Unfocus

[Paulo26]

What´s the best maximum diafragm aperture f? For the effect of unfocusing the backgroud?

[shadowsports · Accepted solution]

Greetings,

The lower the f stop, the wider the aperture is.  This creates a shallower depth of field.  Meaning that the portions of your image in front of or in back of your subject will be out of focus.  In addition, the distance to your subject can affect the degree of blur.  Google "depth of field".

[johnrmoyer]

I suggest https://en.wikipedia.org/wiki/Depth_of_field 

As distance or the size of the acceptable circle of confusion increases, the depth of field increases; however, increasing the size of the aperture (i.e., reducing f-number) or increasing the focal length reduces the depth of field. Depth of field changes linearly with f-number and circle of confusion, but changes in proportion to the square of the distance to the subject and inversely in proportion to the square of the focal length. As a result, photos taken at extremely close range (i.e., so small u) have a proportionally much smaller depth of field.

Sometimes depth of field is based upon an ancient standard for print size, viewing distance, and human ability to see.

When viewing on a screen, it might be very different than a standard print size.

In general, a longer focal length will give a shallower depth of field if one does not stand closer or further.