OOF blur disk factor across formats?

[shaocaholica]

Given the same FOV (different focals) and same subject distance across different formats, how do you calculate the number of aperture stops it takes to render frame relative sized blur disks?

I hope that wasn't too hard to read. Basically we all know that with the same FOV and subject distance, you can render OOF disks on larger formats with slower apertures as compared to smaller formats. My question is, how do you calculate, even if roughly, how many aperture stops there are between two formats to render the same frame relative sized blur disks, ie the same looking image.

 

[i.candide]

Is this the same as 'circle of confusion'?

i.candide
Dangerous to oneself

 

[spijker]

Use a DOF calculator like Dead Link Removed. In Barnack, you can specify the Circle of Confusion / Out of Focus Disk (I assume that is what you mean) size for 35 mm film as a reference and the tool will automatically scale that to the selected format. So you can see what aperture is needed for a certain depth of field in various formats. Barnack lets you add new formats (LF) to the database or edit existing ones (MF).

The standard 0.030mm CoC for 35mm film seems too optimistic for the current state of film and lens resolution. It's more realistic to use smaller number; 0.020 .. 0.010mm.

 

[shaocaholica]

Ah, thanks. That will work. I think I'll play with the actual equation and see if I can find the mathematical relationship between [blur circle size/horizonal format] given a fixed set of FOV and subject distances.