View Full Version : Depth of Field Calculators - Formulae

Honest Gaza

11-07-2006, 02:28 PM

There are many DOF calculators available on the Internet. I have a couple of specific questions relating to these calculators.

Question 1.

To keep it simple, I will use a 50mm Prime lens.

Is the focal length 50, or 50 x 1.5, or 50 x 1.6 (multipliers are different for each brand of camera body).

Question 2. (once Q1 is answered)

Will the calculations be correct for a given focal length even if different lenses are used

(eg. 17-70mm lens at 50mm....and 28-80mm lens at 50mm.....assuming same aperture was used).

I'm assuming that there are specific formulae used to calculate these DOF measurements....are they easily understood so I can create my own chart ?

I realise there are chart creation tools available on the Internet....but am unsure of their accuracy.

gmtech79

11-08-2006, 05:46 PM

I have the expo aperture (same people that makes expodisc) and it gives a conversion chart for 1.5 crop factors which come to the same thing if you multiply the focal length by 1.5. So in your example depth of field would be calculated using 75mm focal length.

Lens type has no effect on the calculations. 50 is 50 no matter what lens you use, just be sure to calculate depth of field with your conversion factor.

bugguy

11-11-2006, 10:16 PM

The focal length will be the actual focal length, so it will be 50mm with a 50mm lens, not 50mm x 1.5 or 1.6. The actual dof for a certain lens at a given aperture will be the same for any camera. The reason it seems different for full-frame cameras vs. ones with cropped sensors is because with the cropped cameras, the images is effectively cropped, so proportionally a larger part of the frame is in focus.

The formula for the distance to the near focus point is

Dn = (d * f^2)/(f^2 + N * c* (d - f))

Df = (d * f^2)/(f^2 - N * c* (d - f))

where

Dn = near limit of the depth of field

Df = far limit of the depth of field

d = the distance which you are focused at

f = the focal length of the lens

N = the f-number

c = the circle of confusion

The circle of confusion is the size of a point projected on the sensor by the lens. The value you choose will basically be the largest amount of blur which you still consider to be "in focus". A good size would probably be around the size of your pixels.

The Wikipedia (http://en.wikipedia.org/wiki/Depth_of_field) article on dof goes into much more detail with more formulas.

gmtech79

11-12-2006, 06:25 AM

The focal length will be the actual focal length, so it will be 50mm with a 50mm lens, not 50mm x 1.5 or 1.6.

Wonder why then with expoaperture they send a focal length conversion table to use to calculate depth of field for 1.5 and 1.6 factors.

bugguy

11-12-2006, 10:43 AM

I'm not very familiar with expoaperture, but one reason I can think of is that given the same lens, you would need to be further from the subject with a cropped sensor than with a full frame camera to have it fill the same percentage of the frame. So if you move further away from the subject with the cropped sensor, so as to frame the subject the same, the dof will be greater than with the full frame camera due to your greater subject distance. However, if you are the same distance from the subject, and use the same lens and f-ratio, the amount of the subject in focus will be the same, assuming the pixel size is equal. This (http://www.cambridgeincolour.com/tutorials/digital-camera-sensor-size.htm) website has some info on the effects of sensor size on depth of field, plus a couple calculators.

bugguy

11-12-2006, 12:19 PM

After thinking about it a little more they probably have the focal length conversion table to adjust the circle of confusion. Generally, the circle of confusion is calculated based on making a 8"X10" print, which is about a 7x enlargement for 35mm film, but for 1.5x and 1.6x crop sensors is a 10.6x and 11.3x enlargement, respectively. For 35mm film the standard value for the circle of confusion is 0.03 mm. Based on this you would want to use 0.03mm/1.5 = 0.02mm for 1.5x crop sensors and 0.03mm/1.6 = 0.01875 mm for 1.6x crop sensors.

Honest Gaza

11-12-2006, 04:39 PM

Thanks Bugguy :)

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