Rifle Scopes Differences between scopes with the same objective size but different zoom ranges?

[shaocaholica]

Sorry if this is a recurring question. I couldn't find anything through searching.

I'm curious how scopes with seemingly similar construction and optics compare in the overlap zoom range. I don't want to deviate from that general question so lets just assume both scopes are same make and same family. So lets say we have:

3-15x50

and

5-25x50

The overlapping zoom range is 5-15. Generally speaking, how will the image compare optically in the overlapping range? FOV, brightness, sharpness, abberations? At 5x (low end), at 10x(middle) and at 15x(high end)?

Of course intended use will dictate one or the other but I'm asking more from an engineering/optics perspective and not really practical.

 

[Jerry R]

The most noticeable difference between higher and lower magnification optics with the same objective lens size is the exit pupil. For example, on 25x the exit pupil would be approximately 2mm while the 15x power optic with the same 50mm objective would have an exit pupil of 3.3mm which would of course transmit more light to the users eye. On the low end the 3x exit pupil would be about 16.6mm while the 5x would be approx. 10mm.
There are a few other factors which affect FOV, contrast and any type of aberration that may be present but generally speaking this would be the most noticeable difference and why most higher powered optics usually have a larger objective lens system. Optics of equal quality and construction will generally have the same image quality over identical ranges of magnification.

 

[NotMalware]

The most noticeable difference between higher and lower magnification optics with the same objective lens size is the exit pupil. For example, on 25x the exit pupil would be approximately 2mm while the 15x power optic with the same 50mm objective would have an exit pupil of 3.3mm which would of course transmit more light to the users eye. On the low end the 3x exit pupil would be about 16.6mm while the 5x would be approx. 10mm.
There are a few other factors which affect FOV, contrast and any type of aberration that may be present but generally speaking this would be the most noticeable difference and why most higher powered optics usually have a larger objective lens system. Optics of equal quality and construction will generally have the same image quality over identical ranges of magnification.

Is the exip pupil a measure of the scopes "eye box"? And what makes the Nightforce Beast image size seem smaller and further away like looking through a tube vs the Kahles image seem bigger and closer to you given the similar magnification and objective lenses?

 

[Jerry R]

Is the exip pupil a measure of the scopes "eye box"? And what makes the Nightforce Beast image size seem smaller and further away like looking through a tube vs the Kahles image seem bigger and closer to you given the similar magnification and objective lenses?

There are other factors involved but to calculate a general diameter divide your objective size in millimeters by the magnification. This can be seen as that dot of light projected on your ocular lens and why it varies in size when you dial from low to high power and vise versa. Remember, light is transmitted in a telescopic sight and not gathered.

Regarding the perceived smaller image vs larger image on equal powers, that is simply the result of image quality or lack thereof depending on how you want to look at it.