Monday, April 20, 2009

Depth of Field

Orphi's asked me to go over this one more time and to be honest I can't blame him it's a bit screwy. So I'll do a simple and complex explanation:

Simple - The smaller the aperture the more will be in focus, now as the aperture appears to be marked backwards from what would seem logical the way to remember this is that the bigger the number the bigger the field.

Complex - Using the DoF calculator for a 1/2.5" sensor plug in a focal length of 6mm (The default of Orphi's camera) and a focal distance of 2.6m now set the aperture to 2.8 (wide) and you'll see that the camera will focus on everything from 1.259m to infinity; now set the aperture to 8 (narrow) and you'll see it's changed to 0.643m to infinity.

For most of Orphi's landscape shots none of this will show up as nothing appears to be closer than 1.2m so everything remains in focus.

Now as I said with his shot what if he took a few steps (say 1m) back and zoomed in (to 12mm)?

Focusing on the same point which is now 3.6m away and flicking between the two apertures we see that at f/2.8 the DoF is 2.631m to 3.069m at f/8 it's 1.754m to infinity. So suddenly the background will be out of focus and possibly some of the foreground.

This is where the hyperfocal distance I dealt with before is required. If you focus on anything after that point then everything after that point will be focus, everything that's roughly half that distance before that point will also be in focus.


Orphi said...

OK, so let me see if I can get this straigh.

Wide apature = more brightness + less focus.
Narrow apature = more focus + less brightness.

Is that the correct way round?


High ISO number = more brightness + more grain.
Low ISO number = less brightness + less grain.

Is that right?

(And if so, why the heck would a camera have “low ISO noise filtration”?)

Finally, is the possible DoF related to the degree of image magnification? Or are they independent?

FlipC said...


High ISO means more sensitivity to light so yes more brightness and more 'grain'

As for "low iso noise filtration" remember although similar to film in output the reasons for noise are different for digital sensors.

Yes DoF is related to image magnification. See the end of my complex description for how it alters if you'd stepped back a metre and zoomed in.