Depth of Field and Focal Length


Today’s Question: I am hoping to get some insight on how Depth of Field is influenced by the variables of Aperture, Subject Distance and Lens Focal Length.

I have recently come across websites that indicate that the common assumption that Depth of Field is influenced by Aperture, Subject Distance and Lens Focal Length is in fact, erroneous.  Specifically, the first 2 variables, Aperture & Subject Distance do affect Depth of Field (DOF), while Focal Length has no effect on DOF.

To further confuse matters, a number of reputable web sites provide online DOF calculators (and also downloadable Apps). Why is focal length included as a factor to be added to the calculation for DOF?

Hoping you can provide ‘clarity’ on seemingly contradictory situation.

Tim’s Quick Answer: I would argue that lens focal length does indeed impact depth of field. A longer focal length will result in less depth of field. That said, I would also argue that distance to subject and lens aperture to be the more important variables to consider.

More Detail: Those who argue otherwise generally stipulate that if you frame the subject the same way, suddenly focal length is not a factor. But in order to accomplish that change in framing you must also change the distance to the subject. In other words, you would be changing one variable (distance to subject) in order to pretend that another variable (lens focal length) was not a factor.

In other words, I could also argue that distance to subject has absolutely no bearing on depth of field, but that instead it is a change in focal length that alters depth of field. If you change your adjust your focal length you obviously must change your distance to the subject to maintain the same framing, so I could argue that it is the focal length (and not the change in distance) that caused the change. But that is obviously (I hope) not a reasonable argument.

In reality, all three factors (distance to subject, lens aperture, and lens focal length) impact the final depth of field you will achieve. And in fact there are additional factors, such as the overall image sensor layout (the size of each “pixel” on the sensor, for example) that also have an impact on depth of field.

To be sure, the calculations involved in determining depth of field for a given photographic situation can be a bit complicated. But as far as I’m concerned there is no question that distance to subject, lens aperture, and lens focal length play the primary (though not exclusive) role in determining final depth of field.