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ED/HDF Glass affecting RBI/Twilight factor

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Old Saturday 8th April 2006, 12:50   #1
cassowary
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ED/HDF Glass affecting RBI/Twilight factor

Does anyone know if ED glass affects either of these parameters? perhaps important if you are looking at travel scopes?


For background I copied the info below

RELATIVE BRIGHTNESS INDEX (RBI)
Relative brightness index or RBI is used as a comparison of image brightness but it does have severe limitations as discussed below. It is determined by square root of the exit pupil. An RBI of 25 or more is considered useful in low light conditions. In bright light conditions an RBI of 9 would have no brightness advantage over one of 50.

RBI = Diameter / Power = exit pupil x square root.
Example : 7x50 binocular 50/7 = 7.14 x 7.14 = 50.98 = relative brightness.


TWILIGHT FACTOR
Twilight factor is a measurement of viewing sharpness and image detail in low light conditions. The larger the twilight factor, the more efficient the low light performance. This indication has some limitations. To calculate the twilight factor, take the square root of the power multiplied by the objective lens diameter.

all input welcome mike
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Old Saturday 8th April 2006, 13:32   #2
Hermann
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Quote:
Originally Posted by cassowary
Does anyone know if ED glass affects either of these parameters? perhaps important if you are looking at travel scopes?
ED glass doesn't affect either of these parameters. In addition, both parameters are of dubious relevance to birding optics anyway.

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Old Thursday 20th April 2006, 09:55   #3
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Felt like a silly question because the parameters of calculation don't take into account the type of glass used.

However I have heard that ED/HDF glass can give a brighter image (per unit area of glass).

Perhaps the parameter I was intrested in is 'effective light transmission'. I am not sure which variables have the most significance on this performance factor. Perhaps the optical variability in brightness between scopes of the same Objective and mag is so similar it is not worth quantifying?


Perhaps this has already been discussed in another thread.

Mike
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