I keep seeing this term in the forums. How can a scope "gather" light? Packrattusnongratus

An optical instrument cannot gather any more light than hits the objective lens. How it transmits that light out the ocular lense makes all the difference.

A scope does not gather light. The lenses transmit light. You can have 2 scopes built the same with the same size and amount of glass, and one can appear signigficantly brighter. Differences in coatings, refinements and degrees of polishing of the lenses, resolving power, contrast and edge-to-edge definition play key roles in the overall perception of the image.

I think this is a term that is used to mean the light that is transferred to the user's eye. In a sense the light that is available to the objestive lens is "gathered" through the optic assembly to the shooter's eye.

The big thing that you need to worry about with scopes is the exit pupil. Ever notice on a spotting scope that when you get to about 30x suddenly the scope clamps down on the size of the image?

Well what really is happening is the exit pupil was reduced by higher power and the column of light transmitted by the scope to the eyepiece is this "exit pupil".

To get the exit pupil size, you divide the objective size by the power setting.

So a 50mm objective divided by 7X (like on Naval Binoculars) give about a 7mm or so exit pupil. This is considered near optimal for low light conditions.

On a rifle scope if you take a 40mm objective and divide it by 9X you get about a 4mm exit pupil which is fine under bright or daytime conditions. Turn the scope down to 5X and suddenly you have a much brighter image and an exit pupil of 8mm or so, which again is optimal for low light conditions.

Now if you take a low power variable with say 5x and a 30mm objective, you have a 6mm exit pupil which isn't bad, under most conditions. (At 4x you have nearly a 7mm exit pupil which is excellent.)

D.

quote:

In a system designed for direct visual observation, the exit pupil should be about the size of the pupil of the observer's eye, and it should be placed a comfortable distance past the eyepiece, so the observer can place his or her pupil in the plane of the exit pupil. This allows the maximum amount of light to reach the retina, and the image is not vignetted.

For example, a set of 7 x 50 binoculars has an exit pupil just over 7 mm, the average human eye pupil size at night. The emergent light at the eyepiece then fills the eye's pupil, meaning no loss of brightness at night due to using such binoculars (assuming perfect transmission). In daylight, when the pupil is only 4 mm in diameter, over half the light will not reach the retina. However, the loss of light in the daytime is generally not significant since there is so much light to start with. By contrast, 8 x 32 binoculars, often sold with emphasis on their compactness, have an exit pupil of only 4 mm. That's just enough to fill a typical daytime eye pupil, making these binoculars better suited to daytime use than night-time.

http://en.wikipedia.org/wiki/Exit_pupil

Exit pupil is the MOST OVERRATED and LEAST UNDERSTOOD of all scope characteristics.

Take a low-end Tasco 3-9x40 and a Nikon Monarch 3-9x40. They will have the SAME exit pupil. Yet which would you want in a low-light situation??? The resolving power, range of contrast and edge-to-edge detail truly minimizes the relevance of the exit pupil numbers. In fact, a good scope such as a Nikon Monarch, Burris Signature, Zeiss Conquest, etc. with a 40mm objective may appear "brighter" than a cheapie with a 50 mm objective (when all are set on the same power).

quote:

Originally posted by Bobby Tomek:

Take a low-end Tasco 3-9x40 and a Nikon Monarch 3-9x40. They will have the SAME exit pupil. Yet which would you want in a low-light situation??? The resolving power, range of contrast and edge-to-edge detail truly minimizes the relevance of the exit pupil numbers. In fact, a good scope such as a Nikon Monarch, Burris Signature, Zeiss Conquest, etc. with a 40mm objective may appear "brighter" than a cheapie with a 50 mm objective (when all are set on the same power).

Well, I am glad to see we have a real expert here. Especially one that mentions the word "brighter", when I personally am more concerned about resolution, contrast, spherical aberration, Chromatic aberration, Curvature of field, coma, etc. and as to those particular brands of scopes you mentioned, I don't own any of them so I can't comment.

quote:

BRIGHTNESS: (not a scientifically valid term) (Resolution) Definition or resolution is determined by;

A. Amount of light present at target,
B. Size, type and quality of glass used for lenses,
C. Amount and quality of light passing through the scope, determined by;
1. Coating type and quality,
2. Restriction of scatter in the interior of the scope,
3. Amount of correction in the design,
4. Magnification (a 2x increase in power, diminishes light and resolution by 1/2)

DMCI-If you will bother to notice, the term "brighter" was intentionally set off in quotes because that is NOT how I would judge a scope, but it's how much of the general public tends to do. It's no use getting into more technical terms than I already mentioned in my earlier post because only a handful of people will follow -- and they ALREADY KNOW the answer to the questions.

This is what you said: "The big thing that you need to worry about with scopes is the exit pupil"

I guess you were offended because I responded that exit pupil is one of the least important of a scope's performance characteristics. To put it even more simply, if you aint got good glass, exit puipil is meaningless. How much clearer do I neeed to be (pardon the pun, please)?

And to give one last example: If you have a $30 Tasco and a Nikon Monarch, Burris Signature, etc., side by side and one the same power setting and try to pick a buck out of the shadows at 200 yards at dusk, will it matter that both scopes have the same exit pupil? I think not...

By the way, I am glad you added the picture to your post. It lends SO MUCH MORE credibility to the whole exit pupil argument