Depth of Field Calculation: Choosing an appropriate COC, Circle of Confusion in UULF

[Asher Kelman]

My "in progress" Camera Obscura will used Ilfochrome, Polaroid film, Transparencies/film; ISO's are 3, 100 and 400 respectively.

The direct to print paper will be viewed as is on the wall from about 10" or 20" with appearance of "reality" and no break up of the image. The camera will shoot at 1:1 using repurposed process camera lenses. I think that 8 lp/mm is needed, but that's just my guess.

How would you rate the paper in the camera for COC in order to do DOF calculation? Not sure which table to use as they include enlarging!

Would it be .15 mm or .3, what do you think?

We have 750, 760mm, 35", 37", 42" and 47 1/2" lenses

Key:

• Have already
  
• Plan to buy
  
• Considering

The pictures will be almost always taken at f 1/22 although it could also be sometimes at 1/16 or 1/32. Image size 20"x 72" to 50"x 96"

So how do we calculate the DOF? There's no magnification of the print/film!

Asher

 

[Doug Kerr]

Hi, Asher,

How would you rate the paper in the camera for COC in order to do DOF calculation? Not sure which table to use as they include enlarging!

The classical view of choosing a COCDL (circle of confusion diameter limit) is based on human visual acuity in some assumed viewing context for the delivered image.

Often, the COCDL will be chosen as the diameter that subtends 1/1500 radian of arc for the assumed viewing context. That's about twice typical human visual resolution (twice as "coarse" as human resolving ability).

Here of course I expect we will assume viewing of the image at full size, so the only parameter we must choose is "from what distance".

As a point of departure, let's assume viewing the image from a distance of 10 feet, or about 3000 mm.

Then the distance on the image that subtends 1/1500 radian at that distance would be 2.0 mm; since we are dealing with viewing the image at full size, that then would directly be a credible value of the COCDL.

If viewing from a distance of 5 feet were to be the predicate, then this would suggest a COCDL of 1.0 mm.

The rationale is discussed in Appendix A of these 70th birthday ramblings of some old Scottish guy:

http://dougkerr.net/Pumpkin/articles/Depth_of_Field.pdf

I assume you have various DoF calculators available. Just be sure that the one you use allows the COCDL to be entered directly with no "help" from the calculator, and allows the focal length to be entered directly, not based on working back from some "full-frame" 35-mm equivalent.

If all else fails, here is a spreadsheet:

http://dougkerr.net/Pumpkin/articles/DOF_calculator.xls

Best regards,

Doug

 

[Bart_van_der_Wolf]

My "in progress" Camera Obscura will used Ilfochrome, Polaroid film, Transparencies/film; ISO's are 3, 100 and 400 respectively.

The direct to print paper will be viewed as is on the wall from about 10" or 20" with appearance of "reality" and no break up of the image. The camera will shoot at 1:1 using repurposed process camera lenses. I think that 8 lp/mm is needed, but that's just my guess.

How would you rate the paper in the camera for COC in order to do DOF calculation? Not sure which table to use as they include enlarging!

Would it be .15 mm or .3, what do you think?

Hi Asher,

A print resolution of 5 to 8 lp/mm is a common range for good to very good resolution at a reading distance of, say, 10 inches. So that would be a COC of 0.1 or 0.0625 mm for inspection at such a short distance. When viewed from a larger distance the COC scales proportionally.

We have 750, 760mm, 35", 37", 42" and 47 1/2" lenses

Key:

• Have already
  
• Plan to buy
  
• Considering

The pictures will be almost always taken at f 1/22 although it could also be sometimes at 1/16 or 1/32. Image size 20"x 72" to 50"x 96"

So how do we calculate the DOF? There's no magnification of the print/film!

To get a 'full-image' overview, we usually have to look at it from a distance equal to the length of the image diagonal. That would give a viewing distance of 74.7 inches to 108.2 inches. That in turn is 7.47 to 10.82 times the 'normal' reading distance, so the COC=0.1 becomes a COC of 0.747 mm (=good) to 0.467 mm (=very good) for the 'smaller' image, and 1.082 mm (= good) to 0.676 mm (=very good) for the larger image. The diffraction blur at f/22 is not an issue at these sizes and viewing distances.

While that may seem comfortingly large COCs, at 1:1 (macro) shooting distance that still only gives a few (2-4) inches DOF (so, multi-row groups are not going to be all in sharp focus).

Cheers,
Bart

 

[Doug Kerr]

For those who may be unfamiliar with my term circle of confusion diameter limit (COCDL), let me elucidate.

In the areas of depth of field, depth of focus, out of focus blur performance, and the like, we often have to speak of three distinct things:

A. The "blur figure" created on the image from a point on the object that is not in perfect focus. This is called a circle of confusion. I call it a circle of confusion (COC).

B. The diameter of some actual or hypothetical circle of confusion. I call this the circle of confusion diameter (COCD).

C. The value of the circle of confusion diameter that we choose to consider as limiting in depth of field or depth of focus reckoning; that is, which defines the amount of blurring we will consider to mark the limits of the depth of field or depth of focus. I call this the circle of confusion diameter limit (COCDL).

In photographic work, we most often encounter item C, which is usually needed as an input to depth of field or depth of focus calculations. Sadly, this is commonly called the "circle of confusion" (COC), the actual name for item A.

Many people will say, "Well, yeah, but what's the harm?"

Well, one harm comes when we wish to, in a technical discussion, speak of item A. What would we then call that? The "fuzzy spot"?

This practice also leads to confusion when we need to mention item B (as we do in actual reckoning of out of focus blur performance, or in the derivation of depth of field or depth of focus equations). For example, in depth of focus blur performance reckoning, if I conclude that for a certain setup a point on a certain background object will generate a circle of confusion whose diameter is 0.1 mm, some people will say, "How can that be - I thought the circle of confusion (diameter) was usually about 0.031 mm?"

So that's why I call the circle of confusion diameter limit the circle of confusion diameter limit.

Best regards,

Doug