Exit Pupil size?

[300meter]

Can someone explain exit pupil size of a scope to me and the importance when selecting a scope?

Leupold LRT **** 6.5-20x 30mm 14.4' L 22 oz 50mm 2.6mm
Leupold LRT **** 8.5-25x 30mm 14.4' L 22 oz 50mm 2mm

The scopes above have an exit pupil size of 2.6 and 2mm. The exit pupil size is smaller 8.5-25x scope. Is there a relationship between the exit pupil size and magnification and does it impact clarity?

Thanks.

 

[NateHaler]

Can someone explain exit pupil size of a scope to me and the importance when selecting a scope? Is there a relationship between the exit pupil size and magnification and does it impact clarity?

Exit pupil is derived by dividing the objective lens diameter by the magnification.

Larger exit pupil = brighter image

A healthy young adult's pupils can dilate to about 7mm maximum. So, for brightest image, like dawn/dusk hunting, you'd want a larger exit pupil.

If you'll be shooting in broad daylight, exit pupil probably isn't nearly as critical. But a brighter image is easier to see than a dimmer image.

 

[Fred Bohl]

Let me begin with a clarification of the Nate Haler comment 'Larger exit pupil = brighter image'. If we are considering the comparative perceived brightness between the two scopes in question under identical conditions, the he is correct. Your visual system responds to both the intensity of the light at a receptor and also the total number of receptors illuminated with a higher total number usually being perceived as being brighter than higher intensity on a small number of individual receptors in day light conditions. However, under low light conditions,dusk or dawn), the type of receptors dominates as the cones are much less sensitive to low light levels than the rods. Since the center 1 degree of the fovea contains only cones and any exit pupil under 3.3 mm will subtend less than 1 degree, then under low light conditions small exit pupil will be perceived as very dim.

The fact that the small exit pupil image will be under 1 degree and be imaged in the center of the fovea does however mean that the scope image will be on the highest density area of receptor cones. This is the area of the retina that yields the highest acuity or clarity perceived by the visual system.

The compromise you have to make is that the best image clarity comes with high magnification with exit pupils under 2 mm which then also require very good target illumination and contrast to make up for the reduce light sensitivity.

 

[JimOliver]

Guys,
Is it not also true that a larger exit pupil diameter results in less critical eye placement, with respect to the centerline of the scope lenses?

Jim

 

[Fred Bohl]

Jim,

Yes, the larger exit pupil will be more tolerant of eye position error,not aligned precisely with the scope axis). However, in bright daylight, depending on age your light adapted eye pupil opening will be 3 to 4 mm diameter and a scope exit pupil larger than that is of no additional advantage.

Additionally, any misalignment of the eye with the scope axis increases the POA error due to residual parallax adjustment error. If the eye is perfectly aligned with the scope axis there will be no POA error due to residual parallax error. Therefore, the smaller the exit pupil size the less potential POA error due to parallax errors.

 

[JimOliver]

Fred,

I understand and agree that the precise placement of the eye on the lens centerline is important when very deliberate shots are made. Would you agree that in some hunting situations, quick sighting is required that will be greatly aided by a larger exit pupil-----even though some precision may be lost?

Thanks,
Jim

 

[Fred Bohl]

Jim,

You are correct for most hunting situations. My concern and emphasis was for target shooting and prairie dog from a bench style varmint hunting.

I've been strictly a target shooter for the last 40 of my 57 years of shooting so please excuse the bias in my perspective.