Lens Field of View

[gareththegeek]

Most lenses you can buy seem to have a field of view of 50+ degrees. I was just wondering if it is ever desirable to have a narrower field of view?

For example if viewing a planet wouldn't it be desirable for the image of the planet to fill a good portion of the viewing area. If you have brought the planet to the maximum magnification usable by the scope based upon seeing conditions and aperture (so that it is still 'contrasty') and it only takes up a small part of the viewing area wouldn't a narrower field of view be desirable?

Cheers,

G

[gareththegeek]

Actually I am confused about field of view.

If a larger field of view is used at same mag then will the view field just be larger? So if the field of view is large enough you would have to move you eye to see it all?

Thanks,

G

[FraserClarke]

Yes, the field of view will just be larger. The magnification will be the same (as it is governed by the relative focal length of the eyepiece and telescope).

Moving your eye around to see the image is related to aligning the exit pupil of the eyepiece with your eye's pupil (so that light coming out of the telescope goes into your eye and hits your retina). That shouldn't depend on eyepiece apparent field of view, I think??

I can't think of an advantage of a smaller field of view eyepiece; other than that they have simpler optical designs, so are cheaper, lighter and contain less glass. The last part means there is less chance of scattered light/ghost images, so you could argue they are better in that sense...

[gareththegeek]

Excellent, thanks very much. That has helped a good deal - I think I understand the whole exit pupil thing now too! Nice 1

G

[brianb]

I can't think of an advantage of a smaller field of view eyepiece; other than that they have simpler optical designs, so are cheaper, lighter and contain less glass. The last part means there is less chance of scattered light/ghost images, so you could argue they are better in that sense...

That's it, in a nutshell. Cheaper by not having to pay patent licences for complex modern designs, too.

The best planetary eyepieces used to be monocentrics which have a working field of 25 degrees. Unfortunately the last manufacturer of these stopped production a few years ago. The Baader Genuine Orthoscopic (40 deg field) is now the best planetary eyepiece ... and whilst it's not bargain basement price, it's priced in the lower mid range, competitive with the better examples of the Plossl design.

[PeterW]

Try spending a day with kitchen roll tubes attached to your eyeballs.... you can still navigate about, but life isn't quite as easy as when you have a much wider field of view. Perfection may come from the narrower fields of view, but I bet you'd be hardpressed to tell the difference with an ethos. I prefer wide field as I use non-tracking mounts and it saves me pushing the scope all the time. I like the view when the moon nearly fills field of view......

PEterW