add lenses after it to flatten the image and project it through the mirror box onto the imager plane as a real image.  I guess I'll have to grab one of my old 50mm lenses and hold it against the focuser to see if I can see anything.

I thought the article was going to recommend using step rings to attach the front filter threads to a 2" barrel from a diagonal or some such.  Epoxying seems so messy and permanent.

[martinl]

I just tried it, using an old Yashica 50mm f/1.9 resting on a 2” extension tube.

It works, it seems sharp (daytime only) and without any obvious optical problems. However, since the lens aperture (50mm/1.9=26mm) acts as a field stop the FOV is very narrow, only barely wider than a 24mm Panoptic (the Pan has a 27mm field stop). A faster lens should give a wider FOV, a 50mm f/1.4 for example has an aperture of 36mm which should translate to an AFOV of around 40 degrees. 

[YKSE]

6 hours ago, Louis D said:

To be honest, I'm surprised it works at all since camera lenses produce a virtual image at the iris location (effectively the eye relief point) and then add lenses after it to flatten the image and project it through the mirror box onto the imager plane as a real image.

Are you sure it's virtual image? My impression from high school is that you can't do anything to a virtual image, either relay or manipulate it at its position, since it's not there, and that's why it's called virtual, in contrary to "real image". OR?

[YKSE]

6 hours ago, martinl said:

I just tried it, using an old Yashica 50mm f/1.9 resting on a 2” extension tube.

It works, it seems sharp (daytime only) and without any obvious optical problems. However, since the lens aperture (50mm/1.9=26mm) acts as a field stop the FOV is very narrow, only barely wider than a 24mm Panoptic (the Pan has a 27mm field stop). A faster lens should give a wider FOV, a 50mm f/1.4 for example has an aperture of 36mm which should translate to an AFOV of around 40 degrees. 

Nice one Marin

I'm interested in reading more of your impression and measurement.

[Alien 13]

I have tried this in the past and works well, I believe there are some zoom eyepieces are actually small CCTV lenses in reverse.

Alan

[martinl]

OK, by popular demand I'll expand on my experiment.

The Yashica Yashinon-DS 50mm f/1.9 is an old M42 mount lens. It is single coated and quite soft wide open when used for photography so I didn't have high expectations of its performance as an eyepiece. The aperture of the field lens is 28mm, corresponding to a maximum possible true field of view slightly larger than a 27mm field stop 1.25" EP and an AFOV of about 31 degrees (AFOV=2*arctan(r/FL)). The eye lens (when used as an eyepiece) is 17mm in diameter, allowing for 30mm of eye relief (ER_max=r/tan(AFOV/2)).

 

 

Add a 48mm-2" adapter from the parts bin and an M42-FX adapter as eyecup and we have this:

 

(Yes, I know that standard operating procedure calls for duct tape, not electrical tape, when joining precision optical components, but I didn't want any sticky residue on the camera lens.)

 

Bizarrely, it was clear this evening (don't worry, it clouded over before nightfall so all is again right in the world) and the 3/4 moon was well placed for some twilight observing with the TV85.

Panoptic 24mm, 68 degrees. 25x magnification.

Yashinon 50mm, 31 degrees. 12x magnification. (Same reproduction ratio.)

 

AFOV aside, I was quite impressed by the Yashinon. It was sharp throughout the FOV, good contrast, no spurious colour and no distortion. By contrast, the Pan 24 does this to the moon at the edge of the field:

 

 

Panoptic 24mm distortion. (No, it's not one of the lost Didcot cooling towers).

 

Yashinon 50mm distortion. (Resized).

 

At this point I replaced the 2" nosepiece with a 1.25" nosepiece. This cut the true FOV slightly to exactly match that of the Pan 24, but also allowed me to directly compare the 50mm Yashinon+2.5x PowerMate to a 20mm Tele Vue Plössl on the moon at 30x. What should have been a walk-over turned into an hour-long quest to find ANYTHING in the 20mm Plössl that didn't show up in the 50mm+PM.

I failed.

So I added a 2x Celestron Ultima barlow to both eyepieces (~60x). Still, the differences were minor. On high-contrast targets the 50mm+PM was, if anything, slightly ahead. It was first to separate the smaller northern mountain in Tycho from the central one, It was also the first to spot the central craterlet in Plato. Only on low-contrast targets were the differences clear. For example, the 20mm Plössl resolved the double rim of Posidonius whereas the 50mm Yashinon+2.5x PM did not. Unfortunately, the clouds rolled in before it was dark enough for any DSO observations.

So, in conclusion. Should you start converting old camera lenses to eyepieces? Probably not. I was impressed by the performance of this lens, but in the sense that one is impressed by a talking dog (it's not what it says that's impressive, but that it talks at all). A 40/32mm Plössl can be had new for not much more than a 50mm camera lens + adapters and make for a smaller, wider AFOV EP with better coatings and fewer air-glass surfaces. On the other hand, if you have a parts bin with all the required items and want a long-FL EP, e.g. for use with filters at large exit pupils, then you have nothing to lose by trying!

 

 

[John]

Interesting, very interesting

I've got an old Pentax SMC F/1.7 50mm lens somewhere - maybe worth an experiment with my longer FL refractors

 

[martinl]

6 minutes ago, John said:

I've got an old Pentax SMC F/1.7 50mm lens somewhere - maybe worth an experiment with my longer FL refractors

 

For a quick test you can just rest it on a 2” diagonal - no mods needed.

At the end of the day an eyepiece is nothing but a loupe for magnifying the virtual image at the telescope’s focus. In the good old days of slide film many photographers used ~35mm camera lenses in preference to commercially made loupes to magnify 35mm slides on light tables. I kinda miss those days. Nothing in digital photography has yet been able to match the brilliance of a Fuji Velvia slide on a light table...

[Rocket_the_Raccoon]

How about using the objective lens if a cheap 8x21 or 10x25 binoculars to make a symmetrical eyepiece?

[martinl]

38 minutes ago, Rocket_the_Raccoon said:

How about using the objective lens if a cheap 8x21 or 10x25 binoculars to make a symmetrical eyepiece?

You could do that. A symmetrical made from 25mm f/4 objectives (typical of binoculars) would give an EP focal length of about 55mm, http://www.astronomyboy.com/eyepieces/ep_calc.shtml, however the AFOV would still be quite small (limited by the 25mm entrance pupil / field stop), about 26 degrees.

To make a symmetrical with more AFOV you’d need faster doublets. I’d estimate that commercial 55mm Plössls probably use something like 50mm f/2 achromatic doublets. 

[John]

I have a 55mm plossl which is similar to the one described above. It's an ex military optic that has been adapted to fit a 1.25" barrel. It works but the AFoV is really narrow - probably around 30-35 degrees. There is no field stop as such and the field edges are defined by the inside of the 1.25" barrel.

It's not a lot of use to be honest - I bought it a long time ago when I didn't know much about eyepieces

[Louis D]

3 hours ago, Rocket_the_Raccoon said:

How about using the objective lens if a cheap 8x21 or 10x25 binoculars to make a symmetrical eyepiece?

Head over to CN and search for posts by Martin Pond.  He's been dissecting binoculars and experimenting with many home-brew eyepieces made from the optics.  He occasionally details his adventures in postings there.

[25585]

I have found an 80mm medium format lens (Bronica) in my cupboard. Must measure it up. 

[martinl]

The downside of medium format lenses is that they are usually quite slow. A typical 80mm f/2.8 lens (29mm aperture) would have an entrance pupil / field stop barely larger than a 50mm f/1.8-1.9 lens and so would give almost the same true FOV. 

[Stub Mandrel]

I made a Plossl using two cheap (but coated dual element) roof prism binocular objectives back to back.

It worked but because I bought some budget EPs I never used it in anger.

[Rocket_the_Raccoon]

Thank you for all the replies :-) For sure will experiment more with homemade EPs when I have time.