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Low light scope?

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keo View Drop Down
Optics GrassHopper
Optics GrassHopper


Joined: April/14/2010
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Post Options Post Options   Thanks (0) Thanks(0)   Quote keo Quote  Post ReplyReply Direct Link To This Post Posted: April/24/2010 at 03:37
Originally posted by Enno Enno wrote:

keo, thanks for the write up.  I am really new at this and had to read thru it a few times to make sense but finally got it.

I should have been clearer, but in case others were confused as much by that explanation, here's a little clearer example (maybe). I was recently testing an inexpensive $10 scope (ebay deal) because it's performing rather well (flat field, holds zero, no CA, 1/4mil click stops, etc.) and wanted to know more about it.

For this 3-9x40 scope the sheet included claims that the exit pupil diameter for this scope ranges from 4.4mm at 9x, and 13.3mm at 3x power.

Let's presume that your eye, in dim light, will only open up the pupil to a max of 7mm dia. (this varies by age and eye). 7mm is usually considered max. 5mm is more typical in older eyes.

(The following numbers are probably off at times by as much as 1mm because I did this hand-held, holding a ruler and scope to a clear sky and measuring its exit-pupil at various powers.)


Scope Power : Scope's Exit-Pupil Dia. (EP) : Visual Result

9x = 4.5mm EP = dimmest image but most magnification.

8x = 5mm EP = image gets brighter as magnification is reduced.

7x = 5.5mm EP = image still seeming to brighten. (unless you have older eyes that only open up to 5mm in dia.)

6x = 6.5mm EP = image still brighter for someone with a 7mm dia. entrance-pupil eye.

5x = 8mm EP = ooops, the scope's exit-pupil is now larger than your own eye's 7mm entrance-pupil diameter. The image no longer is brighter than at about 6x power because that extra light is being dumped outside of the entrance-pupil diameter of your eye.

4x = 10mm EP = no increase in apparent brightness.

3x = 13mm EP = no increase in apparent brightness.


btw: If you want to test your own pupil's diameter, you can make an easy test device for this. Take a strip of thick paper. Use a thin pin to make a series of holes, 2 at a time, spaced apart in millimeter increments. (Or sub-millimeter increments for the really curious.)

Example (periods are pinholes):

.. 2mm

. . 3mm

.   . 4mm

.     . 5mm
etc.

The holes will take on the shape of a long V, close together at one end, and widening at the other. Now in whatever light level you happen to be, hold the paper with the holes close to the eye being tested. Look through those sets of holes at something distant (cover the other eye). Whichever two holes appear to be just touching together that is the diameter of your pupil. If the two little circles of view overlap then that's smaller than your pupil diameter. If two holes are not touching (a space between them), then that's larger than your pupil diameter. Its how we astronomers would measure this for dark-sky observations and matching telescope eyepieces to eyes for optimum performance.

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