Bushnell 7x26 lens system (1 Viewer)

I just cleaned up another Custom 7x26 pair, and they came out
super-clean. It helps to know the way in already. I think I got a deal
due to some paint-rubbing and especially the missing medallion,
but they are unmistakable. Speculators want something set-to-flip,
I think. It had one issue, the right stand pipe unthreaded and blocking
proper focus far away, but that's easy to fix.

Anyway, the oculars are a straightforward Plossl design, no fifth element,
but there is an extra lens at the bottom of the iris/tunnel up front,
just before the prisms. It's concave. Sort of a 'mild-barlow'? not sure.
Like a field lens way up front. The field sharpness is way better than
it should be at the edges, for just a Plossl (ocular with no field lens),
so I was wondering if there are other binoculars with that extra mid-lens.
It's not a focuser: the front lenses still move out and back like other small
Customs. I just hadn't seen that before.

That's very interesting about the Plössl lens design in the Custom 7x26. If that's true, then you could estimate the focal length of the eyepieces from the eye relief. A Plössl's eye relief is about 80% of its focal length (http://www.cloudynights.com/page/articles/cat/articles/how-to/eyepiece-review-pitfalls-r361), and Bushnell lists the eye relief at 16mm, so a 20mm focal length is a good guess. That means the binocular needs a 140 mm focal length to achieve 7 power. You can measure the focal length of the objective lenses easily if they are removed; just focus a far away object onto a piece of paper or other surface, and the distance from objective to paper is your focal length. Anything much less than 140mm, and I suppose that concave piece of glass must be a Barlow.

About those eyepieces: were they 2 elements per group, or 3 and 1? I would have guessed an Ortho or Kellner design just because of the amount of field curvature I see in my Customs, but maybe that curvature comes from the objectives?

It's a bit tricky measuring to the effective centerline of the eyepiece.
The outer face of the eyepiece will always make the measurement short.
Using an 'apparent size' method with a telescope, I did come up with 20mm, though.
So...you've got to be very close.

16mm relief is actually very generous for that objective size.
The Barlow seems like a clever way to get that, though it does cost in field.
The eye relief works for my smaller glasses.

There are 2 groups of two elements in this unit, a classic Plossl. Other Customs have
3 groups (a non-achromatic field lens is added)..and the 7x35 Customs have more variations.

I've been told that 'pincushion' and 'curvature' are two different things, although there is a lot
of ambiguous use out there. I think we are both talking about the same thing, though: there
is a pincushion shaping, but the sharpness covers almost the whole field. The curving
of the field might be from the objectives, or more properly, the fact that they have a short focal
length and the Barlow is gathering that. It's a smoother curving than I would expect from the edge of
a Plossl, and the Plossl isn't really challenged for the Customs' fov.

The focuser lens these days for most expensive roofs is a potent negative lens, so I suspect they are getting
a Barlow-like benefit for their eyepieces and bigger eye relief numbers in a similar way.

Yes, the field curvature I was referring to is the fact that even though the whole field of view is sharp, you have to refocus between center of field and edge of field.

Is it the newer Customs that have the 5-element eyepieces, or the older models?

One of the old models I have does...
I don't know what's in the current Customs.
With the current coatings, more elements are possible without ghosting,
so hopefully they took advantage of that.