Re: Digital Lenses and Andy's comments

Howard <howard.leigh@xxxxxxxxxxxx> · Tue, 24 Aug 2004 12:29:34 +0100

Thanks for your reply (I've cut some of it out here) but it's always 
good to read another person's reasons - makes it easier to accept or 
refute as the case may be.

ADavidhazy wrote:

Howard,

Besides the diagrams there is a statement on that page that is also 
misleading
....
Typically, a lens designed for 35mm film would gain by a factor of 1.5X, a
change that could have a dramatic impact for photographs that necessitate a
wide angle."

Sorry. The focal length of a lens does not change by being attached to a 
camera

that has a smaller or larger format image area.
I agree, though for some people that's surely a much easier concept to 
"understand". In terms of PR, if an ad said the smaller sensor cuts off 
a lot of what the lens sees a less thoughtful purchaser could well be 
put off!

And in practical terms I think the digital image seen by that lens is 
probably better quality than a cropped enlargement to the same size in 
the darkroom.
Now as for the lens diagrams ... they are extermely simplified to the point

that they would lead one to believe that light rays leave the rear 
element of  the D lenses in parallel fashion. If light did indeed leave the lens in 
such a way an image would not be formed

There are always two cones of rays at work. One emanating from every point in the

subject and filling the "entrance pupil" (simplify to diaphragm) and the 
other focused at the corresponing location in the image plane emanating from the

"exit pupil" (simplify to diaphragm) of the lens.
Meaning - rays from a point source of light must hit the image sensor 
(film or otherwise) as a point to produce a point image as in basic 
optics experiments at school?
It is just that the illustrations are oversimplified and they give the "wrong impression". They are also 
designed to overemphasize the defects that might become evident if a non "digital" 
lens is used on a camera equipped with a CCD sensor. Why? PR imho.  
Which is my suspicion - I got excellent results from a Nikon 28-80, a 
Sigma 28-135 and a Nikon 50 mm.
Anyway, I am no optical designer nor an optics teacher or a 
manufacturer. 
Join the clan! But my students often ask me for my advice / opinion 
which is another reason why I like this forum so much.
It is just that advertising hype bugs me when not clearly supported by facts. 
I agree.
If I have offended anyone for expressing my opinion I apologize. I guess I could
keep quiet well, arrivederci for now.
andy

I'm glad you didn't! Thanks again.
Howard