Home Forums Hunting & Shooting Hunting Optics 42mm vs. 50mm objective.......

I recently ordered a 2.5-16x Bushnell 6500 mil-dot scope to use on an AR-15 for varmint hunting, target shooting and general plinking. But, after I'd already made the purchase, I realized that it was a 42mm and not a 50mm scope (duh!! ). I wasn't going to welch on the deal so now I have it and have since purchased a 50mm one as well (the 50mm one should be here this week).

I realize that the 50mm will transmit more light than the 42mm and have heard it's a good idea to have at least 4mm of objective lens diameter for every power level for best performance at high magnification. That being said, if it's even true, I would like to have an idea of just how much light we're talking about and what kind of performance differences I could expect to see between the two.

Thanks for your input.....

How much light would depend on the surface area of the objective and the transmition ability of the scope.
As to any performnance difference, you can use more magnification with the 50mm with the same exit pupil size. Assuming your eyes can use this difference, on a dark night, it would be the difference between 6X for the 42mm scope and a hair over 7X for the 50. On a dark day, at the last of legal light which I've noticed requires at least a 5mm exit pupil, it would be a bit over 8X for the 42mm scope vs. 10X for the 50. E

Nebraska,

I called Leupold and asked if they had some machine which could measure any difference, if any, with the 50 over the 40. The lady said they could measure only 5% better and then only on the highest magnification.

Thanks for the info. That's not a whole lot of difference so I wouldn't be suprised if I ended up going with the 42mm after all.

I dont think you will notice much difference in brightness at lower powers. I do believe that larger objectives for some reason have a bit better resoloution. Ive read that somewhere but not sure where.

Nebraska:
One other thing that is important here, is that the larger objective scope will be heavier, will require taller rings, and with that higher eye placement, etc. So for most, the 42 mm scope will do you just fine. Since you also have the 50 you can see what one you like best.

The average size of the pupil is 5.0 mm. The exit pupil can be determined for a scope or binos by dividing the objective lens diameter by magnification, i.e. 8 x 42, 42/8 =5.25, essentially allowing more light into the scope than the eye allows.
This is important as magnification increases, 16 x 42, exit pupil = 2.625, about 1/2 as much light as your eye can absorb.
It is all a tradeoff, just some food for thought on objective size. The bigger the objective, the more light it will allow to pass at higher magnifications (all other variables must remain constant).
Have a good evening.

The bigger scope reticule lets you turn up the scope ONE whole power in the dim light. Say 6 power, instead of 5. Big deal!

If more people understood this - over-sized scopes would be less frequently encountered in the field.

Yes, big objectives have their place - but in my opinion, the great majority of people who have them - don't need them.

From an esthetics point of view, I think 42 mm scopes look a whole lot better on a rifle than scopes with the 50 mm objectives. And, for practical purposes, there isn't a whole lot of difference between them, for hunting.
Quality of optics is very important as a high end 42 mm objective scope trumps a 50 obj low end scope.

I'll second that, DMB. I found that my Swarovski 42 mm objectives compare with my 50 mm Leupold Vari-X III, while the Swarovski 50 mm really plays in a higher league.

42 is about as large as I want to hunt with. Most of my scopes are 36 and 40.

Since you will have both, I would look through them and see if you can tell the difference instead of asking folks on the internet what they think you should see.

Most of mine are 40 - 44 mm. I like a low mounted scope so I gave up on the larger objectives.

I'm night blind anyway, so I would have to get a scope with an objective the size of a garbage can for it to help me in low light.

Best,

JM.

John,

I like a high mounted scope because it gives the advantage of about 100 feet per second more without the 100 feet per second increase in velocity.

I was thinking about just leaving the 42mm version sealed in the box and selling it but to your point, I'm just to curious now to do that. I'll be comparing them side-by-side this weekend......

+1 If your shooting stuff in the semi-dark and want a specialized rig get a 56MM not enough difference with a 50mm . Glass quality does matter in this equation.

Pupil diameter depend on the amount of light present. In bright light, average pupil size is around 2mm. A young person may have a maximum pupil size of 7mm in full dark. As you get older, your maximum pupil size decreases. And exit 4mm is a good rule of thumb for most shooting including early morning and evening.

If you are only going to be shooting in full daylight, 2-3mm is going to be more than adequate. Unless you are going to be night shooting, 6-7mm is unnecessary and for older people unusable.

To calculate exit pupil, divide objective size by power.

Lastly, note that a large exit pupil means the scope will be less sensitive to head position, even if your eye can't use all the available light.

I have a 50mm Leupold and wish I did not. It just looks goofy. Like a wine bottle on top of the rifle.

I might see if I can work a trade for something else.