Bdtyk

This question is related to this one; I am asking a very similar one because I didn't see in the answers how to arrive to a numeric answer in practice.

Suppose I'm going to shoot a portrait and have most other parameters figured out:

Sensor size: full frame; focal distance: 100 mm; subject distance: 1.5 meters; desired DoF: something like "would like the face to be sharp and the rest look blurred on 8x10 prints".

Now I want to compute the aperture that would help me take that shot. I need to clarify two things for that:

First, I need to make make the notions of "sharp" and "blurred on 8x10 prints" numeric; that is, I have to decide what's the size of the circle of confusion on a full frame that separates "sharp" from "blurred".

Second, I need to assign a numeric value to the depth of focus that would make the face sharp and the the wall behind the subject blurred.

Thus the questions:

1. What numeric values for the circle of confusion correspond to the above scenario?




2. What DoF values are appropriate for portrait shots?

With full frame, use f/8 with appropriate lens (maybe 100 to 120 mm) to allow 8 or 10 foot subject distance, and focus on the near eye, and you'd do fine (with the subject sharpness). That is perhaps 7+ inches of total DOF span, with subject in the middle.

Lenses are "sharp" at only the one focused distance, and become progressively less sharp with distance away from focus.

Full Frame CoC is typically used as 0.03 mm, which in practice has seemed mostly reasonable. CoC only provides a limiting guide for calculating DOF numbers for an 8x10 inch print (meaning, at the DOF extreme near or far range, CoC WILL BE 0.03mm diameter blur there of a point source. The 0.03 mm on the full frame sensor is historically judged to Not be very detectable by the human eye on an 8x10 inch print (viewed at about 10 inches). When CoC grows to 0.03 mm with distance, DOF calculations announce that is the DOF limit distance). Whether that result pleases you or not is your own decision. If it does, then DOF calculations can tell you if your focal length, distance and aperture will provide that range. Deciding a different CoC does not affect the image sharpness in any way. It simply produces different DOF numbers which you may find more appropriate (or not), but it does not change the image in any way. The lens does what it does. DOF tries to compute what it does, based entirely on the decided CoC guideline that you find acceptable.

Cribbing liberally from Wikipedia's Circle of confusion article, then with the information you provided, you can calculate what the CoC should be.

In the following equation,

• 
  ƒ is the lens's focal length;


• 
  N is the f-number (aperture) of the lens;


• 
  S₁ is the distance to the in-focus subject;


• 
  S₂ is the distance beyond which you determine to be not in acceptable focus.

Then the formula for your CoC is (remember to be consistent with units):

So for your 100 mm lens (I'm going to assume ƒ/2.8), focusing on the tip of the person's nose at 1.5 m away, and giving a far depth of field of 2 cm (0.02 m), then the CoC is calculated as (0.02/1.52) * (0.1²/(2.8 * 1.4)) = 0.033 mm. This is somewhat close to the 0.03 mm cited by WayneF in his answer. Note that it sounds close, but that 10% difference in CoC size actually translates to a (relatively) huge difference in depth of field.

For 8" × 10" prints viewed at about 10" away, typical visual acuity results in a CoC of 0.029 – 0.030 mm.