This DOF thang

Vladamir30 wrote:
Most pictures taken with wide angle

lenses DO have different perspective than pictures taken with long
lenses.


Lens focal length has no effect on perspective.
[snip...]

You didn't read my post, did you?? Read it fully before rushing with the
answer, ok. Below is the quote, which you didn't obviously read:

"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."

Severi Salminen

Vladamir30 wrote:
Most pictures taken with wide angle

lenses DO have different perspective than pictures taken with long
lenses.


Lens focal length has no effect on perspective.
[snip...]

You didn't read my post, did you?? Read it fully before rushing with the
answer, ok. Below is the quote, which you didn't obviously read:

"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."

Severi Salminen

Vladamir30 wrote:

I think it actually clarifies rather than
confuses to distinguish between depth of field in the negative as defined by
the size of the circles of confusion and depth of field in the print as
defined by circles of confusion in the film plus three variables
(enlargement, viewing distance, and personal standards of sharpness).

It is hard to believe that the above way of thinking can be good. The
whole process of (mathematically) defining DOF begins from specifying an
acceptable CoC. And one can't specify that without taking the three
issues (enlargement, viewing distance, and personal standards of
sharpness) into consideration. People don't understand this issue
because they usually don't take the three things into consideration.
They just look the DOF scales on lenses and think they are some absolute
values. Guess how many people got confused when thinking how DOF on
Canon D30 (or was it 30D) differed from a 35mm film camera. Many people...

If I look at a 35mm slide from 10 meters, it will be 100% in DOF no
matter how hard I try to look: everything seems to be very sharp.

No, it won't be "100% in DOF " and everything won't "seem to be very sharp"
because your eye won't be able to resolve anything with an object that small
from that distance. ... In fact just the opposite is true, it has no depth
of field and nothing about the image in the slide is very sharp.

First I have to say that the 10m figure was just a throw without any
careful consideration. 1m might have been better - I have to try it
with a real slide first... But that is not the point and here you are
actually contradicting yourself (I'll explain later).

I didn't mean to watch the negative so far that nothing can be seen but
from distance where things don't look blurred at all. Then everything
seems to be in DOF. On the other hand if one looks the negative with a
microscope, everything seems to be blurred. Then there is alko the issue
of smalles possible CoC the lens can draw. If that is larger (after
enlargment) than the larges acceptable CoC, then the images are never
sharp. But that is another thing...

Many people would say that the DOF of the negative is allways
constant regardless of viewing distance. And that is obviously
false statement if we understand the meaning and purpose of DOF and
how it is defined.

Circles of confusion in the negative are obviously always constant in the
negative so I don't know what you mean by your statement..

CoCs are constant in the negative but not in the final print when
looking it at certain distance. And since DOF is defined by the size of
the _final_ CoCs, then there can't be any constant DOF in the negative.

It's the size of the circles of confusion in the negative and in the
print that gives rise to the whole concept of depth of field.

No, only the final print matters - or negative if that is the final
thing to look at. There is and can't be DOF until defining all the
conditions.

Everything in front of or behind that plane is out of focus and is
therefore is represented by circles in the negative and in the print. But if
the circles are small enough our eyes can't distinguish between them and
points so we say those areas are "in focus" or more accurately "appear to be
in focus" in the print even though they aren't.

You are contradicting yourself. What you write above is true and still
you disagreed with it earlier in the post. The above is exactly the same
thing that happens when looking a 35mm negative from great distance:
more and more points became smaller and harder to distinguish from each
other - negative/image becomes sharper - DOF gets larger.

The size of the circles of confusion in the negative is the starting
point for this process and is therefore critical in determining depth
of field in the print..

It is the starting point, of course, but has no meaning _on their own_
in actual perception of DOF.

Somehow I think we both know and agree how DOF is defined, we just seem
to have totally different way of coming to that definition.

Severi S.

Vladamir30 wrote:

I think it actually clarifies rather than
confuses to distinguish between depth of field in the negative as defined by
the size of the circles of confusion and depth of field in the print as
defined by circles of confusion in the film plus three variables
(enlargement, viewing distance, and personal standards of sharpness).

It is hard to believe that the above way of thinking can be good. The
whole process of (mathematically) defining DOF begins from specifying an
acceptable CoC. And one can't specify that without taking the three
issues (enlargement, viewing distance, and personal standards of
sharpness) into consideration. People don't understand this issue
because they usually don't take the three things into consideration.
They just look the DOF scales on lenses and think they are some absolute
values. Guess how many people got confused when thinking how DOF on
Canon D30 (or was it 30D) differed from a 35mm film camera. Many people...

If I look at a 35mm slide from 10 meters, it will be 100% in DOF no
matter how hard I try to look: everything seems to be very sharp.

No, it won't be "100% in DOF " and everything won't "seem to be very sharp"
because your eye won't be able to resolve anything with an object that small
from that distance. ... In fact just the opposite is true, it has no depth
of field and nothing about the image in the slide is very sharp.

First I have to say that the 10m figure was just a throw without any
careful consideration. 1m might have been better - I have to try it
with a real slide first... But that is not the point and here you are
actually contradicting yourself (I'll explain later).

I didn't mean to watch the negative so far that nothing can be seen but
from distance where things don't look blurred at all. Then everything
seems to be in DOF. On the other hand if one looks the negative with a
microscope, everything seems to be blurred. Then there is alko the issue
of smalles possible CoC the lens can draw. If that is larger (after
enlargment) than the larges acceptable CoC, then the images are never
sharp. But that is another thing...

Many people would say that the DOF of the negative is allways
constant regardless of viewing distance. And that is obviously
false statement if we understand the meaning and purpose of DOF and
how it is defined.

Circles of confusion in the negative are obviously always constant in the
negative so I don't know what you mean by your statement..

CoCs are constant in the negative but not in the final print when
looking it at certain distance. And since DOF is defined by the size of
the _final_ CoCs, then there can't be any constant DOF in the negative.

It's the size of the circles of confusion in the negative and in the
print that gives rise to the whole concept of depth of field.

No, only the final print matters - or negative if that is the final
thing to look at. There is and can't be DOF until defining all the
conditions.

Everything in front of or behind that plane is out of focus and is
therefore is represented by circles in the negative and in the print. But if
the circles are small enough our eyes can't distinguish between them and
points so we say those areas are "in focus" or more accurately "appear to be
in focus" in the print even though they aren't.

You are contradicting yourself. What you write above is true and still
you disagreed with it earlier in the post. The above is exactly the same
thing that happens when looking a 35mm negative from great distance:
more and more points became smaller and harder to distinguish from each
other - negative/image becomes sharper - DOF gets larger.

The size of the circles of confusion in the negative is the starting
point for this process and is therefore critical in determining depth
of field in the print..

It is the starting point, of course, but has no meaning _on their own_
in actual perception of DOF.

Somehow I think we both know and agree how DOF is defined, we just seem
to have totally different way of coming to that definition.

Severi S.

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.

--

Stacey

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.

--

Stacey

brian wrote:

Stacey wrote in message
...

the main drawback to larger formats isn't
less DOF when viewing the same size prints, it's slower shutter speeds to
use the smaller fstops needed to get there.


In other words, if you keep the f/# and field of view constant while
increasing the format size, then you reduce DOF. Glad we finally
agree!



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

I said -in practice- I've found that the main issue with larger formats is
the slower shutter speed and with smaller formats it's the image quality
loss at small fstops given the same final print size that -ultimatly-
limits DOF.

--

Stacey

brian wrote:

Stacey wrote in message
...

the main drawback to larger formats isn't
less DOF when viewing the same size prints, it's slower shutter speeds to
use the smaller fstops needed to get there.


In other words, if you keep the f/# and field of view constant while
increasing the format size, then you reduce DOF. Glad we finally
agree!



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

I said -in practice- I've found that the main issue with larger formats is
the slower shutter speed and with smaller formats it's the image quality
loss at small fstops given the same final print size that -ultimatly-
limits DOF.

--

Stacey

"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.)

(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.)

David J. Littleboy
Tokyo, Japan

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