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What is Twilight Factor?

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#1 Guest_**DONOTDELETE**_*

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Posted 25 September 2003 - 03:13 PM

When looking at specs for binoculars, I see a line item called "Twilight Factor" and it has a number like 16, 18.3, 20.5, etc...

What is this number telling me?

Thanks, Tom

#2 KennyJ

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Posted 25 September 2003 - 04:15 PM

Tom ,

Multiply the magnification by the objective size in millimetres ( for example a 12 x 50 = 600 )

Then calculate the SQUARE ROOT of this mumber ( in this case it would be 24.5 )

The HIGHER the Twighlight Factor , in theory at least the BETTER the binocular will perform in POOR or FADING light.

So whilst exit -pupil ( arrived at by dividing objective size by magnification --in this example about 4.2 ) provides increased BRIGHTNESS as it increases ,there comes a stage in lighting conditions where increased MAGNIFICATION helps you see things better in darker situations.

Both of these factors , twighlight factor and relative brightness are very important --but do not tell the whole story .

ACTUAL brightness and TWILIGHT PERFORMANCE are also affected very much by QUALITY of glass and coatings and optical designs incorporated ( e.g prism types )

So a top quality $1000 10x40 for example may very likely in reality outperform a $100 12x50 in terms of TWILIGHT PERFORMANCE , even though the math would lead you to believe otherwise .

Twilight PERFORMANCE is in my opinion a very important and often underated and overlooked factor with binoculars.

Hope this helps - regards -Kenny.

#3 Guest_**DONOTDELETE**_*

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Posted 25 September 2003 - 05:30 PM

So I should pay attention to relative brightness as well and look for a high number there, combined with as high a number as I can comfortably hold in my hands for the Twilight Factor.

I'm kinda of getting it, I think (still wondering about the Rel. Brightness). Definitely understand the Twilight Factor, thanks for the description. Seems to have to do with the "volume" of light transmitted.

Tom



#4 Mark9473

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Posted 25 July 2005 - 11:32 AM

The HIGHER the Twighlight Factor , in theory at least the BETTER the binocular will perform in POOR or FADING light.

So whilst exit -pupil ( arrived at by dividing objective size by magnification --in this example about 4.2 ) provides increased BRIGHTNESS as it increases ,there comes a stage in lighting conditions where increased MAGNIFICATION helps you see things better in darker situations.


Kenny and others,

a browse through the 'Best Of' thread, caused me to dig up this old issue again - I hope nobody minds.

I have to say I really don't quite understand it yet...

I do understand that magnification plays a role in 'seeing things better' in twilight. But comparing to the other parameter "exit pupil size" is where I don't follow.

In daylight, exit pupil plays virtually no role since the eye's entrance pupil will be smaller. At night, we've learned in recent years that magnification is actually more important than aperture, and people have started measuring binocular performance as square of magnification times aperture.

So in summary of binocular performance:
- daylight: magnification
- twilight: magnification x aperture
- night: magnification^2 x aperture
This seems to make sense to me, however it implies that in twilight conditions, magnification is actually LESS important than it would be at night. This also makes sense to me, but it contradicts (in part) Kenny's post - and that's something I'm VERY hesitant about.

Am I making any sense here?

#5 EdZ

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Posted 25 July 2005 - 11:57 AM

This seems to make sense to me, however it implies that in twilight conditions, magnification is actually LESS important than it would be at night. This also makes sense to me, but it contradicts (in part) Kenny's post - and that's something I'm VERY hesitant about.



I think it would be more appropriate to say
it implies that in twilight conditions, aperture becomes more important. I don't think you can reduce the importance of magnification, but (and here we go back to that exit pupil relation) with an equivalent amont of magnification, the larger aperture will give a larger exit pupil and hence a better low light condition performance.

I'm the biggest advocate there is of the use of magnification for improved astronomical performance. But that doesn't always hold true. Examples where that doesn't necessarily work are broad diffuse low contrast extended objects. In these cases magnification takes less importance than most other types of objects.

edz

#6 Rich N

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Posted 25 July 2005 - 11:58 AM

Twilight situations can come on us in the middle of the day. Sometimes you are trying to see a bird in the shadows on an over cast day. You don't have a lot of light to work with. More aperture and magnification is great if you have it with you.

Rich

#7 EdZ

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Posted 25 July 2005 - 12:30 PM

BTW

a browse through the 'Best Of' thread, caused me to dig up this old issue again - I hope nobody minds.


This would be encouraged.

If you have a question or comment about an old topic, the best place for the discussion is attached to the old post.

edz

#8 KennyJ

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Posted 25 July 2005 - 02:22 PM

Mark ,

You've dug up a real " rave from the grave " here my friend !

I must admit that two years down the line , it DOES come across as a little " waffly " -- but I still stand by my words !

But another idea springs to mind !

Mark , I note you have a Leica 8 x 20 and a Swift 8.5 x 44 Audubon in your little collection.

At dusk , compare the images through both of them at a medium distant ( 200 yards ) terrestrial object.

See if you can SEE the difference I tried to explain !

Regards , Kenny

#9 Mark9473

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Posted 25 July 2005 - 02:47 PM

I still stand by my words


and right you are Kenny, and I hope you noticed I tried to approach this with the greatest respect for you. I guess we're looking at the same thing from a different perspective. Actually, EdZ's follow-up was excellent and really made the puzzle pieces fall into place. But I do (humbly) claim some merit because the original 'best of' explanation was incomplete without these recent posts.

But another idea springs to mind !

Mark , I note you have a Leica 8 x 20 and a Swift 8.5 x 44 Audubon in your little collection.

At dusk , compare the images through both of them at a medium distant ( 200 yards ) terrestrial object.

See if you can SEE the difference I tried to explain !

Regards , Kenny


No discussion there! My only complaints on the little Leica's are their twilight performance. I posted a question on this in the Yahoo Binocular Astronomy group a while back (I only recently registered here, though had been lurking a long time). Even EdZ's reply there didn't satisfy me (sorry Ed) because he referred only to exit pupil. But now this is yet another piece of the puzzle falling into place, since indeed this bothers me more in twilight (where aperture is relatively more important) than at night (where magnification is relatively more important).

Basically my only remaining question on twilight factor is whether I am thus correct to conclude that at night but under suburban skies with significant light pollution, binocular performance would then rate as magnification^x times aperture, where x is between 1 and 2.

very best regards,

#10 Joe Ogiba

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Posted 25 July 2005 - 03:07 PM

Here in NYC metro area after dark with significant light pollution I can see more (terrestrial) with both my 7mm exit pupil binoculars (7x50 Celestron Nautica,Pentax DCF 9x63) than my 5mm exit pupil binoculars like my 7x35's,8x42's,10x50's etc.

#11 KennyJ

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Posted 25 July 2005 - 03:09 PM

Mark ,

Thank you for your kind words.

It really does make a refreshing change to see a recently joined member actually take the time to read sections of the " Best of " section before diving in with both feet :-)

As regards your proposed " formula " I think you are absolutely correct to place that all - important "constant" factor as lying SOMEWHERE between 1 and 2.

It's well over a decade now since Roy Bishop announced his mag x obj formula , which always DID seem overly simplified to me personally , but there is no doubt it was a major step in the right direction.

It is an issue I discussed privately with Prof.Ed Zarenski before either of us had even signed up with Cloudy Nights and our virtual community owe it ALL to Ed. for his many hours of research and experimentation to PROVE that not only the Bishop , but the improved ADLER performance indicator formula needed a slight "tweaking" to fit in with what practical observations.

I'm not sure whether they are actually mentioned in the aforementioned CN " Best Of " files , but I seem to recall that Ed and I agreed there WERE other factors that need to be taken into account here -- not least OPTICAL QUALITY.

Given an even -playing field ( if such a thing can ever really exist -- of which I'm not certain given some of the technical complications arising from the design of binoculars of various magnifications ) I think there COULD be something approaching a DEFINITIVE formula which could be arrived at which would satisy all parties.

I've little doubt that there will have to be the odd "square root" and occasional "pi" thrown in before everyone can sleep tight :-)

Regards , Kenny

#12 Mark9473

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Posted 25 July 2005 - 03:23 PM

Here in NYC metro area after dark with significant light pollution I can see more (terrestrial) with both my 7mm exit pupil binoculars (7x50 Celestron Nautica,Pentax DCF 9x63) than my 5mm exit pupil binoculars like my 7x35's,8x42's,10x50's etc.


Makes sense to me, Joe. "Terrestrial" is where you apply the pure twilight factor, where aperture and hence exit pupil is more important than for astronomical use or for daylight viewing.

If we discuss this for a few more years we might be able to draw graphs of the transitions between these cases.

#13 Mark9473

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Posted 25 July 2005 - 03:25 PM

I've little doubt that there will have to be the odd "square root" and occasional "pi" thrown in before everyone can sleep tight :-)

Regards , Kenny


haha, Kenny, many thanks for letting me end my evening with a good healthy laugh :D

#14 KennyJ

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Posted 25 July 2005 - 04:37 PM

Joe ,

It seems our friend Mark might have missed your point in your last post to this thread.

According to " my original post " your 10 x 50 SHOULD let you " see more " than your 7 x 50.

You say it does NOT !

I wonder if that could be due to differences in QUALITY ?

Those Celestron 7 x 50 Nautica and Pentax 9 x 63 DCF are both VERY good binoculars -- yes ? -- no -- ?

What about the 10 x 50 you are comparing them with ?

Which model is that ?

Interested and intrigued ,

Regards , Kenny

#15 Mark9473

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Posted 25 July 2005 - 04:42 PM

It seems our friend Mark might have missed your point in your last post to this thread.


sorry, it's getting late for me.
I'll sign off for today.

#16 KennyJ

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Posted 25 July 2005 - 04:52 PM

Sorry Mark !

I thought you'd signed off almost an hour ago !

-- looking forward to carrying this discussion further !

Wishing you a very good night !

Kenny

#17 Joe Ogiba

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Posted 25 July 2005 - 05:45 PM

Kenny,

I compared them to my 8x42 Pentax DCF WP's (roofs w/Phase coated prisms) and the new 10x50 Pentax WP II's. The 7mm exit pupil does work for me at night (terrestrial) even with my 58 year old eyes.

Joe

#18 KennyJ

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Posted 25 July 2005 - 06:15 PM

Joe ,

Thanks for replying to that.

So -- maybe it's NOT down to a difference in QUALITY then ?

I must say ( and I'll "whisper" this considering the thread it's under ) that both my Zeiss 7 x 42 BGAT and THAT Captain's Helmsman 7 x 50 seem to " show ME more " in twilight than does the Swift 10 x 50 Kestrel.

I still haven't quite worked out whether this is down to :

1. EXIT - PUPIL ( mainly because so few , if any , fellow bino enthusiasts of whom I'm aware , seem to agree with me about ALL the advantages of a larger exit -pupil )

2. WIDER FIELD OF VIEW ( mainly because so few , if any , fellow bino enthusiasts of whom I'm aware , seem to agree with me about the possibility of wider fields "seeming" to give what I call " brighter " images )

3. A serious general misunderstanding of by what degree the human eye pupil actually remains undilated during typical binocular use in a variety of daylight conditions ( mainly because so few , if any , fellow bino enthusiasts of whom I'm aware , seem to agree with me about )

- - - - or - - - -

4. A combination of all three aspects listed above.

Bedtime now !

To be continued ?

Hopefully.

Regards , Kenny

#19 Joe Ogiba

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Posted 25 July 2005 - 06:50 PM

2. WIDER FIELD OF VIEW ( mainly because so few , if any , fellow bino enthusiasts of whom I'm aware , seem to agree with me about the possibility of wider fields "seeming" to give what I call " brighter " images )


I have seen someone say it is just the opposite, that the wider FOV is not as bright because it has to spread the light over a larger area. I never tested that and I don't think the eye can see that little amount of light value difference IMHO.

Joe

#20 Swedpat

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Posted 25 July 2005 - 07:52 PM

My personal opinion is, quite opposite to what some other tell, that the twilight factor is fairly overrated. Why do I think this? Yes, the big lack of the twilight factor index is that the value increases unlimited with higher magnification without taking in consider that the brightness decreases to be unlimited low.

The twilight factor is dependence to adequate brightness to have ANY practical value. In some cases the twilight factor is such irrelevant. For example an advertisment for a 20-60x60 spottingscope. Twilight factor: 34,6-60.

Even with the lowest power this scope has a brightness corresponding to a 7x21 or 10x30 binocular. The twilight factor is highest with the highest power, in wich the scope has 1mm exit pupil. But try to use 1mm exit pupil in any dawn and dask! It's corresponding to look with the naked eye through a needle's eye. Then it doesn't help to have high power, for magnified darkness = darkness. In this case the scope may have the best low light performance around the lowest power there the twilight factor is lowest.

Another example I sometimes experience is the claiming that a 10x25 binocular is better for using in dawn and dusk than a 8x20. But is it really? If they both have equal light transmission the brightness is the same. Of course 10x power means that you see more details than with 8x and the same brightness if the brightness is adequate to see ANY detail. In ANY case there the brightness is adequate to see details you will benefit from higher power. Therefore I just don't agree this is the same as lowlight performance...

My conclusion: the twilight factor can only be used in a limited area and we need always to take in consideration the brightness index and the current outer light conditions.
IF WE DO THAT the twilight factor can be a valuable measure for the ability to see details in low light conditions. IF WE DON’T it can be VERY misleading.

Patric

#21 KennyJ

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Posted 26 July 2005 - 01:23 AM

Great Post Patric !

VERY good points !

I would take my hat off to you , but was never educated to the level where I was awarded a hat :-)

Now we see why !

Actually Bill Cook said many years ago that along with one or two other terms often banded about such as "Relative Brightness Index" and " Relative Light Efficiency" the expression " Twilight Factor " tells you NOTHING that the basic descriptive term such as 7 x 50 CANNOT --

- - and can thus be safely ignored !

Actually , Patric's point reminds me of one I tried to make a few weeks ago , concerning the use of FILTERS in binoculars.

Using filters does NOT affect exit - pupil , objective size or magnification , but it sure as anything reduces BRIGHTNESS -- in the case of a welder's mask to the point of total darkness !

I presume everyone must have thought I was having a bad day when I said that :-)

Regards , Kenny




Regards , Kenny

#22 Swedpat

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Posted 26 July 2005 - 04:53 AM

Thank you Kenny!

Patric

#23 Joe Ogiba

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Posted 26 July 2005 - 05:41 AM

Using filters does NOT affect exit - pupil


I have a hard time understanding that since the eyes exit pupil reacts to brightness and when I use an OIII filter it cuts down the brightness level. :confused:

Joe

#24 KennyJ

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Posted 26 July 2005 - 05:44 AM

Joe ,

EXIT - PUPIL comes from the BINOCULAR -- not the eyes !"

ENTRANCE - PUPIL ( dilation ) is something completely different.

Regards , Kenny

#25 Joe Ogiba

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Posted 26 July 2005 - 05:57 AM

Kenny,

Ok, but the exit pupil is like an aperture stop in a lens and I have seen reflex (mirror based)photo lenses adjusted for f stop use with filters . So your saying you can use a neutral density filter that cuts 99% of the light with no effect to the eyes entrance pupil ? :confused:

Joe


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