How fast is an eyepiece?

[Ags]

I'm guessing around F1 to F2, but does any body know? Obviously its different for different EPs.

[John]

Good question but I don't know the answer !.

Well we know one part of the equation, the focal length but the other part is the aperture and it begs the question - what aperture do we talk about here - the diameter of the field lens of the eyepiece, the field stop of the eyepiece or is it the diameter of the exit pupil that the eyepiece in conjunction with the scope specs creates

I'll be interested to see what others think

While we are at it what is the focal ratio of the average human eye ?.

[newman]

I'm not sure it would be as easy to calculate, especially on ep's with multiple elements.. but even if it's just the focal length divided by the diameter of the field lens, I'm not sure the number itself would serve much of a useful purpose..

[Ags]

I'm just interested in how extreme the optics are compared to the objective.

If using the field stop, then my hyperion 24mm gives 24mm/29mm = F0.83 which is fairly wild, optically.

[TheThing]

About 40mph when dropped over concrete! Just fast enough to evade your desperate grasp.

[John]

About 40mph when dropped over concrete! Just fast enough to evade your desperate grasp.

:D:D:D

[Merlin66]

Whatever the real answer.....the eyepiece works in the same focal ratio as the beam from the objective. An eyepiece in a f10 beam will be working at f10, and in a f5 system it works at f5....

[Ags]

There is the formula:

AFOV x FL = FS x 57.3

where FL is the focal length of the EP and FS is the field stop of the eyepiece. 57.3 is the number of degrees in a radian (not a TV radian :-) and AFOV is of course the apparent field of view in degrees.

The formula rearranges into Jahmanson's suggested EP speed formula:

FL / FS = 57.3 / AFOV

So if that is true the speed of an EP is directly related to the wide-angleness of an EP.

[ollypenrice]

The human eye has a variable f ratio and stops itself down in bright light or opens up in low light (dark adapts). As far as I know the distance from lens to retina doesn't change much so pupil dilation is the key player. I have an eye surgeon friend whom I see occasionally and will ask him more next time I see him! He has a nice direct way of explaining things. One example; 'You can buy laboratory mice genetically progrmmed to become senile prematurely. They're about a Euro each...

Olly

[dmahon]

The eye varies between (approx) F3 to F8.

[palebluedot]

About 40mph when dropped over concrete! Just fast enough to evade your desperate grasp.

Is that the terminal velocity of a Nagler 22mm?

[Ags]

On the other hand, the chromatic ratio is

Focal ratio / Aperture in inches > 3

So a .3 inch lens can comfortably reach an F ratio of 1, from a chromatic aberation point of view - assuming the formula holds up at such extremes.

[great_bear]

Yes it's going to vary wildly, but since the overall system F-ratio will be set by the scope, then all that matters is that the eyepiece can accommodate the fastest of telescopes that it will be put in - so provided that it's (say) somewhat clear of F4 then it'll be fine...

[JamesC]

I think I agree with Merlin 66 that the eyepiece works at the same focal ratio as your telescope.

The stuff I'm going to say will probably make an optical designer's hair curl, but I'm going to try anyway. Eyepieces are not the same as telescopes back to front. I think I'm right in saying that with a simple telescope the entrance pupil is coincident with the objective. In an eyepiece the exit pupil is out from the last lens by some distance which is handy because you want to place your eye there. If you wanted to turn an eyepiece around and use it as a mini-apo you would need to place an aperture stop at the exit pupil that would slow it down to f5 or f4 or however hard you think you can push the eyepiece - that is if you wanted to eyepiece to work as designed, which you probably do.

[Merlin66]

I'm glad someone agrees!

It's a bit like using an f4 collimator lens (on a spectroscope) and then fitting the instrument to an f10 telescope....the emergent beam from the collimator (the exit pupil) will be as a result of the input f10 rather than the lens' f4

An example:

A 30mm diameter, f4 collimator (120mm focal length) will give exit beam of 30mm - by design, but if you input an f10 beam, the output beam will be only (120/10 = 12) 12mm diameter - no matter what you try to do.

HTH