How do I separate light transfer vs exit pupil? (1 Viewer)

[tpcollins]

I understand better coatings promote more light transfer in optics and I understand exit pupil. Let's take a 2-8x32 variable power rifle scope that the manufacture specifies their coatings allow 94% light transmission.

On 2x, there would be an exit pupil of 16mm - can I assume the coatings would transmit 94% of the light entering the objective?

On 8x, there would be an exit pupil of 4mm - what percent of light would be transmitted? Thanks.

 

[Binastro]

Assuming the light transmission is constant, then it will always be 94%.

However, it could be that your eye would be bigger than 4 mm at eight times magnification so you might find the image dimmer.

I suppose it depends what light levels you are using.

Also of course your pupil size cannot be 16 mm at two times magnification, so you are only using part of the objective in this instance.

Do you actually find any difference in brightness when you use the rifle scope?

 

[tpcollins]

Do you actually find any difference in brightness when you use the rifle scope?

I remember hunting a few years ago and I had a Swarovski 3-9x36 scope that I was playing with before daylight. At 3x and in the dark, I could easily see the tree line that was 100 yards away. I started turning up the magnification and somewhere at about 5x-6x the view darken and was nearly black at 9x.

On the same occasion I looked thru my 8x42 Zeiss Victory T* FL ( I know, that was like cheating ) and the Swarovski's 3x (12mm exit pupil) was no brighter than the Zeiss at 8x (with a 5.25mm exit pupil). At my age I realized my pupil probably doesn't open much more that 5mm now - I just wanted to separate the difference between the coating's ability to transfer light versus the amount of light available to enter with a smaller exit pupil - and they are two completely different processes. Thanks.

 

[WJC]

I understand better coatings promote more light transfer in optics and I understand exit pupil. Let's take a 2-8x32 variable power rifle scope that the manufacture specifies their coatings allow 94% light transmission.

On 2x, there would be an exit pupil of 16mm - can I assume the coatings would transmit 94% of the light entering the objective?

On 8x, there would be an exit pupil of 4mm - what percent of light would be transmitted? Thanks.

Coatings in no way over shadow an increase in light grasp. Going from the long-time industry standard magnesium fluoride coatings to multi-coatings, you can, in theory, gain ~ 13 percentage points in light transmission.

When trying to discuss the difference in multi-coatings from one quality manufacturer to another you are splitting hairs with an axe—it’s pure folly.

That folly will, however, take up thousands of words, over several days, several times a year.

Most people don’t consider the mathematical differences they speak of are well below their personal threshold of recognition, or that the difference they are seeing is really the result of:

baffling, blackening of edges, size of field stops, position of field stops, knife edge on field stops, slotted or non-slotted prisms, design of the eyepiece, number of elements in the eyepiece, etc.

But then, over the years I have learned that when it comes to discussing optical performance: if the observer doesn’t know the source of a particular malady, he will eagerly shift the blame to some potential cause that he IS aware of.

Correct? No. Handy? Yes. :cat:

Cheers,

Bill

 

[OPTIC_NUT]

The above posts cover it all, and succintly. Wow.. ;-)
And yeah....a bright scope with less contrast is a problem.

I would only add the caveat that there is additional light lost
in a zoom like that. Not so much the coatings as the grazing losses needed
(or the fringing that happens).
Which is a roundabout way of saying...if you're zooming that much, coatings
and aperture aren't your biggest concern. Reduced or no zoom would yield more,
not by element count these days but by engineering tradeoffs.
Passing through the narrows of a zoom gun scope, the choice of contrast (small-dia diffraction)
or loss (introduce coma before the elements) must be made. No zoom = no such issue.
Chromatics are very hard to keep out of a lens-based image inverter, too.

 

[Red_Shoulder]

Some really good answers here. Don't forget, 2 eyes are brighter than one.

If I remember right, and understood the article correctly, you get a 20-40% increase in aperture using both eyes.

Apparently some people get more than others, it varies.

 

[WJC]

Some really good answers here. Don't forget, 2 eyes are brighter than one.

If I remember right, and understood the article correctly, you get a 20-40% increase in aperture using both eyes.

Apparently some people get more than others, it varies.

You get up to 40% more CONTRAST.

12 UNDERSTANDING BINOCULAR SUMMATION

Another common misconception, especially prevalent among amateur astronomers, is that the light grasp of a binocular is the sum of the light striking both objective lenses. Because so many of the things amateur astronomers want to view are dim asterisms and diffuse nebulae, it is easy to see why they would want to believe this; they want maximum light grasp from any instrument they’re using.

While this may be intuitive, it’s also wrong. There is an increase in contrast and resolution by using binocular over monocular vision. At best, however, the difference in brightness is insignificant.

Avoiding lengthy calculations on binocular summation (most of which I don't understand), you may prove this yourself.

Look out the window and then cover one of your eyes. Did your view diminish in brightness by 50%? Why not? You decreased your light intake by 50%.

The answer lies in how the brain’s part of our optical system works, which is not always the way we think it should. It’s important to realize the brain is far more sophisticated than a piece of mechanical optical gear and, unlike a camera, doesn’t work on mathematical constants and the accumulation of photons. :cat:

Red: don't shoot me; it's Bill

 

[OPTIC_NUT]

Just focus on a fine detail at dusk and look with one eye, then both.
It will seem a little brighter, but much sharper. Noise reduction
due to asychronous signal processing. Apparent brightness and contrast both
get better...and resolution, consequently, if dim.
Maybe not like a camera, but a lot like phased array radar.
It's been described as 'extra brightness' in other forums,
but...just sayin'....it's better than that.
I think the term "autocorrelation" is used when you combine different sources, same signal.

 

[tpcollins]

The above posts cover it all, and succintly. Wow.. ;-)
And yeah....a bright scope with less contrast is a problem.

I would only add the caveat that there is additional light lost
in a zoom like that. Not so much the coatings as the grazing losses needed
(or the fringing that happens).
Which is a roundabout way of saying...if you're zooming that much, coatings
and aperture aren't your biggest concern. Reduced or no zoom would yield more,
not by element count these days but by engineering tradeoffs.
Passing through the narrows of a zoom gun scope, the choice of contrast (small-dia diffraction)
or loss (introduce coma before the elements) must be made. No zoom = no such issue.
Chromatics are very hard to keep out of a lens-based image inverter, too.

Based on that, would I be correct in assuming that a fixed 6 power rifle scope would be brighter (and perhaps clearer) than a 3-9 rifle scope set on 6 power? Thanks.

 

[OPTIC_NUT]

Probably dimmer, yes. I'm sure if it's multicoated throughout, not too bad,
but there is a lot of irising they do in the middle. The main thing is the loss of field.

 

[tpcollins]

Probably dimmer, yes. I'm sure if it's multicoated throughout, not too bad,
but there is a lot of irising they do in the middle. The main thing is the loss of field.

Now I'm confused. The fixed 6 power would be dimmer and have a smaller FOV?

 

[NDhunter]

Based on that, would I be correct in assuming that a fixed 6 power rifle scope would be brighter (and perhaps clearer) than a 3-9 rifle scope set on 6 power? Thanks.

tp:

When comparing rifle scopes and other optics such as binoculars it
does count on the quality of the optics, both lenses, design and coatings.

A fixed rifle scope is no brighter than a variable scope design.
The correct answer, is that it depends, on the above factors.

Jerry

 

[tpcollins]

tp:

A fixed rifle scope is no brighter than a variable scope design.
The correct answer, is that it depends, on the above factors.
Jerry

Thanks Jerry but either that contradicts what Optic Nut says below or I'm having trouble understanding English today.

The above posts cover it all, and succintly. Wow.. ;-)
And yeah....a bright scope with less contrast is a problem.

I would only add the caveat that there is additional light lost
in a zoom like that. Not so much the coatings as the grazing losses needed
(or the fringing that happens).
Which is a roundabout way of saying...if you're zooming that much, coatings
and aperture aren't your biggest concern. Reduced or no zoom would yield more,
not by element count these days but by engineering tradeoffs.
Passing through the narrows of a zoom gun scope, the choice of contrast (small-dia diffraction)
or loss (introduce coma before the elements) must be made. No zoom = no such issue.
Chromatics are very hard to keep out of a lens-based image inverter, too.

 

[NDhunter]

Thanks Jerry but either that contradicts what Optic Nut says below or I'm having trouble understanding English today.

An example would be a high quality optic such as a Leica, Zeiss or
others, compared to a simple Simmons or Bushnell.

There is a difference, zoom or not.

Jerry

 

[tpcollins]

An example would be a high quality optic such as a Leica, Zeiss or
others, compared to a simple Simmons or Bushnell.

There is a difference, zoom or not.

Jerry

I understand that, but what about a fixed 6 power Swarovski versus a Swarovski 3-9 set on 6 power?

 

[henry link]

No difference at all in the light transmission between a fixed and zoom scope. Zooms use the very same design as as fixed magnification scopes. Zooming is accomplished by changing the position of the same lenses that are used for image erection. Also no difference at all between light transmission at 3x ad 9x.

 

[OPTIC_NUT]

Doesn't seem that way, but I've been looking at scopes at the ~$50-100 level.

 

[OPTIC_NUT]

An example would be a high quality optic such as a Leica, Zeiss or
others, compared to a simple Simmons or Bushnell.

There is a difference, zoom or not.

Jerry

Could be....and probably, an even bigger difference than in binoculars.
I know from the bench...two plain achromats as an inverter is not great.
I'd expect the element count and the precision would need a big boost.
OK..it's adding up now. The moral could be....spend!

 

[WJC]

No difference at all in the light transmission between a fixed and zoom scope. Zooms use the very same design as as fixed magnification scopes. Zooming is accomplished by changing the position of the same lenses that are used for image erection. Also no difference at all between light transmission at 3x ad 9x.

Although light transmission is different for a ZOOM scope and Zoom binocular, it should be remembered that the image will grow dimmer with increased magnification as you are spreading out the available light.

Bill