Apparent vs real field of view

[Swissboy]

When looking at specifications for either binoculars or eyepieces for scopes, I often find two data sets for field of view. I think I know that the real FOV means, but what about the apparent one? What does it tell me? E.g. for the SE models, Nikon gives for all three of them an apparent FOV of 6 degrees, whereas only the 8x32 model has a real FOV of 6 degrees.

[henry link]

Robert, I think you misread the specs. All three have 60 degree apparent fields. Divide that number by the magnification to get the real field. Henry

[Swissboy]

Robert, I think you misread the specs. All three have 60 degree apparent fields. Divide that number by the magnification to get the real field. Henry

Well, it shows my limits when it comes to optics, I am afraid. However, I still don't understand what is meant by "apparent field". Maybe, you can help me a bit more, Henry. Thanks in advance! Robert

[henry link]

Robert, OK, this is going to hurt my head, but here goes.
The real field is that slice(in degrees) of the 360 degree real world that the binocular or telescope takes in. I like that system better than the meters at 1000 meters specification because it can easily be converted to apparent field by simply multipling the real field by the magnification. So an 8x binocular with a 7.5 degree real field has an apparent field of 60 degrees(8x7.5=60).

Maybe you can think of it like this. When you're looking through the binocular, all the stuff in the real field appears to be 8 times larger than it really is, so the field itself must appear to be 8 times larger than it really is, thus "apparent field", the size the field appears to be to the eye.

It's useful because it tells you immediately, without the need to refer to magnification or real field or anything else whether the binocular or eyepiece in question has a "narrow", "standard", or "wide" field. Apparent fields of perhaps 38 to 48 degrees are narrow, mostly only encountered at the low magnifications of zoom eyepieces now. 50-55 used to be considered standard, but I think in binoculars most people prefer a wider apparent field than that now. Wide fields go from about 58 to 72 in birding optics. Most wide field binoculars fall between 60 and 65.

Hope that helps. Henry

[Swissboy]

Hope that helps. Henry

Henry, thank you so much. This has really clarified it. Just to double check, let me try another wording of the explanation: If one would draw parallel lines from the edges of the real field in the general direction of the observer, one would then measure the "apparent" field say at one tenth of the distance (for 10x bins). Is this correct, can't draw here, unfortunately? (Your explanation is clearer of course.)

I hope your head does not hurt too much though! Robert

[henry link]

Henry, thank you so much. This has really clarified it. Just to double check, let me try another wording of the explanation: If one would draw parallel lines from the edges of the real field in the general direction of the observer, one would then measure the "apparent" field say at one tenth of the distance (for 10x bins). Is this correct, can't draw here, unfortunately? (Your explanation is clearer of course.)

I hope your head does not hurt too much though! Robert

Robert, Perhaps I'm missreading your explanation, but rather than describing the apparent field of a 10x bin it seems to be describing a real field 10 times wider in diameter than the original real field. A small circle surrounded by a larger one.

Apparent field doesn't exist in the real world so it can't be measured there. You only see it when you look into an eyepiece, the artificial result of the real field being optically magnified.

I'll bet there are other forum members who can come up with a better explanation than mine, but perhaps you've already got the concept and I'm just missreading your post. Henry

[gunvald]

Nikon have an excellent section about the finer details of binoculars on their web site, this might be what you are looking for:

http://www.ave.nikon.co.jp/bi_e/encyclo/tec.htm#fie

[mak]

Interesting about Nikon quoting JIS in its definition of WA.

In Europe, is it not generally accepted by leading manufacturers, that a wide angle field of view is 60 degrees +, and not 65.

[Swissboy]

Robert, Perhaps I'm missreading your explanation, but rather than describing the apparent field of a 10x bin it seems to be describing a real field 10 times wider in diameter than the original real field. A small circle surrounded by a larger one.

Apparent field doesn't exist in the real world so it can't be measured there. You only see it when you look into an eyepiece, the artificial result of the real field being optically magnified.

I'll bet there are other forum members who can come up with a better explanation than mine, but perhaps you've already got the concept and I'm just missreading your post. Henry

Thanks. Henry, I think your description fits what I had meant to say. As English is not my native language, I might get at a certain limit every now and then. Robert

[Swissboy]

Nikon have an excellent section about the finer details of binoculars on their web site, this might be what you are looking for:

http://www.ave.nikon.co.jp/bi_e/encyclo/tec.htm#fie

Yes, thanks gunvald, that's it. It's good to have a visual confirmation.

[AlanFrench]

When looking at specifications for either binoculars or eyepieces for scopes, I often find two data sets for field of view. I think I know that the real FOV means, but what about the apparent one? What does it tell me? E.g. for the SE models, Nikon gives for all three of them an apparent FOV of 6 degrees, whereas only the 8x32 model has a real FOV of 6 degrees.

It's a bit after the fact, but I don't see that anyone gave one rather simple description of apparent field - it is simply the angle your eye would move through if you looked at one edge of the field and then scanned over to the opposite edge.

A minor point is that determining the true field by dividing the eyepiece's apparent field by the magnification is an approximation. It is a pretty good approximation, but the true field is slightly larger than this result, especially at low powers.

Clear skies, Alan