ES82 18mm question.

[Dave scutt]

Hi all.

I just received my ES82 18mm ep , and was wondering does any one else's ep look the same .

There's seems to be something covering the glass and not on the others taken in the picture. 

Dave.

[Louis D]

If it's the middle one, it looks just like the image below, minus the eye cup:

[Dave scutt]

Yes it's the middle one minus the eye cup.

How come it's different, from the rest .

[Don Pensack]

Eye relief.

Given they all have 82° apparent fields, the diameter of the eye lens determines the maximum eye relief obtainable from the eyepiece.

It has less eye relief than the other two.

Eye relief (maximum) = 0.5 eyelens diameter / Tan 0.5 Apparent field angle.

As you can see from the formula, the larger the eye lens diameter, the longer the eye relief.

 

[Louis D]

3 hours ago, Dave scutt said:

How come it's different, from the rest .

Either it's a scaled down version of a longer focal length 82 degree eyepiece (think 16mm NT5 vs 31mm NT5), and thus everything but the AFOV gets smaller, ER included; or it's a different design achieving the same AFOV, but with a smaller ER.  As Don says above, a smaller eye lens at constant AFOV leads to less ER.

Why didn't ES choose to maintain constant ER and AFOV throughout the 82 range?  Only they would know.  I'm guessing it was to keep the line more compact and affordable.  Look at how big the ES-92 line is with constant 92 degree AFOV and 17mm usable ER.