Eye relief

[domstar]

I've just been thinking that I don't properly understand eye relief. I don't wear glasses and I like a nice screw-up eye cup, but I find I can focus perfectly wherever my eye is (a UHC filter might make things difficult). I can move my eye back several centimetres and the image in the middle doesn't change although the field of view obviously gets smaller. 

So what is eye relief? It doesn't seem to be about focus so is it how far the eye needs to be away from the lens if I want to see the whole field of view? 

[westmarch]

You have summed it up in your last sentence.  Its the maximum  the eye needs to be away from the eyepiece if you want to see the whole field of view.  Typically it is a problem for spectacle wearers. Although even for those lucky enough to have perfect vision, when you get to 6mm and below, eye relief is so short that your eyelashes start to smear the lens and moisture from your eye can cause fogging at low temperatures.

Too much eye relief can also  be a problem since its difficult to get your eye at the optimum distance from the eyepiece  - you end up with kidney beaning where the edges of the fov gets blacked out.

John

 

[domstar]

24 minutes ago, westmarch said:

eye relief is so short that your eyelashes start to smear the lens and moisture from your eye can cause fogging at low temperatures.

Oh yes. Unfortunately, I do smear my smaller lenses.

Thanks for the info, John. Now, what's kidney beaning (or for that matter pin cushioning) or should I start another thread?

[vlaiv]

13 minutes ago, domstar said:

Oh yes. Unfortunately, I do smear my smaller lenses.

Thanks for the info, John. Now, what's kidney beaning (or for that matter pin cushioning) or should I start another thread?

Kidney beaning is sort of "blackout" at the part of FOV - in the shape of kidney bean - which tends to move around (coming and going) when you move your eye about. I'll try to find diagram or image of it to make it clear:

Here is good example:

Left is diagram of how it is created, and right - visual appearance of the effect - note that it is in bottom position, but it can be all around, and will move around as you move your eye.

Pin cushion is distortion of geometry of the field, and can be understood with help of following image:

[Louis D]

1 hour ago, domstar said:

but I find I can focus perfectly wherever my eye is (a UHC filter might make things difficult).

Even when below the eye relief point?  Generally, most folks get terrible blackouts all over the image when they push in too close to the eye lens.  This is especially noticeable with long eye relief eyepieces without adjustable eyecups to keep you the minimum distance away from the eye lens.

[domstar]

Thanks @vlaiv a very clear explanation (as always). I won't even mention what I thought they were ?.

[domstar]

20 hours ago, westmarch said:

 Its the maximum  the eye needs to be away from the eyepiece if you want to see the whole field of view.

Does that mean eye relief is simply determined by the diameter of the glass at the top part nearest to your eye? For example, my 6mm ortho has a tiny hole and little eye relief but a different design with a bigger hole would let me see the whole field of view from further back. Or is it more complicated than that?

[vlaiv]

10 minutes ago, domstar said:

Does that mean eye relief is simply determined by the diameter of the glass at the top part nearest to your eye? For example, my 6mm ortho has a tiny hole and little eye relief but a different design with a bigger hole would let me see the whole field of view from further back. Or is it more complicated than that?

It's more complicated than that, but you have a very good point - if optical designer is aiming for certain eye relief and designs the rest of eyepiece in that way - they need to put suitable size eye lens to make it work. It depends on both desired eye relief and AFOV of eyepiece (larger AFOV - larger eye lens for given eye relief).

[rwilkey]

I always find the following diagram from Wikipedia useful (The eye relief. 1 Real image 2 - Field diaphragm 3 - Eye relief 4 - Exit pupil) :