How to Calculate Depth of Field

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

You can find online complex formulas and tables, but best solution I found is – Depth of field calculator and table view here: DoF table.

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.

 

dof-calculator
DoF Calculator: http://www.dofmaster.com/dofjs.html

Hyperfocal distance

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:

– Smaller aperture you set, closer hyperfocal distance gets (example f22 will have closer hyperfocal distance than aperture set at f2.8)

– Small focal length lenses will have a closer hyperfocal distance value (example 10mm will have at f22 hyperfocal distance at a subject distance of 1 feet while 300mm with the same aperture at around 400 feet)

– While behind focus point can be infinity in front there will be different, there is “rule" that says Hd/2, meaning that everything from the camera up to Hd/2 is not in focus

It is good to know hyperfocal distance; it’s good to know how much of the background and how far from the background your subject has to be in order to separate the subject from the background.

Circle of Confusion

In calculator above, there is one more term: “Circle of confusion“. But I found better explanation here: https://www.youtube.com/watch?v=Pdq65lEYFOM

Conclusion about DoF calculator and other tools

Many photographers practice trial and error technique as they want to show their knowledge. I choose to go to my set prepared and armed :) Knowing what to expect and how to get desired effect. Using calculator, you will learn fast and soon enough you will get to the point where you would know where to stand and what would be in the focus for sure. There is no shame in using tools that will help you produce what you need and want faster, use DoF calculator or DoF table if you need to :)

About Tommy

Photography allows me to be what I want to be, to be where I want to be, and to do what I want to do ... I'm not professional photographer and I don't need a title, I love to take photographs and that is what I do, I love to learn and I always try to do it better ...

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