Depth of Focus

Depth of focus (DoFocus) and Depth of Field (DoField) are often confused with one another, and used inappropriately. This article will try to explain DoFocus and how it relates to your camera and varying circumstances, such as focal length and your point of focus. DoField is explained in a seperate article.

DoFocus? DoField? You Say Potatoe, I Say Potato?

These two terms may be difficult to comprehend at first, but after thinking about them over some time, it will all make sense.

DoFocus - What does it mean

This is the point where your film, or CCD sensor, should fall between in order to get a sharp image (on your recording medium - redundatn?). So what it all comes down to is the tolerances of your lens, assuming focusing is spot on, becoming your "constant" for the explanation.

Your Focal Length

The longer, aka more telephoto, the lens, the more DoFocus you have to capture a sharp image on your film plane. Meaning you have more tolerance for misalignment (in lens, rangefinder, mounting, etc...) to get a "correctly" focused negative. In converse, by using a shorter, aka wide angle, lens, you're effectively decreasing your DoFocus on the film plane. So in the real world, precision becomes of the utmost importance. So the slightest misalignment in your setup, like film buckling, rangefinder alignment or lens element misalignment, can lead to a soft negative.

Your Point of Focus

At infinity, all lenses have the same DoFocus. But depending on the focal length, the rate (factor) at which the DoFocus increases varies as you focus on a closer subject. Longer lenses' DoFocus increases at a faster rate (or is it 'bigger factor'), compared to Wider lenses. So the closest focus on a telephoto lens would have a much larger DoFocus than that of a wide angle lens. Again, the telephotos are more forgiving to mechancial tolerances compared to wider lenses.

Your Apperture Setting

The smaller you set your apperture, the more DoFocus you have. (NOTE: Smallest apperture is not to be confused with your widest aperture (smallest number))

Larger DoFocus

The most DoFocus will be when your telephoto is focused at it's closest distance, with it's smallest apperture.

What It Means to Your Camera

If you like your wide angle lenses, make sure your equipment is serviced from time to time. And since most rangefinder shooters prefer wide angles, make sure to pay close attention to your enlargement from time to time. Instead of blaming the make of the lens or the camera, make sure your equipment is within its original specifications and tolerances.

NOTES: The reason being, that DoFocus is based on magnification, so Telephotos have a higher magnification ratio than wide angles.