40mm or 50mm Objective


Apr 14, 2004
Los Angeles
Last decision before I order the new Leupold today. It's going on a .280AI for deer/antelope out to 400yds. The difference in weight is next to nothing. The only thing that I can think of is the higher position of my head on the stock due to high rings vs. med rings. Which would you go with?

I would personally go with the 40mm objective. I know some people would disagree, but I believe that the higher ring height is a bigger disadvantage over the smaller fraction of more light.
I love the 40mm over the 50. Thats why I'm ****ed at nightforce for not making a 40mm scope. Instead their new model has a 30mm straigt tube objective.

[ 05-21-2004: Message edited by: John M. ]
Yep, its optical quality that counts, ive been trying to get S&B to build a variable PMII with a 42mm Obj for about the last 10 years, unfortunately all these optics folk seem to want to go bigger and bigger on the objective side of things, a 50mm is ok, it doesn't mean major mounting problems and yes light gathering etc is good, but i would be happy with a 4-16x42.. what i would do is see where you can get a better deal and buy whichever size of objective you can at a price that is right, you will be ok with either,. Pete
Well, after having 40mm scopes for over 30 yrs now, I am begining to lean towards the 50mm ends. Mostly because the 50 will give me more light in early morning hunting when the scope is set above 10x.

I have been using a 4.5x14 50mm for about 2 yrs now and I like it much better than the other 4.5x14 40mm that I have.

By this fall I will have a new 4.5x14 50mm Boone & Crocket - 30mm w/side focus, Leupold.
I have found that, at least in the state in which I hunt, by the time it is dark enough for the 50mm to matter, it is past LEGAL shooting time. A good fit to my face means more to me.

I go for the 50mm, 40's are old technoligy... worthless in my opinion.... it depends on if your just a target shooter or a hunter, 99.999% of hunting is done in low light conditions, Most Deer are seen Early in Morning or late in Evening just as it get light or just before it gets dark..
I Hunt Coyotes, most of them are out at night or Early and late afterneed on LOW light..
The 50's shine over the 40's in that area, now for daytime hunting, say Chucks or Prarie Dogs, they are equel.
I often wander why they even make 40mm's, I wander why they don't make 60-70mm scopes...
having the scope sit higher is an atvantage (as long as it is rigid), as your point of impact at 100-200-300 etc will be closer than with the scope close to barrel. in Therory it makes your rifle shoot flatter at longer range with the scope higher.

Guys... do you know what the advantage of utilizing a 50mm objective is if you are hunting deer over a 40? Quick answer... during low light hours (magic hour) the use of a 50mm obj. will begin to loose light gathering capability when you power up any higher than 8 power. A 40mm objective is less...
To finish what "W" just posted; the 40mm will begin to loose light when the power ring is turned higher than 6.5x, rather than 8x with the 50mm.
Ideally, you want an exit pupil of about 6mm when the light is low.

IMO, 50mm scopes are WAY over rated, just like magnum cartridges used for shooting whitetails.
And yes, for those of you who may be interested, I have both.

Up in this paart of the world we are allowed to hunt 1 hour before sunrise and 1 hour after sunset-the 2 most productive times when game is on the move-50mm really perform at this time.
In low light conditions the 40mm will provide as much light as the 50mm if the magnification is reduced slightly.

The real benefit of the 50mm-56mm scopes is that it allows you to use slightly greater magnification while still achieving the magic 6mm exit pupil for maximum usable light.

This is a rather arguable benefit. Bigger isn't always better even though it seems that way.


I am going to stick my head over the parapit and put in my reckoning on this.

Flame me if you think I am wrong. You might be right. I am not an expert.

As I understand it,two factors define the capability of a scope at twilight.

Twilight Factor (square root of (mag x objective diameter ))


Relative Geometric Brightness (square(exit pupil))

Twilight Factor indicates resolving power and RGB indicates ability to gather light.

Multiplying these gives a combined measure which I will call Twilight Capability.

If you calculate the Twilight Capability of a 50mm scope and a 40mm scope at various magnifications, and allow for the stopping down caused by the human eye pupil, some interesting numbers appear.

We can easily compare the numbers by dividing the Twilight Capability of the 50mm scope by the Twilight Capability of the 40mm scope. Lets call this the Relative Twilight Capability.

We can see that at low magnification, there is very little difference between the scopes. The Relative Twilight Capability is
1.118 giving a slight edge to the 50mm.

This is because the eye pupil (say 6mm) becomes is limiting factor at low magnification when the scope exit pupils are large.

At high magnification, the scope exit pupils are small and the eye pupil is not a limiting factor in both scope.

When this happens, the ability of the 50mm scope to gather more light gives it a distinct advantage over the 40mm scope.

At 10x mag. and greater, the Relative Twilight Capability is 1.747, indicating that at higher magnification, the 50mm scope is 1.747 times more useful than the 40mm scope.

It is the median area between say 5x and 10x strange things happen as the eye becomes a limiting factor. Different people will have diffent experiences with different size scopes depending on their own individual twilight eye pupil size. Also eyes differ greatly. Some might benifit more from resolving power than brightness and visa versa.

I have posted the numbers at the link below if any body wants to see them.

I will email an excel spread sheet to anybody who wants it,detailing full calculations.



The Numbers
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