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DOF Qustion

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Ashley23310

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Recently, I read the following statement in an old book from the days of film:

"If the image size on the film remains the same, and if the aperture remains the
same, the depth of field, from near to far, will remain the same regardless of
focal length of the lens used. Of course, the camera to subject distance must
change as the focal length of the lens is changed."

I have run some math on this and I am satisfied that the statement is correct, but I was wondering if our experts agree, or if someone might have a contrary opinion. Any comments?

--
Ash
 
Ashley23310 said:
Recently, I read the following statement in an old book from the days of film:

"If the image size on the film remains the same, and if the aperture remains the
same, the depth of field, from near to far, will remain the same regardless of
focal length of the lens used. Of course, the camera to subject distance must
change as the focal length of the lens is changed."

I have run some math on this and I am satisfied that the statement is correct, but I was wondering if our experts agree, or if someone might have a contrary opinion. Any comments?

--
Ash
Click to expand...
 
Admirable that you have done the math, but you still might find this on-line DOF calculator useful since it will do the math for you given the relevant inputs:

http://www.dofmaster.com/dofjs.html

And you can read about DOF here, as well as other places on the net:

http://en.wikipedia.org/wiki/Depth_of_field

"The DOF is determined by the camera-to-subject distance, the lens focal length, the lens f-number, and the format size or circle of confusion criterion."

For most practical purposes one just needs to know that a wider aperture (lower f number) gives a shallower DOF than a narrower aperture (higher f number). But both the aperture and the focal length are factors in determining DOF as you can easily see using the on-line calculator.
 
Ashley23310 said:
Recently, I read the following statement in an old book from the days of film:

"If the image size on the film remains the same, and if the aperture remains the
same, the depth of field, from near to far, will remain the same regardless of
focal length of the lens used. Of course, the camera to subject distance must
change as the focal length of the lens is changed."

I have run some math on this and I am satisfied that the statement is correct, but I was wondering if our experts agree, or if someone might have a contrary opinion. Any comments?

--
Ash
Click to expand...
It isn't exactly true but is really close when depth of field is small.

It fails when so-called hyperfocal conditions are approached (that's when the rear dof extends to infinity). In such cases larger focal length yields smaller dof.

The (nearly) exact equation is

dof.rear=(CN/m)(1+1/m) / (1-CNm/F)

C=Circle of Confusion, N=F-Number, F=Focal length, m=magnification (ratio of sensor to scene width)
 
osage_archer said:
Admirable that you have done the math, but you still might find this on-line DOF calculator useful since it will do the math for you given the relevant inputs:

http://www.dofmaster.com/dofjs.html

And you can read about DOF here, as well as other places on the net:

http://en.wikipedia.org/wiki/Depth_of_field

"The DOF is determined by the camera-to-subject distance, the lens focal length, the lens f-number, and the format size or circle of confusion criterion."

For most practical purposes one just needs to know that a wider aperture (lower f number) gives a shallower DOF than a narrower aperture (higher f number). But both the aperture and the focal length are factors in determining DOF as you can easily see using the on-line calculator.
Click to expand...
Thanks for the reply. Calculators like dofmaster and Barnack are fine for getting the answer to a problem, but they do not give me the depth of understanding that I seek I enjoy doing the math.

--
Ash
 
Dave Martin said:
Ashley23310 said:
Recently, I read the following statement in an old book from the days of film:

"If the image size on the film remains the same, and if the aperture remains the
same, the depth of field, from near to far, will remain the same regardless of
focal length of the lens used. Of course, the camera to subject distance must
change as the focal length of the lens is changed."

I have run some math on this and I am satisfied that the statement is correct, but I was wondering if our experts agree, or if someone might have a contrary opinion. Any comments?

--
Ash
Click to expand...
It isn't exactly true but is really close when depth of field is small.

It fails when so-called hyperfocal conditions are approached (that's when the rear dof extends to infinity). In such cases larger focal length yields smaller dof.

The (nearly) exact equation is

dof.rear=(CN/m)(1+1/m) / (1-CNm/F)

C=Circle of Confusion, N=F-Number, F=Focal length, m=magnification (ratio of sensor to scene width)
Click to expand...
Oops make that...

dof.rear=(CN/m)(1+1/m) / (1-CN/mF)
 
Dave Martin said:
Ashley23310 said:
Recently, I read the following statement in an old book from the days of film:

"If the image size on the film remains the same, and if the aperture remains the
same, the depth of field, from near to far, will remain the same regardless of
focal length of the lens used. Of course, the camera to subject distance must
change as the focal length of the lens is changed."

I have run some math on this and I am satisfied that the statement is correct, but I was wondering if our experts agree, or if someone might have a contrary opinion. Any comments?

--
Ash
Click to expand...
It isn't exactly true but is really close when depth of field is small.

It fails when so-called hyperfocal conditions are approached (that's when the rear dof extends to infinity). In such cases larger focal length yields smaller dof.

The (nearly) exact equation is

dof.rear=(CN/m)(1+1/m) / (1-CNm/F)

C=Circle of Confusion, N=F-Number, F=Focal length, m=magnification (ratio of sensor to scene width)
Click to expand...
Thanks for your comments and thanks for the equation - I will enjoy working some math. I am sure that there are many hidden facts about DOF that I will discover as I explore the subject. Already, I have disproven, to my satisfaction at least, the old rule of thumb that DOF extends one third nearer and two thirds farther than the plane of focus. The "rule" is true only over a small range of distances. DOF is a slippery subject but an interesting one.

--
Ash
 
Graystar said:
gisle said:
The statement is true for longer focal lengths (above 100 mm for FX), but not for shorter focal lengths.
Click to expand...
Oh yes it is. See this LL study...
http://www.luminous-landscape.com/tutorials/dof2.shtml
Click to expand...
To quote from the link I posted:

"His [Reichmann] experiment is unfortunately badly set up, and he also makes the mistake of comparing apples and oranges. For some reason, he sets up his scene so that this main reference target (the hand puppet) is no longer visible when he goes wider than 50 mm. To remedy this, he takes a crop from his 17 mm shot (showing the distant tower), and enlarges the cropped portion much more than the other photographs in his comparison set. Since we know that the degree of enlargement is one of the factors that determines the perceived DOF, it doesn't make sense to compare this enlarged crop to other photos that has not been subject to the same cropping and enlargement."

--
– gisle [ See profile/plan for equipment list ]
 
gisle said:
Graystar said:
gisle said:
The statement is true for longer focal lengths (above 100 mm for FX), but not for shorter focal lengths.
Click to expand...
Oh yes it is. See this LL study...
http://www.luminous-landscape.com/tutorials/dof2.shtml
Click to expand...
To quote from the link I posted:

"His [Reichmann] experiment is unfortunately badly set up, and he also makes the mistake of comparing apples and oranges. For some reason, he sets up his scene so that this main reference target (the hand puppet) is no longer visible when he goes wider than 50 mm. To remedy this, he takes a crop from his 17 mm shot (showing the distant tower), and enlarges the cropped portion much more than the other photographs in his comparison set. Since we know that the degree of enlargement is one of the factors that determines the perceived DOF, it doesn't make sense to compare this enlarged crop to other photos that has not been subject to the same cropping and enlargement."
Click to expand...
All that proves is that the author only looked at the pictures and didn't read the article.

.
 
Graystar said:
gisle said:
Graystar said:
gisle said:
The statement is true for longer focal lengths (above 100 mm for FX), but not for shorter focal lengths.
Click to expand...
Oh yes it is. See this LL study...
http://www.luminous-landscape.com/tutorials/dof2.shtml
Click to expand...
To quote from the link I posted:

"His [Reichmann] experiment is unfortunately badly set up, and he also makes the mistake of comparing apples and oranges. For some reason, he sets up his scene so that this main reference target (the hand puppet) is no longer visible when he goes wider than 50 mm. To remedy this, he takes a crop from his 17 mm shot (showing the distant tower), and enlarges the cropped portion much more than the other photographs in his comparison set. Since we know that the degree of enlargement is one of the factors that determines the perceived DOF, it doesn't make sense to compare this enlarged crop to other photos that has not been subject to the same cropping and enlargement."
Click to expand...
All that proves is that the author only looked at the pictures and didn't read the article.

.
Click to expand...
The concept of Depth of Field covers those areas of the scene which are considered acceptably sharp .

On the other hand, the Luminous Landscape article is talking about something else. It discusses the degree to which objects are out of focus .

Regards,
Peter
 
Ashley23310 said:
Recently, I read the following statement in an old book from the days of film:

"If the image size on the film remains the same, and if the aperture remains the
same, the depth of field, from near to far, will remain the same regardless of
focal length of the lens used. Of course, the camera to subject distance must
change as the focal length of the lens is changed."

I have run some math on this and I am satisfied that the statement is correct, but I was wondering if our experts agree, or if someone might have a contrary opinion. Any comments?
Click to expand...
This is only true under some circumstances . Try this article:

http://toothwalker.org/optics/dof.html

The simplified statement of that article is that your statement is true only when the focus distance is "much smaller" than the hyperfocal distance; when the focus distance is similar to the hyperfocal distance, the wide angle lens has more DoF. And again, that's the simplified statement; the article goes on to qualify it further for the case of macro photography.

For a quick counterexample of a different sort, compare Figure 3 with Figure 5 in the article, which show closeup photos taken with a 28mm and a 105mm lens at f/4 and f/22. In Figure 3, taken at f/4, neither of the pictures demonstrates more DoF than the other; in figure 5, taken at the same positions, f/22 and shown at a smaller size , the one with the 28mm lens has more DoF. (And yes, display size is a significant DoF parameter; larger prints = less DoF.)

Depth of field is complicated.
 
Graystar said:
All that proves is that the author only looked at the pictures and didn't read the article.
Click to expand...
Huh? What part of the Reichmann's "article" do you think have substance?

I did of course read all of it (after all there is very little text). It is obvious that Reichmann simply postulates based on a bungled experiment.
--
– gisle [ See profile/plan for equipment list ]
 
--

It is the mark of an educated mind to be able to entertain an idea without necessarily accepting it. -Aristotle

The one serious conviction one should hold is that nothing should be taken too seriously.
...oh, and I see by the lack of responses that I am right yet again.
 
Ashley23310 said:
Jeff_Donald said:
There are three exceptions to the rule the original poster states. Here is a good analysis of the exceptions, http://toothwalker.org/optics/dof.html
Click to expand...
Thank you. Although I have not yet read the article thoroughly, I like the manner in which it has been organized, separately examining the various factors involved in Depth of Field.
Click to expand...
Yes. It is definitely the best article on the topic of DoF anywhere on the net. I note the sidebox with the important point that... "DOF should not be judged from background blur."

The closing statements are worth quoting as well....

""Finally, although the depth of field is a primary discipline of the photographic craft its importance should be seen in the proper perspective. It does not require familiarity with equations to confront the world with stunning photography. IMHO, from the theory of DOF, the most important ingredients would be the quest for the true hyperfocal distance to avoid backgrounds that are just not sharp, and the art to disengage the subject from the background. The latter, however, is more a matter of blur and FOV control than DOF control.""

[I also approve of the staring role played by Gromit, who's ability to deliver a line I've found riveting in everything he has ever done.] ;-)
--
Regards,
Baz

"Ahh... But the thing is, they were not just ORDINARY time travellers!"
 
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