Testing bins out.

[Aaron1996]

Hi,

Am I right in thinking that it is possible to check the true aperture of bins by shining a torch into the eyepiece and measuring the circle of light emerging using some tracing paper?

Tonight's a clear night and Jupiter and Venus were interestingly(?) in line with the moon. Finances come and go but soon I will make my decision, it varies week by week but, so far, it could be the Visionary 12x60. Does anyone know them? I hope so.

Thanks for your time.

Regards,

Bill.

[Mr Q]

The aperture size is the diameter measurement in mm of the objective lens (the front lens). What you are describing sounds like exit pupil size.

[Aaron1996]

The aperture size is the diameter measurement in mm of the objective lens (the front lens). What you are describing sounds like exit pupil size.

Hi Mr Q,

Thanks for your reply. It's a question of seeing if the bins are stopped down to any extent. Someone said shining light INTO the eyepiece and measuring the width of the emergent beam would show the true aperture. I was just checking, the members give some great info.

Regards,

Bill.

[Merlin]

All things being equal, we get the exit pupil diameter by dividing the aperture by the magnification, so 10X50 bins have an exit pupil of 5mm.

[Flying Eye]

Hi Mr Q,

Thanks for your reply. It's a question of seeing if the bins are stopped down to any extent. Someone said shining light INTO the eyepiece and measuring the width of the emergent beam would show the true aperture. I was just checking, the members give some great info.

Regards,

Bill.

You know, that's a really good point. I wonder how one does establish stopping down in binoculars and quantify by how much this may be present.

It's not something I have read about so far. Obviously there are reports of it being the case, but not how the fact was established.

Very interesting.

[BinocularSky]

One way of describing the exit pupil is that it is the image of the objective, formed by the eyepiece. A piece of paper can be used to locate this (tracing paper is easiest) -- if the objective is illuminated (e.g. by daylight), the exit pupil is where the smallest (i.e. best focused) circle of light is behind the eyepiece. To establish its size, either measure it with a vernier caliper (accurate), or use millimeter graph paper to project the image onto (rough).

With budget binoculars, you will often find that the exit pupil is smaller than what is given by the formula: Exit Pupil = Aperture/magnification when you plug in the nominal values for aperture and magnification. The two possible reasons are that the effective magnification is greater than the nominal aperture, or the effective aperture is less than the nominal aperture. I've never found a case of the former (in fact, the magnification is sometimes slightly less than stated).

Another way of establishing this is that described on Cloudy Nights by Ed Zarenski. He determines how much of the exit pupil is illuminated by different zones of the objective. This method also reveals the degree of vignetting.

(When you determine magnification, do remember that it is normally for "objects at infinity". It changes slightly as you focus on nearer objects -- the old 'O' Level lens formula should reveal why )

[Aaron1996]

Hello Steve,

Thank you for your comments. I'm glad that I haven't made my choice yet as the skies haven't been great but that's going to change soon.

Regards,

Bill.

[BinocularSky]

Re vignetting, you may be interested (or bored ) by this ancient thread on CN (my username there is sftonkin). In particular, see post #150063 by Bill Cook near the bottom of the page this image (and similar) by Barry Simon (from post #87956, about 2/3 way down page):