>
> Warren Kato wrote:
> >I just bought an XA with flash as a carry-around camera. I was looking at
> the
> >tiny lens, about 1.25 cm. in diameter, and comparing it to the front
> element
> >of the SLR Zuiko 35m f 2.8, which has a diameter of 2.9 cm.
> >
> >According to my calculations, over 5 times as much light hits the front
> >element of the SLR lens as does the XA. Where does it all go?
>
> OK, as no one else has posted an explanation, I'll have a go with my limited
> knowledge.
> In order to avoid collison with the mirror assembly a SLR wide-angle has to
> incorporate a totally different design than a view-camera as the XA. The XA
> has much of it's optics where an OM-camera has it's mirror mechanics.
> So distance to the film is much shorter (less loss of light) on the XA.
> I'm sure some other list-members can explain this in a much better way.
>
> Ulf Westerberg
Good enough. There are two ways to get a wide angle of view, a short focal
length lens or a reverse telephoto. The short FL lens has its optical
center the same distance from the film as its focal length. Look how close
the lens is to the film in the XA. It's diameter is roughly FL/f, so
a 35mm f/2.8 would be about 12mm across. This does not work for SLR
lenses under about 50mm or so, because you need about 40mm clearance for
the mirror. With a few exceptions (like the 40mm Zuiko) all lenses under
45mm are reverse telephoto. These have a negative group in front and a
positive group in the rear, sort of like a giant one of those door
peephole things in front of a normal lens. One property of this lens
type is that only part of the front element participates in forming the
image at a point on the film, but it's a different part for each point.
As a result they have a much larger front element than a short focal length
lens would.
Paul
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