Resolution 32 vs. 50 (2 Viewers)

[Jefrs]

A 12” telescope gathers more light than a 6” telescope, and has greater resolution no matter what eyepiece you use.

Light gathering and resolution are invariant.

The image gets dimmer as the magnification is increased, but the instrument does not gather less light.

As the magnification increases to the field of view decreases, which reduces the amount of light making the image.
Where a telescope has a primary folal length of say 650mm and a 130mm diameter tube it will have an aperture value of f/5, and this will be printed on the side or in the spec sheet.
Where another telescope has a focal length of say 1300mm in the same 130mm diameter tube, it will have an aperture value of f/10, and again will be in its spec sheet.
Where we add an eyepiece (ocular) that will alter the magnification and hence the overall focal length, and again reduce the field of view, and the aperture value.
Do not confuse the iris diameter (objective size) with the aperture value. Although they are often used interchangeably in conversation they mean different things. The former is a diameter measurement, the aperture is dimensionless ratio. The f-number is a measure of how much light passes through the instrument from subject to image. The aperture f-number is /defined/ as focal length divided by iris diameter (the iris is usually the objective). That means a short FL but narrow instrument can have the same f-number as a long FL but wide one e.g. 100/25 = f/4, 200/50 = f/4

If you don't believe me then believe Nikon. They gave the formulae in the links above. You can easily rearrange the equations and work this out.

 

[kimmik]

So my question to you:

In a simple convex lens of 100mm diameter,
With focal length 1000mm,
What is the focal ratio?

Now add a 10mm eyepiece at the focal point,
What is the magnification?

At no point does focal length or focal ratio change in the above example.

 

[jafritten]

Good day,

I have people heard saying that a 10x50 has more resolution compared to, let's say, a 10x32 because of the lens diameter. Is that true? And if that is true, how will you notice that?
Let's say it is a very bright day, will the 10x50 outperform a 10x32 if you spot an object far away in regard of the resolution? Will the 50's show more pixels / show a sharper image?
I just wonder...
(NL Pure 10x32 vs. EL 10x50)

It seems to me that a larger objective diameter (10x32 vs 10x42) will show more fine detail. I used to think that the reason for this was higher resolution, but this is not true. In theory, both configurations should perfom equally well in bright daylight when the pupils of your eyes stop down the flux of light to, say, 2.5 mm which is roughly where one's eyes have the greatest resolution. Holger Merlitz states that in dim conditions larger diameters of the exit pupil will give more usable contrast, and thus, more detail (cf. Handferngläser by Holger Merlitz, Verlag Europa-Lehrmittel, 2013). My impression is that even in bright daylight (i.e. when my pupils are stopped down to 2.5mm) binoculars with larger objective diameters show more detail. I find this especially noticable when viewing at longer distances. I am not educated enough to give an explanation for this; I can only describe what I see. I think contrast (light and dark) and colour contrast (difference between colours) play a role here. This is mere conjecture, of course.

Very interesting topic, though. Someone should write the book "Mysteries of binocular vision"...

 

[Maljunulo]

As the magnification increases to the field of view decreases, which reduces the amount of light making the image.
Where a telescope has a primary folal length of say 650mm and a 130mm diameter tube it will have an aperture value of f/5, and this will be printed on the side or in the spec sheet.
Where another telescope has a focal length of say 1300mm in the same 130mm diameter tube, it will have an aperture value of f/10, and again will be in its spec sheet.
Where we add an eyepiece (ocular) that will alter the magnification and hence the overall focal length, and again reduce the field of view, and the aperture value.
Do not confuse the iris diameter (objective size) with the aperture value. Although they are often used interchangeably in conversation they mean different things. The former is a diameter measurement, the aperture is dimensionless ratio. The f-number is a measure of how much light passes through the instrument from subject to image. The aperture f-number is /defined/ as focal length divided by iris diameter (the iris is usually the objective). That means a short FL but narrow instrument can have the same f-number as a long FL but wide one e.g. 100/25 = f/4, 200/50 = f/4

If you don't believe me then believe Nikon. They gave the formulae in the links above. You can easily rearrange the equations and work this out.

Okay, let’s try again.

A telescope of a given aperture has the same light-gathering ability and resolution irrespective of its f-ratio.

 

[Jefrs]

Okay, let’s try again.

A telescope of a given aperture has the same light-gathering ability and resolution irrespective of its f-ratio.

wrong

 

[Maljunulo]

Educate me.

Please explain to me exactly how a 4” f/4 telescope gathers more or less light, and has more or less resolving power than a 4” f/16 telescope.

I’ll wait right here.

 

[Binastro]

Jefrs is repeatedly wrong.

He is using shorthand notation for camera use.

What aperture are you using?

Answer f/2.8.

The question should be.
What aperture stop or what relative aperture are you using?

This is asking for a fraction or ratio, which indeed has no physical size.

An aperture can be any size or shape.
It could be a triangular, square, oblong or an irregular shape.

The Kite 16x42 IS has an irregular shaped front aperture.
The Minolta 6x18/8x18 has a rectangular front aperture.

Elliptical aperture masks are sometimes used on obstructed telescopes to give refractor like unobstructed performance.
Although usually these masks are circular.

As to post #45 Jefrs is completely wrong.

As to post #46 that is basically correct with caveats.
A 4 inch f/4 telescope has more aberrations or optical errors than a 4 inch f/16 telescope.
In practice the fast 4 inch f/4 has slightly less resolution than the slow 4 inch f/16 telescope.
The light gathering of both is basically the same, but there are uncorrected errors in the 4inch f/4 telescope that reduce the quality of the image.

But post #46 is basically correct and post #45 is totally wrong.

But if people live in different parallel universes these situations occur.

Regards,
B.

 

[tenex]

But if people live in different parallel universes these situations occur.

What I never understand is how people so badly misinformed can possibly remain so convinced that they know what they're talking about that they actually patronize and lecture others. How is this possible in the first place, and how do they manage to ignore corrections of their errors? (Focal ratios really are "rather simple" material, hardly rocket science.) Some studies have been done (Dunning-Kruger) but they only scratch the surface.

 

[Jefrs]

Educate me.

Please explain to me exactly how a 4” f/4 telescope gathers more or less light, and has more or less resolving power than a 4” f/16 telescope.

I’ll wait right here.

The f-number tells you that. f/16 is four stops slower than f/4. You cannot compare them without knowing the focal length. And that will determine angle of view which dictates the light gathering. This is not an opinion or patronising but a simple statement of fact, which you can verify from reliable sources online. Wikipedia has most of the information needed f-number - Wikipedia.
Educate, please take a university course in physics, astro-physics or ophthalmic optics.
Maybe read the The Ilford manual of photography or Hedgecoe ISBN: 9781405334761 (the late John Hedgecoe, Professor Emeritus Royal College of Art)

 

[tenex]

You cannot compare them without knowing the focal length. And that will determine angle of view which dictates the light gathering.

You're the one who can't identify the focal length of a scope's objective. But in any case it does not determine the angle of view, and angle of view does not determine the amount of light gathered. This is total confusion. What sort of "retired scientist" did you claim to be?

Educate, please take a university course in physics, astro-physics or ophthalmic optics.

More patronizing rubbish. Quite unpleasant and likely to grow more so. I suggest that we avoid provoking further "simple statements of facts" from this individual that only dance around the point at which his understanding fails.

 

[kimmik]

4” f/4 - focal length is 4x4=16"
4” f/16 - focal length is 4x16=64"

I guess you didn't know about this equation.

edit: if @Jefrs would like to share the name of the college where he took the physics module, I will consider applying to their fine arts department, as they seem to have a creativity vibe.

 

[Scott98]

Okay, let’s try again.

Better you than me! I haven't the patience for flat-earthers

It's baffling to me - how can one argue with physics? Physics is what makes the sun rise, it keeps your feet planted on the ground....don't mess with it!

 

[Binocollector]

Quite a funny thread I must say .

 

[Maljunulo]

What I never understand is how people so badly misinformed can possibly remain so convinced that they know what they're talking about that they actually patronize and lecture others who demonstrably do.

The funniest example of this which I have seen occurred in a forum from which I was subsequently banned.

During a discussion involving some rather elementary physics there was a poster who delivered a rather scathing and condescending put-down of another poster.

What Poster A did not realize was that Poster B was one of the guys who was in on the Higgs Boson thing.

Much hilarity at the expense of Poster A ensued.

 

[William Lewis]

Well. On another note!

Hi Reinierbos

I've got a package in the way in the form of a old Japanese 10x50 porro "carton" is the maker (£36!) I've not really used 10x binoculars for a few years so I'll post up how I get on in comparison to the new '32 porro and '56 roof I'll be using over on the Norfolk marshes in a few weeks although it must be said any resolution differences may be due to the likely 50 year difference in the date of production...

 

[ReinierB]

Does anyone know if the eyecup of the EL50 is exactly the same as the eyecup of the EL42?

 

[dries1]

Does anyone know if the eyecup of the EL50 is exactly the same as the eyecup of the EL42?

Yes I believe it is, I know the 8.5X42 is the same as the SV x50, so I would say the 10X42 would also be the same.

 

[ReinierB]

Yes I believe it is, I know the 8.5X42 is the same as the SV x50, so I would say the 10X42 would also be the same.

Do the EL 10x50 and the EL 12x50 have the same eyecups? The eye relief is different (20mm vs. 19mm) so I assume the eyecups of the EL 10x50 are 1mm longer?

 

[dries1]

The eye relief is based on the ocular lens configuration not the eye cup length. However, I placed the eyecup of the 10X50SV/2014 on the 2016/12X50 and yes they are slightly longer, 1mm perhaps. So the eyecups do fit.

 

[ReinierB]

The eye relief is based on the ocular lens configuration not the eye cup length.

But doesn't that imply that the eye cup length is different when the rest of the binoculars is the same? If the eye relief is shorter, than you need shorter eyecups.

That's why I assume all the SCL 56's have different length eyecups, for instance. The EL 50's have also different lengths. The EL 42's have the same eyerelief, so have the same size eyecups as well (I think).

Sorry, just trying to understand how it al works.