What is Reddit's opinion of Lens2scope 10mm 1:4 Eyepiece for Nikon F(DX) Lens, Straight View, Black Color?

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TL;dr: Using a rifle scope is probably not a good idea. You probably don't need to know this stuff, but if you're interested here's a very deep dive on camera lenses, resolution, and sports optics then you may appreciate this. FWIW, you nerd sniped me with this question. Quite the interesting rabbit hole.

> I think I understand the implications of what you're saying: a smaller sensor requires more resolving optics to produce images with comparable detail to those produced by a larger sensor.

Yes, and to be clear, resolving isn't just magnification.

>> diffraction level below the resolution of your camera.

> Is this something that can be somehow calculated or is it an inherent property of the lens that depends on the quality/size of the glass and can only be measured?

https://www.cambridgeincolour.com/tutorials/diffraction-photography.htm

A diffraction limited lens is one where the quality of the glass is good enough that the glass isn't the limit of the resolving power of the lens. There are also lenses where the resolving power is limited by optical flaws called aberrations. Aberrations can come from the fundamental design of the lens, from poor tolerancing, or from low-quality glass.

Diffraction depends on the size of the exit pupil of the lens. This is usually called the ƒ-ratio of the lens. The ƒ-ratio is the focal length of the lens, divided by the entrance aperture of the lens. But for telescopes and camera lenses, the ƒ-ratio is provided so you don't actually need to do the math yourself.

Binoculars internally have a focal length, aperture, and ƒ-ratio. According to this page, 10x50 binoculars typically have around a 200mm focal length and are ƒ4. A 200mm ƒ4 lens sounds great, but there are problems will discuss later.

> Can this be understood as the resolution rating of a lens?

Yes, although we often don't measure it that way in conventional photography. The absolute resolving power of a lens is more typically discussed in microphotography.

> I sometimes see lenses rated or advertised with a certain resolution number: 1.3 to 8MP for small M12 or CCTV lenses and 12MP or higher for DSLR/mirrorless camera lenses... Higher resolution lenses are always more expensive and smaller lenses tend to have lower f-numbers.

Generally, better lenses require much more complex optical designs and more expensive glass. But the measurements you're reading about here are discussing the output of the lens, not the ability to resolve fine, distant details.

E.g. a 20mm lens that can produce 8mp of detail can't capture nearly as much detail of the moon as a 200mm lens that can produce 8mp of detail.

These measurements frequently take into account the resolving power of the sensor as well. In an extreme example, a nearly flawless lens like the Zeiss Otus 55mm ƒ1.4 can resolve 20MP of detail on a Nikon D7200, 20MP of detail on a D750, and nearly 50MP of detail on a D850. The lens is so good that it's almost completely limited by the camera.

> It's starting to look like rifle scopes are inherently worse for my purposes by design...

Rifle scopes are designed for their specific purpose; be durable enough to handle a bit of rough treatment and the shock produced by firing the rifle... Be light enough and small enough to maintain the balance of the rifle. Magnify distant objects so that you can accurately hit your target. Produce a useful eye-box for the shooter, without sacrificing a lot of the other requirements.

You can use a camera lens as a sports optic. I've actually tried this adapter, and didn't like it; the eyebox/exit pupil was far too small, making the lens uncomfortable to use. I strongly suspect that a camera lens wouldn't last long on top of anything much bigger than a 22, either.

> How can one calculate/find out the focal length of a scope?

Beware: Some supposition is ahead; I haven't studied sports optics in any detail.

A scope doesn't have a focal length, because it doesn't actually focus light. Light exiting the scope is at best going to be columated, or at worst diverging. The scope is designed so that your eyes actually focus the light on your cornea.

Because of this design, you would probably need a camera lens behind the optic in order to actually use it, and obtain focus on the sensor. This could be the integrated lens of a small camera, or just a camera lens on your DSLR.

> I don't see such specs for spotting/rifle scopes.

It can't have a focal length, because it doesn't actually focus light on it's own. Internally, there are focal lengths, but they won't be particularly useful to you.

The objective, magnification, and eye-relief are going to be the most important things if you want to calculate the overall resolving power of the system.

In that sense, binoculars may be more like teleconverters than they are like camera lenses.

Major supposition warning: The following is based on a semi-limited understanding of sports optics, and the use of magnifiers in front of a lens (not normally done in photography.) I haven't tested this, but it passes a basic sanity check.

I believe you may be able to calculate the resolving power and aperture of a sports optic and camera lens system using the entrance aperture and magnification factor.

First, you need a camera lens. The ideal camera lens is one where the focal length of the lens by the exit pupil of your scope is roughly the diffraction limit of your sensor as an f-ratio.

E.g. For a crop sensor camera (diffraction limit ƒ8) and a 3x30 scope (exit pupil 10mm) a good lens would be an 80mm lens (80mm/10mm = ƒ8.)

Next, take the focal length of the lens, and multiply that by the magnification factor of the scope. E.g. 80mm x 3 = 240mm.

Next, sanity check it. 240mm (combined focal length) / 30mm (entrance optic of scope) = ƒ8.

Here's the gotcha... You're out $240 for the scope. $100-$200 for a cheap lens that can zoom to 80mm. And you have a lot of glass in your optical path that is all introducing a bunch of optical aberrations. You definitely won't reach the diffraction limit with this setup (you will be limited by optical aberrations.) Plus, you need to figure out how to mount it all (this setup would be extremely sensitive to calibration and vibration.)

Or, you could spend $250 on a Kenco 600mm ƒ8 reflector lens. Or buy an inexpensive telescope.

Keep in mind that the cheep zoom itself should go to at least 250mm @ ƒ5.6.

Do not spend money based on my calculations here. This is theoretical, I haven't tested it. It's very possible I made some fundamental mistake in my logic or calculations.

Might be fun to play with the rifle optic if you already have that, the lens, and the camera. But I wouldn't recommend spending money on it. There are lots of cheap options designed for cameras.