>John Petrush wrote:
>
>> sorry, but I just can't help much on glass - I too would like to know more
>> about that. All I know for sure is when the glass is special enough to get
>> an acronym, the price of the lens in question goes **way** up.
>
>Hey Olympians! We are two now, who wants to know more about ED and UD glasses.
>Anyone got an interesting site to tell? Or any information at all?
>
>> This one I think I know, but if I'm incorrect, I'm sure the right answer
>> will come forthwith. A "normal" lens is focused by moving the elements
>> closer or farther apart by some means, generally a helicoid thread
>> mechanism. In this case, the physical distance between the rear-most
>> element and the front-most element varies. With an internal focussing lens,
>> an element, or group of elements, in between the front and rear move,
>> shifting the place of focus but the physical distance from front to rear to
>> not change.
>
>Then Foxy wrote in another thread:
>
>> I thought the normal lens just extended. Put a 300mm on a 25mm tube
>> (focussed at infinity), or wind it out 25mm, same result.
>>
>> Internal focussing move elements and can focus closer (generally), but they
>> reduce their focal length. This isn't a problem for most situations, but
>> once you focus closer and on small things the focal length reduction becomes
>> greater. Or so I'm told.
>
>I guess internal focus is a newer design than "normal lens focusing", if
>so they
>must have developed it to give lenses improvement. Now, how much better in
>performance is an IF lens compared to a "normal"?Just curious.
>
>Thanks, both!
>
>--
>Regards/
> Ingemar Uvhagen
> Gislaved, Sweden
>
>
My impression was that IF lenses arose out of the need to reduce to size
and load on the tiny motors for autofocussing cameras and lenses. It did
not have_anything_ to do with an inherent superiority of one system to
produce an excellent image at the film plane. It was an engineering
solution to the limitations of batteries and electrics. Just think about
resting the barrel of an EF lens on a support to get a sharp image and burn
out the little motor trying to push out the lens barrel. With IF the little
motor can be quite small and light, housed in the lens, and the whole lens
suddenly becomes a what some might think is a reasonable alternative to
manual focus lenses with EF. You then have room for other motors to jiggle
a new lens element in order to stabilize the image.
I think that probably I would wonder, when given the choice, do I want to
shoot outdoors with a manual version of an IF lens that was really designed
for the autofocus version of the lens with its inherent compromises to be
exceptionally light and to have extra space inside for the components that
are not there, or would I rather have the lens elements housed in an EF
mount that is brass or light alloy and is designed for strength and
durability. I think that if you look around at some of the acknowledged
best lenses where price is no object like Zeiss, Leitz, and even Oly
supertelephoto, you will find EF. If there were any great superiority to
IF, except for weight and power considerations in autofocus, there would be
no EF lenses. Most, probably not all, IF manual lenses are like a base
Chevie sedan--no power accessories and no Corvette either.
My two cents.
Winsor
Winsor Crosby
Long Beach, California
mailto:wincros@xxxxxxxxxxx
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