DoF may not exist...

Started 3 months ago | Discussions thread
Greg7579
Greg7579 Forum Pro • Posts: 14,041
Re: The reason for this thread...

Erik Kaffehr wrote:

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.

Not anymore. That's the whole point. Photographers then were not seeing what we see now. Plus they were not shooting GFX and they were not doing post on powerful computers with these new fantastic monitors which are only getting better every year.

Erik, it is time. Fuji, it is time. Forget the math and CoC and do the work by shooting in a lab and in the wild and stating when the pixels start to lose it. Please.

Someone (Fuji) please do DOF tables for all of the GF lenses and please do not even mention CoC in them. Just do the tables and state the distances at all the apertures for each lens at each focal length of the zooms.  That's a lot of stuff, but easy to plug in on an app.

II don't need it because I have shot it so much.  But it would get the base-line right and stop these threads from popping up on this Board every day. 😁

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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