Depth-of-Field Calculator - NS 2.0 MSIE 3.0 or later Required

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The subject of this column is based partly on an article that appeared last Summer in one of the popular camera magazines and depth of focus lens charts that appear on the web. Many times we find ourselves in the field and don't have the information at our finger tips. Here's an easy way to determine the depth of focus in the field.

When photographing landscapes, we are often faced with a decision as to what will be "in-focus" and what will have to be sacrificed to "out-of -focus." There's a simple formula that can be used in the field to ensure a distant object (infinity) will be sharp and at the same time, let you know how much in front of infinity will also be in sharp focus. After you select the focal length of the lens you will be using, you can determine your point of focus (at F/11) with the following formula: FD = FL divided by 10 squared, where FD is your focus distance and FL is the focal length of the lens you are using to photograph the scene. Example: Using a 50mm lens, 50/10 X 50/10 or 5 X 5 = 25 feet. Set the distance scale on the lens at 25 feet. With the lens aperture at F/11, we can be assured infinity will be in-focus. To determine how much foreground will be in-focus simply divide 25 by 2 = 12.5 feet. In other words, with a 50mm lens at F/11, focused at 25 feet, everything from 12.5 feet to infinity will be in focus. Most auto-focus lens today have very few markings to help you set the distances manually. I suggest you view the scene, select a spot about 25 feet away and auto-focus on this spot. Then switch off the camera's auto focus feature and take the picture.

Should the scene require more foreground in focus, increase the depth of field by stopping the lens down to a smaller aperture. For F/16, multiply 25 (the distance we determined in the above example) by 2/3. The new focusing distance is 17 feet. With a 50mm lens focused at 17 feet and F/16, now everything from 8.5 feet to infinity is in focus. More depth of field is required? Stop down another stop to F/22 and multiply 17 again by 2/3. The new distance is now 11 feet. Set the 50mm lens at 11 feet and everything from 5.5 feet to infinity will be in focus.

If all of this seems a little too complicated, the table below should help. I suggest you cut out this table, laminate it, and carry it in your camera bag. An inexpensive way to laminate is to buy clear, self sticking shelf paper and put it on both sides of the object to be laminated, leaving a 1/8 " border. This works great for protecting important papers and booklets such as your camera manual.

                  
                   F/11  F/16    F/22    F/32
  28mm              8'     6'      4'      3'
  35mm             11'     8'      5'      4'
  50mm             25'    17'     11'      8'
  70mm             49'    33'     22'     15'
 100mm            100'    67'     45'     30' 
 135mm            182'   121'     81'     54'
 200mm            400'   267'    178'    119'
 500mm           2500'  1667'   1111'    741'  




From: charu@iced.com
Newsgroups: rec.photo.equipment.medium-format
Subject: Re: 645 vs. 35mm depth of field question
Date: Mon, 02 Mar 1998 

The same conclusion was reached in an earlier discussion with a more
technical and rigorous explanation by Ben Weiner:

From: bweiner@muon.rutgers.edu (Ben Weiner)
Newsgroups: rec.photo.equipment.medium-format
Subject: Re: MF depth of field
Date: 18 Dec 1996 

Garry Lee glee@iol.ie writes:

>The point remains that the DOF is the same, view for view in any format.
>For the same sharpness, the Circle of Confusion is bigger in a bigger
>format BUT that is because the negative is bigger and needs less
>enlargement.                                   
>It is a MYTH that larger formats have less DOF.
>They don't.

With all due respect to the participants in this discussion, it is
a confusing subject, but the above is simply not true.  If you have
ever struggled to get enough DOF with a reasonable f-stop while
using a "normal" length lens on 4x5 or 8x10 you will know that
larger formats DO have less DOF.

Let's look at hyperfocal distance as an indicator of DOF.
There is a simple formula for hyperfocal distance:

     h = F^2 / (N*c)

  h = hyperfocal distance
  F = focal length
  N = f-number                  
  c = diameter of circle of confusion

This formula comes from the Lens FAQ which David Jacobson posts
to rec.photo.moderated and can also be found in any number of books.
It is really simple to derive from similar triangles, but it
is difficult to show this in a text medium, without pictures.

OK.  Suppose we compare 35mm, and 6x7, which is about twice as big
in linear size.  (No arguments over "4x as big in area", please!)
Take a 50mm lens on 35mm, and a 100mm lens on 6x7.  If you stand in
the same place with the two cameras, these give roughly the same
"normal" perspective and angle of view.  I want to compare how
the two formats do when taking the SAME picture.

Let's suppose my subject matter extends from 5 meters to infinity.
To get it all in sharp focus, I need to set the lens so it
has a hyperfocal distance of 10 meters.  I want to know what    
f-stop I need.  I can solve the above formula for N, the f-number:

      N = F^2 / (h*c)

BUT I need to know the circle of confusion.  Let's take c = 0.025 mm
(1/100 inch) for 35mm.  Since I only need to enlarge the 6x7 neg
half as much, I can use a twice-as-big circle-of-confusion for
6x7; c = 0.05 mm.

OK, plug in the numbers (don't forget to convert h from meters to mm):

For 35mm, N = (50*50) / (10,000 * 0.025).  N = 10, i.e. use f/10 (or f/11).

For 6x7, N = (100*100) / (10,000 * 0.05).  N = 20, i.e. use f/20 (or f/22).

See?  You need to stop down two extra stops, from f/11 to f/22, to get
the same depth of field.                                      

Why is this?  Basically, it's because of the F^2 in the formula.
The hyperfocal distance gets larger - and the depth of field gets
smaller - by a factor of 4 when you double the film size and
lens size.  The circle of confusion also gets larger, but only
by a factor of 2.  There's a factor of 2 left over.  In order
to get back the depth of field you've lost, you have to make
the f-number go down by a factor of 2 (which is 2 f-stops).

Another way to look at it is this: out-of-focus areas are blurred
because of the physical diameter of the aperture.  That's why
stopping down any lens (smaller diameter aperture) gives more depth
of field.

If you stand in the same place and photograph the same scene with
two lenses that give the same angle of view, you will get the same
DOF if the lenses have the same aperture diameter - physical      
diameter in mm, (NOT the same f/number).

The 50mm lens at f/10 has an aperture of 5mm.

The 100mm lens at f/10 has an aperture of 10mm, hence less DOF.
To get the same DOF as the smaller lens, you need an aperture
of 5mm, hence 100m lens at f/20.

If you don't believe me, try it.  If you try taking a picture with a
35mm camera and normal lens at f/8 or so, you will get respectable
DOF.  With a 6x7, normal lens, f/8, you will get less DOF.  With a
4x5 or 8x10 camera and its normal lens at f/8, you will get so
little DOF it will make your head spin.  (Large format photographers
rarely use apertures as fast as f/8 for this reason, but sometimes
it can be put to creative use.)

I have spoken.                     
{end quote by Ben Weiner}


  "Charles Petzold"  wrote:
>
> Let's pretend that you own a lens that you can attach to either your 35mm
> camera or your MF camera.  You place both cameras on tripods side by side
> pointed at the same scene.  As you switch the lens between the two cameras,
> what happens to the depth of field?
>
> Answer: The DOF obviously remains the same.
>
> HOWEVER, the problem is that the particular scene you're shooting is
> rendered in the *same size* on the 35mm negative and the MF negative.
> You're wasting part of the larger MF negative.  So what do you do?  You have
> two choices:
>
> You could switch to a longer lens on the MF camera, and the depth of field
> decreases.
>
> Or, you could move closer to your subject with the MF camera, and the depth
> of field also decreases.
>
> So, what you're observing is normal and natural and an unavoidable
> consequence of using a larger film format.
>
>         -- Charles Petzold
>
                                    

Related Postings on DOF and calculations & formulas can be found at our comments page for those wanting more discussions on DOF....