Thread: Scopes low light performance - link to scopecalc

In the best internet tradition of linking rather than original contribution, this site scopecalc has a good explanation of factors affecting how well a telescopic sight will perform in twilight conditions. It has an online calculator which you can put your own figures into to illustrate and compare scopes.

It explains why the capability of your own pupil to dilate can strongly affect how much benefit you'll get from a higher spec scope. Older people whose pupils can't dilate so far get less benefit from big objectives. Also why some brands like Swarovski quote smaller effective objective diameters at low power. No, its not just optical corner cutting - at powers below 5x, it really doesn't matter how big the objective is because the exit pupil is bigger than the human eye's pupil. They have a correction for "realistic light gain" which takes into account reduced resolution at higher magnification due to diffraction (bending of light around edges). I don't think "perceived brightness" is directly useful except for comfort but there is a table giving absolute illuminance values for common situations (bright sun, indoors, moonlight etc I would be interested to see what it is in NZ native bush under canopy) and relates these to the lower limit of high resolution colour vision in the human eye. Essentially, you need at least 3 - 10 lux at your pupil to see well enough to shoot. A 50mm objective can pull up full moonlight to this level at 8x but 32mm can't do it. Big objectives come into their own above 10x. Also, this shows that higher powers do give a brighter image. The idea of using low power for low light is just not right. I find I can see better in dark bush if I wind the power up and now a number says its true.

Here's a comparison of my current scope and two contenders. The site has a cursor to give you numerical values across the graphs.




Here's the brightness table.