Home Forums Hunting & Shooting Hunting Optics 50MM obj scope brighter, given same exit pupil size, as a 42mm?

Would this be an accurate statement.

Given equal quality glass, and assuming my 54 year old eyes only open to 5mm, a 50mm lense at 10X would appear just as bright as a 40mm lense at 8X. At any power less than 8X the 50mm lense has a theoretical advantage, but I cannot use it so they would appear equal. At powers greater than 10X neither scope has any real advantage.

I understand some eyes will open to greater than 5mm, but chose that number to keep the math simple.

No matter how much light the scope transmits, you eye is limited by it's maximum pupil size. However, scopes with large exit pupils are more forgiving in eye placement when looking through them.

I sure appreciate all the posts. I think my conclusions are:
For sure: a 50mm objective will allow a 2 power increase and give the same amount of light as a 42mm objective.
For maybe, or maybe not: a 50mm objective "might" give more light at the same exit pupil size as a 42mm. How noticeable this might be is up in the air.
For sure: a larger exit pupil allows more latitude in eye placement, which can be gained by using a lower power.

The larger exit pupil can make the image brighter because it can "flood the eye with light." But as afar as eye placement goes, that would only be side to side, not back and forth eye placement, and it would be a very small amount at that. E

The 50mm is effectively brighter.

There is something else to consider here. The exit pupil is practically impossible to illuminate equally across its entire area. The illumination differences are greater illumination in the center and lesser illumination at the edge. So if you have a 5mm eye pupil peering through a 7mm exit pupil in the scope, your eye will be looking through the more evenly, and better, illuminated center of the exit pupil.

The plain and simple fact is that a larger objective puts more light energy through the optical system than a smaller one. We like to use the "all else being equal", but all else can't really be equal. Too much difference in eyes, and the perceptions behind the eyes tend to negate that.

It sort of comes down to having more light can't hurt anything. Unless you have to get a big enough scope to unbalance your rifle.

At the end of the day, I'm not sure what practical difference there is in a 40 vs 50 mm comparison. Technically it is there, what (or how much difference) you see might be another matter

I spoke with Ilya about this topic and he and I did agree that the larger objective will provide more light energy than a smaller objective will. What that means is not all 5mm exit pupils are not created equal.

That's my thinking in an earlier post.

Yes. It was your post that got me thinking about that and it made perfect sense.

Thanks. I was "thinking out loud" in a hopefully logical way, but I'm sure as heck not an optical scientist.

I would think that the # of photons per square MM in the focal plane would be greater with a 50mm scope set at 4x than with a 24mm scope at 4X all else being equal. You let more light in the light density at the focal plane must be higher.

..Exactly

That's kinda how ILya explained it to me that the concentration of light would be greater with a 50mm objective lense than say a 4x32mm Leupold would have even though the exit pupil diameter is the same. He also said that the FOV would have to be the same with both scopes (meaning the ER would have to be equal).

I believe that Leupold will tell you that the collection of light falls off after 42mm unless you increase the diameter of the tube.

Tube diameter is pretty much a myth with regards to light transmission. That was something that was started by the euro scope makers and apparently it stuck as people still believe a larger diameter tube increases light transmission.

Tell us this. Which has a larger tube diameter? A Bushnell 6500 4 1/2-30X50 with a 30mm tube or a Minox 13X56 with at least a 40mm tube. My Bushnell lasts two minutes longer after sunset for showing detail than the Minox does. So much for increase tube diameter passing more light.

That is a good question, and shows why you cannot compare a simple rifle scope to a complex roof prism
optical design used in a binocular. They are 2 different animals.
A binocular does not use a tube size, but prisms, and the objective size in both are what
is important, and that means light gathering. What gets to your eye is what is important.

I know I can see longer with my Kahles 50mm objective than my Night Force 24 mm objective. The thin center reticule in the Kahles is the first to go however. Wish I had the $$ for a Zenith 3-12 x 50 with flashdot.

A big surprise, a Kahles 50mm, brighter than any other 24 mm.

I really like Kahles scopes, but their prices are really getting out of sight.