Eyepiece measurements

I've tried quite a few eyepieces since I bought my first real telescope, always in the hope of wide fields, bright images, a wide range of magnifications and little cost. I sell on those which I no longer have a use for, so I don't currently own all the EPs listed below.

I measure focal length by projecting a grating through the eyepiece. Note that I am not an optics expert, nor do I possess proper laboratory equipment. These measurements are not guaranteed to be accurate, they are merely my best attempts. I would welcome advice on how to improve my methods.

The "field stop diameter" of an eyepiece is the best measure of how much sky you will be able to see in a given telescope. This true field of view is calculated from the field stop diameter and the telescope focal length as follows:
tfov = (fsd / fl) x 180 / π

Some eyepiece manufacturers state the field stop diameter, although it may be hidden away on an obscure part of their web site. On some eyepieces the field stop is a narrowing of the barrel and can be directly measured. In other cases it can only be estimated from the stated apparent field of view and focal length.

The "focus position" is a measure of how parfocal the eyepieces are. I measure it using a small refractor. Each eyepiece is used to focus on a distant TV aerial, and then the focuser drawtube position is measured with a vernier caliper. The "zero" position is measured with a screen in place of the eyepiece. Positive values mean the focuser is further out. This can be "corrected" with parfocalising rings. Negative values mean the focuser is further in. This can make the eyepiece unusable with some telescopes.

Another effect of the "focus position" of an eyepiece is to vary the gain of a Barlow. A positive value means the eyepiece's focal plane is nearer the Barlow lens, which reduces its gain. A negative value increases the Barlow's gain. The effect is greatest with the popular "shorty" Barlows. Stephen Tonkin has written a good web page on Barlow gain calculation.

Eyepiece
(Thumbnails link to larger images)		Focal length (mm)Apparent field of view
(stated)		Field stop diameter (mm)Focus position (mm)
statedmeasuredstatedinferred1measured
William Optics DCL-52 camera adapter7539
50mm Rini T-thread camera adapter5046.9-48.960°52.440.534
42mm SuperView
(GSO)		4240.3-41.465°47.7-16
40mm SWAN
(William Optics)		4036.1-37.570°48.946-5
40mm SuperView Plössl T-thread camera adapter
(Teleskop Service)		4037.6-38.926.90
38mm 2" Erfle
(Surplus Shed)		3831.3-31.760°39.8
32mm GSO Plössl
(Hands On Optics)		3232.1-32.651°28.51
30mm ST80 widefield
(1rpd)		3030.6-31.880°4141.90
27mm Panoptic
(Tele Vue)		2725.6-26.768°30.532.07
25mm W70
(Antares)		2524.0-25.267°29.2-3
25mm Plössl
(Hands On Optics)		2524.2-25.152°22.73
25mm Super MA
(SkyWatcher)		2525.2-25.752°22.70
25mm MA(?)
(Celestron)		2524.8-25.20
24mm Panoptic
(Tele Vue)		2423.1-24.268°27.028.55
20mm Wide Angle
(OWL Astronomy)		2018.8-19.565°22.73
20mm Kellner
(Antares)		2020.8-21.816.11
20mm Kellner
(Skylux)		2019.4-20.115.36
18mm orthoscopic
(BC&F - Telescope House)		1817.2-18.512.611
17mm Hyperion
(Baader)		1716.0-17.268°20.920.2-4 (1¼")
20 (2")
14mm Wide Angle Nikon camera adapter
(ScopeTronix)		1413.7-14.466.2°16.2-6
13mm Hyperion
(Baader)		1368°17.715.40 (1¼")
24 (2")
12.5mm orthoscopic
(Antares)		12.512.6-13.040°9.48.9-7
12.5mm Plössl
(Antares)		12.512.3-12.910.11
10mm XW
(Pentax)		1010.1-10.370°12.412.20
10mm Super MA
(SkyWatcher)		1010.2-10.952°9.13
10mm MA(?)
(Celestron)		1010.0-11.53
9mm UltraWide
(Synta)		99.5-10.066°10.41
6mm UltraWide
(Synta)		66.0-6.766°6.91
6mm Plössl
(Eclipse Optics)		66.1-6.852°5.51
5mm Long Eye Relief
(Sky-Watcher)		54.6-5.245°3.91
4mm Kellner
(Skylux)		46.1-6.53.45

[1] The field stop diameter of an eyepiece can be estimated from its focal length and apparent field of view as follows:
fsd = fl x AFOV x π / 180