Why is there depth of field?

[James G. Dainis]

Sandeep, Your first paragraph is accurate.

 

If you can use f/18 without diffraction losses or inconvenience then why not use it? At f/11 the circle of confusion from the tree may fall just on the acceptable 0.03mm on the film or sensor. But with f/18 it will be even smaller, say 0.20 mm. That difference may not be perceived or noticeable on small prints but on big enlargements or cropping out it will be a factor.

 

The acceptable circle of confusion size is not fixed. That depends on the gear used and the size of the ultimate print.

0.03 may be fine for 35mm film or full frame sensors making up to 8x10 inch prints but for larger prints or cameras with smaller sensors, then the circle of confusion used in calculations of DOF should be smaller, perhaps 0.02 mm. If using larger formats then the acceptable circle of confusion can be larger.

[george_ghio]

<p>Damn, I thought that DOF was so I wouldn't walk into walls. On the serious side, many thanks to all for better information. While I am not a beginner, and have always known what to do about DOF it really is nice to get a good visual demo to aid understanding.</p>

[AlanKlein]

<p>Joe, your daughter might not be asking for the physics explanation of DOF but rather a question of first causes. Not how it happens but why it was set up to work that way in the first place and doesn't work some other way. Of course, I don't know what she had in mind, but it could well be a deeper question of why the laws of nature were set up to operate in a particular method rather than a scientific explanation of the law itself. </p>

<p>As a kid I asked questions like this but couldn't ask it in a way that really meant, well, how come it works that way, and not another way so that people would understand my question. It wasn't until I got older that I realized my question was more fundamental. You mught wish to explore this with your daughter and you may be surprised where the conversation leads both of you. Alan</p>