Depth of field, and blurring the background

[ajf]

Suppose I have a subject which is 2ft (60cm) deep, and I want it all to be in focus. The depth-of-field tables tell me that there are an infinite number of ways in which I can do that. But suppose also that I want the background to be as out of focus as possible. If the background is 20ft (6m) behind the subject, and I can get 20ft from the subject, what combination of focal length/aperture/distance gives me the depth of field that I want while putting the background as out of focus as possible? Do I use a short focal length close up, or a longer lens at a greater distance, and do I need to use as wide an aperture as possible?

I realise that I can go out and test these things, but I am thinking that someone here must know the theory!

 

[neuroanatomist]

Try this:

http://howmuchblur.com/#compare-1x-50mm-f1.4-and-1x-85mm-f1.8-on-a-0.9m-wide-subject

 

[ajf]

Try this:

http://howmuchblur.com/#compare-1x-50mm-f1.4-and-1x-85mm-f1.8-on-a-0.9m-wide-subject

Thanks, Neuro. That's what I need.

 

[tolusina]

Suppose I have a subject which is 2ft (60cm) deep, and I want it all to be in focus. The depth-of-field tables tell me that there are an infinite number of ways in which I can do that. But suppose also that I want the background to be as out of focus as possible. If the background is 20ft (6m) behind the subject, and I can get 20ft from the subject, what combination of focal length/aperture/distance gives me the depth of field that I want while putting the background as out of focus as possible? Do I use a short focal length close up, or a longer lens at a greater distance, and do I need to use as wide an aperture as possible?

I realise that I can go out and test these things, but I am thinking that someone here must know the theory!

Most modern cameras have a 'Stop Down Preview' or 'Depth of Field Preview' function.
Have you tried that yet?
Viewfinder will darken, Live View with exposure simulation darkens less.

Pushing that button is a whole lot faster and easier than measuring distance to subject, then desired depth of field of subject, then consulting a book, table, chart, calculator.

Scene in view, camera in hand; take a photo or consult a reference?

 

[martti]

There is an app for iPhone called OptimumCS you can use to put your distance and f-stop to get the sharpest possible picture. I do understand that some people take pictures at night so that the focus is not easy to judge on the screen or the LiveView either. You dial in the closest distance and the furthest distance, the focal length and the sensor. It gives you the optimum aperture.
Avoid diffraction, get the best DoF.

 

[surapon]

Yes, Sir
I use this great program too.

http://www.dofmaster.com/dofjs.html

Enjoy.
Surapon.

 

[wsmith96]

Here is another Bokeh simulator.

http://dofsimulator.net/en/