Parfocal Formula ?

I was out observing last night with some others and had a friends 31mm Nagler in my scope. After giving it back I popped in a meade 26mm plossl (with adapter) and was suprised to see the image was still perfectly focused.

Is there some formula I can use to calculate if eyepieces of different focal lenghts produced by different manufacturers are parfocal, or close to it ?

Also is it parfocal or parafocal ? I see both used freely on the net.

Thanks ... Fred

There is no "formula" as the location of the focal plane is determined not only by focal length and eyepiece type/construction:
E.g. take they very same eyepiece and mount it in a longer case, so that all lenses are 1/2" further away from the EP barrel and its no more parfocal with its unmodified sibling ...

For you that means: Every eyepiece requires a different focuser travel, which can be measured for every EP (its called the focal plane location), but I can't be predicted from other EP data.
Once you have this location, any two EP with the same focal plane location are parfocal. IMO only Televue specified this number for their EP.

Michael

The correct spelling is parfocal.

I've always believed that to make eyepieces parfocal, the distance from the aperture stop in the eyepiece to the mounting flange must be the same.

The mounting flange is the shoulder or step that keeps the eyepiece from sliding farther into the eyepiece holder. Where the shiny 1 1/2" or 2" diameter part stops.

The aperture stop sets the apparent field of view, so it's location must coincide with the image plane formed by the telescope.

The aperture stop is a metal disc usually found in front of the lenses (i.e. farthest away from the eye), so it'd be easy enough to measure the distance between this and the flange on two parfocal eyepieces and see if they are the same.

-Jason