mwsilver wrote in post #18545813
Like some here I have used the Dofmaster online site for DoF calculations. Over time I have also downloaded and tried out a variety of DoF apps for my Android phone. What I had quickly discovered was that the results from the Android apps almost all differ from each other as well as from Dofmaster, sometimes by large amounts. Most of the time, the difference is not significant enough to cause any problems. However, I wonder if others have seen the same thing, can identify the best phone app to use, and explain why the calculations would differ from app to app and to Dofmaster. In my ignorance, I would assume that these calculators are using some sort of standard DoF calculations and should be almost identical. I have searched online on several occasions but have not yet found a useful answer
Here is the REASON for the lack of consistency between various DOF programs...
First begin with the understanding that the CofC value is based upon an ASSUMPTION of using an 8x10" print which is viewed from 12" away, and the actual DOF calculator CofC valueassumes some level of magnification of the in-camera image to arrive at final 8x10" print size... APS-C image is enlarged about 13.5X, FF image is enlarged about 8.5X; the CofC for each format size is simply
CofC (at 8x10) / mag factor used to make print
Now the important part (pertinent to your question), you need to understand that the CofC value (in the 8x10" print) makes assumptions about the viewer being able to SEE a 'blur circle' which is the out-of-focus representation of a perfect point.
Most DOF programs assume 'manufacturer standard' and that is far less visual acuity than the 20/20 goal that optometrists try to get our corrected vision to equal! And even then there is NOT a single universal value used by all...
- The DOFmaster that you linked assumed APS-C CofC of 0.019mm
- The so-called 'Zeiss formula' is a supposed formula for computing a circle of confusion (CoC) criterion for depth of field (DoF) calculations.
The formula is c=d/1730,
where d is the diagonal measure of a camera format, film, sensor, or print, and c the maximum acceptable diameter of the circle of confusion, so 'Zeiss formula' value of CofC is 0.01563
- the common 'manufacturer standard'... "All the camera lens manufacturers in the world including Carl Zeiss have to adhere to the same principle and the international standard that is based upon it (0.03mm for the 35mm format), when producing their depth of field scales and tables." or an APS-C CofC of 0.0187mm
... yet Zeiss itself gives the values d/1000 as the traditional standard and d/1500 as the modern standard. If d/1500, CofC is 0.0180
The fundamental issue is also what is assumed to be the quantification of human visual acuity ... which is often stated as detail as small as 'one minute of arc',
and yet you also read it described '30 seconds (one half
minute) of arc'