BetterPhoto Q&A
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Understanding Depth of Field

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May 17, 2001

The smallest aperture is not always the ideal f-stop for DOF. A lens is sharpest near f8 so the more you stop down the less critical sharpness you have. IOW if you can stop down to f11 and get enough DOF to cover what you are shooting that is better than cranking it down to f32. Some modern cameras will even calculate it for you and focus at the hyperfocal distance.

There's that phrase again - "hyperfocal distance"(hd). hd is the point at wich you focus so your DOF covers the area you want it to. IOW if I want the rock 2 feet in front of me in focus and the tree 10 feet away from me in focus I need to know where within that 8 foot span to focus. Generally its about 1/3 of the way in. Which would be at about 4 feet in my example. This point is the hyperfocal distance. Are you confused yet? Find a DOF chart and play around with it.

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May 17, 2001

Thanks
Stephen

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May 18, 2001

http://www.photocritique.net/dof.html

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May 18, 2001

Jeff's description of Hyperfocal Distance needs a minor correction (sorry Jeff).

Hyperfocal Distance is the lens focus setting at which the rear boundary of the DOF is at "infinity." This is _not_ a focus setting of infinity, but a distance less than that, sometimes _much_ less. It is also the focus setting that will give you the _greatest_ DOF because it is the closest distance at which you can set lens focus and still have "infinity" appear to be in focus.

Hyperfocal Distance is dependent on three things:
1. lens focal length
2. lens aperture setting
3. maximum diameter circle of confusion

#3 is is what gives understanding hyperfocal distance (HD) and depth of field (DOF) some difficulty because it's based on acuity of the human eye and perception of what is sharply focused and what is not. The rest is pure optics and its geometry.

The "maximum circle of confusion" is the largest size a circle can be _on_the_film_ before the human eye can detect that it's a circle and not a pinpoint, on a print or projected slide at an average viewing distance. Fortunately, some "standards" for its value have evolved based on film format (size), average viewing distances, and the average acuity of the human eye. Each film format has its own maximum circle of confusion value. For 35mm format film, it's 0.025 millimeter; for 6x6 medium format, it's 0.060 millimeter.

Therefore this setting depends on the film size (for your Canon Elan 7 it's 35mm format) the lens focal length and the aperture setting you are using. Change lens focal length or lens aperture and the Hyperfocal Distance changes.

The utility of setting lens focus at the hyperfocal distance is getting the greatest DOF. It is most often used for making photographs with great depth in them such as landscapes. It's also a very old trick (with an aperture of f/8 or f/11) used by photojournalists in days of yore to keep from having to adjust focus in fast moving situations (before auto-focus camera bodies came along). "f/8 and be there" was not just a joke for them!

The really cool part (Good News):
When lens focus is set at the hperfocal distance, the _front_ boundary of the DOF is _half_ the hyperfocal distance making the DOF a no-brainer to calculate. Example: if the HD is 20 feet for a particular focal length and aperture, and you set the lens focus at 20 feet, the DOF is 10 feet to infinity.

The Downside (Bad News; yes there had to be some):
Using hyperfocal focusing _assumes_ any image made using it will be viewed at an "average" viewing distance. For a 4x6 inch print, that's about 18 inches or so. For an 8x10 print it's several feet (typically hanging on a wall). If you make an 8x10 print of a hyperfocal focused image, and view it from two inches using an 8X loupe, you will see that details in very distant objects are slightly fuzzy. Put it back on the wall and view it from 2-3 feet or farther, and you cannot detect it.

If you want the equations for finding hyperfocal distance and DOF for 35mm format, just reply to this thread and I'll post them with a brief explanation of how to use them. You can build your own spreadsheet hyperfocal and DOF calculator using them. If you're saavy with a spreadsheet program, it's not difficult.

-- John

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May 18, 2001

	Hyperfocal Distance Table Jim sets this up to show thumbnails. Click on the thumbnail to see the table full size. John A. Lind

-- John

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May 19, 2001