# Thread: Eyepiece focal length

1. ## Eyepiece focal length

is there a way to measure the focal length of an eyepiece directly? I have an eyepiece that is not labelled. I think it is either a 25mm or a 32mm, not sure which.

2. Hi Greg,
I have never dealt with it in practice but it's what I would do:
I would take a tungsten bulb (the old type one, or the car one) and use the EP to project the image of the spiral onto a piece of paper. the distance from the lens to the paper would be the focus distance of the EP.
Cheers,
Michael

3. Hi Greg,
I have never dealt with it in practice but it's what I would do:
I would take a tungsten bulb (the old type one, or the car one) and use the EP to project the image of the spiral onto a piece of paper. the distance from the lens to the paper would be the focus distance of the EP.
Cheers,
Michael

4. Greg, try this:

Put the eyepiece (EP) on your scope and focus on a distant day time object. Now step back a little from the EP, you will see a bright circle at the middle of the EP eye lens, this is called the "exit pupil." Measure that in millimeter, like I tried it out with an EP and the exit pupil is 3.5mm.

Next, measure the diameter of your telescope's objective lens/mirror (if you know the specs from the manufacturer, better) in mm's. Like the small refractor I used for measuring the exit pupil above is 66mm.

Divide the objective diameter by the exit pupil, 66/3.5 = 18.8 which is the magnification.

Next, if you know the focal length of the telescope you're using, divide this by the magnification above, like for my scope, its focal length is 388mm/18.8 = 20.6mm which is the derived focal length of the eyepiece.

The eyepiece in this example is a Telvue 20mm plossl so the result is quite near. Hope this helps.

Best,

Hernando
Last edited by ibase; 09-22-2010 at 05:08 AM.

5. ## The Following 2 Users Say Thank You to ibase For This Useful Post:

greg1234 (09-22-2010),Miciurin (09-24-2018)

6. Originally Posted by ibase
Greg, try this:

Put the eyepiece (EP) on your scope and focus on a distant day time object. Now step back a little from the EP, you will see a bright circle at the middle of the EP eye lens, this is called the "exit pupil." Measure that in millimeter, like I tried it out with an EP and the exit pupil is 3.5mm.

Next, measure the diameter of your telescope's objective lens/mirror (if you know the specs from the manufacturer, better) in mm's. Like the small refractor I used for measuring the exit pupil above is 66mm.

Divide the objective diameter by the exit pupil, 66/3.5 = 18.8 which is the magnification.

Next, if you know the focal length of the telescope you're using, divide this by the magnification above, like for my scope, its focal length is 388mm/18.8 = 20.6mm which is the derived focal length of the eyepiece.

The eyepiece in this example is a Telvue 20mm plossl so the result is quite near. Hope this helps.

Best,

Hernando
Hey Hernando..i just tried this. It worked like a charm!! First i tested it with my 12mm Meade MA. The exit pupil measured approx. 1.7mm, which made the magnification 90, which estimated the eyepiece FL at 13.3 (pretty close to 12).

Then i measured the UNKNOWN eyepiece exit pupil at 3.0mm. This put the magnification at 50, which estimated the eyepiece at 24. So, it must be a 24 or 25...definately NOT a 32.

Thanks

7. ## The Following User Says Thank You to greg1234 For This Useful Post:

ibase (09-22-2010)

8. Interesting bit of information here.
Thanks guys..

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