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How do you do optical comparisons?

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Horsemany View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Horsemany Quote  Post ReplyReply Direct Link To This Post Posted: February/24/2009 at 13:18
Originally posted by John Barsness John Barsness wrote:

I'll be making an extensive post next week on not just that scope but a couple of others that were recently tested. Names will be named.
 
Booyah!!!  Names will be named, feelings will be hurt, and the B.S. will be cut through.  I eagerly await.  Nobody wants to hear how poorly the scope they just bought performed in  a test.  It's always funny how personal some people take their optics.  Can't wait.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Lorne Quote  Post ReplyReply Direct Link To This Post Posted: February/24/2009 at 15:34
That less than satisfactory scope to be named is probably that expensive one I just bought to see what the hype was Big Smile...... sucks to be me
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Sgt. D Quote  Post ReplyReply Direct Link To This Post Posted: February/24/2009 at 15:56
Originally posted by Lorne Lorne wrote:

That less than satisfactory scope to be named is probably that expensive one I just bought to see what the hype was Big Smile...... sucks to be me
 
 
Welcome to the OT, yes many of us wait with you. But don't dispair, Some of us may have got it right.
 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote huntingaddict Quote  Post ReplyReply Direct Link To This Post Posted: February/24/2009 at 16:42
One thing that surprises me about forums is how different the impressions are at different sites.  One that I check out is just all over themselves for Nightforce Optics.  If it's not a NF, it's junk in their eyes.  Here the name that comes up the most is the 4200.  However, I see the 4200 as kind of a practical choice.  Kind of like yes, that other scope is terrific, but is it terrific enough to justify 3 times the price of the 4200.  I like practical quality.  On the other site they rave about the clarity of NF, here opinions seem to be that they are solid and reliable, but clarity is just ok compared to other scopes costing the same.  Some places you go people just want to tell you that what they own is the best so they can feel good about their purchase and anything else stinks.  Luckily here it seems like a lot of people  own a variety of optics and call it a bit more fairly without just being over run with one brand.  I'm sure you know what I'm talking about, some sites are nothing but Leupys, etc.  Personally I'm not married to any one brand.  I have a good handful of scopes and the funny part is that they are all different brands. 

Thanks for the great site guys.

Tom  
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Horsemany Quote  Post ReplyReply Direct Link To This Post Posted: February/24/2009 at 16:51
huntingaddict.
 
I'm kind of a hobbiest I suppose.  I have many interests and I have noticed the shooting/hunting world is sometimes short on facts compared to other hobbies.  I don't know why this is but it's something I've noticed since I was young and just getting started.  There's a lot of nostalgia and brand bias I think.  Some folks trying to justify their purchase by telling everyone else how great it is.  That's what I like most about this forum is there are plenty of members who give unbiased opinion and try to boil down to the facts.  And speaking of "boiling" just wait until Mr. Barsness reveals which high priced scope didn't fare well in testing.  It's important to remember quality is NOT proportional to price when dealing with optics.  Especially with terrible foreign exchange rates right now.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Gunshow75 Quote  Post ReplyReply Direct Link To This Post Posted: February/24/2009 at 17:37

I test rifle scopes for optical characteristics using several methods. I begin by mounting the scope on a tripod so it is stable, but can be rotated and tilted reproducibly.  

I check for resolution and light transfer with a high contrast target. I use a Snellen E chart placed at 60 ft indoors. The E chart is a series of black Es on white paper that gives a contrast ratio of about 16:1. The Es are rotated up, down, left and right, and the letters range in size from a bit more than 1 inch to less than 1/8 inch. The series of black bars (3) and white spaces (2) are of uniform widths and spacing for each letter.  

I work in a darkened space and illuminate the chart with a lamps controlled with a dimmer device. For the upper limit, using a light meter, I set the brightness at the chart to a level of about 100 lux, which is about the light level on a dark, overcast day. Similarly, for the lower limit, I set the light level to about 3.5 lux, which is the dark limit of civil twilight. Some scopes do not make it to the 3.4 lux limit. I test the resolution at the center of the field, at 50% field width, at 75% field width, and at 90% field width.  

I use the smallest E visible as the letter E to calculate the resolution in line pairs per millimeter, or in arcminutes. The smallest E that can be identified as the letter E, at a given low light level, establishes the "light transfer," or brightness, value for a scope.  

I do not work at a fixed magnification or exit pupil size. I change the magnification to optimize the result for the scope. Under some conditions, large, though dim, objects can be more readily identified by the brain than smaller, brighter ones.  

Because I hunt whitetails, primarily, I also look at a red/brown interface on a black background. I begin at a light level where the colors are obvious and reduce the light level until I can no longer distinguish red from brown. I record the light level at which that happened. It is an effective, though not overly scientific, approach to getting a handle on color contrast.  

I then take the scope outside during the night and look at stars to identify coma, among other things. When available, I use the moon to identify chromatic aberration. Otherwise, I use other methods.  

I will describe how I check for flare, astigmatism, and other things when I have more time tomorrow.  





Tom



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Post Options Post Options   Thanks (0) Thanks(0)   Quote John Barsness Quote  Post ReplyReply Direct Link To This Post Posted: February/24/2009 at 20:09

Yeah, I frequently use similar techniques for testing binoculars and spotting scopes. In fact a friend (who writes about optics for far more publications than I do) and I get together about once a year to test binoculars in a set-up very similar to your "darkened space." The chart, however, is the same bar-chart used by the U.S military for their optical tests.

A good tripod is nice, but to be absolutely still a heavy weight (like a flat bar of lead) also needs to be attached to the tripod head to keep things as still as possible. Also, to really tell the difference between different optics, a quality multiplying lens of 4-6x power should be attached to the objective lens. This really separates things.
 
Then there is individual eyesight. We do the test with two people because there are always some variations, and often invite other people to join in.
 
You don't have to use the moon to find chromatic aberrations. A stop sign also works very well, or anything else with a "hard" visible edge. 
 
Flare is another subject altogether, but astigmatism in a hunting scope is irrelevant, unless it affects the center of the field of view.
 
These are all the reasons I came up with a simpler test for riflescopes, and one more applicable to general hunting use. It is fun and interesting to make tests like the ones you are doing (and I like to make them myself) but 95% of them do not really make any difference in hunting scopes. I like to watch the moons of Jupiter in my spotting scopes, but they are rather irrelvant when hunting.
 
I will, however, be interested in how you check for flare. I have talked to many optical engineers about this problem, and they use varying methods for dealing with this problem.
 
 
 
 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Gunshow75 Quote  Post ReplyReply Direct Link To This Post Posted: February/25/2009 at 06:54

As regards your comments, I will respond to them in order:

1.  I choose to do some testing as I have described because it enables me to control the level of dark adaptation for my eyes without having to wait until a certain time of day.  As I am sure you know, it takes up to 30 minutes to dark adapt, and you can't test low light performance without being dark adapted.  

2.  Lots of different "charts" can be used to test resolution.  All serve the same purpose.  The US military chart is not uniquely suited to the purpose.

3.  How stable the setup is depends upon the situation.  I have not found that adding weight increases the stability of my setup.  

4.  I, too, use a magnifier to boost the setup, but I do not attach a magnifier to the objective lens as you describe.  I disagree with that procedure.  I magnify the image.

5.  I described my process, what I do.  Another person doing the same thing is not a different process.

6.  I did not say that one had to use the moon, only that I do so when it is convenient.  

7.  Even if, as you describe, astigmatism is only relevant in the center of the fov, then it is relevant and should be evaluated.  I do.

8.  Clearly, you and I have different opinions about the importance of some of the optical characteristics of a rifle scope.  Things that are not important to you might be important to me, and vice versa.  Importance, like better, is a subjective term.  While the moons of Jupiter might be irrelevant, as objects in and of themselves, their absence might be indicative of an aberration that I consider to be important.  

I prefer a scope with an image that is "sharp" from edge to edge, because I find the image to be less confusing; more like my normal vision.  One could use a scope whose image in the center of the field isn't sharp, just as one could eat scrambled eggs with a toothpick.

I test for flare by using a four (4) inch silver Christmas ornament that I illuminate with a lamp.  The ornament is placed in front of a black background with a series of equally spaced letters.  I rotate the scope towards the silver ball until I note the presence of flare.  The letters indicate how close I was able to get before flare appeared.



Tom



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Post Options Post Options   Thanks (0) Thanks(0)   Quote Horsemany Quote  Post ReplyReply Direct Link To This Post Posted: February/25/2009 at 08:28
Gunshow
 
Do you perform these tests for your own data or are you in "the biz" of optics? 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote John Barsness Quote  Post ReplyReply Direct Link To This Post Posted: February/25/2009 at 08:56
Gunshow75,
 
Thanks very much for your comments. In reality I also place the magnification on the eyepiece of the optic to be tested; my fingers got separated from my brain when I typed "objective."
 
I would disagree that weight added to the head of the tripod isn't needed, but didn't mean to imply that the military chart was the only valid test-chart. In fact I mentioned earlier in the thread about using my own chart for dim-light testing. Perhaps you missed that.
 
I use it the way I do primarily because my job involves comparing various scopes of similar sizes in order to make recommendations to readers. This is also why I try to use a magnification that eliminates exit pupil as a factor. Yes, we may use a scope at a magnification high enough to create an exit pupil smaller than 7mm but that is not what I am after in this test. I am testing the glass itself, in order to get an idea of how a certain scope's optics compare to another's in the same class. This is the same reason the German product testing organization known as DEVA tests scopes of roughly the same magnification and objective size against each other, not against all other scopes.
 
Your flare test is quite interesting. Thanks for describing it.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Gunshow75 Quote  Post ReplyReply Direct Link To This Post Posted: February/25/2009 at 11:37

Horsemany, I do the tests to satisfy my curiosity.  I am not "in the optics biz." 

John;

I understand the "brain to fingers disconnect."  I, too, often say one thing when I mean something else. 
 
I use the tripod in, and on, a building that is massive, weight wise, and tied to bedrock.  For example, three rooms, each 24'x24' inside, sit side by side.  The walls and ceiling, which are of high-density concrete, are seven (7) and five (5) feet thick respectively.  Vibration is not a problem for me, but if it is for you, then adding weight will certainly dampen the system.
 
I consider a rifle scope as tool to be used to its fullest extent, which includes the range of magnifications available.  I form an opinion about the scope, not the scope set at 5.7X, for example.  I do not put rifle scopes having 40mm or 50mm objectives into different categories.  IMO, if they are to be used for hunting, they can be compared on that basis. 
 
I think one should do tests under controlled conditions.  Then, within limits, the data can be compared to other scopes.  The data are just descriptive statistics, and they do not require accuracy to five significant digits to be meaningful. 
 
Often, it isn't the descriptive statistics that explains performance characteristics that influences one's choice.  For some, assuming the image is "good enough for them," whatever that means, subjective attributes like, "... it is lighter," or "... it has rainguard," or something similar becomes the deciding factor.  


Tom



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Post Options Post Options   Thanks (0) Thanks(0)   Quote John Barsness Quote  Post ReplyReply Direct Link To This Post Posted: February/25/2009 at 13:29
Tom,
 
I do the binocular comparisons in a house basement with a concrete floor. It is pretty steady but not like your building!
 
I do test scopes at other magnifications than one with a large exit pupil, but those are other kinds of tests. I just sent an article in to one magazine about 6.5-20x scope, and mentioned that it had some chromatic aberration around the edge of the FOV at higher powers, but also that "it wasn't enough to be noticed by anybody except an anal-retentive gun writer."
 
But I find the 6x test to be very revealing with scopes in the 3-9x range. Aside from doing that test in the hour after sunset, I also make one on very dark nights with a 100-watt bulb 25 yards from my chart (and yes, I have found that only one brand of bulb will do, because it is the only brand I've found locally that is consistent from bulb to bulb).
 
I have written elsewhere on this site that the ability of a scope to retain zero is actually far more important than minor differences in optical quality. Unfortunately, I have found in other tests that retaining zero isn't all that common, even among some very high-priced scope brands. Hence my limited obsession with those minor differences in optical quality.
 
John
 
 
 
 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Roy Finn Quote  Post ReplyReply Direct Link To This Post Posted: February/25/2009 at 13:42
Gunshow, if you are comparing scopes with different sized objective lenses, how do you make a fair comparison of optical quality. As John Barsness suggested earlier, if you set your scopes at a magnification that would essentially cancel out exit pupil differences you are still looking at scopes set at different power settings. In other words, if you set all the scopes to have an exit pupil of say 6mm, one scope might be set at approx 5x and another might be set at 8x. In my opinion, that really won't give you equal comparisons from scope to scope, or brand to brand.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Horsemany Quote  Post ReplyReply Direct Link To This Post Posted: February/25/2009 at 15:34
Good point Roy.  I await the answer.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote HOLLOWPOINT Quote  Post ReplyReply Direct Link To This Post Posted: February/25/2009 at 15:38
The best but alas the most expensive method of testing the optical quality of a scope is with collimator and MTF-meter.
 
 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Gunshow75 Quote  Post ReplyReply Direct Link To This Post Posted: February/25/2009 at 18:30

John, IMO, holding zero is a requirement, not an option.  I have only owned one scope that would not hold zero - a Leupold.  I am sure it happens with all manufacturers, perhaps with some more than others.  

Roy,

I know that it isn't appropriate for me to respond to a question with another question, but why do you think different sizes of objectives should have different optical qualities?  What do I gain by setting the exit pupils to the same size?  

If I can see a maximum of 4 objects (Es) with one scope, and a maximum of 3 Es with another, what difference does it make if one scope is set at 4X and the other at 5X, or both at 2X for that matter?  I dont' care how big the Es are, or how "bright" they are.  I want enough resolution to tell they are Es, and enough brightness to see them.  More Es are better than fewer Es.       

To be sure, diffraction affects should be less pronounced, and brightness should increase as lens diameter increases, for example. I say should be because factors other than size are contributing factors to sharpness and brightness.  I have a scope with a 42mm objective that has a much sharper image than many 50mm scopes I have seen, and it's brightness doesn't always lag as far behind as the ratio of the square of the diameters would have predicted.  

I don't own a scope with an objective larger than 44mm, but it isn't because larger diameter objectives don't measure up.   Some are excellent, and I am strongly considering acquiring a scope with a 50mm objective.  



Tom



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Post Options Post Options   Thanks (0) Thanks(0)   Quote Roy Finn Quote  Post ReplyReply Direct Link To This Post Posted: February/25/2009 at 18:59
My point was that you can't set scopes in your comparisons to the same exit pupil size and power if they have different sized objectives. Unless you are comparing scopes that vary quite a bit in optical quality, the scope (s) with the most X's will show you more detail almost exclusively. When I said "optical quality" I meant that in a generic sense, not that 50's will show less distortion, CA, fringing, flare etc. than similar quality 40's would reveal.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Horsemany Quote  Post ReplyReply Direct Link To This Post Posted: February/25/2009 at 19:02
I think what Roy's question was getting at is valid.  To compare scopes of different obj. and compare them identically would give the larger obj. an advantage all other things being equal.  His original question spelled it out clearly. 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote RifleDude Quote  Post ReplyReply Direct Link To This Post Posted: February/25/2009 at 20:24
While performing the kind of extreme, in-depth optical tests described above to micro-evaluate the optical quality of 2 competing quality riflescopes may be interesting to an optical physicist, I generally think such an endeavor is overkill for testing a hunting scope.  I can see the value of such hyper-critical testing with premium binoculars and spotters, but one isn't typically scanning the countryside and viewing scenery with a riflescope, or at least one shouldn't be.  A scope is an aiming device used to acquire a target already positively identified by other means, even to further evaluate the target in greater detail before squeezing the trigger, but only after knowing for certain it is indeed a target the shooter is willing to kill.  Ideally, it should have good enough, even superb resolution, contrast, and light management sufficient to resolve targets and place a precision shot in all environmental conditions likely to be encountered.  With this in mind, how would some slight aberration only visible in extreme test conditions really detract from its usefulness as long as it's an otherwise quality instrument that's perfectly capable of fulfilling its intended purpose?   I can see the value of quantifying slight aberrations if the scopes in question are so closely matched in all other respects that one needs some justification to choose one over the other.  But, in my experience, that's seldom the case.  The reason I say this is simple.  Unlike binoculars, where the sole purpose of the optic is extensive, detailed observation, other physical characteristics of a riflescope -- reticles, turrets, power range, size, etc. -- usually play a more vital role in its selection than whether or not it has some slight aberration observable only in very specialized conditions that in reality won't ever influence its suitability as a dependable aiming device.  Given the option, I would prefer that my scopes be as free of optical flaws as possible, but when viewed in light of how a scope is used, I don't think evaluating them in the same manner appropriate for alpha class binoculars ultimately yields much practical value.  I don't mean to imply I don't think image quality is important in a riflescope; I do, but to what degree is the question.  I want my scopes to deliver as superb an image as possible, as long as the pursuit of optical perfection doesn't overshadow other practical features or come at too high a price tag. 
 
I compare riflescopes in the environment where I'll actually be using them -- outdoors, in varying lighting conditions, while mounted on rifles.  In this context, I can usually see readily apparent differences between two competing riflescopes sufficient to positively say I prefer one over the other without extensive testing with specialized equipment required.  Before I get to the level of evaluating traits like coma and slight astigmatism, usually visible differences in FOV, center field clarity, low light performance, tunnel vision, etc. are sufficient enough to trump other less conspicuous optical flaws.  If, after a reasonable time spent in field conditions, I can't easily discern which of two given scopes are optically superior, then for all practical purposes, whatever optical differences may exist are insignificant enough to me to warrant concern or base a buying decision on.  I then focus on other features that determine a scope's suitability for my intended use, which are usually more critical than ultimate image quality.  Of course all of this assumes that the scopes in question are mechanically sound and sufficiently durable to begin with, or they are worthless even if the optics are world class.
Ted


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