Exit Pupil

[josh1022]

I am trying to get a better handle on this whole exit pupil thing. My understanding is that the exit pupil is the diameter of the light,
(in mm) coming out the back of the scope. So does this mean that the light in the daytime is also only this wide? Also, if you have a 5 power scope that has a 50mm objective lens & your getting a 10mm exit pupil but your eye is only going to dialate to 5 or 6mm then what good is the other 4mm going to do you? Or does this just mean you can move your eye around a little bit more than if it was a smaller exit pupil? What I'm getting at is if the extra light is not going to do me any good I might as well go with the smaller objective lens or more magnification. Right?

 

[Jim Watson]

I think you have it. A "light pencil" larger than your pupil gives a larger "eye box" and makes alignment easier. Are you going to target shoot with time to carefully line up behind a small exit pupil or are you hunting or shooting "tactical" and need to get up and going in a hurry?

 

[josh1022]

I am mostly hunting from a box blind on a power line that only has 2 maybe 3 places that I can shoot, and should have plenty of time to line up the shot." If I didn't have very much time I'm not sure I could make a quality, long range shot anyway." The rest of the time it is just shooting at the range so that doesn't really matter. I still need to be able to use the gun for a close,fast,surprise offhand shot if I need to though.

 

[josh1022]

So a 50mm scope (in low light) does't give you a brighter picture than a 40mm, it just gives you a wider area for your eye to view the site picture.

 

[rcmodel]

It depends on the power of the scope.

Exit Pupal = objective lens dia / power.

A 4x-33mm = 8.25mm exit pupil, or more then your eye can use.
A 12x-50mm = 4.2mm EP, or a dim image in low light.

A typical 3x-9x-40mm would give 13.3 at 3x, and only 4.4 at 9x.

Anything over 7mm is not any brighter, but is much easier to see through when mounting the scope quickly.

rc

 

[al123]

So does this mean that the light in the daytime is also only this wide?

Exit Pupil for a given optical system stays the same regardless of light conditions.

The iris of the human eye can, in low light conditions, open to 7-to-8 mm in diameter. As one gets older, for many folks this gets less and less, maybe 5-to-6 mm in diameter. In daylight conditions the iris opening can be much smaller.

 

[ranger335v]

The exit pupil is a pure math thing. Brightness of the image is not. No scope can transmit 100% of the light that strikes the front (objective) lens. But, fact is, the human eye cannot tell the difference between a light transmisson of 92% vs. 98% so that figure is vastly over-rated.

A second factor often un mentioned is the effect of magnification, or apperant distance for dim light shooting. We may easily see a deer at 20 yards in twilight but, without magnification, that same deer may be very hard or even impossible to pick out at 100 yards with the un-aided eye. A 5x magnification makes the deer as easy to see as if he were 20 yeards away. So, for dim light shooting, both the magnification AND exit pupil, up to about 5mm, does make it easier.

The real world twilight time difference is so brief that many of us prefer to use smaller, less powderful optics for a more compact rig and better all-around usefullness.