Asking about eyepieces

qu1xs1lv3r · Post by **qu1xs1lv3r** » Tue Oct 25, 2005 1:02 pm

when they say long eye relief... what does this refer to??

is this different from exit pupil diameter?

or can somebody jz explain to me the parts of an eyepiece.. hehe thanks!

rlow · Post by **rlow** » Tue Oct 25, 2005 1:25 pm

Eye relief is a measure of the distance from the eyepiece lens to your eyes where you have placed your eye in the optimum distance to see the whole field of view in the eyepiece. For a good eye relief of 20mm, this means you can see the whole field of view within the eyepiece from a distance of about 20mm from the eypiece without sticking your eye on the eyepiece itself. This is very convenient to use, especially for those who view while wearing spectacles.

Exit pupil is a measure of the diameter of the cone of light which exits from the eyepiece lens. It is obtained by dividing the objective diameter of the telescope (or binocular) from the magnification of your eyepiece, eg, for a 50mm binocular with 10x magnification, then the exit pupil is 5mm (50mm/10x). Ideally, the lowest magnification to use need not give an exit pupil greater than 7mm (for youths<30 years old) or 5mm for older folks(>50years old) as the pupil in our eyes only open to a maximum of 5-7mm under dark condition and our pupil usually does not open as big as we gets older.

Post by **weixing** » Tue Oct 25, 2005 1:49 pm

Hi,

Exit pupil is a measure of the diameter of the cone of light which exits from the eyepiece lens. It is obtained by dividing the objective diameter of the telescope (or binocular) from the magnification of your eyepiece, eg, for a 50mm binocular with 10x magnification, then the exit pupil is 5mm (50mm/10x). Ideally, the lowest magnification to use need not give an exit pupil greater than 7mm (for youths<30 years old) or 5mm for older folks(>50years old) as the pupil in our eyes only open to a maximum of 5-7mm under dark condition and our pupil usually does not open as big as we gets older.

If you look at the lens of your eyepiece when inserted in a telescope or the eyepiece of a binocular when pointing at a bright light source, you'll see a bright circular image that seem to "float" above the eyepiece... that is the exit pupil. You can measure the exit pupil by using a rule on this circular image.

Have a nice day.

qu1xs1lv3r · Post by **qu1xs1lv3r** » Tue Oct 25, 2005 2:29 pm

how about the diameter where i actually look through? what is that called? im back to wearing contact lens again! woohoo!

Canopus Lim · Post by **Canopus Lim** » Tue Oct 25, 2005 3:19 pm

Diameter? It is just called the exit pupil. Also, another part of the EP is the field stop. It is located at the end of the EP where the barrel meets the lens holder. The field stop can be either a lens or a ring. This lens/ring size determines the AFOV of the EP. The bigger the field stop, the light at steeper angles (and hence wider field) can enter the EP. The true field of view can be calculated by using this field stop (you can just measure it with the ruler) divided by the Focal length of your telescope. It will be in radians so convert it to degrees by multiplying 180/pi. This I believe is more accurate than using the TFOV=AFOV/magnification; there is a slight difference in the calculation between the former and the latter.