Hyperfocal Focusing With APS Sized Sensors

The question is often asked whether the depth of field (DOF) for a 35mm camera lens remains the same when it is used on a digital camera with a smaller APS sized sensor. It would seem that it shouldn't change just because a smaller part of the image circle is used, but in reality it is different. This is because DOF calculations are based upon three factors: the degree of enlargement of the final image, the viewing distance, and the smallest discernible point at that viewing distance.

This last item is known as the "circle of confusion" (COC) and is based upon a point 1/100" in size on an 8x10" print viewed at normal reading distance. Basically, COC is a value that helps determine what appears to be in focus to the human eye. What varies with sensor size is the degree of enlargement required to make an 8x10 print, and therefore the COC value also varies. The greater the degree of enlargement, the easier it is to see if something is out of focus, so focus becomes more critical. If we make a print larger than 8x10, anything on the edge of the DOF range may appear soft, but normally a larger print is viewed from a greater distance. So under normal circumstances a given DOF calculation can be used in the field without concern for print size.

Back to practical matters, how are we to adjust to the new reality of different DOF values for our trusty 35mm lenses? Fortunately there are ample resources on the internet which allow us to calculate new DOF charts. The first thing we must do is to get the COC value for the camera we are using. Here is a link to a good web site for that. It has a long list of camera models and the COC value for each (BTW, the COC value for a 1.5 f.o.v. APS sensor is .020, for 35mm it is .030):

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

Then go to any one of the good DOF calculator sites (Google it) and plug in the numbers, including the COC value. This is my favorite DOF site:

http://www.dudak.baka.com/dofcalc.html

because we can run calculations for 5 different lenses at once (or 5 different settings for the same lens). I usually print these out and save them. From the sheets I make small charts that I carry in the camera bag. What follows is a practical example and application of these tools.

Landscape Photography and Hyperfocal Focusing

For many landscape photographers who try to squeeze every inch of DOF out of a lens, this is an important issue. A common practice is to use what is called hyperfocal focusing. The hyperfocal focus point is the focus distance that gives the greatest depth of field, from infinity to the closest possible distance, or what is called the "near point". This focus distance varies with focal length and aperture, and the lens must be focused at this point to get the greatest DOF. There are two ways to do this.

The old tried and true method is to use the DOF scale on the lens. Suppose, for example, we are using 35mm film and have composed a scene with a 50mm lens and we have foreground elements that we want to be in focus. We know that f/16 is the smallest aperture that produces an acceptably sharp image on this lens, so we select that aperture. Then we turn the focus ring until the infinity mark lines up with the f/16 DOF point on the left side of the center focus mark. Now the f/16 DOF point on the right side is lined up just to the left of the 8' distance mark. So we know that anything from about 8.5' to infinity will be in focus, and we can adjust our composition if necessary. Using this method, we don't actually need to know what the hyperfocal distance is, but we can easily tell by seeing what distance mark on the focus ring is lined up over the center mark in the DOF scale. In this case it is about 17'.

Pentax A50/1.4

However, because the DOF calculations are different with APS sized sensors, we can no longer rely on these lens DOF scales. We will have less DOF than they show us. But while we get less DOF from each lens, the smaller sensor is actually giving us greater DOF at equivalent fields of view (FOV).

For example, using the same 50mm lens on a Pentax APS sensor camera with a 1.5 FOV factor, it's hyperfocal distance at f/16 is now 25.6' and the DOF is now 12.8' to infinity. This may seem like a significant loss, but we must remember that this 50mm lens is now the equivalent of a 75mm lens on a 35mm camera. The hyperfocal distance for a 75mm lens with 35mm film at f/16 is 38.4' and the near point is 19.2'. So at equivalent focal lengths, the smaller sensor is giving us increased DOF.

On the digital camera we can use a 35mm lens to get a "normal" FOV (equivalent to a 52mm lens). The hyperfocal distance for this lens at f/16 is 12.6' and the near point is 6.3':

       
Sensor   Normal,f/16   Hyperfocal Dist.    Near Point
-------------------------------------------------------
35mm       50mm          17.0'               8.5'  
APS        35mm          12.6'               6.3'  

Sensor   Tele,f/16     Hyperfocal Dist.    Near Point
-------------------------------------------------------
35mm       75mm          38.4'              19.2'  
APS        50mm          25.6'              12.8'

So when considering equivalent focal lengths, our smaller sensor is giving us greater DOF, but we can't rely on the lens DOF scale to tell us what it is.

So we must resort to the second method for hyperfocal focusing. This method requires knowing what the hyperfocal distance is and specifically focusing the lens at that point.

Here is a typical workflow for using this technique:

- Choose focal length and compose the scene

- Focus on the closest object in the scene

- Determine the distance of the closest object using the distance scale on the lens focus ring (this will be the near point)

- Consult the DOF chart to determine what aperture is required to include this near point and what the hyperfocal distance is

- Refocus the lens to the hyperfocal distance, set the aperture and take the picture

For example, suppose we are using a 35mm lens and we determine the near point to be 15'. According to the DOF chart for 35mm (see below), we get 12.5' to infinity at f/8, and the hyperfocal distance is 25.1'. So we refocus to 25', set the aperture to f/8 and take the picture.

In order to use this method of hyperfocal focusing we must know the hyperfocal distances for all our lenses (or zoom focal lengths) at all apertures (or at least those we use). This sounds like a daunting task, but it's actually quite easy to make up small charts and carry them in the camera bag. I have used the Dudak calculator to make up hyperfocal charts for commonly used focal lengths and apertures, and am reproducing it here for the reader's convenience. I hope this article has been helpful.

Best Regards,
Clayton Jones

Hyperfocal Charts For APS-C Sensor (1.5 fov factor) Based On COC .020
Used By Pentax And Other Makers
Various Focal Lengths And Optimum Apertures
Includes The Pentax 31mm, 43mm and 77mm Ltd Lenses
(All Distances Are In Feet)

  Focal Length: 18    35mm Equiv: 27
Aperture   Hyperfocal dist.   Near point
-----------------------------------------
  4.0          13.3             6.6
  5.6           9.5             4.7
  8.0           6.6             3.3
 11.0           4.8             2.4
 16.0           3.3             1.7


  Focal Length: 24    35mm Equiv: 36
Aperture   Hyperfocal dist.   Near point
-----------------------------------------
  4.0          23.6            11.8
  5.6          16.8             8.4
  8.0          11.8             5.9
 11.0           8.5             4.3
 16.0           5.9             3.0


  Focal Length: 28    35mm Equiv: 42
Aperture   Hyperfocal dist.   Near point
-----------------------------------------
  4.0          32.1             16.0
  5.6          22.9             11.5
  8.0          16.0              8.0
 11.0          11.6              5.8
 16.0           8.1              4.0


  Focal Length: 31    35mm Equiv: 47
Aperture   Hyperfocal dist.   Near point
-----------------------------------------
  4.0          39.4            19.7
  5.6          28.1            14.1
  8.0          19.7             9.9
 11.0          14.3             7.2
 16.0           9.9             4.9


  Focal Length: 35    35mm Equiv: 52
Aperture   Hyperfocal dist.   Near point
-----------------------------------------
  4.0          50.2             25.1
  5.6          35.9             18.0
  8.0          25.1             12.5
 11.0          18.3              9.1
 16.0          12.6              6.3


  Focal Length: 43    35mm Equiv: 65
Aperture   Hyperfocal dist.   Near point
-----------------------------------------
  4.0          75.8             37.9
  5.6          54.2             27.1
  8.0          37.9             19.0
 11.0          27.6             13.8
 16.0          19.0              9.5


  Focal Length: 50    35mm Equiv: 75
Aperture   Hyperfocal dist.   Near point
-----------------------------------------
  4.0         102.5             51.3
  5.6          73.2             36.6
  8.0          51.3             25.6
 11.0          37.3             18.6
 16.0          25.6             12.8


  Focal Length: 65    35mm Equiv: 98
Aperture   Hyperfocal dist.   Near point
-----------------------------------------
  4.0         138.6             69.3
  5.6          99.0             49.5
  8.0          69.3             34.7
 11.0          50.4             25.2
 16.0          34.7             17.3


  Focal Length: 77    35mm Equiv: 115
Aperture   Hyperfocal dist.   Near point
-----------------------------------------
  4.0         243.1            121.6
  5.6         173.7             86.8
  8.0         121.6             60.8
 11.0          88.4             44.2
 16.0          60.8             30.4


  Focal Length: 90    35mm Equiv: 135
Aperture   Hyperfocal dist.   Near point
-----------------------------------------
  4.0         332.2            166.1
  5.6         237.3            118.6
  8.0         166.1             83.1
 11.0         120.8             60.4
 16.0          83.0             41.5


  Focal Length: 105   35mm Equiv: 157
Aperture   Hyperfocal dist.   Near point
-----------------------------------------
  4.0         452.1            226.1
  5.6         323.0            161.5
  8.0         226.1            113.0
 11.0         164.4             82.2
 16.0         113.0             56.5


  Focal Length: 135   35mm Equiv: 202
Aperture   Hyperfocal dist.   Near point
-----------------------------------------
  4.0         747.5            373.7
  5.6         533.9            267.0
  8.0         373.7            186.9
 11.0         271.8            135.9
 16.0         186.9             93.4


  Focal Length: 150   35mm Equiv: 225
Aperture   Hyperfocal dist.   Near point
-----------------------------------------
  4.0         922.8            461.4
  5.6         659.1            329.6
  8.0         461.4            230.7
 11.0         335.6            167.8
 16.0         230.7            115.3