DoF may not exist...

Started 3 months ago | Discussions thread
Erik Kaffehr
OP Erik Kaffehr Veteran Member • Posts: 6,177
The reason for this thread...

One reason is that DoF (Depth of Field) is a concept of acceptable sharpness. It is a very simple concept that had served photographers well for over hundred years.

At the same time, photographers knew that DoF was a simplification, based on human visions and expected viewing conditions.

Some of the basic assumption were:

  • Closest viewing distance would be around 25 cm or 10", that is based on normal human vision. 
  • Prints were not very large and larger pictures would be viewed at longer distance.

That led to a formulation where the acceptable blur was 1/1500-th of the diagonal of the negative. Using that figure, the acceptable blur circle for 24x36 mm is 43 mm/ 1500 -> 0.029 mm, this has been rounded to 0.030 mm.

Doing the same math 6x7 cm film, assuming a real frame size of 55x69 mm, the figure would be 88/1500 -> 0.06 mm.

0.030 and 0.060 mm were used for calculating DoF scales for 24x35 mm and 6x7 cm lenses.

Now, if we would look at nearest and most distant points of acceptable focus when using the DoF scales on the lens and printing at the same size, those points would be equally sharp.

But, I would assume that most photographers moving from 24x36 mm to 6x7 cm wanted sharper images.

One way of achieving that is to stop down more. So, shooting 67 cm we may use twice the f-numbers as indicated on the lens barrel.

But, stopping down increases blur, so at some point blur from diffraction exceeds the size of the geometric blur. Stopping down more does not gain more sharpness. Germans call it förderliche blende.

Jim Kassons curves are calculated from several hundred images, so it could be said that the curves say more than hundreds of images.

In a way, DoF tables served photographers well.

But, the digital era is a bit different. In the film era we did not print that large so often and at least I usually checked slides using a 15X loupe. That was not very convenient.

With digital, print sizes may have increased and we often view images on screens that are large compared to the 8"x10" prints we often had in the film era.

Also, we can zoom into images to great depth. That essentially means that DoF parameters based on film era print sizes need to be modified.

Best regards

Erik

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Erik Kaffehr
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