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04-08-2010, 01:11 PM   #1
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Bokeh Blur Measurement

I have come across a couple GREAT Depth of Field calculators which have an excellent quantitative measurements but how do you quantify how "blurry" the background is?

If I snap a picture at 15' at 50mm and f3 and the background is 20' behind subject a calculator will tell me all kinds of things like front focus back focus infinity something, circle of confusion and this and that, but it won't tell me how blurry the background is and how to get the same level of blur out of a f5.6 shot at 75mm with background 20' behind subject how close do I need to be to subject.

If there is a numerical equivalent to this that I can use these calculators for and I just don't know the term for "Blurriness" please fill me in.

Thank you,

04-08-2010, 01:20 PM   #2
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Bokeh is qualitative, not quantitative, and there are as many variable as there are lenses and pictures.
I doubt very much if it's something that will be pinned down with a numbers game.
04-08-2010, 01:33 PM   #3
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Original Poster
I highly agree with you. It is qualitative and not subject to calculation which is why I worded it as I did. Hoping someone could give me a launching point because such a measurement requires human opinion if you will.

For instance, someone could say....
In MY opinion, the blur from a tamron 28-75 at 40mm f3, 10ft from subject, background 10ft behind subject is comparable to the kit lens at 65mm f5.6, 15ft from the subject, move the subject 5ft more from background.
04-08-2010, 02:50 PM   #4
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Unless you're a conjoined twin, your dialogue with yourself is confusing me.

I don't think one can quantify blur in any meaningful way. Yes, it would be possible to find how much one (theoretical) point was smeared across the image, but this would not tell you anything about the quality of the blur. And it is the bokeh quality that matters more than how much or how little there is.


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