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Tubes and Objectives

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Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: March/15/2010 at 10:22
wjm308 View Drop Down
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With so many different options out there, so many manufacturer's to choose from, do you buy German glass or Japanese glass, do you get a larger tube or a larger objective or both?  It can be quite overwhelming trying to decide on the "best" scope for each individuals preference.  The good news is that we have many options today and there is some great glass to be had for some really good prices.
 
My question today is focussed on tube and objective size as I am still a little unclear about the full advantages of both.  I would prefer responses from those who actually "know" rather than those just giving their opinion, but I realize this is a public forum and I'm bound to get people's "opinions" anyway Big Grin
 
So with tube size, we have 1" (25.4mm), 30mm and 35mm tubes and with objectives I'll simply focus on the 40 - 50mm range as this will cover by far the majority.  I spoke with another shooter recently about the advantages of a 35mm tube and he indicated that the only advantage was increased windage and elevation adjustment.  Well that makes sense; but I know a little something about optics and it would seem to me that a larger tube will allow more light to pass through and also allow a larger field of view than would a smaller tube size, is this not the case? 
 
Traditionally we have seen larger objectives that lay claim to better light gathering capabilities and while it's certainly true that if your front objective is larger it has the potential to gather more light, much like a larger drain will allow more water to fall through.  But given this drain to water analogy, if you have two drains with the same openings but one funnels down to a smaller pipe diameter, physics would dictate that the drain with the largest diameter pipe would allow the most water to drain at any given time.  This being said, I understand that the properties of light differ from the properties of water but the physics I believe would still apply would they not?  I guess you can see why I was hoping for someone who actually knew the answers (like an optical engineer) could explain the physics involved and help clarify the situation. 
 
I realize too that there are other factors at play, like the quality of the glass used (if it's poor quality glass that has lower light transmission then that will make a difference), but all things being equal (glass wise), if you had two scopes, let's say a 4-16x40mm scope with a 35mm tube, and then you had a 4-16x50mm with a 1" tube, would both scopes provide the same amount of light to the eye even though the objective size has a 10mm difference, or would the scope with the larger main objective still provide more light to the eye even though the smaller objective scope has a 10mm advantage in tube size?
 
My last question is this, how can we (as hunters, competition shooters, LE & Military, etc.) determine the best combination of tube vs. objective size for the situations that we'll be shooting in?  Would the spec for "Exit Pupil Diameter" allow us to determine how much light is actually getting to your eye, or are there other spec's we should be watching that is a good sign of light gathering capabilities?  Thank you.

Bill
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: March/15/2010 at 13:37
RifleDude View Drop Down
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First of all, despite what it would seem, tube diameter has absolutely no effect on light transmission.  I know that sounds counter-intuitive, but it's true.  The advantages of a larger tube diameter are that it potentially (but not always) provides more W/E adjustment range, it accommodates higher zoom ratio erector assemblies better, and it allows for larger diameter erector lens elements, which theoretically can provide slightly better center field resolution.  All else being equal, larger main tubes are also stronger, but unless you are VERY hard on your scope, that's probably a moot point.  In some cases, 30mm and larger main tube scopes leave more room for a more robust erector spring system, providing better mechanical reliability.

The main reason for the different tube diameters is simply the fact that Europe and all countries besides the US uses the metric system exclusively, and the US has traditionally used the inch system but now uses both.  Therefore, some of the finest scopes you can buy have mm size tubes because that's the standard in their country of origin.  The American shooter had been led to believe the 30mm tube provides better light transmission, and the Euro scope manufacturers of course did nothing to dispel that myth because it helped their sales.  In actuality, some of the best low light scopes just happened to have 30mm tubes because for a long time, most of the truly high end scopes came from companies offering only 30mm models. 

The 1" tube scopes are all aimed at the US market, and the Euro scope manufacturers later decided to capitalize on the 1" tube trend in the US.  However, most of the companies that make primarily 30mm scopes but offer 1" scopes for the US market still lavish their best technologies in their 30mm scopes, not because of the tube size, but because they sell more 30mm scopes.  Zeiss is a good example of this.  Though the Conquest line are fine scopes, they aren't as good as the Victory series.

The 34mm, 35mm, 40mm, etc. main tubes are all primarily found on tactical model scopes because the larger main tube provides additional adjustment range and room for more robust erector assemblies and erector support springs.  It's also easier to cram the new higher zoom ratio mechanics into a larger tube.

Concerning objective sizes, larger objective diameters do indeed provide more light transmission.  But only to a certain point.  Whether a larger objective is beneficial or not depends on the magnification and focal length of the optic.  Larger objectives combined with longer focal length can provide slightly better resolution along a greater portion of the field of view because the objective lens used would have less of a parabolic surface.  A larger objective provides a larger exit pupil at a given magnification.  Larger exit pupil = more light that reaches the eye.  An optic's exit pupil diameter is determined by the objective size (mm) divided by the magnification.  So, a 50mm objective scope at 10X would have a 5mm exit pupil.  The general rule of thumb is that a 7mm exit pupil provides all the light the average human eye can use, because that is at or above the maximum dilation of the iris of a healthy young person.  For most adults, 5mm is plenty, though.  So, all else being equal, a larger objective allows you to use more magnification and still have good light transmission and in the process provides less critical eye placement behind the optic.  But, once an optic provides the magic 7mm/8mm exit pupil, you aren't gaining much, if any advantage with a larger objective.  For most hunting, I see no advantage to going beyond a 50mm objective.  If I hunted primarily at night from an elevated blind, as is commonly done in Europe, I can see the advantage of a 56mm objective, as it would allow you to take advantage of excellent low light performance up to 10X or so. 

All optics involve a series of trade-offs, and there are practical considerations here as well.  Is the extra light transmission worth the increased bulkiness of the scope?  That is a personal decision.  Personally, I don't like for my scope's objective bell to be wider than the width of the rifle's forend, because the rifle doesn't fit as well in scabbards and gun cases.  I also think such a scope looks out of proportion to the rifle.  A larger objective diameter requires the use of higher scope rings, so the scope might not line up with your eye naturally when you shoulder the rifle.  For these reasons, I've found that limiting max objective size to 50mm in most cases provides a good compromise between excellent low light performance at the practical magnifications I use and physical size of the scope.  For most hunting scopes on most rifles, I prefer a 32mm - 42mm objective.  I prefer scopes in the 1.5-6X, 2-7X, 3-9X, and 2.5-10X variable range, and these objective diameters are more than adequate for these scopes.

Objective diameter and exit pupil by themselves don't determine optimal low light performance.  Fully utilizing the potential of a large objective requires good optics.  A cheap, low quality scope with a big objective bell won't provide the low light performance of a high end scope with smaller objective but with superb optics.  The overall optical design, number of lens elements, types of glass used, internal baffling, and lens coatings all contribute to overall light transmission.  Lens coatings optimized for light transmission in the blue spectrum provides the best low light performance as well.


Edited by RifleDude - March/15/2010 at 13:45
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: March/15/2010 at 16:07
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An exellent write up RD !
 
My own experience (espesially as I have become an grown up at 64 years) is more or less summed up in your last paragraph, and I do a lot of low light hunting.
About bulkiness of the scope, I have on my light weight Kimber Montana 84 M in 308 Winchester, a Zeiss Victory 2,5-10x50 with the #4 reticle.
 
This rifle is used mainly for combined stalking and high seat for deer.
Some my find this scope to big and cumbersome on a light weight, but again one has to take into consideration what kind of hunting the rig is ment for.
For me, this set up is a perfect light rifle which have seen use on red and roe deer in Norway, as well as fallow, fox, roe and muntjac in England.
 
Total weight on the rig without ammo is 3,0 kilogram sharp = 6,63 lbs
And yes, there is clearence between scope and barrel Wink
 
 
 
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: March/15/2010 at 16:32
RifleDude View Drop Down
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Nice rig, Seawolf!

A lot depends on how you use your rifle too.  If you do a lot of walking in challenging terrain, for example, in steep, mountainous country and high altitude, you may appreciate being able to shed every ounce you can from your rifle weight, and losing a bit of low light performance might be a good trade-off for shedding a few ounces.  In Seawolf's hunting for red deer in a blind at night, a premium 50mm or 56mm scope makes sense, because he will be spending a lot of time in a stationary position where he will usually have time to take a carefully aimed shot, so every bit of extra low light performance he can get is beneficial.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: March/15/2010 at 17:54
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+1 on what RD said. I will add that, in my experience, glass gets to a point where good enough will do and then I am more concerned with tracking and reliability or things like zoom ratios and compact form factors.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: March/15/2010 at 18:04
wjm308 View Drop Down
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Thank you very much Ted, that was an oustanding write up.  Apparently my theory about larger tube size = more light transmission is wrong (which is why I'm not an optical engineer).  Obviously the properties of light are much different from the properties of water and therefore the difference must be neglible for the given tube diameters we find on modern rifle scopes.  However, at some point my theory must hold as you couldn't have a 50mm objective scope with a 5mm tube and still get the same light transmission as a 50mm objective with a 30mm tube, at some point the tube has to become a limiting factor in the light that ultimately reaches the eye, basically the tube is acting as an aperture like that from a camera lens, narrow down the hole for the exiting light and you limit the amount of light reaching the sensor, but with a rifle scope it would appear they are actually increasing the lens size at the rear, so it is a design of a large front objective, a small tube and a larger rear objective.  It would seem the same optical principles that affect camera lenses would also affect rifle scopes when it comes to the aperture or shrinking the size of the light path, but I don't know enough about these principles to make an anology between the two.  I suppose the tube of the rifle scope is really only acting as the body of a lens, it contains internal glass elements that help with focus and correction (of certain optical distortions), the primary lens element is the front objective which determines how much light the lens is capable of capturing, much like the front objective of the rifle scope determines its light gathering capabilities.  Sorry for rambling here, I'm trying to grasp this whole concept and write it out as I think of it.  I'll stop now and let others chime in if they'd like.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: March/15/2010 at 19:05
RifleDude View Drop Down
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Wjm, the reason the tube diameter doesn't affect light transmission is not because your assumptions about aperture sizes are incorrect.  It's because the column of light passing through the erector assy doesn't get anywhere close to the diameter of the lens elements in the erector assy of a typical scope.  In addition, a 30mm scope doesn't always have larger erector lens elements than a typical 1" scope; sometimes the 30mm scope only has more adjustment room. 

While it's theoretically true that you could have a main tube that is small enough that it would restrict light transmission, it's a non-issue with the common rifle scope tube diameters.  Since the diameter, shape, and position of the erector lens elements are dependent on the focal length of the objective and ocular lens systems, the erector assy will never be a limiting aperture to light transmission.  The form and placement of all lens elements are dependent on all other components of the overall optical design, so constricting the tube diameter to the point it becomes a limiting factor in light transmission would also render the optic useless.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: March/15/2010 at 22:34
wjm308 View Drop Down
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Originally posted by RifleDude RifleDude wrote:

It's because the column of light passing through the erector assy doesn't get anywhere close to the diameter of the lens elements in the erector assy of a typical scope.  In addition, a 30mm scope doesn't always have larger erector lens elements than a typical 1" scope; sometimes the 30mm scope only has more adjustment room. 
Thank you Ted, this makes sense to me and is as I suspected based on your previous comments.  Obviously, like you mention later, there will come a point where you would have a tube that does limit the amount of light, but the amount of light that is passing through the erector assembly is much smaller than the lens elements inside that assembly for a 1" tube, this makes sense for why it wouldn't matter to have a 1", 30mm or 35mm.  Thank you for taking the time to help educate me, I just wasn't figuring out how the tube diameter couldn't affect the light, but your response helps me understand this optical characteristic in the rifle scope. 
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