objective lens diameter size ?

[Frankr]

Is there any real benefit to a bigger objective lens when comparing say a 44MM to a 50MM at the same power or even a 56MM . Naturally they say a bigger objective lets in more light to start with but what about resolution or clarity of the sight picture in low light. Once we tested a Leupold 3-9x50 to a bushnell 4-12x44 at dark and really did not see much if any difference in them right before and at dark. I had a Leupold VX6 3-18x44 and I thought it was quite good in low light to dark I wonder how a 50mm would compare.. Should I buy a 50MM over a 44mm ?

 

[pacificman]

If there is any chance for more light like they claim I would buy the bigger front objective
especially when the magnification is turned up. Of course my eyes aren't getting any younger either...

 

[Turbulent Turtle]

Let me see if I can present it in a way that makes it easy to understand.

The size of the exit pupil (the picture that is presented to the eye) is easily computed and it's predictable by dividing the objective lens diameter by the magnification. This will be constant regardless of light transmission. So for the Leupold at 9X and 50mm objective the exit pupil is 5.55mm. This allows the human eye to easily get behind the riflescope and acquire a good picture at maximum magnification. In bright sunshine where your pupil will be very small, you will get behind the scope very easily. When it gets darker and your pupil dilates and opens up, it's still very easy to get behind the scope. If the light transmission is good in the scope, the picture will be very bright because it's large (more light) and well lit.

The 12X at 44mm will provide a decent size exit pupil at 3.66mm. In bright sunshine, it will also be easy to get behind the scope and the image will be very bright is the light transmission is decent. At low light, it will be a little more difficult to get a good image behind the scope, but still quite easy to do. You may detect some difference but it's not going to really jump out at you; we're talking a half-stop difference here.

Jumping to a 56mm lens at the same maximum magnification will present a greater jump is exit pupil size and brightness in low light and you will be able to tell the difference but it will not be drastic, just easily noticeable.

The resolution is more a function of the glass, but for the same glass quality, bigger is always better in optics.

 

[Mr.Knowit all]

There is a GREAT article in Guns & Ammo SNIPER magazine(page 41). Several good articles in there this time. IMO

 

[carlsbad]

I have a background in astronomy where we push the limits of resolution. I'll add a little bit to Turtles's description above which I fully agree with. In shooting, atmospheric effects almost always prevent you from getting even close to Dawes limit. So the larger objective does nothing to help with resolution.

In fact, in turbulent air (as we almost always have at the range) large telescopes are often stopped down to a smaller diameter.

In low light, bigger diameter objective is noticeable.

In bright light, I'd say it doesn't matter at all, just the quality of the glass, quality of the optical design, and quality of the coatings.

Frankly, most of the time we are looking at a fuzzy image, badly degraded by atmospheric conditions.
The larger exit pupil is considered a negative in astronomy where your pupil is wide open and approximately 8mm diameter max. We don't want to waste light in astronomy. However, small exit pupils are easy to view with your pupil wide open. In daylight, your pupil is small and we don't mind wasted light that never makes it through your iris. So a larger exit pupil is easier to get your eye behind.

--Jerry

 

[bsekf]

I understand the quality of the coatings have a great deal to do with clarity.... and price. In equal scopes Denys is 100% correct. I think different eyes see the clarity differently and not always the same. I am nearly 80 and some days the vertical crosshair looks double, on other days it is fine.

Bill

 

[scubohuntr]

The difference between 40 mm and 56 mm may not sound like much, until you realize that the light gathering is a function of area, not diameter. A 40 mm objective has an area of 1256.6 square mm; while a 56 mm has 2463.0 square mm. Very nearly double.

The best coyote scope I ever used was the old Weaver K-856. In low light the brightness and especially the color definition were in a completely different league from anything else. Granted, lens coatings were fairly primitive 35 years ago, but the physics haven't changed. More light coming in means better vision in marginal conditions, all else being equal.

Of course, the K856 needed very high rings to clear that monster bell, so there are drawbacks too.

 

[Turbulent Turtle]

I have a background in astronomy where we push the limits of resolution. I'll add a little bit to Turtles's description above which I fully agree with. In shooting, atmospheric effects almost always prevent you from getting even close to Dawes limit. So the larger objective does nothing to help with resolution.

In fact, in turbulent air (as we almost always have at the range) large telescopes are often stopped down to a smaller diameter.

In low light, bigger diameter objective is noticeable.

In bright light, I'd say it doesn't matter at all, just the quality of the glass, quality of the optical design, and quality of the coatings.

Frankly, most of the time we are looking at a fuzzy image, badly degraded by atmospheric conditions.
The larger exit pupil is considered a negative in astronomy where your pupil is wide open and approximately 8mm diameter max. We don't want to waste light in astronomy. However, small exit pupils are easy to view with your pupil wide open. In daylight, your pupil is small and we don't mind wasted light that never makes it through your iris. So a larger exit pupil is easier to get your eye behind.

--Jerry

I've not been able to come back here and spend quality time to respond. First off, thank you for the kind words; I enjoy this subject so the back and forth is great. Always learning.

Second, I agree that we do not get close to Dawe's Limit looking at targets in F-class, especially during the middle of the day. My calculations show that limit to be about .21 arcsecond for a 56mm objective and that represents about .03 inch at 1000 yards. I sure hope I didn't drop or add a decimal in there. That represents about a 5% difference from a 52mm objective, BTW.

On very clear days, early in the morning on "winter" days, we probably get closer to that limit than on normal days. However when we are inspecting with 50X or 80X, there's probably something there and that's also where ED or Fluorite glass would come in, removing the color fringing. (Think rings.)

I get what you are saying about astronomy and pupil being wide open, but that's contrary to what we encounter in F-Class. When I'm in the field in bright sunlight, I don't think my 60+ year old pupil is at 8mm, probably closer to 1mm, and that's when the exit pupil size comes into play for me. The larger diameter lens up front gives me a larger exit pupil at the back and since my own pupil is very small, it's easier for me to get behind the scope. I could wear sunglasses to shoot and thus increase the size of my eye's pupil, but that also makes the picture darker.

There's always a trade-off in optics.

 

[carlsbad]

I get what you are saying about astronomy and pupil being wide open, but that's contrary to what we encounter in F-Class. When I'm in the field in bright sunlight, I don't think my 60+ year old pupil is at 8mm, probably closer to 1mm, and that's when the exit pupil size comes into play for me. The larger diameter lens up front gives me a larger exit pupil at the back and since my own pupil is very small, it's easier for me to get behind the scope. I could wear sunglasses to shoot and thus increase the size of my eye's pupil, but that also makes the picture darker.

There's always a trade-off in optics.

I guess what I was saying it that although it is counter to my astronomy training, I'm starting to understand why shooters prefer a bigger exit pupil allowing easier alignment with the smaller daytime pupil. --Jerry

 

[Turbulent Turtle]

Ah, now I get you. I'm slow so you have to explain things to me.

And, are you saying that astronomers are always in the dark?

 

[XTR]

For me application has much to do with what size objective I'm willing to mount.

In a competition rifle which can be adjusted 8 ways to Sunday to get a correct position then I'm good with just about anything, though if you are shooting in a weight limited class bigger is heavier, and that's a squared function of the diameter of the objective.

In a hunting rifle I will not consider an objective greater than 44mm for both weight and height above the bore. Tall scoped are more difficult to get into position behind when you are trying to shoot from the improvised positions that you can see hunting.

 

[carlsbad]

Ah, now I get you. I'm slow so you have to explain things to me.

And, are you saying that astronomers are always in the dark?

Except for solar astronomy which is relatively small portion. An astronomer will spend 30 minutes getting eyes totally adjusted and pupils wide open. Of course no lights are allowed and keeping exit pupil below dilated astronomer pupil diameter is a design limit for wide field eyepieces. --Jerry

 

[Turbulent Turtle]

Except for solar astronomy which is relatively small portion. An astronomer will spend 30 minutes getting eyes totally adjusted and pupils wide open. Of course no lights are allowed and keeping exit pupil below dilated astronomer pupil diameter is a design limit for wide field eyepieces. --Jerry

Thanks, the stuff I learn.

Thank you very much for the info.

 

[Fred Bohl]

TT,

I think the battery in your calculator must be dying --- For a 56 mm objective, the Dawes resolution limit is 0.035 MOA or 2.1 arcsecond which is 0.361 inch at 1000 yards.

 

[carlsbad]

I get .303 in at 1000 yards. So a 30 cal bullet hole is right at the theoretical resolution limit for the scope in perfect seeing conditions. As a kid in MO with perfect skies and no polution and looking at double stars that are white on pitch black background, I was able to achieve approximately Dawes limit once.

At the range, with a perfectly blackhole on white paper, no turbulence and clear air, you have a slight chance of seeing a bullet hole at 1000 yards.

I can see .243 bullet holes at 600 yards with my 5" spotting scope on a good day but not very often. Dawes limit is .015 moa which is .094".

--Jerry

 

[Turbulent Turtle]

TT,

I think the battery in your calculator must be dying --- For a 56 mm objective, the Dawes resolution limit is 0.035 MOA or 2.1 arcsecond which is 0.361 inch at 1000 yards.

Argh, I used millimeters when the formula was for centimeters.

It just goes to prove that Americans can't handle the metric system.

I would have crash landed the Mars Explorer.

 

[Fred Bohl]

For those that may be interested in scope resolution standards and testing -- from an old post:

First be assured that there are in fact quite objective and very exacting testing methods and equipment for the lenses, assemblages of lens and complete optical devices (like telescopic sights) to verify most of the design parameters intended. Also be assured that most optical products we buy are in fact tested to insure some measure of compliance with the intended result (more testing and tighter compliance values for the better quality devises).

All real optical devises are diffraction limited by definition since the objective lens diameter or other aperture restriction is of finite rather than infinite size. As you read in your cited links the question is what value should be used for the diffraction limit. The cited values include:

Raleigh's Resolution Limit (MOA) = 2.33/Objective Diameter(mm), approximate 1/4 wave error, Strehl Ratio = 0.82

Dawes's Resolution Limit (MOA) = 1.93/Objective Diameter(mm), approximate 1/5 wave error, Strehl Ratio = 0.88

Sparrow's Resolution Limit (MOA) = 1.17/Objective Diameter(mm), approximate 1/8 wave error, Strehl Ratio = 0.95

All three are based on observational experiments reflective of the time frame and quality of instruments used by the experimenters. I chose to use the Dawes value in my calculations as being the most representative of amateur level optics of today. Sparrow’s very optimistic value has only been verified by the very big and super quality major observatory all mirror telescopes using high end cameras or digital image sensors not human eyes. Note, excellent individual mirrors and lenses are being made at 1/10 wave error, Strehl Ratio = 0.97, but actual telescopes have more elements so that an overall 1/5 wave error is very good. For example, even the simplest fix power telescopic sight has 4 lenses so 0.97 x 0.97 x 0.97 x 0.97 = 0.88 overall. Also all mirror telescopes with direct film or digital image sensors have at least 2 mirrors so 0.97 x 0.97 = 0.94 overall.

Modern optical test equipment uses either high resolution film or high resolution digital imagers for the observation (minimum imager resolution is 10 times better than the required device resolution). When us mere humans test optical instruments we use our own visual system as an imager. As wondrous as that is, it is not very consistent or predictable individual to individual or even with a single individual observer from observation to observation. That is the reasons for my defining my observer as having "normal" human visual acuity.

 

[rifleman700]

Is there any real benefit to a bigger objective lens when comparing say a 44MM to a 50MM at the same power or even a 56MM . Naturally they say a bigger objective lets in more light to start with but what about resolution or clarity of the sight picture in low light. Once we tested a Leupold 3-9x50 to a bushnell 4-12x44 at dark and really did not see much if any difference in them right before and at dark. I had a Leupold VX6 3-18x44 and I thought it was quite good in low light to dark I wonder how a 50mm would compare.. Should I buy a 50MM over a 44mm ?

Pick up the latest copy of Sniper magazine. They have an article about objective lenses and what they really do and dispels quite a lot of the commonly repeated light transmission theories.

 

[Turbulent Turtle]

I made a special trip to the store in order find the current issue of Sniper, from Guns and Ammo. I’m now sorry I did that and rifleman700 owes me $9.73 and I’m not including the cost of me writing this post.


The article is written by Tom Beckstrand. I’ve seen his name before, and probably even read some of his earlier prose but after reading his article “I Object!” I think I’ll give him a wide berth from now on.


His opening paragraph is where he commits his biggest mistake when he says that the statement “a bigger objective lens gathers more light” is wrong. He does not take exception to the use of the word “gather,” which I could understand and agree with, he just says the statement is wrong, but he doesn’t quite come out and say WHY he thinks it’s wrong. Certainly not in the opening paragraph and not in the second paragraph.


In the third paragraph he starts talking about the fact the objective lens size determines the size of the exit pupil but then brushes that aside for now. Strange, because he just contradicted himself. A bigger objective lens produces a larger exit pupil. That would mean that a larger objective lens let in a more light than a smaller objective lens because you need light to create a bigger exit pupil. I guess he was absent that day in optics 101.


From that point, the rest of the article is aimed at (pardon the pun) snipers and snipers-wannabees. He talks about how you actually want the smallest possible exit pupil because you’re more certain to be properly behind the rifle to take the shot. He also talks about glass quality and he seems to have recently discovered something called ED glass. He insists that nothing under $2,500 is worth anything (not that I disagree completely with that,) but the thing of it is, I am not a sniper. I never was a sniper and I will never be a sniper nor do I have any delusions or illusions of being a sniper. I am a long range F-TR competitor, period, end of story.


As a competitor, I will spend quality time looking through my riflescope and my spotting scope. My March-X 5-50X56 produces an exit pupil of between 1.4 and 1.12 millimeter depending on whether I’m at 40X or 50X (my two commonly-used magnifications.) I would not enjoy smaller exit pupils. I spend most of my time on the line looking through my spotting scope (exit pupil size of 3.04 @ 27X,) or the general conditions. When I finally make my wind call, I need to quickly transition to my riflescope and get the shot off. An even smaller exit pupil would make that more difficult.


Mr. Beckstrand also makes allusion to the fact looking through a quality riflescope for hours on end (as he says snipers do, but I have no clue, so I’ll take him at his word,) does not cause them to get headaches. That I can understand, lousy optics will strain your eye and can lead to headaches. But I would think that having a very small exit pupil as he recommends, would cause some strain on the shooter looking through that scope. A bigger exit pupil allows you to relax a little bit for the long haul. But again, I’m not a sniper, never have been and never want to be. Besides, I’m way too old.


Since this magazine is for snipers, the magnifications he discusses are 25X and under and several of his comments and statements do not apply or are iffy when we get into 40X and above.


All in all, once you get through the massive self-contradiction with respect to the amount of light, the article is probably ok for snipers, but falls short for long-range competition with high magnification scopes.


You still owe me $9.73, rifleman700.

Oh, and TBW, I didn't see anything about "commonly repeated light transmission theories," but I may look again.

ETA: Cross posted verbatim to the light transmission thread.

 

[rifleman700]

Stumbled across this today on Optics Planet. Very short and to the point. Comment on tube size I am sure will cause some to blow a gasket.

http://www.opticsplanet.com/howto/h...pe-objective-lens-size-and-tube-diameter.html