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Objective Lens Diameter

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Optics Apprentice
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Post Options Post Options   Thanks (0) Thanks(0)   Quote South Pender Quote  Post ReplyReply Direct Link To This Post Topic: Objective Lens Diameter
    Posted: June/15/2019 at 00:23
I guess that it seems somewhat obvious that the larger the objective lens diameter the more light the scope will transmit (if that's the word).  But is this necessarily the case?

If we compare two 10 power scopes, one with a 24 mm. objective lens and the second with a 42 mm. objective lens, does it necessarily follow (assuming equal lens quality) that the 42 mm. model will produce a brighter image?  Would the 42 mm. scope necessarily be better in low-light conditions?

I realize that the exit pupil will be wider with the larger objective lens diameter, and that will make it more flexible vis-a-vis eye placement, but will the image quality necessarily be better as well?

I guess I'm showing my grasshopperness with this question....Big Grin
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 3_tens Quote  Post ReplyReply Direct Link To This Post Posted: June/15/2019 at 18:00
In the case stated of equal glass and coating quality the 42 mm scope will be much better in low light than the 24mm scope. Exit pupil is the key.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote BlackSox16 Quote  Post ReplyReply Direct Link To This Post Posted: July/28/2019 at 12:37
Maybe I'm wrong but exit pupil assuming glass and design is equal has nothing to do with "eye box"....so brightness to the human eye directly correlated to exit pupil I beliieve.  Koshkin or others could really nail the facts down though...
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Post Options Post Options   Thanks (0) Thanks(0)   Quote BlackSox16 Quote  Post ReplyReply Direct Link To This Post Posted: July/28/2019 at 12:38
Eye box is a function of scope design in other words....
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Post Options Post Options   Thanks (0) Thanks(0)   Quote EvaUnit Quote  Post ReplyReply Direct Link To This Post Posted: March/31/2021 at 11:02
I always had great results whenever I used a 4amm. Lens on my Cannon, but that really depends on your eyesight in general as well. I have a real despise when it comes to glasses as they make me look like an owl. I prefer wearing EDITED TO REMOVE SPAM LINKS which are blue and they look fantastic on my eyes. I do it mostly to attract more customers and look better. Image is important when you are a photographer and I have use contact lenses a few times on models as well. The result is extremely realistic.

Edited by SVT_Tactical - April/12/2021 at 07:33
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Scrumbag Quote  Post ReplyReply Direct Link To This Post Posted: March/31/2021 at 12:55
Yes, exit pupil is the key.

Your eye can open to 7mm max (in a young person, tightens with age) and this leads to the "Rule of 7"

You determine exit pupil size by Objective / magification.

So, for an

8x56, exit pupil = 56/8= 7mm
7x50, exit pupil = 50/7 = 7mm
6x42, exit pupil = 42/6 = 7mm

(Ever wondered why particularly "Classic German" scopes come in those sizes? There's your answer!)

Scrummy
Was sure I had a point when I started this post...
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Post Options Post Options   Thanks (0) Thanks(0)   Quote koshkin Quote  Post ReplyReply Direct Link To This Post Posted: April/01/2021 at 12:00
A larger objective scope, all else being equal, will perform better in low light and, within reason, better in good light as well.  Even if your eye pupil is not fully dilated, there are benefits to a larger exit pupil even especially for prolonged observation.

ILya
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Post Options Post Options   Thanks (0) Thanks(0)   Quote WJC Quote  Post ReplyReply Direct Link To This Post Posted: April/01/2021 at 12:57

I rarely show up on this forum, anymore. So, I hope you guys will bear with me while I try to throw my 2 cents worth into the conversation.

First, let me say that in over 50 years of optical design, manufacture, repair, research and tech writing, moments ago, and on this forum, I came across the term “eye box” for the very first time. Have I just been sheltered or is there a science-based reason for that discrepancy?

Secondly, the size of the CLEAR APERTURE of the objective lens (less the width of the retainer) is responsible for 100% of LIGHT GRASP. But while that is a huge factor in image brightness, there are several mitigating factors involved in determining image brightness.

1) Physiological factors. No two observers have exactly the same sensitivity to the same optical stimulae and aberrations.

2) Baffling

3) Blackening

[Glare—light from un-blackened or unbaffled surfaces—can reduce APPARENT brightness from light that SHOULD be reaching the retina.]

4) The size of the field stop

5) The position of the field stop

6) The knife-edge on the stop

7) Blackened edges on lenses and prisms

8) Slotted prisms—where applicable

9) Meeting of the focal planes of the objective lens and the EP

10) Design of the EP

11) Number of lenses in the EP

12) Glass types in the objective and EP

Finally, and on another subject, Chris Farris said the parallax is—in his very first sentence on the subject—essentially an “optical illusion.” However, while he has been quite accurate, throughout, that first sentence is erroneous. Parallax is a very real anomaly. I only mention this because the prior can give the impression that parallax is something the observer can just get over or get used to. This philosophy can lead to poor animals suffering and dying in the bushes and a dearth of meat in the freezer.

Please see my Binoculars: Fallacy & Fact (The Instruments, The Industry, and You)

The bottom line? Unless you're God or the world's greatest magician, buy another scope.


Edited by WJC - April/01/2021 at 13:04
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Cienapotr Quote  Post ReplyReply Direct Link To This Post Posted: July/25/2022 at 06:09
The illumination of the image of an extended object increases with an increase in the diameter of the lens and with a decrease in its focal length. It might seem that this way, you can also increase the brightness of the image of an extended object and get images, for example, brighter than the source itself. However, this is not the case.  I have been doing photography for a long time, so I managed to study some laws and read articles about the best lenses for Nikon cameras. An optical system with no losses does not change the luminous flux, but reducing the image area increases the solid angle to which the luminous flux is directed the same number of times.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote cbm Quote  Post ReplyReply Direct Link To This Post Posted: August/12/2022 at 20:14
No optics expert here. But I have a lot of alpha class scopes and binoculars. In my opinion, if the glass is on the top tier, there is marginal difference between 42mm-56mm scopes. Bigger is slightly better but not worth the size and weight to me! 

I believe there could be examples of lesser 56mm scopes performing as well as top tier 42mm scopes in low light comparing only. 

Bottom line, the lower tiered scopes are really good these days! You have to weigh your own cost benefit. For me it is the best 42mm-44mm out there. 
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