Question about effect on Depth of Field of Zoom vs Macro

[fdog]

Just did an experiment in my apartment (will be glad when wetsuit season gets here) and it seemed as if I got better depth of field when I was about 4 ft away and used zoom vs 8 inches away using macro (with these settings, object was scaled to same size). Does this make sense?

Assuming you used the same f-stop; yes, this makes sense.

Without getting into the mathematical stuff, DOF is not a fixed % of the focus distance. Example:

60mm lens, 35mm film, f 2.8

Focus distance 10': Near @ 9.4' , Far @ 10.6', Usable depth 1.2'
Focus distance 20': Near @ 17.9, Far @ 22.7', Usable depth 4.8'

In the example above, you would expect that doubling the distance would double the usable Depth of Field. Instead, there is 4 x as much.

So, the closer you are, the "thinner" your usable DOF becomes, even more than is implied by the smaller distance.

All the best, James

 

[dlwalke]

Thanks so much for the very informative posts! The aperture was the same at the two distances. I've recently conducted a few more experiments in which the results were not as clear cut. I think that the opposing influences of zoom and distance, at least at the range I was shooting from, come close to canceling each other out and other factors may exert an influence nearly or more so as great.