One Day...
AH elite
Twilight Factor (???)
I do not know the origin of the "twilight factor" measurement. It is said to be a measure of how much detail can be seen in low light. The formula to calculate it is: square root of magnification x objective diameter in mm. For example the great Swarovski Z8 1.7-13.3 x 42 scope has a "twilight factor" of 23.63 at 13.3x (√ 13.3 x 42), 15.87 at 6x (√ 6 x 42) and 8.44 at 1.5x (√ 1.5 x 42).
I fully understand that magnification is needed to see details, but I personally do not think that twilight factor is a really useful measure. For example in the above example for the exceptional Z8 1.7-13.3 x 42 scope, the "twilight factor" leads you to expect that you will see more detail at dusk and dawn with the scope set on 13.3x ("twilight factor" of ~24) than you will if you set it at 6x ("twilight factor" of ~16).
I personally think that it is much more realistic to establish the fact that you want a 7 mm exit pupil light beam at dusk and dawn and a 3 mm exit pupil light beam in full daylight, and that if you need more magnification, you simply need a larger objective, period. For examples:
In the end, this is why really powerful magnification binoculars (and scopes) MUST have huge objectives...
For example... when guiding chamois hunters in France once upon a time, I used to carry Steiner 20 x 80 binocs. Very unwieldy maybe, but they provided BOTH enough magnification (20x) and enough light in full day light (4 mm exit pupil light beam) to identify a grand old Class III Chamois trophy (Hint: the penalty for killing erroneously a Class II adult Chamois when in possession of a coveted Class III tag was forfeiting of trophy, heavy 4 figure $x,xxx fine, and 3 year suspension of hunting license... Ouch if as a guide you say "shoot"...). These mammoth Steiner 20 x 80 binocs worked very well during the day but they were useless at dusk or dawn. I therefore ALSO carried in the backpack a pair of Zeiss 8x56 BGA...
In full daylight the 20x "very good" glass Steiner showed things that the 8x "exceptional" glass Zeiss simply could not show, even though the quality of the Zeiss glass far exceeded the quality of the Steiner glass. But even though the Steiner 20 x 80 had a twilight factor of 40 (√ 20 x 80), which was twice that of the Zeiss 8 x 56 twilight factor of 21 (√ 8 x 56), I guarantee you that the 7 mm exit pupil light beam of the 8 x 56 made them great at dawn and dusk, while the 4 mm exit pupil light beam of the Steiner made them all but unusable at dawn and dusk...
The only way to glass comfortably for hours, hunting Class III grand old Chamois in the French Alps with mammoth 20 x 80 binocs...
Relative Brightness (???)
Another concept that floated at a time was "relative brightness" to measure how bright a scope or binoculars image is. Same story, I do not know what is the origin of "relative brightness" but the formula for it is to square the exit pupil. So, to continue the same example as above, the Steiner 20 x 80 have a "relative brightness" of 16 (80 / 20 = 4 then 4 x 4 = 16) and the Zeiss 8 x 56 have a "relative brightness" of 49 (56 / 8 = 7 then 7 x 7 = 49). I personally never grasped what comparing a "relative brightness" of 16 to 49 would indicate that comparing an exit pupil (i.e. light beam diameter) of 4 to 7 would not convey...
This is likely a bit too long of a post (again!) but these are difficult concepts to explain in 3 sentences. I hope this was of interest
I do not know the origin of the "twilight factor" measurement. It is said to be a measure of how much detail can be seen in low light. The formula to calculate it is: square root of magnification x objective diameter in mm. For example the great Swarovski Z8 1.7-13.3 x 42 scope has a "twilight factor" of 23.63 at 13.3x (√ 13.3 x 42), 15.87 at 6x (√ 6 x 42) and 8.44 at 1.5x (√ 1.5 x 42).
I fully understand that magnification is needed to see details, but I personally do not think that twilight factor is a really useful measure. For example in the above example for the exceptional Z8 1.7-13.3 x 42 scope, the "twilight factor" leads you to expect that you will see more detail at dusk and dawn with the scope set on 13.3x ("twilight factor" of ~24) than you will if you set it at 6x ("twilight factor" of ~16).
If you do this, the amount of light that reaches your 7 mm fully dilated pupil at dusk or dawn with the scope set at 13.3x will be a 3.15 mm exit pupil light beam (42 mm objective / 13.3 magnification = 3.15 mm exit pupil light beam). My personal experience is that with such a narrow exit pupil light beam, I personally could not care what magnification is involved because the light beam does not bring to my eye even half of the light it needs to see anything other than a black blob that could be, to use CoElkHunter's earlier very good example, a bush (?), a cow (?), or a 6x6 elk (?) ...I personally think that it is much more realistic to establish the fact that you want a 7 mm exit pupil light beam at dusk and dawn and a 3 mm exit pupil light beam in full daylight, and that if you need more magnification, you simply need a larger objective, period. For examples:
If 6x magnification with a 42 mm objective is not enough at dusk and dawn, which I totally agree with to see fine details (e.g. elk tines), then 8x magnification with 56 mm will certainly provide more details because it provides additional magnification while still providing a 7 mm beam.
Similarly, if 10x magnification with a 40 mm objective is not enough in full daylight, which I totally agree with to see very fine details (e.g. mountain goat trophy characteristics), then 20x magnification with a 80 mm objective will certainly provide more details because it provides twice the magnification while still providing a 4 mm beam.
In the end, this is why really powerful magnification binoculars (and scopes) MUST have huge objectives...
For example... when guiding chamois hunters in France once upon a time, I used to carry Steiner 20 x 80 binocs. Very unwieldy maybe, but they provided BOTH enough magnification (20x) and enough light in full day light (4 mm exit pupil light beam) to identify a grand old Class III Chamois trophy (Hint: the penalty for killing erroneously a Class II adult Chamois when in possession of a coveted Class III tag was forfeiting of trophy, heavy 4 figure $x,xxx fine, and 3 year suspension of hunting license... Ouch if as a guide you say "shoot"...). These mammoth Steiner 20 x 80 binocs worked very well during the day but they were useless at dusk or dawn. I therefore ALSO carried in the backpack a pair of Zeiss 8x56 BGA...
In full daylight the 20x "very good" glass Steiner showed things that the 8x "exceptional" glass Zeiss simply could not show, even though the quality of the Zeiss glass far exceeded the quality of the Steiner glass. But even though the Steiner 20 x 80 had a twilight factor of 40 (√ 20 x 80), which was twice that of the Zeiss 8 x 56 twilight factor of 21 (√ 8 x 56), I guarantee you that the 7 mm exit pupil light beam of the 8 x 56 made them great at dawn and dusk, while the 4 mm exit pupil light beam of the Steiner made them all but unusable at dawn and dusk...
The only way to glass comfortably for hours, hunting Class III grand old Chamois in the French Alps with mammoth 20 x 80 binocs...
Relative Brightness (???)
Another concept that floated at a time was "relative brightness" to measure how bright a scope or binoculars image is. Same story, I do not know what is the origin of "relative brightness" but the formula for it is to square the exit pupil. So, to continue the same example as above, the Steiner 20 x 80 have a "relative brightness" of 16 (80 / 20 = 4 then 4 x 4 = 16) and the Zeiss 8 x 56 have a "relative brightness" of 49 (56 / 8 = 7 then 7 x 7 = 49). I personally never grasped what comparing a "relative brightness" of 16 to 49 would indicate that comparing an exit pupil (i.e. light beam diameter) of 4 to 7 would not convey...
This is likely a bit too long of a post (again!) but these are difficult concepts to explain in 3 sentences. I hope this was of interest
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