A Photography forum. PhotoBanter.com

If this is your first visit, be sure to check out the FAQ by clicking the link above. You may have to register before you can post: click the register link above to proceed. To start viewing messages, select the forum that you want to visit from the selection below.

Go Back   Home » PhotoBanter.com forum » Photo Equipment » Large Format Photography Equipment
Site Map Home Register Authors List Search Today's Posts Mark Forums Read Web Partners

roll-film back: DOF question



 
 
Thread Tools Display Modes
  #1  
Old July 20th 04, 06:00 PM
RSD99
external usenet poster
 
Posts: n/a
Default roll-film back: DOF question

DOF is related to

- Lens Focal Length

- Lens Aperture

- Distance

Film size does *not* enter into the equation(s).


  #2  
Old July 20th 04, 07:52 PM
Hemi4268
external usenet poster
 
Posts: n/a
Default roll-film back: DOF question

DOF is related to

- Lens Focal Length

- Lens Aperture

- Distance


I think your missing magnification. DOField on a contact print is different
then say a 10 enlargment.

Larry
  #3  
Old July 21st 04, 02:49 AM
Leonard Evens
external usenet poster
 
Posts: n/a
Default roll-film back: DOF question

RSD99 wrote:
DOF is related to

- Lens Focal Length

- Lens Aperture

- Distance

Film size does *not* enter into the equation(s).


That is not quite right. The way film size enters into the
"equation(s)" is that for the same size final print, you have to enlarge
more for a smaller format, less for a larger format. Since you mention
equations, let's look at one. The formula for hyperfocal distance is

f^2/Nc

where f is the focal length, N is the f-number and c is the diameter
of the maximal acceptable circle of confusion in the film. For a small
format, you would have to choose a smaller c because of the greater
degree of enlargement. A smaller c in the denominator means a larger
hyperfocal distance, which in turn means less depth of field if you
focus exactly on a subject at the same distance. So if the three
things you mention are the same, you get less depth of field in the
smaller format than in the larger format.



  #4  
Old July 21st 04, 02:50 AM
Leonard Evens
external usenet poster
 
Posts: n/a
Default roll-film back: DOF question

Hemi4268 wrote:
DOF is related to

- Lens Focal Length

- Lens Aperture

- Distance



I think your missing magnification. DOField on a contact print is different
then say a 10 enlargment.


Correct.


Larry


  #5  
Old July 21st 04, 02:50 AM
Leonard Evens
external usenet poster
 
Posts: n/a
Default roll-film back: DOF question

Hemi4268 wrote:
DOF is related to

- Lens Focal Length

- Lens Aperture

- Distance



I think your missing magnification. DOField on a contact print is different
then say a 10 enlargment.


Correct.


Larry


  #6  
Old July 21st 04, 03:07 AM
Leonard Evens
external usenet poster
 
Posts: n/a
Default roll-film back: DOF question

Mike wrote:
Ah. I see. Hence the reason digicams have such large DOF (with their
tiny sensors) is that in order to fit everything into the frame at a
reasonable distance, a very short focal length must be used.


The answer to your original question is that you would actually get less
depth of field with the roll film back if you used the same lens, the
same aperture, and the subject were at the same distance from the lens.
The only way to understand this is through the formulas, but one way
to help you understand it is as follows. 90 mm is a wide angle lens
for 4 x 5 but is a normal lens for 6 x 7. Going from a wide angle lens
to a normal lens, with aperture fixed, reduces depth of field for a
subject at the same distance. If that were the only issue, it could
explain why you get less depth of field. Unfortunately, you are also
changing formats, going from 4 x 5 to 6 x 7. With everything else
equal, that tends to increase depth of field. But now you have to
worry about quantititative matters. The first effect is significantly
more pronounced than the second effect, so the net result is that you
end up with less depth of field.

Similarly, you can't understand the reason why digicams have so much
depth of field without thinking about it quantitatively. If you want a
good explanation, see Bob Atkins article at www.photo.net, which is one
of the few places I've seen without mistakes in explaining DOF.

It is often enticing to use qualitative discussions to try to explain
one particular thing you've found out to be true. But such explanations
are often wrong when applied to some other related situation. The
advantage of a scientific explanation is that it is clear when it works
and when it doesn't and most important why it does or doesn't.



On Tue, 20 Jul 2004 17:17:36 +0000, Vladamir30 wrote:


It will be the same as using a 90mm lens on any camera at the same
lens-to-subject distance and with the same aperture. What may, as a
practical matter, sometimes increase depth of field using the roll film back
as compared with 4x5 film is the possibility that with the roll film back
you will set up farther from the subject than you would have if you were
using it as a 4x5 lens since the angle of view will be so much narrower when
the image area is only 6x7. . The greater the lens-to-subject distance the
greater the depth of field, all other things affecting depth of field
remaining the same.

"Mike" wrote in message
news
When using a 6x7 rollfilm back on a 4x5 camera (say with a 90mm lens), my
understanding is that you end up with a crop. What about DOF? Will it be
the same as using a 90mm lens designed for 6x7 on a MF camera body? Or
will it be shallower because the image circle is much bigger?







  #7  
Old July 21st 04, 01:09 PM
Vladamir30
external usenet poster
 
Posts: n/a
Default roll-film back: DOF question

Leonard Evens said:

This is wrong. See my other response. It depends on the format since a
smaller format has to be enlarged more. In fact, the way the math
works out, you get less depth of field with the 90 mm lens and the roll
film back, assuming the subject distance is fixed and the final print is
the same size.


I don't think it is wrong. Depth of field relates to the size of the circles
of confusion in the negative, which in turn is affected by only three things
as I said before, lens focal length, aperture, and lens to subject distance.
"Enlarging more" (i.e.image magnfication) is one of the factors relating to
"acceptable sharpness" in the print, not to depth of field. If you wish to
introduce image magnfiication into the discussion then you also should talk
about the viewing distance from the print that you're assuming and explain
what you consider to be an "acceptably sharp" print at any given
magnification and any viewing distance. But those things shouldn't, IMHO, be
confused with depth of field.

"The factors affecting depth of field are governed by the following
principles: (1) The depth of field doubles if the f number is doubled . . .
(2) if you double the subject distance the depth of field increases by four
times . . . (3) if you reduce the focal length by one half, the depth of
field increases by four times. . . " Adams, "The Camera," p. 49.

Note the absence of any mention here of film format or image magnification
from this explanation of how the three factors affecting depth of field
work. Adams then goes on to discuss image magnification and print viewing
distance as two of the factors, along with depth of field, that relate to
"acceptable sharpness" in the print (he doesn't mention personal standards
of "acceptable sharpness" but obviously that is relevant also). So I think
that if we're talking about depth of field we are talking about the size of
circles of confusion in the negative and that is affected only by three
factors of which image magnification isn't one. If you're talking about
image magnficiation you're talking about a factor that doesn't affect depth
of field but that rather affects acceptable sharpness of the print.


"Leonard Evens" wrote in message
...
Vladamir30 wrote:
It will be the same as using a 90mm lens on any camera at the same
lens-to-subject distance and with the same aperture.


This is wrong. See my other response. It depends on the format since a
smaller format has to be enlarged more. In fact, the way the math
works out, you get less depth of field with the 90 mm lens and the roll
film back, assuming the subject distance is fixed and the final print is
the same size.

What may, as a
practical matter, sometimes increase depth of field using the roll film

back
as compared with 4x5 film is the possibility that with the roll film

back
you will set up farther from the subject than you would have if you were
using it as a 4x5 lens since the angle of view will be so much narrower

when
the image area is only 6x7. . The greater the lens-to-subject distance

the
greater the depth of field, all other things affecting depth of field
remaining the same.


Just what happened would depend on how the different quantitative
factors compared. In the specific case you describe, it does in fact
end up that moving further back so the image size in the final print is
the same, you end up with more depth of field. That would be where
there is a primary subject not too far from the camera as in
portraiture. But had the equations been different, or the underlying
assumptions different, that might have worked out differently. The
thing to keep in mind is that different factors may affect DOF at
different rates. So just knowing that one thing increases and another
decreases doesn't tell you what the net effect will be. It depends on
how fast each increases or decreases.




"Mike" wrote in message
news
When using a 6x7 rollfilm back on a 4x5 camera (say with a 90mm lens),

my
understanding is that you end up with a crop. What about DOF? Will it

be
the same as using a 90mm lens designed for 6x7 on a MF camera body? Or
will it be shallower because the image circle is much bigger?










  #8  
Old July 21st 04, 03:03 PM
Leonard Evens
external usenet poster
 
Posts: n/a
Default roll-film back: DOF question

Vladamir30 wrote:
Leonard Evens said:


This is wrong. See my other response. It depends on the format since a
smaller format has to be enlarged more. In fact, the way the math
works out, you get less depth of field with the 90 mm lens and the roll
film back, assuming the subject distance is fixed and the final print is
the same size.



I don't think it is wrong. Depth of field relates to the size of the circles
of confusion in the negative, which in turn is affected by only three things
as I said before, lens focal length, aperture, and lens to subject distance.


You can, if you wish, define depth of field in your own unique way, but
that is not the way it is usually defined.

The usual definition assumes a normal user who is looking at a certain
size final print at a normal viewing distance. A typical standard for
the print would be an 8 x 10 print viewed at 10 (250 mm) to 12 inches.
It is the maximum acceptable circle of confusion in the print that is
relevant. The choice of print coc depends of course on how discerning
the viewer is. One plausible choice is 0.2 mm or thereabouts. But
some people can see better than that and would choose a smaller value.

Once you choose the coc in the print, then the coc in the film is
obtained by dividing by the enlargement factor. For 4 x 5, that is
about 2, so the coc in the film would be about 0.1 mm (or less for
fussier viewers). For 6 x 7, the enlargement is about 3.6, so the coc
in the film would be about 0.2 divided by that or about .05 mm.

"Enlarging more" (i.e.image magnfication) is one of the factors relating to
"acceptable sharpness" in the print, not to depth of field. If you wish to
introduce image magnfiication into the discussion then you also should talk
about the viewing distance from the print that you're assuming and explain
what you consider to be an "acceptably sharp" print at any given
magnification and any viewing distance. But those things shouldn't, IMHO, be
confused with depth of field.


See above. You choose a standard for print size and viewing distance.
As I said, an 8 x 10 print viewed at 10-12 inches is a good choice.
Most viewers are not comfortable viewing something at closer than 10
inches, and it is usually assumed that people will try to view a print
at about the diagonal distance. For an 8 x 10 print, that is a little
over 12 inches. If the print is larger, then people will generally get
proportionately further back. For example, a 16 x 20 print might
normallybe viewed at about 2 feet. If so, a coc of size 0.4 in such a
print would be acceptable corresponding to a coc of 0.2 mm in a print
half the size viewed at half the distance.

Of course, there always will be people who will insist on getting closer
to the larger print than the diagonal distance. For such people, a
smaller print coc would be appropriate and hence a smaller coc in the
film.

I think you are making the assumption that depth of field is an absolute
characteristic just of the lens. What you say would be a good way to
proceed if we only viewed contact prints, but that is not the case in
modern photography. In photography as practiced today, depth of field
is not an absolute quantity but is relative to what is needed for the
final image.


"The factors affecting depth of field are governed by the following
principles: (1) The depth of field doubles if the f number is doubled . . .


I'm not sure what you mean by that. Depth of field depends on a variety
of factors, one being the subject distance, so a simple statement like
that doesn't make sense. One way to quantify such statments about how
depth of field changes is to ask how much you have to change the
f-number to obtain the same depth of field. From that perspective, your
(1) is a tautology.

(2) if you double the subject distance the depth of field increases by four
times . . .


Within certain ranges, that is approximately true, but it isn't
generally true. For example, by doubling the distance, you could go
from finite depth of field to infinite depth of field. In terms of
f-number change, the statement is approximately true.

(3) if you reduce the focal length by one half, the depth of
field increases by four times. . . " Adams, "The Camera," p. 49.


Again, in terms of f-number change, the statement is literally true.



Note the absence of any mention here of film format or image magnification
from this explanation of how the three factors affecting depth of field
work.


But those factors are implicit. In all these statements, Adams is
assuming a fixed format. Remember that Adams is talking as a
practicing photographer, not as an optical scientist. As such, his
statements are relative to his typical way of working with his typical
equipment. If you had questioned him further, he would of course have
told you that with different equipment and with different aims, the
rules would be different.

By the way, Adams does make some rather obvious mistakes in places, so
he isn't the best reference in some of these matters.

Adams then goes on to discuss image magnification and print viewing
distance as two of the factors, along with depth of field, that relate to
"acceptable sharpness" in the print (he doesn't mention personal standards
of "acceptable sharpness" but obviously that is relevant also). So I think
that if we're talking about depth of field we are talking about the size of
circles of confusion in the negative and that is affected only by three
factors of which image magnification isn't one.


You are confusing two things here. For any point in the scene which is
not in the exact subject plane, the image of that point in the film
plane will be a disc, called a circle of confusion. The closer the
subject point is to the plane of exact focus, the smaller will be the
size of the image disc or circle of confusion. But depth of field is
calculated by specifying the maximal possible circle of confusion which
can not be distinguished from a point. But that term is clearly
subject to assumptions about who is doing the distinguishing and under
what conditions. If you viewed a contact print at 10 inches you would
choose one value for the maximum. If you were viewing a 2 x enlargement
also at 10 inches, you would choose the same value for the enlargement,
but necessarily half that value for the film.

If you're talking about
image magnficiation you're talking about a factor that doesn't affect depth
of field but that rather affects acceptable sharpness of the print.


So how do you choose the maximal acceptable coc in the film? Do you
use the same value for an 8 x 10 camera and a 35 mm camera? If you do
that, you are going to get values very different from what you see in
DOf tables. You are making a valid distinction, but I don't think you
have really thought it all through. In particular you are ignoring the
need to choose a maximum allowable coc for the negative and how that
choice depends on a variety of assumptions. If you prefer, you can
restrict the term 'depth of field' to refer only to depth of field of
contact prints viewed at 10 inches, and distinguish that from "adequate
sharpness" in enlargements, but that would be a rather unusal way to
use the terms and would not be consistent with what most other people
are doing.




  #9  
Old July 21st 04, 03:17 PM
jjs
external usenet poster
 
Posts: n/a
Default roll-film back: DOF question

"Leonard Evens" wrote in message
...

[...]
Of course, there always will be people who will insist on getting closer
to the larger print than the diagonal distance. [...]


I find this to be more and more the case lately and I have a tentative
theory that it is due to experience of the same persons with digital
imaging. When they can magnify, they will in order to explore deeper and
deeper into an image. Maybe we will see the day when shows have a roped-off
distance.


  #10  
Old July 21st 04, 03:17 PM
jjs
external usenet poster
 
Posts: n/a
Default roll-film back: DOF question

"Leonard Evens" wrote in message
...

[...]
Of course, there always will be people who will insist on getting closer
to the larger print than the diagonal distance. [...]


I find this to be more and more the case lately and I have a tentative
theory that it is due to experience of the same persons with digital
imaging. When they can magnify, they will in order to explore deeper and
deeper into an image. Maybe we will see the day when shows have a roped-off
distance.


 




Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

vB code is On
Smilies are On
[IMG] code is Off
HTML code is Off
Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
Graphic 120 roll film back for 4x5 Massimiliano Spoto Large Format Photography Equipment 2 May 24th 04 02:01 AM
FS: 6X8 ROLL FILM BACK FOR 4X5 Massimiliano Spoto Large Format Photography Equipment 0 May 20th 04 06:06 AM
6X8 ROLL FILM BACK FOR 4X5 Massimiliano Spoto Large Format Photography Equipment 0 May 20th 04 05:55 AM
6X8 ROLL FILM BACK FOR 4X5 Massimiliano Spoto General Photography Techniques 0 May 20th 04 05:55 AM
6X8 ROLL FILM BACK FOR 4X5 Massimiliano Spoto Fine Art, Framing and Display 0 May 20th 04 05:55 AM


All times are GMT +1. The time now is 10:13 AM.


Powered by vBulletin® Version 3.6.4
Copyright ©2000 - 2024, Jelsoft Enterprises Ltd.
Copyright ©2004-2024 PhotoBanter.com.
The comments are property of their posters.