ES 24mm in 68 or 82?

[YKSE]

10 minutes ago, cloudsweeper said:

I recently saw this too, and it does seem to work quite well.

A bit of algebra applied to it gives us:

AFOV = (FS dia x 57.3)/(FL of EP)      -  which gives it in terms of EP parameters only. 

Again, this appears to work to a reasonable accuracy.

For example, using data for the ES 68* series, we have, for the 20mm EP, an AFOV of 65.3* instead of 68*.

Doug.

Exactly.

Here's a picture I took with my over Maxvision 20mm, the Pincushion is slightly more than I see through the eyepiece, but you get the idea of streched outer area.

16 minutes ago, Peter Drew said:

Surely the 88 degrees must be the apparent FOV?, a true FOV would need a "fisheye" lens.   

Nt quite sure about the "fisheye" lens. I would like to measure to 88° AFOV, but I just couldn't reach closer to under 85°. And my drift timed TFOV reached the 19.2mm FS.

[Ben the Ignorant]

3 hours ago, John said:

To deliver an 82 degree AFoV in a 24mm eyepiece the field stop should be 33.5mm in diameter. A 37mm field stop in that focal length would give an AFOV of around 90 degrees I think. Maybe yours is an UUWA ?

No, but maybe mine is an ultra narrow ruler:

[Ben the Ignorant]

3 hours ago, John said:

To deliver an 82 degree AFoV in a 24mm eyepiece the field stop should be 33.5mm in diameter. A 37mm field stop in that focal length would give an AFOV of around 90 degrees I think. Maybe yours is an UUWA ?

No, but maybe mine is an ultra narrow ruler:

[John]

I'm not sure that is actually the field stop. I think that might be the retaining ring for the lower lens elements. The field stop for many multi-element / group designs is often within the eyepiece body and not easily accessible without taking the eyepiece to pieces.

 

[SpaceWalker]

21 hours ago, Ben the Ignorant said:

My 24/82 is a Meade.

It doesn't matter. The older 2" Meade 82*AFOV eyepieces as well as the ES82s are made by the same manufacturer, JOC, so the optics are identical.

 

15 hours ago, Ben the Ignorant said:

No, but maybe mine is an ultra narrow ruler:

It's NOT the field stop. As John has correctly noticed the FS in many modern ultra wide angle eyepieces is buried inside, so your choice would be either to trust manufacturer's info if available or to disassemble the eyepiece for measuring the FS. Although, you can figure out the real AFOV in a different way. Hold the EP at arm's length directing it toward light source like a lamp  and take a caliper into the other hand and measure the exit pupil. Than do math.

[YKSE]

17 hours ago, Peter Drew said:

Surely the 88 degrees must be the apparent FOV?, a true FOV would need a "fisheye" lens.   

Here's a picture of a view through a docter UWA's, not trace of barrel distortion(is it fisheye?), just slightly less pincushion than than many UWA or SWA eyepieces.

 

[Peter Drew]

I think my post was misunderstood , my understanding is that eyepieces have two "FOV's", apparent and actual(true). The apparent being the angle presented to the eye by the field stop and the actual (true) being the angle of the portion of sky that can be encompassed. As far as I know there is no eyepiece that can deliver a 88 degree actual (true) field of sky view hence my referral to a "fisheye" lens that could.

[YKSE]

Thanks for the clarification, Peter, appreciated

The definitions I was referring to is better explained with example, I think. Let's have a look at three 14mm EPs (assuming all the numbers are exact of course):

1. Televue Delos 14mm, FS 17.3mm, specified AFOV 72°

2. Pentax XW 14mm, FS 17.6mm,  specified AFOV 70°

3. Nikon SW 14mm, FS 17.6mm, specified FOV 72° (my measured AFOV 67°)

My understanding (can be wrong of course) is the FS decides how much sky (at the same FL) we're going to see, the different distortions in EPs determines how the features near the edge look like, but same FS will show exactly the same amount features in the FOV, depite the AFOVs are different. And this actually seen sky is what I called true field of view(TFOV).

 

This results in the conclusion that

1. That Nikon's specified 72° FOV is TFOV calculated with the formular mentioned above.

2. 14mm XW, with larger FS, will show slightly more sky than Delos, despite smaller AFOV.

3. 14mm SW will show slightly sky than Delos too, despite clearly smaller AFOV.

This is of course chat when

[jetstream]

My 23mm Luminos has a 37mm FS despite its 82 deg AFOV... which is too bad really because if they kept the FS to 33mm or so most all the edge issues people complain about would be gone.

Is it correct for me to assume that AFOV and TFOV(determined by the FS dia) are independent of each other? ie if I put a 20mm FS in the 23mm Luminos- does it keep the 82 deg AFOV?

Everyone should hear Zeiss's explanation of my zooms "virtual" field stop... but no need, I just use it lol!

[jetstream]

From Televues website

"The Field Stop and Apparent Field of View

The field stop is the metal ring inside the eyepiece barrel that limits the field size. It's projected by the eyepiece so that it appears as a circle out in space when you look through the eyepiece. The angular diameter of this circle is called the apparent field of view (AFOV) and is a fixed property for each eyepiece design. For example, Plössl eyepieces have an AFOV of 50°, DeLites have 62°, Panoptics have 68°, Delos have 72°, Naglers have 82° and Ethos eyepieces have 100° or 110°. "