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Jim Kaye

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This isn't a Nikon-specific question, but since I use a D100, I'll ask it here...

I know that for a given aperture, DOF varies directly with camera-to-subject distance and inversely with lens focal length, but do these effects exactly cancel each other out for a particular subject? For example, suppose I shoot a portrait (head shot) using a 50 mm lens at f/2.0. Then I switch to a 200 mm lens, keep the aperture the same, and back up so the head appears the same size in my viewfinder as in the first shot.

Will DOF be the same in the two pictures? If not, which image with have greater DOF? Does anyone know a theoretical explanation for how this works?

Thanks,

Jim Kaye
 
Jim Kaye said:
This isn't a Nikon-specific question, but since I use a D100, I'll
ask it here...

I know that for a given aperture, DOF varies directly with
camera-to-subject distance and inversely with lens focal length,
but do these effects exactly cancel each other out for a particular
subject? For example, suppose I shoot a portrait (head shot) using
a 50 mm lens at f/2.0. Then I switch to a 200 mm lens, keep the
aperture the same, and back up so the head appears the same size in
my viewfinder as in the first shot.

Will DOF be the same in the two pictures? If not, which image with
have greater DOF? Does anyone know a theoretical explanation for
how this works?

Thanks,

Jim Kaye
Click to expand...
If you want a looong and detailed study about DOF, here you can get a full book (PDF format) http://www.trenholm.org/hmmerk/download.html

--

Saludos,
Rafa Barberá
 
Richard,

Well if so, the online DOF calculator says that for a 50mm lens on the D100 at f/2.0 and 10 ft camera-to-subject distance, the near limit of acceptable sharpness is 9.5 ft and the far limit is 10.5 ft (DOF=1 ft), while for a 200 mm lens at f/2.0 and 40 ft camera-to-subject distance, the limits of acceptable sharpness are 39.5 ft and 40.5 ft (DOF=1 ft). So it seems that the two changes (focal length and camera-to-subject distance) do cancel each other out.

Now, if only there really were a 200 mm f/2.0 lens...!

Thanks,

Jim
RichardLaing said:
Could you just multiply by focal length? E.g. twice as far back
for each doubling of focal length?
Click to expand...
 
The depth of field of both lenses will be the same "as long as the subject fills identical porportions of the frame". This even works with your 28 mm vs your 200 mm at the same ƒ-stop using the example above.

The difference will be the field of view which can make a difference to the look of your shot.

Theoretical reason, I'm sure exists, I prefer practical.
Jim Kaye said:
This isn't a Nikon-specific question, but since I use a D100, I'll
ask it here...

I know that for a given aperture, DOF varies directly with
camera-to-subject distance and inversely with lens focal length,
but do these effects exactly cancel each other out for a particular
subject? For example, suppose I shoot a portrait (head shot) using
a 50 mm lens at f/2.0. Then I switch to a 200 mm lens, keep the
aperture the same, and back up so the head appears the same size in
my viewfinder as in the first shot.

Will DOF be the same in the two pictures? If not, which image with
have greater DOF? Does anyone know a theoretical explanation for
how this works?

Thanks,

Jim Kaye
Click to expand...
--
35 to 4 x 5
 
Yes, perspective will certainly differ a lot when the camera-to-subject distance changes to maintain the subject the same size on the sensor with the two different lenses in my example.

Jim
JBP said:
The depth of field of both lenses will be the same "as long as the
subject fills identical porportions of the frame". This even works
with your 28 mm vs your 200 mm at the same ƒ-stop using the example
above.

The difference will be the field of view which can make a
difference to the look of your shot.

Theoretical reason, I'm sure exists, I prefer practical.
Click to expand...
 
Jim Kaye said:
This isn't a Nikon-specific question, but since I use a D100, I'll
ask it here...

I know that for a given aperture, DOF varies directly with
camera-to-subject distance and inversely with lens focal length,
but do these effects exactly cancel each other out for a particular
subject? For example, suppose I shoot a portrait (head shot) using
a 50 mm lens at f/2.0. Then I switch to a 200 mm lens, keep the
aperture the same, and back up so the head appears the same size in
my viewfinder as in the first shot.

Will DOF be the same in the two pictures? If not, which image with
have greater DOF? Does anyone know a theoretical explanation for
how this works?

Thanks,

Jim Kaye
Click to expand...
Technically, DOF is a function of image magnification. Focal length and camera-to-subject distance are just factors that determine magnification. Therefore, theoretically, as long as image magnification remains constant, DOF should remain constant. However, I have seen the results of tests done by Ctein published in one of the photo-oriented magazines that seem to dispute this. I don't know the validity of his tests.
 
Thanks, Don, those graphs are really helpful. Something that I didn't appreciate, but which all your graphs seem to indicate, is that for a given aperture and image size, the shorter focal length lens has more rear DOF than the longer focal length lens, while the longer focal length lens has more front DOF than the shorter lens. So if one wanted to throw a distracting background out of focus, for example, given a particular aperture and image size, it appears that it would be better to use a longer focal length lens from a greater distance away than a shorter lens from a closer distance (assuming that the difference in perspective is acceptable).

Am I interpreting the information correctly?

You have a great web site, BTW -- thanks!

Jim Kaye
Don Fleming said:
Don Fleming said:
Does anyone know a theoretical explanation for
how this works?
Click to expand...
http://www.dofmaster.com/dof_imagesize.html
Click to expand...
 
I was told that out-of-focus blur doesn't simply depend on DoF (or lack thereof) by another DPR forum member, Helmuth Schumann, and indeed, depends on focal length (besides lens bokeh). Here is the link to his page explaining this:

http://www6.brinkster.com/58vp/blur/

I have not been able to verify the truth of the matter myself.

Don,

Since you are an apparent expert on DoF, can you shed some light on this matter about out-of-focus blur?

Also, another question for you that some of us wondered. Why is the CoC on 35mm-based DSLRs apparently based on crop factor?

Based on your calculator, a 1.5x crop DSLR (like all Nikon's) w/ a lens that yields a certain 35mm equiv FOV would have 1.5x greater DoF than fullframe 35mm body w/ a lens that yields that exact FOV. For example, your calculator shows that the D70 using 50mm lens would yield 1.5x greater DoF than a F100 film body using a 75mm lens -- both having same FOV.

Thanks for your insights...

Man
Jim Kaye said:
Am I interpreting the information correctly?

You have a great web site, BTW -- thanks!

Jim Kaye
Don Fleming said:
Jim Kaye said:
Does anyone know a theoretical explanation for
how this works?
Click to expand...
http://www.dofmaster.com/dof_imagesize.html
Click to expand...
Click to expand...
--
Just another amateur -- see profile for more + some basic photog resources.
As usual, YMMV + caveat emptor.
Contact me at [email protected]
Indulge my fancies at http://www.pbase.com/mandnwong
 
acccording two a nikon handbook I had a long time ago, when you're focused close up (as in macro work), DOF is about 50/50. When you get to 10-20 ft range, it's more like the oft quoted 1/3 / 2/3. As you get nearer to infinity, the ration is closer to 20/80. If I remember correctly, the book made no mention of lenses.

But...if I'm thinking of this correctly--a wide-angle lens would be focused much closer to achieve the same image size (which is to say, the camera would be much closer) than a telephoto would. So in that sense, for a given image size , the telephoto would always produce more rear DOF compared to the wider angle.

does that seem right?
TyKo
Jim Kaye said:
Am I interpreting the information correctly?

You have a great web site, BTW -- thanks!

Jim Kaye
Don Fleming said:
Jim Kaye said:
Does anyone know a theoretical explanation for
how this works?
Click to expand...
http://www.dofmaster.com/dof_imagesize.html
Click to expand...
Click to expand...
 
Because you back up, you change the lens-to-subject distance, and depth of field will not be the same. To keep depth of field the same, shoot from the same distance with the same f/ stop, and then enlarge the wide angle photo so the head size is the same.

In your example, the shot with the telephoto lens will have greater depth of field because the distance to subject increased.
It's the distance and the f/stop.
Variations undoubtedly come because f/ stops on lenses are not precise.
Jim Kaye said:
aperture the same, and back up so the head appears the same size in
my viewfinder as in the first shot.

Will DOF be the same in the two pictures? If not, which image with
have greater DOF? Does anyone know a theoretical explanation for
how this works?
Click to expand...
--
^^^Bob^^^
 
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