Choosing objective size for rifle scpes

[JLS]

AWS posted the following in a recent post on rifle scopes:

a few thoughts on scopes.  A 50mm objective will have no clearer or sharper image than a 40mm scope.  Clarity is strictly governed by quality of glass and coatings.  A 50mm objective will transmit more light in the upper powers of the scope if the quality of the glass and coatings are equal, meaning if you have both scopes set at at 10x the 50mm lens will be useable for a little longer in the evening or brighter in dim light.  Move them to 5x and they will be equal.   A lesser quality scope with a 50mm lens can be dimmer than a better 40mm scope due to the quality of the coatings and glass that transmit the light/image.  I shoot guns with 20mm scopes for coyote hunting and they have been more than bright enough and I have no restrictions on shooting hours, if I can see a coyote coming with my eye I can see him in the scope in theory a 20mm scope is just as bright at 3x as a 50mm scope at the same power, again glass and coating being equal.

The draw back of 50mm scopes is that they have to be mounted higher than a 40mm scope,  a 1/4" or more.  I tried one and had to build up the comb of the stock so high to get a good cheek weld that I couldn't remove the bolt any longer.

To expand a little further for those who don't understand exit pupil, what it is, and how it affects light transmission.

The human eye pupil is capable of dilating to 7mm.  This the maximum size it can expand to, and at the peak of your youth.  As you age and/or you have eye issues, the ability to expand to 7mm is lost.  Your maximum pupil size may only be 5 or 6mm.

The exit pupil on optics is calculated by dividing the objective lens by the magnification. 

40mm obj lens/10x = exit pupil of 4mm
50 mm obj lens/10x = exit pupil of 5mm

40 mm obj lens/6x = exit pupil of 6.67
50 mm obj lens/6x = exit pupil of 8.3

So, as you can see the benefit of a larger objective is largely determined by the magnification you are using.  At 10x, most human eyes will benefit from the 50mm objective, which gives an exit pupil of 5 mm vs. 4mm for the 40mm objective.  However, at 6x magnification, the advantage of the 50mm objective is completely lost.  A human eye cannot benefit from an exit pupil larger than 7 mm, and many will not benefit from an exit pupil greater than 6.67 (that of the 40mm objective).

Also, as AWS pointed out, quality of glass and quality of coatings will also come into play.  A scope with higher quality glass and higher quality coatings that offers an exit pupil of 4mm at 10x may very well be brighter than a lesser quality scope that offers an exit pupil of 5mm at 10x.  Imperfections in the glass create image distortion and lost light transmission.  Poor quality coatings do the same.

Bottom line, think about the distances you'll be shooting at and the magnification you'll use in low light situations and let that be your guide in choosing the objective lens size you want and need.  There is no free lunch, and the money you'll spend on a larger objective could very well buy you better quality glass and coatings instead.  Keep in mind what AWS said about larger objectives.  I like a small, compact scope that mounts low to the rifle for a number of reasons.  I rarely use more than 6x in the field, particularly in low light because I don't plan on shooting over 250-300 yards as the end of shooting hours approach.  For me, a 36mm objective lens is perfect as I'm getting a 6mm exit pupil at 6x, which is probably all that my eye can realistically accommodate. 

[JDHasty]

That is all true, but it leaves out a few other factors like how critical it is to have your eye perfectly lined up behind the scope etc.   I use some small and compact scopes, but I also use some that are larger.  And I use some that are just huge. 

[JimmyHoffa]

I seem to remember that transmission tube diameter was important too.  That for a 1" diameter tube, there was no need to go above 44 mm objective.  But if you went to the 30 mm tube, you were good for a larger objective diameter--56 maybe?

[Curly]

tube dia doesn't matter for light transmission

[JDHasty]

tube dia doesn't matter for light transmission

To a degree, but that is essentially an accurate statement.   

[Bob33]

I am personally a fan of smaller objective lens. I have two compact Leupold 3-9x33mm lens on two different rifles that I've owned for several decades. They have been around the world and killed lots of different critters. As JLS pointed out, at lower power magnifications there is little disadvantage to a smaller objective in terms of light transmission.  I've always believed that if it is too dark for me to see well enough through those scopes, it's too dark to ethically shoot.

Smaller objective scopes are lighter, less prone to damage on higher recoiling calibers, and can be mounted closer to the bore.

[JLS]

That is all true, but it leaves out a few other factors like how critical it is to have your eye perfectly lined up behind the scope etc.   

Yep, that's why I wrote this:

I like a small, compact scope that mounts low to the rifle for a number of reasons

.

[JDHasty]

Lots on the tube dia question here:

http://www.opticstalk.com/30mm-vs-1inch-tube-for-light_topic1696.html

[Bob33]

Lots on the tube dia question here:

http://www.opticstalk.com/topic1616&KW=30.html

I've read numerous articles on this topic, and the debate continues. The opinions of individuals I respect the most are that tube diameter makes no difference in light transmission. Some argue otherwise. If it does make a difference, it is very slight and not enough justification for a larger tube. The primary advantage of larger tubes is a greater range of vertical adjustment.

[JDHasty]

Lots on the tube dia question here:

http://www.opticstalk.com/topic1616&KW=30.html

I've read numerous articles on this topic, and the debate continues. The opinions of individuals I respect the most are that tube diameter makes no difference in light transmission. Some argue otherwise. If it does make a difference, it is very slight and not enough justification for a larger tube. The primary advantage of larger tubes is a greater range of vertical adjustment.

I agree.

I have north of two dozen scopes and to the best of my recollection I think I have a whopping one that has a 30mm tube.  I have the scopes I like too, the decisions to purchase were not made lightly.   

[Bob33]

They have their place, but I think high magnification riflescopes with large objective lens do little for the average rifleman. They're the "magnums" of optics - bigger is better, every time. The reality is that most hunters could do just fine for the vast majority of shooting situations with a fixed 4x or 6x scope.

[Bean Counter]

I just close my eyes and slam the trigger 

[JDHasty]

They have their place, but I think high magnification riflescopes with large objective lens do little for the average rifleman. They're the "magnums" of optics - bigger is better, every time. The reality is that most hunters could do just fine for the vast majority of shooting situations with a fixed 4x or 6x scope.

You and I are on the same page there.  I have a bunch of scopes because I don't like moving them around - so except a couple rifles that I use for big game and varmint shooting they have a scope put on when I acquire them and that is how they are set up for as long as I own them.  On a couple I have a couple different scopes that I change out, Weaver bases and Burris Zee Rings make that an almost seamless process.   

[JLS]

The reality is that most hunters could do just fine for the vast majority of shooting situations with a fixed 4x or 6x scope.

The other advantage of going fixed power is you can spend additional money on glass and coating quality relative to what you would spend on a variable power scope.

[JDHasty]

The reality is that most hunters could do just fine for the vast majority of shooting situations with a fixed 4x or 6x scope.

The other advantage of going fixed power is you can spend additional money on glass and coating quality relative to what you would spend on a variable power scope.

The Leupold 6x42 FX-3 is a real hum dinger of a great scope for open country hunting.