What does this Number mean? (1 Viewer)

bsifrit · Sep 28, 2004

I was looking at the Zeiss 8x40 victory BT’s specs and there are two numbers that I am not too sure about.

1. Twilight performance. The BT’s scored a 17.9. Is this a good score? Is a higher number better than a lower number? What exactly does this number represent?

2. Relative Geometric Brightness. The BT’s scored a 25-same Q’s as above.

Thanks for the knowledge,
Brett.

gorank · Sep 28, 2004

check out:
http://www.meditec.zeiss.com/C1256E2700349659/Index/?ReadForm&84

click on T..as in Twilight factor..

salty · Sep 28, 2004

the higher the twighlight factor, the brighter the image. usually.

iporali · Sep 28, 2004

bsifrit said:
I was looking at the Zeiss 8x40 victory BT’s specs and there are two numbers that I am not too sure about.

1. Twilight performance. The BT’s scored a 17.9. Is this a good score? Is a higher number better than a lower number? What exactly does this number represent?

2. Relative Geometric Brightness. The BT’s scored a 25-same Q’s as above.

Thanks for the knowledge,
Brett.

Hi Brett,

Welcome to a great Forum with lots of optics-nut people!

Those are IMO two useless figures.
1. Twilight factor should somehow describe the performance of the binoculars in low light. It depends on the objective size and magnification and it practically measures the "density" of light at the exit pupil. This is useless because this density never reaches the retina as such. Both magnification and objective size help the eye to see things (excuse the pun) in low light, but I very much doubt that a 10x40 behaves identically to a 40x10.

2. Relative Geometric Brightness is simply the exit pupil multiplied by itself - maybe trying to show that the exit pupil is actually two-dimensional... just silly (it is not a square).

The exit pupil (objective diameter/magnification) and light transmission are IMO the best indicators of the twilight performance. BTW, the Zeiss Victories have very good light transmission.

Ilkka

Bill Atwood · Sep 28, 2004

Hi Brett,

I more or less agree with Ilkka. Those numbers will be identical for all 8x40 binos. FWIW, the Victories do have a good reputation for brightness. Try to get the Victory II model if you can, the I's apparently had some issues.

Alot of folks (in the US) making their first foray into higher priced bins usually buy from Cabela's, Bass Pro Shop, or other general "outdoor" stores. FYI, there are alternative oulets (i.e. Eagle Optics) that usually have lower prices.

Pileatus · Sep 29, 2004

bsifrit said:
I was looking at the Zeiss 8x40 victory BT’s specs and there are two numbers that I am not too sure about.

1. Twilight performance. The BT’s scored a 17.9. Is this a good score? Is a higher number better than a lower number? What exactly does this number represent?

2. Relative Geometric Brightness. The BT’s scored a 25-same Q’s as above.

Thanks for the knowledge,
Brett.

Brett,

My advice is based on personal observations and information gleaned from the experts.

If you're looking for brightness in the 40-42mm class then the accepted order is (highest light transmission at the top)

Zeiss Victories/FL's
Leica Ultravid
Nikon LX/HG/Premier roof prisms
Swarovski EL

Some may disagree with the ordering because all are wonderful in daylight, and it is only when the light fades or you're in a dark forest that low-light capability becomes a serious issue. Color, contrast, resolution all affect the final image, so don't put too much faith in the numbers.

If brightness in low light is your primary goal, then there's no debate. The 42mm Zeiss FL's are the brightest roof prism I've ever seen and they are BRIGHT. However, you may not detect this in a well-lit store because your pupils aren't adapted to low light.

There are some excellent daylight bins for a lot less money than the top-end models, but you probably won't get the low-light capability you're looking for.

Good luck.

John

bsifrit · Sep 29, 2004

Thank you all for your help!

I have found a couple bins in my price range (all US $):
Zeiss Victory 8x40 (assuming I) for $752
Zeiss 7x45 B Design Selection for $688

Leica Trinovid 7x42 (BA or BN, not sure of the difference) for $610
Leica Trinovid 8x42 (BA or BN) for $635
Leica Trinovid 8x50 BN for $690
-I was thinking this would give me better low light output, but I am conserned that they will be too bulky.

I don't know much about the Trinovid, but the price seemed nice. Any help would be greatly appreciated.

Thanks again,
Brett.

Bill Atwood · Sep 29, 2004

AFAIK those are very good prices, maybe suspiciously low. They may not have US warranty. The Victory will be the least "bulky". The Design Selection and Leica 8x50 will be hmmmm...monstrous is probably going overboard, but they will be a handful.

Generally speaking the bigger objective lens will be brighter and obviously bulkier.

Pinewood · Sep 29, 2004

bsifrit said:
Thank you all for your help!

I have found a couple bins in my price range (all US $):
Zeiss Victory 8x40 (assuming I) for $752

Thanks again,
Brett.

My dear Brett,

I am guessing that the quoted prices are for used items or demos.
In the case of the Zeiss Victory, if it was imported by Zeiss USA, then it is still guaranteed. If you call Zeiss, and give them the serial number they can verify if it was imported by them. Same goes for the older design selection, reputed to be a nice optic but nose heavy.

I am one of the few happy owners of the Zeiss Victory 8x40 I. It is brilliant and lightweight. Although it has excellent central resolution, it is bettered by several glasses, including the Zeiss 7x42 BGAT* Dialyt. It has nice edge sharpness and good contrast. Four nights, ago, I compared it to an inexpensive 8x42 Porro. With the Porro, I could not see any stars, through a humid, light polluted sky, but the Zeiss revealed a few. Last night, the Zeiss was resolving stars at least one magnitude dimmer. I am going to lay this on improved contrast from better coatings and from better internal baffling. This is the sort of advantage high priced optics should bring to the user.

Relative geometrical brightness has some use. A 5mm exit pupil is not 25% brighter than a 4mm exit pupil, but more than 50% brighter. The former has a geometrical brightness of 25 and the latter one of 16.

Happy bird watching,
Arthur Pinewood :scribe:

iporali · Sep 29, 2004

Pinewood said:
...
Relative geometrical brightness has some use. A 5mm exit pupil is not 25% brighter than a 4mm exit pupil, but closer to 50% brighter. The former has a geometrical brightness of 25 and the latter one of 16.

Happy bird watching,
Arthur Pinewood :scribe:

That is true - it tells the relative difference of the exit pupil area. But why not rather use the real values (pi*r^2) - and call it simply "the area of the exit pupil"?

Ilkka

Pinewood · Sep 29, 2004

iporali said:
That is true - it tells the relative difference of the exit pupil area. But why not rather use the real values (pi*r^2) - and call it simply "the area of the exit pupil"?

Ilkka

Dear Ikka,
[pi times r squared] works nicely, but relative geometric brightness may suit those who have difficulty with maths.
An interesting discussion of binocular brightness, for astronomy, including twilight factor, may be found at

http://w1.411.telia.com/~u41105032/visual/limiting.htm

Happy bird watching,
Arthur Pinewood

iporali · Sep 29, 2004

Pinewood said:
Dear Ikka,
[pi times r squared] works nicely, but relative geometric brightness may suit those who have difficulty with maths.
An interesting discussion of binocular brightness, for astronomy, including twilight factor, may be found at

http://w1.411.telia.com/~u41105032/visual/limiting.htm

Happy bird watching,
Arthur Pinewood

Thanks Arthur - an interesting link indeed. I guess I have to specify my words about the twilight factor being useless - it *may* help determine the performance of a binocular in astro-use in conditions when the iris is completely dilated. I still have my reservations in daylight viewing, but surely I cannot deny the practical observations made by several astronomers and Carl Zeiss

.

Ilkka

Pinewood · Oct 3, 2004

iporali said:
Thanks Arthur - an interesting link indeed. I guess I have to specify my words about the twilight factor being useless - it *may* help determine the performance of a binocular in astro-use in conditions when the iris is completely dilated. I still have my reservations in daylight viewing, but surely I cannot deny the practical observations made by several astronomers and Carl Zeiss .

Ilkka

Dear Ilkka,

I apoligise for not writing your name correctly.

I have taken up an interest in bird watching to extend the use of my binoculars, which I had employed primarily for astronomy and for looking at river traffic near my home. The concerns of bird watchers and stargazers are not the same.

Happy nature observing,
Arthur Pinewood

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