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Objective Lens...50 or 40mm

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Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: March/17/2007 at 00:04
rain252 View Drop Down
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What is the reason a larger objective lens can give a better  image (assuming the same scope, just bigger obj. lens) ?

I thought it was because a larger area can gather more light.  I was told that has nothing to do with it...it is because it has better resolution.   Whats the deal?

Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: March/17/2007 at 00:19
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rain252, interesting thought.....   If you dont have the proper light gathering capabilities, then I wouldnt think anything else would be up to snuff either....????  In optics everything should be associated for top performance..        30
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: March/17/2007 at 07:18
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One reason would be that for a given magnification the larger objective provides a larger exit pupil do would be better suited to low light usage.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: March/17/2007 at 10:03
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check out the post by chris in the beginning of the riflescopes section. passive optical devices can't gather light, in order for that to occur some type of amplifer must be used, such a rhodium coated laser pumped fiber optics tube. Dogger put it best-- also the larger exit of the physical device causes less work on the eye muscles. In consumer optics everything is associated with the particular price range the manufacuter wants to sell into
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: March/17/2007 at 10:52
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I'll go read that...thx.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: March/17/2007 at 23:16
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Dale, are you saying the larger obj.  doesnt project more light through the scope..??

 

     30

Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: March/18/2007 at 02:49
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Optical devices do not project or gather or make light.  They transmit light. 

That means that when there is a certain amount of light incident on the objective lens, a portion of that light (over 90% for quality scopes) will be transmitted all the way through to the ocular end.  Naturally, if the objective lens is bigger there will be more light incident on it, and consequently, there will be more light going through the scope. 

This has been discussed in some detail earlier and there is a lot of information collected in the FAQ on this forum.


And just for the sake of accuracy:

Dale, are you sure you did not mean erbium dopes externally pumped fiber amplifier?
I've never heard of a fiber coating being useful for amplification.  Typically it is a dopant of some sort.  Most often erbium or, more rarely, thullium and praseodymium for other wavelengths.  Also, I am not aware of any fiber amplifiers working in visible light.  Typically, these are designed for telecom wavelengths which are primarily in near infrared region.

ILya
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: March/18/2007 at 19:14
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No, I was simply offer an explanation as to a amplification scheme rather than a passive scheme. Not necessarily one used in optical devices for visual use, in my case soliton pumps. Or without the laser, wavelengths in the cosmic ray region for pumping hydrogen in thermo devices. Fiber optics being used to direct the "stream" or carry the digital code as you mention in telecommunication wavelengths in micro wave region. In either case both ignore the amplifications scheme of photo mulitplers in the visible region.

As you clarified the surface area of the objective lens is the qualifing factor in amt. of light reaching the observer. I was hoping the reader would go to FAQ to gain some backround, as you well know the , the term light gathering is used in various advertising media for rifle scopes and as such misunderstood.

It is sometimes difficult for a reader to understand that the amount of energy coming out cannot exceed the amount put in-- unless a pumping scheme is utilized.

Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: March/18/2007 at 20:56
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Hot30 can correct me here but I think that transmission is what he meant.
ILya and DC, since most glasses have metals in them, is this light "absorption" due to photoelectric effect? Since it appears that some lense manufacturers are moving away from lead in their glass is this effect the reason, to increase efficiency? It seams that the efficiency levels being obtained with present glasses are pretty high. I have to wonder how far consumers are willing to go to pay for 98% vs say 94% in regards to efficiency.


Edited by tahqua
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: March/18/2007 at 21:05
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In well corrected optics with everything else being equal, a larger aperture (objective) will provide better 'angular resolution'. 

 

F ratio (focal length ratio) is the focal length divided by the aperture, where a 50mm lens of 300mm focal length will have an f ratio of f6. The f ratio on typical camera lenses can be adjusted for setting exposure. With optics of equal correction, the faster the f ratio the better the 'linear resolution', an example being stopping down a camera lens too much results in a fuzzier image.

 

An objective forms an 'airy disc' at the focal plane, where one way of looking at image formation is that one kind of ends up painting an image with dots, the dots being the airy discs.  

 

A simple way to think of optical correction is to think of the objective directing light rays, and the the more one can get the rays to all fall witin the airy discs at the focal plane, the better the correction. If you have light scatter you end up with some light not falling within the airy discs and you will have lower contrast. If you have 1 wavelength of spherical aberration a lot of light doesn't fall into the airy discs, and the objective may only perform ok at low magnification.

 

An objective with a long focal length will have a 'slow' f ratio, which means that it will have bigger airy discs, which means that it's easier to correct. The aperture defines the angular resolution, so a large, 'slow' objective will be easier to get good image quality and also provide better image detail.

 

A large objective with a short focal length with have a 'fast f ratio', be better suited for taking pictures as it has better linear resolution, but be harder to correct as well as slower f ratio optic as the airy discs are smaller.

 

Seeing detail when looking thru an optic depends upon angular resolution (larger aperture) and how well corrected it is (easier with slower f ratio), so faster optics will be more expensive, require more exotic glass, etc., to do as well.

 

Magnification is the focal length of the objective divided by the focal length of the ocular.

 

A faster optic will tend to provide bigger exit pupils, while a slower one will provide smaller exit pupils and higher magnification.

 

'Diffraction limited' is a vague term, often associated with '1/4 wavelength' of spherical aberration, and is a common criteria for a well corrected system. Since most binoculars, rifle scopes, and spotting scopes seem to be 'fast', they aren't likely to be 'diffraction limited', so resolution will depend more upon overall optical correction than on objective size. 

 

In summary a larger objective could provide better resolution, it will provide a bigger exit pupil at the same magnification.

 

A bigger exit pupil and faster f ratio will be harder to correct well for viewing as the eyepieces (oculars) are being asked to work more at the edges of the field instead of just the center. I would expect a larger objective to also have softer edges, everything else being equal.

 

It's all about tradeoffs, most often trading money for something that you want :^).  

Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: March/18/2007 at 21:08
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Photoelectric effect has little if anything to do with absorption of light in glass.

Generally, when trying to increase the transmission through a scope or binocular, absorption is the least of your problems.  Reflection at glass/air interfaces is the primary contributor here.  Absorption is at best a third order effect.

When choosing a scope, light transmission is a realatively unimportant parameter to look for.  Human eye is very good at adjusting to different light levels.  You want to look for image fidelity: resolution sharpness, etc. 

Coatings are important not for how much light they transmit, but rather for how little light they reflect.  If coatings are of bad quality you get a significant enough amount of light bouncing between different surfaces inside the scope to mess with image fidelity, create ghost images, etc

ILya
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: March/18/2007 at 22:53
hot30 View Drop Down
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Tahqua, I thought it right, didnt say it right....  Gentlemen, is the large obj. just a simple mechanical advantage to cure an angular problem in the design of some scopes..  Like in the angular transmission of light as it enters the tube behind the objective.....   Bushnell still has these "LONG" 4200 class scopes which are the "brightest" supposidly built this way to perform as they do...   RE:  small obj. and long  tube to promote the Straightest and Brightest picture..??

 

 30

Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: March/18/2007 at 22:58
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Originally posted by hot30 hot30 wrote:

Tahqua, I thought it right, didnt say it right....  Gentlemen, is the large obj. just a simple mechanical advantage to cure an angular problem in the design of some scopes..  Like in the angular transmission of light as it enters the tube behind the objective.....   Bushnell still has these "LONG" 4200 class scopes which are the "brightest" supposidly built this way to perform as they do...   RE:  small obj. and long  tube to promote the Straightest and Brightest picture..??

 

 30



You've got me a little confused here.  What is angular transmission of light?  How can light enter the tube behind the objective?

The term "brightness" really has no meaning pertaining to passive optical devices like scopes or binoculars.  It is frequently used and it really should not be.

Longer tube on an Elite 4200 is primarily there in order to house a 4x erector system.  As for the large objective lens, it is not there to cure any angular problems.  It is beneficial on high magnification scopes since it provides for a larger exit pupil.

ILya
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: March/18/2007 at 23:06
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Koshkin, light entering the tube after it passes through the obj....  Mechanically does the light have to be "straightened" if you will, as it proceeds futher down the tube twords the ocular end..??

 

    30

Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: March/18/2007 at 23:22
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Light does not get "straightened" in any way shape or form that I can think of.   The objective is a lens, hence it takes in light from a scene of a certain size and focuses it onto a spot.   At some point the beam of light goes through some additional lenses, but there is no straightening of any sort.  The light that enters the objective lens at too wide of an angle is supposed to be blocked/baffled, so it doe snot get too far.

ILya 
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: March/18/2007 at 23:45
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Koshkin, "entering at too wide of an angle", thats where im dabbling..  You say it gets baffled or blocked, OK....  Can I use the term refraction for a minute..?? That spot you mentioned is at the center of the obj..??  So this baffling is used to "clean up" the light and permitts the light from this "spot", being in the center of the obj., to continue futher into the tube on its way to the ocular end..?

 

    30

Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: March/19/2007 at 00:28
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This is not straightforward to explain.  I'll give it shot, but this is really something that I can explain in two minutes with pen and paper and may not be able to explain with words at all.  If it does not make sense, I'll have to find the time in the future to draw a few pictures and scan them in.

Anyhow, you are thinking of light in terms of straight lines, right? you are thinking that each part of an image goes through a particular part of the lens, right? 

That is a VERY wrong way of thinking about.  Every part of an image goes through every spot on the lens.  That is due to the fact that in the plane of the lens the image is absolutely defocused.  That is why, for example, you can put a small round aperture immediately in front of your objective lens that is half the size of the objective and you will still get the same field of view.  The object will appear a little dimmer, but the field of view will not change.

Now, imagine you are looking through a scope at particular object.  Imagine that the object you are looking at fills you field of view completely.  Now imagine that there is a light bulb just to the side of the object you are looking at.  Because of where the light bulb is positioned, it is not in your field of view.  However, it is a bright light source, and  the light from it also falls onto the scope.  However,  it is at too wide of an angle and you are not supposed to see it.  If the scope is built properly, it is not supposed to have any effect on the image quality.  In this case "too wide of an angle" means that the object is too far from the center line (or optical axis) of the scope to be visible.  Internal baffling of the scope is supposed to prevent all the light entering the scope off-axis (at too wide of an angle) from reaching your eye.

One of the tests I do when I test scopes in low light, is putting too bright light sources off axis both in front and behind the scope and see how much it effects the image.

ILya
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: March/22/2007 at 14:44
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here goes........

 

i always learn something from koshkin, good stuff......

 

my 2 cents on "big bell" objective lenses...................the are brighter for MY uses, without a doubt.

 

same scenario.................2 scopes................exact product models

 

lowlight, almost dark / a buck steps out 120 yards or so away / you try to make out the points (game managment land - 8 points or better)

 

3-10x40 - set on 10 power to look for horns - exit pupil of 4 mm

 

3-12x56 - set on 10 power to look for horns - exit pupil of 5.6 mm

 

there IS a difference in brightness - if 7mm is the MOST any pupil can digest......then the 20% increase in usable light (due to the larger objective) is pretty significant, isn't it?

(1.4mm out of 7mm eye capabilities = 20%)

 

that's like comparing a 78% light transmission scope w/ a 98% light transmission scope, right?

 

koshkin, am i wrong in thinking this.  i HAVE been wrong before.

 

i never said the bigger scopes LOOKED better

 

J

 

 

 

Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: March/22/2007 at 15:44
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Originally posted by jonbravado jonbravado wrote:

here goes........

 

i always learn something from koshkin, good stuff......

 

my 2 cents on "big bell" objective lenses...................the are brighter for MY uses, without a doubt.

 

same scenario.................2 scopes................exact product models

 

lowlight, almost dark / a buck steps out 120 yards or so away / you try to make out the points (game managment land - 8 points or better)

 

3-10x40 - set on 10 power to look for horns - exit pupil of 4 mm

 

3-12x56 - set on 10 power to look for horns - exit pupil of 5.6 mm

 

there IS a difference in brightness - if 7mm is the MOST any pupil can digest......then the 20% increase in usable light (due to the larger objective) is pretty significant, isn't it?

(1.4mm out of 7mm eye capabilities = 20%)

 

that's like comparing a 78% light transmission scope w/ a 98% light transmission scope, right?

 

koshkin, am i wrong in thinking this.  i HAVE been wrong before.

 

i never said the bigger scopes LOOKED better

 

J

 

 

 



You are generally correct, but numerically a little off.  Everyone's eye is a little different and not everyone's pupil can dilate to 7mm.  If you are comparing a scope with a 40mm objective lens and 56 mm objective lens, the large scope will bring 93% more light than the smaller one (the amount of light that gets to the eye is proportional to the area of the lens, not diameter.  It is kinda important to remember what is the point of reference here.  I always use the smaller objective lens scope as a point of reference when I compare two scopes.  If you use the larger one as the point of reference, for example, a 40mm objective lens passes through 49% less light than 56mm one). 

However, this difference will only be apparent at higher magnifications.  At 7x and below, for example, you should not see any difference.  Yeah, the exit pupil is bigger, but you really can't use it unless the magnification is set pretty high.

You have to be pretty careful talking about the amount of light, since it is not everything.  It is a valid comparison when talking about two scopes of identical quality.  If the scope is not very good, than you may get a bright unresolved blob with a larger lens, but it is still an unresolved blob.  I'll take dimmer, but sharp image over a bright, but soft one any day.

Then there is an issue of apparent brightness.  It just occured to me that I talked about this on a different forum a while back, so here is a copy of that post (ignore a few acerbic remarks, it was written for a different audience):


>>>>
A few comments.

First of all, all this arguing about how much total light gets to the eye is not very important at all. Human eye is remarkably good in utilizing even minuscule amounts of light. You do want to keep the exit pupil of the scope reasonably big, so that the eye only sees the picture coming through the scope and concentrates on that. Here I am assuming that we are discussing scopes of equal or similar exit pupils.

I have elaborated on this a few times before, but I think it all fell on deaf ears. There is brightness (sometimes confused with transmission) and there is apparent brightness.

Brightness refers strictly to the total amount of light that gets through the scope.
Light transmission is the ratio of the light that gets out of the scope and the light that gets into the scope. For example, a light transmission ratio of 90% means that

(Light Out / Light In ) * 100% = 90%

The amount of light that is incident on the scope (we assume that all of the objective lens is utilized and there is not vignetting) is proportional to the area of the objective lens. For example, a 30mm objective lens is subjected to 125% more light than a 20mm objective lens. Since the area of a lens is proportional to a square of the radius of it, here is a brief table of how much light is incident on lenses of different size with respect to a 20mm lens

Lens Diameter, mm vs brightness ratio, % (with 20mm lens being 100%)
20 100%
24 144%
30 225%
32 256%
36 324%
42 441%
44 484%
50 625%
52 676%

Apparent brightness, is basically how bright the image appears to your brain. How bright the image appears to your brain does not have a whole lot to do with the actual amount of light that gets to the eye. If the image is clearly resolved, it will appear bright to your brain. If the image is not very clearly resolved, it will appear comparatively dim. The amount of light that gets to the eye has very little to do with how well the image will be resolved.

Magnification has a lot to do with how well you can resolve details, since the details are magnified a little more. However, it is not as simple as it sounds because when you magnify the image you also magnify a lot of the imperfections and flaws in the scope optics and make various opicall aberrations play a much more visible role in distorting the image. Still, with the same optical quality and the same exit pupil, you’ll obviously see better through a 6x scope than through a 4x scope. However, not as much better as you would think since to diminish the effect of flaws and aberrations a higher magnification scope actually has to be of a higher optical quality. Once the magnification gets sufficiently high, of course, you also run into a problem of shaking (bouncing crosshairs) in almost any field position. That degrades apparent image quality as well. Another additional factor is that with higher magnification you are looking at a smaller part of the scene. Although with higher magnification and larger objective lens you may be theoretically capable of letting more light through the scope, you may end up having less total light incident on the front lens due to less light being available since you are looking at a smaller part of the object.

When I say optical quality, I mean the quality of the polish of the lenses and the precision of the grinding of the lenses. That has the most effect on how well the image is resolved.

Another significant factor in the image resolution/quality is coatings. Multiple reflections between different surfaces inside the scope wreak havoc in image quality. This is where high quality coatings are most important.

Some personal experiences that may relate: back when I had both, I could see better (resolve more detail) with a IOR 4x24 than with Leupold M8 6x42, despite Leupold's higher magnification (to forewarn the next question I am quite familiar with focusing scopes).
>>>

Let me know if this makes sense.

ILya




Edited by koshkin
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: March/22/2007 at 15:53
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It's clear to me why his Sig is "Dark Lord of Optics"  If you think you understand optics all you have to do is read some of this and then you realize that you know very little.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: March/22/2007 at 15:57
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Thanks Cajun Hunter.

Actually the term "Dark Lord of Optics" was coined by cheaptrick.  I think it least half of cheaptrick's and mine combined 5000 posts on this forum are dedicated to making fun of each other, and somewhere in there he came up with this phrase and it stuck.

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Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: March/22/2007 at 16:40
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That's some good stuff, Koshkin!  I learn a great deal from you!  I had some of the very misconceptions about the nature of light entering an optic that you address.  Thanks for your posts!  You are a tremendous asset to this forum!

Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: March/23/2007 at 07:30
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it's quite obvious koshkin has NO idea what he's talking about ............

 

wait.....yes he does......

 

good stuff. and thanks.

 

J

Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: March/23/2007 at 09:13
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"A top quality scope will give you all the light you need without a huge objective. Only 36mm but it's a Zeiss!"

 

Well, I disagree, as people's needs are different. Trying to use higher magnification for longer distances and/or smaller targets in dim light will usually require a larger exit pupil, which means a larger objective. For others they may find that a 36mm 3x9 works fine for all their needs. I prefer a 1.5x6, for me it's a better low end that duplicates the 'instant' target aquisition that I'm use to with iron sights, and at 6x I still have a decent exit pupil that allows for some misalignment.

Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: March/26/2007 at 09:33
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yeah, i need to just break down and buy a S&B - it's one of the only scopes i don't have experience with.

 

is it NOTICABLY better than the diavari's? i find it hard to believe.

 

based on price, they should, but i am skeptical.

 

J

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