# OMG...tell me thiis guy is wrong about micro four thirds

Started 6 months ago | Discussion thread
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 Format, Viewing Size, Viewing Distance Independent Background Blur Metric In reply to clengman, 6 months ago

clengman wrote:

... The maximum blur disk diameter depends on the absolute aperture of the lens and the subject distance and is most usefully (I think) expressed as a percentage of the frame height. This will tell you how blurred the background looks.

I decided to try ditching this messy Hyperfocal Distance and Depth of Field stuff altogether, and used Merklinger's "Object Field" model in order to derive a mathematical identity that states the percentage of the total image-frame diagonal dimension that the blur-disk (Merklinger's "Disk of Confusion") represents (along the lines of what you are describing above).

B = ( (100) / ( (F)(H) ) ) x ( ( (L)/(D) )^(2) ) x ( Df - D ) / ( 1 + (M)/(P) )

where:

B is the percentage of the image-frame diagonal that the blur-disk represents;

F is F-Number;

H is the diagonal dimension of the image-sensor;

L is the Focal Length (when focused at infinity);

D is the Camera (front nodal-plane) to Subject (plane-of focus) Distance;

Df is the Camera (front nodal-plane) to Background Subject Matter Distance;

M is the Image Magnification;

P is the Pupillary Magnification.

In most situations, M is close to zero, so even if P is less than one (as in the case of a telephoto lens-system), the divisor of ( 1 + (M)/(P) ) will have a value that is close to one, so:

B ~ ( (100) / ( (F)(H) ) ) x ( ( (L)/(D) )^(2) ) x ( Df - D )

So, there you go, my friend. Number-crunch away. Last one in the pool is a "rotten egg" ...

Edited 6 months ago by Detail Man
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