New but probably wrong insight about lowlight (1 Viewer)

[ReinierB]

I have been rethinking this over and over and maybe I have a new insight. I could be wrong though. So please correct me. I am here to learn.

I have always been asking myself which one is better in lowlight: 8x42 vs 10x42, 8x32 vs 10x32, 8x25 vs 10x25, etc. when all other specifications are the same, such as transmission.
My conclusion was, and maybe still is, that the exit pupil is most important. So 8x42 is brighter and more suitable for use in lowlight compared with 10x42, provided that the eye pupil is > 4.2mm.

Some say that the objective lens diameter is most important. It is all about aperture, astronomy fascinados say. So in that case 8x42 and 10x42 do both have the same light gathering and it's the objective lens diameter that counts.

My experience is that 8x42 appears to be brighter than 10x42 in low light, although I see more details with 10x42.
So what I am saying might be wrong. I am just trying to understand.

8x42 and 10x42 both have 42mm objective lens diameter and the same light gathering. The beam of light narrows down at 5.25mm for the 8x42 and 4.2 for the 10x42. Am I wrong when I say that both beams are containing the same amount of light? So the same amount of light hits your eyes? In that case, when your eye pupils are 4.2mm, the view of the 10x42 must appear brighter than this of the 8x42, because you will spill 1.05mm of the light beam of the 8x42. You use the full amount of light with the 10x42 but you spill some of the 8x42s.
Only when your own eye pupils are >5.25mm, both the 8x42 and the 10x42 must appear as bright, because the light gathering is also the same, since the objective lens diameter is 42mm for both.

What error am I making?

 

[Maljunulo]

“Light gathering” is determined by the objective diameter.

Higher magnification “spreads the light over a larger area” so a higher magnification produces a dimmer image.

(sort of)

 

[Ignatius]

You're almost there, ReinierB. But it isn't solely the objective aperture that decides how much light reaches your eye, but also the magnification.
Both combine to give you something called the exit pupil, and the closer that correlates to the diameter of your dark-adapted own pupil, the better you will be able to see in dawn or dusk situations. The EP is calculated simply by dividing the objective diameter by the magnification, which in your case gives you 5.3 mm for your 8x42 and 4.2 mm for your 10x42. If the EP is larger than your pupil, light is lost and the 'efficiency' of the bino is reduced. When, conversely, your pupil is larger than the EP, contrast is reduced, and again the bino loses efficiency. You can have your dark-adapted pupil size measured by an optician, and there are some DIY ways to be found online.
One caveat though: as people get older two things happen - their eyes dilate less as it gets darker (from around 7 mm at ca. age 20 to around 4 mm at ca. age 60 - there are several studies online which do give varying numbers), and secondly the resolution power of the eye gets less. This leads to the situation, that as a young man you'll happily use an 8x56 at dawn and dusk, but as an older man you might prefer to use a 10x56.
As to your question: the 8x42 will always appear brighter no matter whether your own pupil's aperture stops it down, but as an older man the 10x42's better resolution might give you a clearer image.

 

[Binastro]

Actually it depends what you are looking at.

If you want to see fainter stars then 10x42 is better than 8x42 by about 25%.

If you want to see extended faint detail then 8x42 is better than 10x42.

In addition, eyes vary a lot.

At age 60 my pupils were 6mm and my friend's still 7mm at that age.
Also one of my eyes saw 50% fainter than the other.

In addition, if really dark, dark adaptation varies.
As one ages this gets less efficient and takes longer to achieve.
Normally, about 20 minutes gives good dark adaptation, but full dark adaptation can take hours or even a day.
Deep sky observers don't go out in the sun for 24 hours.

However, bird watchers probably don't try to spot birds, say owls, in really dark conditions, so the actual view may be a compromise.

The best thing is to have both 8x42 and 10x42 and see which is better in various light levels.

Also IS binoculars do better than equivalent non IS binoculars.

In daylight, the pupils get down to 2mm and in some cases 1.5mm.

Regards,
B.

 

[jan van daalen]

Reinier,

What you have to take in account is the fact that the humans pupil doesn't stand still but moves tens of times per second. In theory will a 8 and 10x42 be the same when the pupil is 4.2 at the max. Still the 8 times looks brighter because of the fact the 'moving' pupil catches the light up to 5mm instead of 4.2.
A 8x50 will show more brightness and more ease of view etc etc.

Jan

 

[Scott98]

Being an astronomer I am guilty of aperture-elitism but I would agree with the statement "larger exit pupil is brighter" but with the following caveat "for a given aperture, larger exit pupil is brighter"

 

[jring]

Hi,

for astronomy aperture is king for many reasons, but the main one is that it limits resolution so you can see smaller objects. Provided that your eye's pupils still can widen enough and your skies are dark enough, you also want a wider exit pupil (and thus less magnification) with that aperture for the ability to see fainter large objects. Not so dark skies can be cheated with filters...

If you want to observe animals at dusk or dawn, things are a bit different and quite a few papers have been written on this. In general, if you have two similar pairs with the same exit pupil but different sizes like an 8x32 and a 10x40, the one with larger aperture and thus magnification will win, if you can still hold it steady enough (or use a tripod or stabilized pair). I would really recommend Holger Merlitz book on binoculars (now finally available in english, iirc) which has several chapters on this and of course all with citations of the relevant papers...

Joachim

 

[ReinierB]

I would really recommend Holger Merlitz book on binoculars (now finally available in english, iirc) which has several chapters on this and of course all with citations of the relevant papers...

I have just ordered it (after your recommendation). Maybe that will give me some light bowls.

 

[ReinierB]

Higher magnification “spreads the light over a larger area” so a higher magnification produces a dimmer image.

(sort of)

Can you please clarify this? "Spreads the light over a larger area"? Which larger area?

 

[ReinierB]

As to your question: the 8x42 will always appear brighter no matter whether your own pupil's aperture stops it down

Always? Also in bright daylight when my eye pupil is just 2.5mm?
Or only when my eye pupil > 4.2mm? (8x42 compared with 10x42)

 

[ReinierB]

What you have to take in account is the fact that the humans pupil doesn't stand still but moves tens of times per second. In theory will a 8 and 10x42 be the same when the pupil is 4.2 at the max. Still the 8 times looks brighter because of the fact the 'moving' pupil catches the light up to 5mm instead of 4.2.

That is interesting. I didn't know that. What is the range of movement? Is that really as big as from 4.2 to 5mm and back, and than from 4.2 to 5mm again?

 

[ReinierB]

if you have two similar pairs with the same exit pupil but different sizes like an 8x32 and a 10x40, the one with larger aperture and thus magnification will win

Are you saying that the 10x40 will win in brightness, or only in seeing details with the same brightness?

 

[Maljunulo]

Can you please clarify this? "Spreads the light over a larger area"? Which larger area?

If you take an image and magnify it by 2X, the area of the image is then four times what it was at 1X.

It cannot be as bright as it was at 1X, because it is the same quantity of light, but spread out over a larger area.

I don’t know any other way to say it.

 

[Swedpat]

I like this topic. I will give my try to complement.
Total light gathering power is linear to surface area of the objective.
Brightness is determined by exit pupil.
Brightness is not to be confused with light amount/light gathering power.
For example: if you compare following configurations: 10x25, 10x50 and 20x50.

10x25 and 10x50: same magnification
10x50 and 20x50: same light gathering
10x25 and 20x50: same brightness

If you observe an even illuminated surface which covers the entire FOV, 10x25 and 20x50 provide the same light amount into your eyes, while 10x50 provides 4 times the light amount.

If you observe an illuminated object(like a lamp or the moon) with dark surrounding, 10x50 and 20x50 provide the same light amount into your eye, while 10x25 1/4 of the light amount.
Here 10x25 and 20x50 provide the same surface brightness while the surface brightness with 10x50 is 4 times higher.

If you observe point sources like stars, the light amount and perceived brightness, with 10x50 and 20x50 is the same, while with 10x25 it is 1/4.

Brightness is often mixed up with light amount. For example when watching light sources, more light will reach your eye with a larger aperture despite exit pupil is smaller, because magnification is higher.

It's also common that people believe a binocular can be brighter than the naked eye. It can never be without electronical amplification. Otherwise naked eye provides brighter image by a few percents than the highest light transmission optics.
This misunderstanding is caused because people become much more blinded of light sources through the binocular. But the brightness can actually become significantly lower, because the eye pupil contracts more due to the increased light amount. It's just not able to do it enough so the light amount reaching the eye is still much higher.

 

[jring]

Are you saying that the 10x40 will win in brightness, or only in seeing details with the same brightness?

Hi,

actually both. Same exit pupil but larger objective aperture means more photons per area unit. But the increases magnification helps too.

Joachim

 

[Binastro]

Not really.

Much fainter stars are visible in a 20x50 compared to a 10x50 because the sky background is darker.
In fact, stars twice as faint are visible in the 20x50 or about 0.7 0r 0.8 magnitude fainter.

In addition, many binoculars are vignetted, so 10x50s are typically 10x47.
The Optolyth 12x50, much loved by nature watchers is actually 12x42, with a very strange shaped exit pupil.

Additionally, dark adaptation is much more significant than aperture variation within reason.

A person going to a telescope from indoors will see very little regarding stars or nebulae.
After 20 minutes or 30 minutes totally invisible objects are seen as bright.

I had quite good unaided vision of faint objects, but with telescopes some saw fainter than me, say the Merope nebula in the Pleiades.

The actual results in practice are far more complicated than some realise.

Also tripod mounted or IS binoculars show more.

Furthermore, using the Astro cards for Messier and other objects, very faint objects whose position is very accurately known are seen that are invisible without the cards.
The gain is about two times.

Also skilled use of averted vision shows much fainter objects.

Regards,
B.

 

[Swedpat]

Hi,

actually both. Same exit pupil but larger objective aperture means more photons per area unit. But the increases magnification helps too.

Joachim

I mean that is not correct. Photons per area are already included in the exit pupil, which is based on the relation of aperture and magnification.
All configurations with same exit pupil have same brightness providing equal light transmission. Otherwise 20x80 would be brighter than 10x40. It is not. The same with comparing an 8x20 to an 80x200 telescope.
Brightness is the same, you just comes closer.

 

[jring]

Hi,

@Binastro I agree, but I did not mean astro use.

@Swetpat I think that this discussion has taken place before, not sure if with you or how it went. My argument was and is that the objective lens is illuminated with a certain number of photons per area unit. Thus a larger objective lens area means proportionally more photons hit that area. If we assume an ideal instrument, all of those photons get refracted into the exit pupil.
So if we have two instruments with the same exit pupils but different apertures, the number of photons illuminating the exit pupils will be proportional to the objective aperture area. In real instruments a certain percentage of the photons is lost due to material effects, of course.

Joachim

 

[Ignatius]

Since one can measure transmission as a percentage of light exiting versus entering an optical system as per DIN ISO 14490-5, one can also take a photodetector to measure light quantities, as photons trigger an electrical signal. That signal is measurable or countable. Ergo one could compare different optical systems with the same exit pupil for brightness.

 

[Maljunulo]

Stars are self-luminous point sources.

Virtually everything else we look at is extended, more or less reflective objects.

These two things are not the same, although I suppose one could argue that the image of the extended object is made up of a large number of (overlapping?) airy discs, so they really are the same.

Perhaps I’m just confused.