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lonepal
Joined: 28 Jan 2017 Posts: 217 Location: Turkey

Posted: Fri Nov 10, 2017 2:28 pm Post subject: MPE 65 DOF Calculation For f/2.8 and f/4.0 


Hi;
I used http://coinimaging.com/calculator.html? for DOF calculations and just asking to approve my calculations.
For example;
DOF@f/2.8, COC:0.018 for 5X = 0.0242mm
DOF@f/4.0, COC:0.018 for 5X = 0.0346mm
Somebody please approve the calculator is right
Thanks. _________________ Regards.
Omer 

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rjlittlefield Site Admin
Joined: 01 Aug 2006 Posts: 18858 Location: Richland, Washington State, USA

Posted: Fri Nov 10, 2017 6:29 pm Post subject: 


Those numbers match the classic ray optics geometry model: 2*C*F*(m+1)/(m*m) for C = circle of confusion, F = nominal Fnumber, and m = magnification, assuming focusing by extension using a lens with pupil factor = 1.
That formula ignores diffraction, which seems like a pretty fundamental mistake because we're always working in the regime where diffraction is important. But in this case (and many others) it works out OK because the Fnumbers are in the critical band where the Airy disk spot size is about the same as COC. That condition makes the geometryonly formula give numbers that are close to what the fundamentally more accurate wave optics model says.
To be specific, the wave optics numbers for 1/4lambda wavefront error at 550 nm are 0.0248 for f/2.8, and 0.0507 at f/4. The larger DOF predicted by wave optics for f/4 is because F/4 at 5X gives effective f/24, which has an Airy spot size significantly larger than 0.018 mm so the classic formula is pessimistic. For smaller fnumbers it goes the other way and the wave optics formula becomes pessimistic because it does not account for the limitations of sensor resolution.
The reality is that there is no crisp line between "sharp" and "not sharp". The image just becomes gradually more blurred as you get farther away from perfect focus. If you are focus stacking, this means that larger steps produce gradually worse focus banding. For sufficiently small steps you can't see the banding at all; then it becomes maybe detectable with careful study; then clearly detectable with careful study; then immediately obvious to an expert; and so on, until at some point even a firsttime viewer would say there's something wrong.
As a result, the judgement of what is an acceptable step size will vary from one photographer to another. Even with the same photographer it can vary from one combination of subject/lighting/lens/application to another.
Use the calculations for guidance. If in doubt, go smaller. If you need a precise number, determine it by experiment.
Rik 

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lonepal
Joined: 28 Jan 2017 Posts: 217 Location: Turkey

Posted: Fri Nov 10, 2017 11:51 pm Post subject: 


Thanks for the explanations Rik!
Then why we do not create a DOF sheet for database for the most common lenses?
When somebody need the average DOF values, for example for MPE 65, they can use the sheet as a reference.
That's just an idea. _________________ Regards.
Omer 

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rjlittlefield Site Admin
Joined: 01 Aug 2006 Posts: 18858 Location: Richland, Washington State, USA


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