1-4x scopes objective lens size

[rhino62]

I am in the process of looking into possibly getting a better 1-4x scope than the one I currently have (Millett DMS which are pretty good scopes for the money!), since I would like better low light performance. In considering the Leupold scopes in VX-3 and VX-R ranges which offer illuminated reticules they do not offer a true 1x also what is more puzzling is why they have 20mm objective lens, were as all other scopes with 30mm tubes seem to have 24mm objective lens. I was wondering if anyone knows why Leupold would do this, other than the fact it is the same objective lens size they use for their 1 inch tube 1-4 scopes, however I would assume the lens quality is different and it is not the same one used with the 30mm scopes.

 

[Mike Webb]

I think they stick with a 20mm objective lense because that is all that is required to get enough usable light into a low magnification. It is all about exit pupil size. To find the exit pupil divide the magnification into the objective lense diameter e.g., 20 divided by 4x magnification= 5mm exit pupil diameter. The human eye will dilate the pupil to 7mm diameter in total darkness but only to around 5.5 mm diameter maximum in dim light(late evening, early morning) so an exit pupil size of around 5mm will admit all the usable light into the eye assuming good quality lenses. Higher magnifications require larger objective lenses e.g., 42mm divided by 6x =7mm exit pupil. At 2x magnification your 20mm objective lense will have a 10mm exit pupil, more than enough. Hope this helps, no one attempted an answer so thought I'd give it a shot.
You can see the principal yourself with a 3x-9x with 40mm objective or similar scope. At 9x the scope is sharper than 3x but noticably less bright. At 9x the exit pupil is only a little larger than 4mm.

 

[sheephunter]

What Mike said. Additionally, as our eyes age the pupil's ability to dilate decreases. Many of us older shooter are in the 4-3.5mm range. 7mm is the maximum and it seems most prevalent in teenagers. 5mm is kind of the accepted standard in hunting optics today. Any more objective lens size is a waste. You can gain far more brightness from a scope with quality glass and quality coatings and this is why many older shooters need higher quality optics..

 

[rhino62]

I think it is very true that as one ages that the need for better scopes becomes more apparent. In regards to exit pupil size I found an interesting comment about using the formula Mike quoted for calculating exit pupil size in this review of the Vortex 1-4X PST http://opticsthoughts.com/index.php/rifle-scopes/4-rifle-scope-reviews/114-vortex-viper-pst-1-4x

 

[Dr. Hunter]

My Schmidt and Bender 1-4x has a 20mm and it's fine. I think you can trust that they know what they're doing.

 

[BASG]

I had a 1-4 Leupold vari x II a bunch of years ago thinking with the field of view and very short shots on the east coast I had the world by the ass. NOPE! With the 20mm obj in the thick woods deer I could see with my naked eyes were all but invisible through that scope, I didn't have it very long.