Re: film & digital slr lenses revisited

Shiva Das wrote:
In article <gdiun1$b11$1@xxxxxxxx>,
"\(used to be\) Fat Sam" <samandjanet@xxxxxxxxxxxxxxxxxxxxx> wrote:

"Blinky the Shark" <no.spam@xxxxxxxxxxx> wrote in message
news:pan.2008.10.20.20.27.59.545725@xxxxxxxxxxxxxxxxxxxxxxxxx
(used to be) Fat Sam wrote:

Hi, sorry about this but I have another question that's bugging me.
I'm looking at some lenses on Ebay, Minolta fit for a sony A200.
I know that the sony A200 has a crop factor of 1.5, so a 300mm
lens for a film slr would effectively perform as a 450mm.

So far so good.

This would be wonderfull for what I'm wanting it for, but I have a
problem.
How can I tell from the listings whether a 300mm lens is 300mm in
film terms
or 300mm in digital terms?

You can't. Unless the manufacturer specifies it as a "digital-only"
lens, and that has nothing to do with focal length.


300mm is 300mm.

So am I overcomplicating this, and just confusing myself?
It wouldn't be the first time I've done that to be honest.

Yes


I was told that if I bought a DLSR lens that was labelled up as
300mm, and attached it to my DSLR, it would perform as a 300mm lens.

It will, but not the way your soon-to-be-ex-friend explained it.

But if I bought an older film SLR lens that was labelled up as a
300mm, and fitted it to a DSLR, it would effectively perform as a
450mm given that the DSLR sensor has a 1.5 crop factor.

Yes, but _any_ 300mm lens will act this way.

I know that 300mm is 300mm regardless of what they're being used on,
but my understanding was that the smaller sensor makes the lens
behave as if it was longer because its length is greater in relation
to the size of the imaging plane.

Only because it is cropping from the potential area the image would be
if it were used on a 35mm film camera. Different angle-of-view.


From this, I sort of assumed that there is a physical difference in
the way an older film SLR lens and a DSLR lens is made.
Or is the difference just in the way they're labelled?

Neither.


So assuming there is a difference between film SLR and DSLR lenses,
either in the way they're made or the way they're labelled, is there
an easy way to tell if it's been designed with film or digital in
mind?


There is no difference except that lenses designed specifically for
digital aps-c sensors probably will vignette on a full-frame or 35mm
film camera.


Or is all that I've just said nothing more than a load of confused
nonsense from some deranged Irishman who really needs to spend some
time reading Wikipedia?

Give yourself some credit, Sam, after all perhaps you had a few pints
at the local before trying to wrap your mind around Newtonian optics!

Sadly, I didn't have that excuse.
It just sounded like I was drunk ;-)

A lens is labeled with its actual focal length. period. the end. full
stop.

The differences have to do with coverage, which is to say that the
circle of light projected by the lens to the film/sensor has to cover
all four corners.

Here's an example: I have a 40mm lens for a 35mm camera. It cost about
$100.00, is physically very small and light, and has a "normal" angle
of view.

I also have a 40mm lens for a 6x6cm camera. It cost about $5,000.00,
is physically very big and heavy, and has an "ultra-wide" angle of
view.

Both of these lenses are identical in focal length. Were I to mount
the small lens on my 6x6, the film would show an image within a
circle of approximately 36mm diameter, surrounded by black.

Were I to somehow mount the big lens on the 35mm body, I would not be
able to tell the difference between images produced by either lens.

Don't worry. Be happy. All 300mm lenses are 300mm focal length. What
people are talking about when they say "makes the lens behave as if it
was longer" is that there is a factor of 1.5 applied to the
angle-of-view due to the smaller sensor. Unfortunately most people
apply that to the focal length instead, using it as a shorthand for
how wide or narrow the image will appear.

Google "angle of view", "image circle", and "focal length" and look at
lots of pictures and diagrams. Visual presentation is better than
verbiage for this stuff.

Here is Ernst Wildi's definition of focal length:

"Distance from the principal plane in the lens to the point at which
the lens forms an image of a subject at infinity. The focal length is
always engraved on the lens. The focal length of a lens is the same
no matter where or how used -- and regardless of what image format it
is to cover."


I think I'm starting to get it.
Give me time. It will sink in eventually. Thanks for being patient with me.


.

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