Anonimo
•

Contributing Member
•
Posts: 583

Long answer (better DoF calculator, math, etc.)

Hi Ed!

Links

Excelent article about lens equivalence:

http://www.josephjamesphotography.com/equivalence/

Better DoF calculator.

http://www.cambridgeincolour.com/tutorials/DOF-calculator.htm

The answer:

can_ka_no_rey
wrote:

Example:

24mm f/1.4 10 feet distance:

5DII: total DOF = 4.69ft.

1DmkIII = 3.52ft.

50D = 2.88ft.

I'm lost. This means "the smaller the sensor, the shallower the DOF"?

The math

From the optics, if s is the focusing distance, f is the focal lenght, N is the f-number, C is the circle of confusion at the print and E is the enlargement factor (i.e. the ratio between print size and sensor size) then the DoF is given by the distances Dn (near) and Df (far) from the lens, calculated as follows:

1/Dn = 1/s + (CN/Ef)(1/f - 1/s)

1/Df = 1/s - (CN/Ef)(1/f - 1/s)

Now the trick when comparing croped versus full frame is to understand that:

Now let's compare the DoF of prints of the same size in different scenarions:

a) 50mm f/10 shoot from a 50D versus 50mm f/10 shoot from a 5D croped to the same angle of view as the 50D. We have the same s, f, C, N , E.
There is no difference
.

b) 50mm f/10 shoot from a 50D versus 80mm f/10 shoot from a 5D. We have the same s, C, N and Ef but f is different.
The 5D has a shallower DoF
.

c) 50mm f/10 shoot from a 50D versus 80mm f/16 shoot from a 5D. We have the same s, C and Ef. We stick with N(1/f - 1/s) = N/f - N/s, where N/f is the same but N/s is different. The 5D has a shallower DoF.
However
: if s is large compared with f (i.e. if you are not doing macro) then 1/f - 1/s ~ 1/f; once N/f is the same the difference is neglicenciable.
This is the
lens equivalence
.

Notes about DoF Master:

DoF Master adopts an arbitrary circle of confusion C = 0,030mm for full-frame sensors and E=1, correcting that for the crop factor of the selected camera. This number comes from an international norm for the calculation of DoF scales in lenses. Carl Zeiss claims that this norm was set long before the WW-II and was based on the resolution provided by film emulsions at the time (check page 3 of newsletter below).

http://www.zeiss.de/C12567A8003B8B6F/EmbedTitelIntern/CLN01e/
$File/CLN1.pdf

For comparision: the pixel-pitch of an EOS 5DMkII is 0,0064mm.

I recommend the DoF calculator from Cambridge-in-Colour, which uses print size, viewing distance and visual accuity to estimate a circle of confusion.

Regards,

Anonimo