Over 20X Scope, Under $200, with good Light Transmission?

[Matt304]

Yes, I know, those requirements sound a little demanding for under $200.

I am looking for a scope, at least 6-24x, with good light transmission at full power. I have a new 17HMR, and I don't want to spend a whole lot on its scope.

Any suggestions? I was looking at a Bushnell Banner 6-24, then I read the light transmission is poor at 24X. So I'm steering clear of it.

Thanks

 

[Guns and more]

The laws of physics take over. Take the diameter of the objective lens (the front) and divide it by the power. If you had a 60mm. lens at 20 power, the exit pupil is 3mm. The exit pupil is the diameter of the beam of light leaving the eyepiece. In bright sun, that is fine. But in low light your eye might open to 7mm., so that 3mm. beam of light will appear dark. Same with binoculars. 7x50 are brighter than 7x35 (in low light). If you try to achieve an exit pupil of 7mm, then objects in the eyepiece will appear as bright as they do to your eye.
Why are binoculars usually 7 power? Because it's hard to hand hold anything more powerful. I have some 10x70's but they're meant for a tripod.
In a spotting scope, I doubt you will find one with much more than 80mm. lens. So at 11 power you'll be close to the limit. Of course if you only use it in bright sun, the pupil can be smaller because your eye may be only 3mm.
Does this make sense?
If you are asking about a rifle scope, the lens is smaller (due to physical limitations) so it's harder yet.

 

[Matt304]

I was asking about a rifle scope, but either way, you explained something to me I did not understand before; pupil exit!

I know that a 24x at 40mm objective is not going to be very good on light transmission now. So I will need to find a 50-56mm for what I seek it sounds, maybe even with 30mm main tube. It helps knowing what to look for now.

Thanks a lot.

 

[rodregier]

As you age, your eye's pupil cannot open as wide, so 5mm exit pupil in an optic for most uses is ok. A larger exit pupil value in an optic *does* make eye positioning less critical. I try not to go below 3mm for an optic that would be used on a "heavy overcast" day. For night or heavy woods usage, 5mm exit pupil for an optic as a minimum is a good guideline. ...thus the popularity of 8x40 or 7x35 binoculars - a fair tradeoff between weight and brightness.