This DOF thang

David J. Littleboy wrote:


"Stacey" wrote in message
...
Severi Salminen wrote:

Most pictures taken with wide angle
lenses DO have different perspective than pictures taken with long
lenses. Still one should understand that the reason is not the focal
length but the practical need to usually alter the shooting position
when changing focal length.

What helped me figure this out was doing macro work. When you start
working
at high magnification of small objects, all this "Short lenses have more
DOF" etc quickly are seen for what they are, untrue.

"Short lenses have more DOF" is quite true for normal photography _at the
same f stop and same subject distance_. When you do macros, you don't keep
the subject distance the same. (And the definition of "subject distance"
becomes problematical at close distances.)


Eaxctly, you aren't limited by infinity so it's easy to understand what's
really going on. Plus it's easier to move 6 inches rather than 60 yards!
:-)


(I seem to recall discussion to the effect that DOF stays the same for the
same magnification. So if you switch from a 110mm lens to a 55mm lens at
the same subject distance, you get four times the DOF at the subject
location, but if you halve the subject distance for the 55mm shot, the DOF
should remain the same. I think. Corrections welcome for this _same
format_ comparison.)


You're exactly right. The lens focal length makes no difference or else
everyone would want short length macro lenses. If you fill the frame with
the subject, the focal length doesn't change the DOF, just the subject to
camera distance. That's why a short tele makes for a good macro lens, you
get some room for the lighting. 120mm on medformat works great.

--

Stacey

"Gregory W Blank" wrote in message

Vladamir30 wrote:

But then that's the whole idea behind telephoto lenses,....is it not?
To flatten and compress distant subjects? Rather than making expansive
wide angle images.

"Telephoto lenses" (by which I assume you mean a long focal length lens,
not a true telephoto lens) don't compress and flatten distant subjects and
wide angle lenses (i.e. short focal length lenses) don't make "expansive"
images. The compression and "expansiveness" you observe when using these
lenses is a result of your camera position

And the magnification/FOV they provide. In landscape use they do compress
the image and this become confusing because infinity isn't something that
can be changed i.e. moved closer or further from the camera.. It can be
magnified with a lens change which does end up compressing the scene.



--

Stacey

"Gregory W Blank" wrote in message

Vladamir30 wrote:

But then that's the whole idea behind telephoto lenses,....is it not?
To flatten and compress distant subjects? Rather than making expansive
wide angle images.

"Telephoto lenses" (by which I assume you mean a long focal length lens,
not a true telephoto lens) don't compress and flatten distant subjects and
wide angle lenses (i.e. short focal length lenses) don't make "expansive"
images. The compression and "expansiveness" you observe when using these
lenses is a result of your camera position

And the magnification/FOV they provide. In landscape use they do compress
the image and this become confusing because infinity isn't something that
can be changed i.e. moved closer or further from the camera.. It can be
magnified with a lens change which does end up compressing the scene.



--

Stacey

Stacey wrote:

"Gregory W Blank" wrote in message

Vladamir30 wrote:

But then that's the whole idea behind telephoto lenses,....is it not?
To flatten and compress distant subjects? Rather than making expansive
wide angle images.

"Telephoto lenses" (by which I assume you mean a long focal length lens,
not a true telephoto lens) don't compress and flatten distant subjects and
wide angle lenses (i.e. short focal length lenses) don't make "expansive"
images. The compression and "expansiveness" you observe when using these
lenses is a result of your camera position

And the magnification/FOV they provide. In landscape use they do compress
the image and this become confusing because infinity isn't something that
can be changed i.e. moved closer or further from the camera.. It can be
magnified with a lens change which does end up compressing the scene.

When discussing about this "compression" you should define whether you
mean it in engineering or artistic sense. For an engineer, no such thing
happens as a funcition of focal length. You can get the same effect by
enlarging a part of a wide angle shot. The quality will be poorer,
because there is more magnification, though.

For an artist the effect is real, but the artist is composing in real
time through the viewfinder, and then the wide/tele difference can be
seen easily. But it's just an artistic impression.

-- Lassi

Stacey wrote:

"Gregory W Blank" wrote in message

Vladamir30 wrote:

But then that's the whole idea behind telephoto lenses,....is it not?
To flatten and compress distant subjects? Rather than making expansive
wide angle images.

"Telephoto lenses" (by which I assume you mean a long focal length lens,
not a true telephoto lens) don't compress and flatten distant subjects and
wide angle lenses (i.e. short focal length lenses) don't make "expansive"
images. The compression and "expansiveness" you observe when using these
lenses is a result of your camera position

And the magnification/FOV they provide. In landscape use they do compress
the image and this become confusing because infinity isn't something that
can be changed i.e. moved closer or further from the camera.. It can be
magnified with a lens change which does end up compressing the scene.

When discussing about this "compression" you should define whether you
mean it in engineering or artistic sense. For an engineer, no such thing
happens as a funcition of focal length. You can get the same effect by
enlarging a part of a wide angle shot. The quality will be poorer,
because there is more magnification, though.

For an artist the effect is real, but the artist is composing in real
time through the viewfinder, and then the wide/tele difference can be
seen easily. But it's just an artistic impression.

-- Lassi

"Stacey" wrote in message
...

I never said it wasn't. John was the one who said at the -same F/stop- the
DOF was the same.

Now let's put this back into context. That was not exactly what I said. I
made particular requisites concerning print size (and by inference, same
COC). Apparently I was wrong, but in a more complex proposition that simply
the above.

"Stacey" wrote in message
...

I never said it wasn't. John was the one who said at the -same F/stop- the
DOF was the same.

Now let's put this back into context. That was not exactly what I said. I
made particular requisites concerning print size (and by inference, same
COC). Apparently I was wrong, but in a more complex proposition that simply
the above.

jjs wrote:

"Stacey" wrote in message
...

I never said it wasn't. John was the one who said at the -same F/stop-
the DOF was the same.

Now let's put this back into context. That was not exactly what I said. I
made particular requisites concerning print size (and by inference, same
COC). Apparently I was wrong, but in a more complex proposition that
simply the above.


And also more complex than Brain is making it. I think you are closer to
the truth in practice than he is.
--

Stacey

jjs wrote:

"Stacey" wrote in message
...

I never said it wasn't. John was the one who said at the -same F/stop-
the DOF was the same.

Now let's put this back into context. That was not exactly what I said. I
made particular requisites concerning print size (and by inference, same
COC). Apparently I was wrong, but in a more complex proposition that
simply the above.


And also more complex than Brain is making it. I think you are closer to
the truth in practice than he is.
--

Stacey

Lassi Hippeläinen wrote:

Stacey wrote:

"Gregory W Blank" wrote in message

Vladamir30 wrote:

But then that's the whole idea behind telephoto lenses,....is it not?
To flatten and compress distant subjects? Rather than making expansive
wide angle images.

"Telephoto lenses" (by which I assume you mean a long focal length
lens, not a true telephoto lens) don't compress and flatten distant
subjects and wide angle lenses (i.e. short focal length lenses) don't
make "expansive" images. The compression and "expansiveness" you
observe when using these lenses is a result of your camera position

And the magnification/FOV they provide. In landscape use they do compress
the image and this become confusing because infinity isn't something that
can be changed i.e. moved closer or further from the camera.. It can be
magnified with a lens change which does end up compressing the scene.

When discussing about this "compression" you should define whether you
mean it in engineering or artistic sense. For an engineer, no such thing
happens as a funcition of focal length. You can get the same effect by
enlarging a part of a wide angle shot. The quality will be poorer,
because there is more magnification, though.

For an artist the effect is real, but the artist is composing in real
time through the viewfinder, and then the wide/tele difference can be
seen easily. But it's just an artistic impression.



Yep what he said.. This is a subject that can be WAY over analized and it's
better for someone to just shoot some film and figure out how to use DOF to
their advantage from actual use.
--

Stacey