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How far to the blur?

I can't seem to find an app or a website that will tell me how distant my subject must be from the background to get an adequate blur.  Is this knowable?  Let me see if I can be more clear.  Lots of apps will tell me my depth of field, but they're all concerned with what's absolutely sharp, in focus.  We portrait photographers often want to know not just what's not in focus, but how far out of focus it is.  How far behind the focal point will an adequately blurred background begin to appear?  It would be nice, before I pick my lenses and head to a new location, to have an idea of how much I can blur the background.  I mean, we all know a 200mm lens at f/2.8 will create a nice bokeh of anything a hundred feet behind it.  Or closer. . .but how much closer?   A more reasonable example: say I'm shooting in a backyard and only have 50 feet to stage in.  I know I can position one person 30 feet from the nearest object, stand ten feet in front, zoom in to 200mm at f/2.8 and get great blur and hence isolation.  But suppose five more people jump into the picture--now I have to step back, widen out to, say, 70mm and stop down my lens to f/8 for the way they're grouped.  Have I lost my separation from the background?  Of course, I have something of an intuitive feel for it from my own experiences, but I'd like to have a better grasp of it.  It seems like this is all something that could be calculated; it's just a matter of physics and optics.  Anybody know of an app, a webpage or even a <gasp> book that would help?  Thanks, all--and thanks, Neil, for giving me a place to post an obscure question like this!

Comments

  • ranythebardranythebard Member
    edited June 2015
    What a coincidence. Gina Milicia brought up the topic on Bokeh on her
    podcast yesterday. She mentioned http://www.billionbokehs.com/
  • dbrunodbruno Member
    That's really great! I didn't look too closely, but is there some link to the math?
  • Math? Oh I thought people use these sites / apps because they dont want to do the maths? I cant do the maths. :p
    Dont see link to math though.
  • dbrunodbruno Member
     I just like to see where the results come from. I feel a little more comfortable plugging in numbers if I know a little of what's really happening.
  • Hey, thanks, Ranythebard.  I just (finally) checked on and saw your post.  I think I've found a site that's more helpful, at least for what I want to understand: 

    http://dofsimulator.net/en/

    This has a picture in which the (cartoon) model is separate from a selectable background.  You can vary the lens, f-stop, distance to subject and distance to background.  Playing with it a little, I found that for a 200mm f/2.8 lens, with my model at 11 feet for a headshot, I need the background at least 45 feet back for full bokeh (or at least, full bokeh as this site simulates it).  If I had to widen out to include a second person, giving me a lens length of 120mm or so, I'm never going to get the same smooth bokeh.  Alternately, I could move my subjects back to about 22 feet and get better bokeh at 200mm, but only if I had 150 feet or more behind.  I also played with changing lenses--if I switched from the 70-200 to an 85 f/1.4.  That gives similar bokeh with the subject at 6 feet and the background at 35--so maybe a good strategy for a smaller yard.  I haven't test it against my own gear and yard--but that was the whole point! 

    Anyway, pretty handy for us geeks who need a visual aid. 
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