### DOF and circle of confusion

greenspun.com : LUSENET : Large format photography : One Thread

A while back I read a chart wich would give the correct F- stop setting based on near to far standards movement. In other words focusing on the nearest point then measuring the distance on the monorail to the farthest point entering that distance to a scale which would gine me the proper F stop. Since purchasing the Rodenstock Studio calculator I have learned that monorail angle and repro ratio are also factors. My problem is that the Rodenstock calculator is"based on a Circle of C.. of .03mm for 35mm.IYO is this sufficent for large enlargement? Are there other scales out there? I use 4X5

-- Roy Feldman (royfel@hotmail.com), August 28, 1998

#### Response to DOF

The circle of confusion for 4x5 is usually set at .15mm or 0.09 mm. You will find some online calculators here: http://home.sol.no/~gjon/depth.htm

-- Jon Grepstad (gjon@online.no), August 28, 1998.

#### Response to DOF

Thanks for the response, I had no idea there were so many DOF scales out there but none really answer my question. Based on measurment of movement of lens standards ONLY is there a DOF scale out there? Mr. Grepstad's answer was overwhelming, could you narrow it down?

-- Roy Feldman (royfel@hotmail.com), August 28, 1998.

#### Response to DOF

An article in Photo Techniques (Mar/Apr 96) on view camera focus used this method. Basically, you tape or glue a mm scale on your bed, and then focus on the nearest object of interest, read the position on the mm scale, then focus on the farthest object of interest, and note its position on the mm scale, and then, using this focus spread (in mm), refer to a chart for optimum f stop to use, based on line pairs per mm resolution. I photocopied the chart and taped it to the back of my camera for reference. I hope this helps.

-- Ron Shaw (shaw9@llnl.gov), August 28, 1998.

#### Response to DOF

The scale mentioned by Ron sounds as if it is based on the formula t=2.C.N', where t=depth of focus (not field), C is diameter of Circle of confusion, and N' is effective aperture, probably assumed here to be equal to the marked aperture. Thus the scale is independant of focal length (neat, huh?). The repro factor is relevant for closeups.

Of course, one advantage of LF is movements, especially puting the lens at an angle to the film. See http://fox.nstn.ca/~hmmerk/ for the gory details.

-- Alan Gibson (gibson.al@mail.dec.com), August 28, 1998.

#### Response to DOF

Mr.Shaw has gone "above and beyond" by offering to snail mail the article mentioned in his response. Mr. Gibson your formula sounds great but having attended art school I'm afraid I don't have the where with all to figure it out.I know that in 35mm a COF of .03 is low end and .01 equals what film can be resolved at,I have no idea how or why this translates to 4x5.Is it possible,Mr. G. to give a example a art school grad can follow? ( I know I should have listened to Dad and taken some "real" classes, but that was 20 years ago).

-- Roy Feldman (royfel@hotmail.com), August 28, 1998.

#### Response to DOF

If you look at the DoF marking on a 35mm lens, it will look something like this:

```+------+------+------+------+------+------+
|      |      |      |      |      |      |
22     16     8      V      8      16     22

```

I've only shown marks for f/8 and f/16.

On a 35mm lens, the scale is on a fixed part of the lens, and distance marks on the focusing ring can be compared to the f/numbers to see the DoF.

On a 5x4 camera, you may not have a distance scale. Suppose you focus by racking the front back and forwards. With a ruler and pen, you can create a paper scale that looks much like the one above, and hold it against the camera as you rack back and forth. If the total racking distance is the same as the distance between the two f/8 marks, then you have to stop down to f/8 or smaller.

Now for a bit of theory. Ignore this paragraph if you like. Depth of focus refers to the total distance we could move the film backwards and forwards, while the image still looks sharp. Call this distance "t". For distant objects, the value of t is approximately twice the diameter of the circle of confusion, multiplied by the aperture number. "t = 2.C.N". The value of C is often taken to be 0.03mm for 35mm cameras, or 0.1mm for 5x4. These numbers are not set in stone: Nikon used different values for their cheap (consumer) "E" series lenses than their more expensive regular lenses.

Using C = 0.1mm, here are the value of t for various values of N.

```N    t (mm)
===  ======
5.6  1.1
8    1.6
11   2.2
16   3.2
22   4.4
```

t is the total distance, so the two f/8 marks should be 1.6mm apart, so each of them is 0.8mm from the central mark. So your scale will look like this:

```+------+------+------+------+------+------+
|      |      |      |      |      |      |
22     16     8      V      8      16     22

<0.8mm ><0.8mm >
<--- 1.6mm ---><--- 1.6mm --->
<------- 2.2mm ------><------- 2.2mm ------>
```

Note that these distances are small; you will need a small pen.

Of course, if you are photographing a landscape, or other plane object, you would probably tilt the lens or film, rather than relying on depth-of-field to keep things in focus.