In article , Paul Farrar <farrar@xxxxxxxxxxxx> wrote
>>
>> 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
>
Just to aid in the understanding of this - although most people refer to
any lens that has a long focal length as a telephoto, the correct
definition is a lens which is shorter than its focal length. So when
Paul talks about a reverse telephoto lens he is describing a lens which
is longer than its focal length.
Just in case any beginners out there think a reverse telephoto is a wide
angle lens! ;-)
--
Kennedy
Yes, Socrates himself is particularly missed;
A lovely little thinker, but a bugger when he's pissed.
Python Philosophers (replace 'nospam' with 'kennedym' when replying)
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