More on DOF

Using a very shallow Depth of Field (DOF) is problematic. Often it can result in one eye of the subject being out of focus. And it can result in out of focus images if the "focus and recompose" technique is used. 

So, how do the different factors affect DOF? As a general rule-of-thumb:

 

Doubling the aperture number also doubles the DOF (2x)

Doubling the working distance quadruples the DOF (4x)

Doubling the lens focal length quarters the DOF (1/4x)
**Note: these rules start to fall apart when working close to the hyperfocal distance where you have a large DOF.... but then that's not usually when DOF is of concern.

 

Doubling the aperture does not affect the field of view captured and it also (generally) increases the lens' sharpness, so it seems like the obvious choice. But it also requires an ISO 4x (2 stops) higher or a shutter speed 4x (2 stops) slower (or one of each...you get the idea).

 

Doubling the working distance does not increase the lens' sharpness and it also doubles the captured Field of View (FOV) while giving 4x the DOF. However, if you use a lens that is twice as long to get 1/2 the FOV (i.e. to maintain the original FOV) the lens increase then cuts the DOF back by .25x and the final DOF remains the same. 

To change the DOF by changing focal length or subject distance you *MUST* compromise on the composition (captured FOV).

 

But often this is not a big deal. Shallow DOF is often problematic due to short subject distances and in those cases small increases in working distance do not affect the FOV that significantly and the final image can readily be cropped to the desired composition. For example, if you are working from 7ft increasing the working distance to 10ft (~50% increase) will not greatly increase the FOV but it will double the DOF. Try it for yourself!

 

Personally, I don't care about "Bokeh." I care much more that what I want in focus is in focus. And, as a photographer your job is to "control" the background, not "throw it out of focus." Shallow DOF to throw the BG out of focus is a "technique" to help you when you cannot control the BG, but it is "more correctly" used to "create a feeling."

 

I created this image by placing black faric behind the subject.

Thistle PS'ed

 

And I created this image by choosing a position that put the BG (trees) about 70yds away.

PPTHHPTHPFFTHPPPT!!!

 

Neither was taken with a lens/camera particularly known for it's "bokeh" nor with a shallow DOF chosen to blur the BG.

 

 

DOF calculators will tell you how much DOF you will have for a certain lens/camera/aperture/distance based upon some "constants," the circle of confusion (COC). The only thing to note is that if you are not using the constants (i.e. display size/ viewing distance) then the DOF will be different, you need to use a different COC. And if you have no control over the display size or viewing distance then the DOF will vary for different situations....DOF is *not* a fixed aspect of an image.

 

Another thing to know about DOF and DOF calculators is that they tell you nothing about the out of focus (OOF) portions of an image. A shallow DOF means the image has a hard/rapid transition into and out of focus. The harder/quicker the transition is to OOF, the more OOF that area will be. Even small changes to the DOF can make a significant difference in the way the OOF areas appear.

 

 

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