How to Calculate Depth of Field
You may probably know it already, but depth of field, in simple words, describes how much of your subject or background will be in focus. Wiki link.
I had a little discussion with my friend, fashion photographer. He always shoots his studio photos with apertures from f9 down to f13. I told him that the lens he uses is sharpest around f6.3, but then he said, OK, but my subject would not be in focus as I want it to be.
He shoots with Canon 5D Mark II camera and Canon EF 135 f2L USM lens. His distance from the model is around 13 feet (4 meters), his background (backdrop) is another 10 feet (3 meters) behind the model. So I wondered if my suggestion of using f6.3 was good or wrong.
Where, When and Why
Wherever and whenever you can. To save time and look professional :)
Missing aperture setting to get whole model in focus means you need to shoot over and over again. Off course that with super L lenses and great top end cameras you could do it easily, just put f22 when you can and that is it. But for us “mortals” that have lower end gear where every single f stop means more sharpness it is very useful to know which are best settings to get in focus what you need and not more.
Trial and Error
It is not bad to test it on the spot, but keep in mind that camera’s LCD is not always the most accurate tool to check DoF as smaller displays tend to show images sharper than they really are. It would be best to carry your laptop to check your DoF and then proceed with shooting using the best settings.
Fore example: when I just got macro 100mm f2.8 lens, I was trying to get bugs clear at f2.8, and from start I was wondering how come that nothing is in focus, or so little. By all calculations when you shoot close with 100mm on FF camera, if you are around 30 centimeters (12 inches) from the subject, only 1mm will be in the focus. I tried it and at first in viewfinder it looked like more is in focus, while zooming in or check on laptop showed me that true in focus range was really around 1 mm.
To see how far or how close you need to be from the subject and with which settings, it is best to use Depth of Field Calculator.
Depth of Field Calculator
Calculator has everything, focal length (reel length written on your lens), selected f-stop and subject distance in feet, inches, centimeters or meters.
So I input his data:
Lens Focal Length: 135mm
My f-stop advice: 6.4 (it has no 6.3 but 6.4 will be just fine)
Subject distance: 13 ft (4 meters)
The result is that it will be 1.06 feet or 33 cm of focus spread, in front of the focus point 0.51 ft (0.16m) and behind the 0.55 ft (0.17m).
So it seems that my suggestion was not good enough. Everything except face and body that sticks out would be out of focus. Better results would be at f9 but to be sure that the whole model is in focus, f13 would be the best choice. Of course you can shoot on which ever settings you like, but it is nice to know where boundaries are.
Hyperfocal distance is basically the distance that you can focus to at any given aperture, where the resulting depth of field will retain sharpness in the scene from as far away as infinity to the (hyperfocal distance / 2) point.
Read about it: http://en.wikipedia.org/wiki/Hyperfocal_distance
Calculator and wiki indeed match, if you put 50mm at f8 you would get Hd at 34.3 feet (10.5m). Meaning that everything from that 17.15 ft (34.3ft/2) or 5.25m (10.5m/2) spot up to infinity will be in the acceptable focus.
Facts about Hyperfocal distance: