Playing around with depth of field and I think I finally get it?

Started 4 months ago | Discussions thread
Gerry Winterbourne Forum Pro • Posts: 18,619
Re: Playing around with depth of field and I think I finally get it?

LillyRoseAvalos wrote:

I didn't know the focusing distance was calculated from the sensor.

Note that I said that this is in lens specifications (which you can see if you look at the spec sheet for any lens). But, as Tom has pointed out, DOF calculations might not use the same datum.

The quote from DoFMaster says that there it is measured from the front of the lens. As long as one knows which method is used it really doesn't matter. As it happens, what DoFMaster uses as the focus distance is called the working distance in lens specs.

How do I calculate the focusing distance then?

You don't. You either measure it, if it's close enough. Or you estimate it. Unless you are a cinematographer it never matters what the exact focus distance is because all DOF calculations are approximations at best.

I haven't followed this whole thread so someone else may already have said what follows; if so I apologise for the duplication.

The important starting point is to understand that DOF isn't and can't ever be absolute. It is the range of things in a photo that to the viewer look sharp. There are immediately some uncertainties here: how good are the viewer's eyes (we're all different)? What does the particular viewer consider sharp? If you follow this Beginners Questions forum you'll see that every few days or weeks someone asks "Does this photo look sharp?" and there is never a unanimous yes or no reply.

The next thing is that what looks sharp depends on the size of the picture (compare the first on-screen sight you have of a photo here to the original size version) and how close one is to it (a poster looks fine from across the street but very fuzzy from close up).

DOF calculators can't take account of al these factors so they adopt a set of assumptions. What follows is from memory; I might be a bit out on some of them. Human vision is assumed to be 20:20; sharpness is measured by the Circle of Confusion (assumed to be 0.03mm for FF and different values for other formats); picture size is taken as 10x8"; viewing distance as 18".

So even if two people look sat the same photo standing next to each other they'll see different DOF if their eyesight differs; sand if they peer more closely it will change … and so on. So, with all the variation there the exact focus difference doesn't matter.

Diffraction softens images; it is caused by light passing through an opening (aperture). It's effect is always present but at wide apertures it is negligible. When DOF calculations were first devised it was very rare for the apertures used to cause significant softening so the calculations just ignored it - and almost without exception they still do. With today's smaller sensors small apertures are quite common so diffraction can reduce DOF from what's calculated. One more reason not to bother too much about exact distances.

Despite all the uncertainties, for most purposes the standard calculations are near enough to what we see. As long as you remember that they aren't exact they are fine for most everyday purposes.

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Gerry
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I'm happy for anyone to edit any of my photos and display the results
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First camera 1953, first Pentax 1985, first DSLR 2006
http://www.pbase.com/gerrywinterbourne
gerry.winterbourne@ntlworld.com

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