Depth of Field Part 2…

Alright, time for math class…wait, I thought this was a photography blog? It is, but, with most things in life, math is here too. So, today we are going to learn how to determine our depth-of-field. (range of acceptable sharpness)

Webster defines depth-of-field as:

the range of distances of the object in front of an image-forming device (as a camera lens) measured along the axis of the device throughout which the image has acceptable sharpness

With cameras, when you focus on something at a given distance, with any aperture, only one plane is in focus at a given time. Now, certain things can adjust over what distances this appearant focus drops off, and this is what gives us depth-of-field.

This is one of the most profound effects in all of photography. I allows the photographer to control the way everything looks in a photograph. Want a razor-thin depth-of-field for food photography? No problem, need everything in focus to infinity? No problem.

Let us start with the hyperfocal distance. This is the distance to focus on when you want the maximum range of sharpness. When you focus on the hyperfocal distance for a lens, everything from half that distance from the camera to infinity will be in acceptable focus. How do you determine this hyperfocal distance you ask, well, lets find out.

On older film cameras, they had aperture rings and hyperfocal scales that made it somewhat easier, and even some more expensive lenses still have the scales, but, most entry and mid level lenses do not. We will see how to determine this for any lens, though.

Remember: every calculation in this tutorial is overly simplified, meaning that most dof and hyperfocal calculations leave out exacting specifications, because every lens is manufactured differently, and therefore not quite the same as any other. So, these formulas are approximations, albeit good enough for most people.

  where:

H is the hyperfocal distance of a lens, in mm

f is lens focal length, in mm (actual, not accounting for sensor crop)

N is the f/stop

c is the circle of confusion, in mm

Circle of confusion, what is that? Glad you asked. It is an optical spot caused by a cone of light rays from a lens not coming to a perfect focus when imaging a point source. You don’t have to remember that, just know what it is for your particular camera format.

35mm=.029

APS-C=.018

4/3 (Olympus)=.015

I am not going to give them for medium and large format cameras, because if you are using one of those, you are probably not reading this blog.

Now, you can use that formula at any time to determine what distance to focus on to get maximum range of sharpness out of your lens. What about any other time? What about when you want a defocused background? No problem. You can always determine the range of near and far acceptable focus for your chosen settings (aperture, subject distance, and focal length) Heres how to find the near distance of acceptable sharpness:

 where:

Dn is the near focus distance of acceptable sharpness

s is the focal (subject) distance

H is the hyperfocal distance, in mm

f is the lens focal length, in mm

And to find the far limit of acceptable sharpness:

 where:

Df is the far focus distance of acceptable sharpness

s is the focal (subject) distance

H is the hyperfocal distance, in mm

f is the lens focal length, in mm

So, using the numbers you get from these formulas, you can determine if you have, say, 5.24 feet of sharpness when focusing on a subject at 15 feet, with a lens focal length of 80mm, and an aperture of f/8. This will allow you creative freedom by freeing you to decide if you want another object to appear in focus with the main subject of your photograph, or just as a blur in the background.

These may seem tough, and they can be, but, it is just another tool in the photographers arsenel to use to create their masterpiece.

Thank you for reading, and have a great day!!!

This entry was posted on 9. April 2009 at 16:09 and is filed under Photographic Techniques. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response or trackback from your own site.