To APO or not to APO, that is the question (1 Viewer)

Eye relief is the space (usually given in millimetres) between your eye's surface and the front surface of the eyepiece such that you will see a full field of view. It only really matters when you use a scope wearing specs. or whn you choose a scope for digiscoping. For myself, I cannot use any scope comfortably wearing my specs, so I invariably remove them.

That said, if you do wish to wear specs, I would absolutely try a scope out before buying as the published figures do not bear out in practice in my experience. There are too many variables.

Depth of field / focus? Well, I don't notice it changes much with different make scopes and binoculars myself, but it certainly changes as magnification changes. Certainly, using a scope at high magnification produces a very narrow depth of focus. Also, I prefer 7-8.5x binoculars, as 10x binoculars have a narrower depth of focus.

romancitizen said:

Many thanks for all of your replies, the advice seems to be, try before I buy. Will get Ace Optics on the case.

I have an optician appointment to get through first, will see what she says.

btw, what does 'eye relief mean', seen that on a lot of other posts ? I have managed to understand most of the jargon, APO etc, but this eludes me.

Final question, is depth of field important ? I can understand people saying that fov is important, but why dof ? surely if you are looking at a special bird the depth of field is irrelevant. On the other hand, I guess if you are looking at a group of birds you want to see thjose 100 yards away and those 120 yards away.

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Eye relief is a measure of how far away from the eye piece your eye can be whilst seeing all of the FOV. Small eye relief means that an eye glass wearer might not be able to see the entire FOV while wearing eye glasses. I find that I need at least 17mm for a comfortable view.

DOF is imporant for various reasons. Large DOF makes focussing easier, especially for moving birds, and because more objects are in focus over a greater range of distances, the eye has to work less and so you get less eye strain.

Do indeed try before you buy. At Ace Optics try and look at dark aerials on roofs against a bright sky. If you see objectionable colour fringes around them then you have too much CA.

scampo said:

Depth of field / focus? Well, I don't notice it changes much with different make scopes and binoculars myself, but it certainly changes as magnification changes. Certainly, using a scope at high magnification produces a very narrow depth of focus. Also, I prefer 7-8.5x binoculars, as 10x binoculars have a narrower depth of focus.

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That's a good point. There's no point comparing the DOF for scope A at 20x and scope B at 30x as DOF drops rapidly as magnification increases.

Leif said:

Eye relief is a measure of how far away from the eye piece your eye can be whilst seeing all of the FOV. Small eye relief means that an eye glass wearer might not be able to see the entire FOV while wearing eye glasses. I find that I need at least 17mm for a comfortable view.

DOF is imporant for various reasons. Large DOF makes focussing easier, especially for moving birds, and because more objects are in focus over a greater range of distances, the eye has to work less and so you get less eye strain.

Do indeed try before you buy. At Ace Optics try and look at dark aerials on roofs against a bright sky. If you see objectionable colour fringes around them then you have too much CA.

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Leif - hi! Hope you're well. Correct me if I'm wrong, but isn't DOF is a physical characteristic of a lens that is related to aperture and magnification. I don't notice it changes much between makes - do you?

scampo said:

Leif - hi! Hope you're well. Correct me if I'm wrong, but isn't DOF is a physical characteristic of a lens that is related to aperture and magnification. I don't notice it changes much between makes - do you?

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Yes I do see differences in DOF between binoculars with the same aperture and magnification. I too have heard that the DOF should be the same.

Maybe differences in transmission are the explanation. Low light transmission means the eye opens up more leading to less DOF from the eye. This might be nonsense but it's the only explanation I can think of. If it is nonsense, I hope that someone knowlegeable will enlighten me with the correct reason.

I think you're right. Also, I suppose aperture comes into it as the wider the aperture, the less the dof - including that of the eye, I suppose.