F-16 Isn’t Magic

I’ve been giv­ing photo courses lately and I’m com­ing across a few points that peo­ple are reg­u­larly hav­ing trou­ble with. The lim­its of depth of field (or how sharp objects should be in gen­eral) is one of the things that many pho­tog­ra­phers don’t com­pre­hend. This is often because they are aware of only one of the three fac­tors that deter­mine depth of field, namely the aper­ture. Many of us know that when we use a small aper­ture we get good sharp­ness from fore­ground to back­ground ver­sus large aper­tures. But this is true only up to a cer­tain point because two other fac­tors are miss­ing.  A small aper­ture like F-16 isn’t a magic one that will give you great sharp­ness from fore­ground to back­ground in all cases.

Take the fol­low­ing image called Rust for exam­ple. It was cre­ated by Crash­cat from our forum for our monthly assign­ment called - June 2012 — f16 or smaller– Shoot­ing with a small aper­ture. Thx Crash­cat for the use of this image.

This image was shot at ISO 1600 f/16 at 1/20 using a 105mm lens.
As we can clearly see the depth of field here is shal­low and this is because there are two other fac­tors besides the cho­sen aper­ture that influ­ence depth of field. These fac­tors include the dis­tance from the object we are pho­tograph­ing and the focal length we use.  As we  approach an object, depth of field dimin­ishes. The longer the lens we use the less depth of field we will have ver­sus using a shorter one.

The image we are look­ing at is a macro image and so the cam­era is very close to the object. Had the cam­era been far­ther way, we’d see more sharp­ness from the top of the screw to the bot­tom of the screw. Not tons more sharp­ness mind you, but more. The side effect is that the screw wouldn’t have the mag­ni­fi­ca­tion that it does and would look less ‘close-up’.

Had this lens been wider, we’d also see a small increase in sharp­ness from the top of the screw to the bot­tom of the screw, but again the screw’s per­spec­tive would seem smaller.

There is no easy answer here. It’s just a mat­ter of prac­tis­ing and know­ing what to expect.

For those that are look­ing for fab­u­lous pre­ci­sion, feel free to use a depth of field cal­cu­la­tor which will show you the depth of field you can expect under any shoot­ing condition.