"Depth of field is insufficient"

Started Dec 17, 2011 | Discussions thread
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lnbolch Senior Member • Posts: 2,012
Re: Depth of fields are like going around in circles—confused...

mateo goodman wrote:

After reading this whole thread, I still am not sure exactly how depth of field is determined. Are they property of a give f-stop and focal distance or like bokeh - different on every lens?

Some things in photography are absolute but a lot are not. 1/30th of a second is 1/30th of a second until the shutter is out of calibration. Apertures can be calculated with high mathematical accuracy, but f/8.0 from one lens may not pass the same amount of light as another. Movies are shot with lenses calibrated in t-stops which consider light transmission. A t/8.0 gives exactly the same exposure as another t/8.0.

To most still photographers, this makes no difference, since we can use highly accurate through-the-lens light metering (which can be fooled). With a hand-held meter, it can be a disaster, and working photographers spend a whole lot of time sussing this out. I have an f/8.0 mirror lens made by Perkin Elmer of Hubble fame, that I had to meter as an f/11. Mathematically, it was f/8.0 but there was so much loss with mirrors and glass that it transmitted the equivalent of and f/11 lens.

Now when depth of field is discussed things really go Zen. Calculations are based upon something called Circle of Confusion. The best definition I have heard, is eight optical engineers sitting around a table arguing about how to explain it in words real people can understand.

Wikipedia defines it thus:

"In optics, a circle of confusion is an optical spot caused by a cone of light rays from a lens not coming to a perfect focus when imaging a point source. It is also known as disk of confusion, circle of indistinctness, blur circle, or blur spot.

In photography, the circle of confusion (“CoC”) is used to determine the depth of field, the part of an image that is acceptably sharp. A standard value of CoC is often associated with each image format, but the most appropriate value depends on visual acuity, viewing conditions, and the amount of enlargement. Properly, this is the maximum permissible circle of confusion, the circle of confusion diameter limit, or the circle of confusion criterion, but is often informally called simply the circle of confusion.

Real lenses do not focus all rays perfectly, so that even at best focus, a point is imaged as a spot rather than a point. The smallest such spot that a lens can produce is often referred to as the circle of least confusion."

Thus depth of field depends upon the assumptions you make about the acceptable size of the circle, the characteristics of the film or sensor, and the size of the print. Common depth of field tables are often of historic origin, based upon characteristics of long-ago films enlarged to some arbitrary size those who calculate them chose as the target.

My AI-S lenses all have hyperfocal scales on them. Supposedly, if you select an aperture, the lens will show you were to focus to have everything within the required distance parameters sharp—assuming the film that you are shooting has the characteristics of the film Nikon used for the calculations and you are printing at whatever they decided was the proper size. By the time AI-S lenses came along, film characteristics were pretty standard, unless you shot a lot of Kodak SO2475 Recording like I did. Then along came sensors, and Nikon stopped putting hyperfocal scales on their lenses.

Perhaps the most significant term in the above definitions is "acceptably sharp". To a calibrationist, Nikon's holy trinity just make it—under their standard. No other lenses may be acceptable. (See the Nikon SLR Lens Talk forum on this site.) To a web-photographer, if it looks sharp at 800×600, it is sharp. To a photojournalist, if it looks sharp on the printed page, it is sharp. To a family snapshooter, or a kid posting phone pictures on Facebook, the ability to see enough characteristics in the subject to pretty much recognize who it is, may be quite sufficient.

Thus a shot at f/4.0 may look sharp over the whole range when printed snapshot size at 4×6 inches, but a disaster when enlarged to 40×60 inches—until you step back to the point that you can see the whole print, and it looks sharp again. Go figure.

Welcome to the circle of confusion. As digital shooters, we do have an immediate review, and my D700 will enlarge details enormously. If depth of field is important, I can do an immediate visual inspection. On the other hand, there is a cult who glory in achieving the most minimal of depth of field and buy very expensive glass just to kill sharpness outside of the narrowest of slices.

So if under your own set of circumstances, if you find the image to be "acceptably sharp", then it is "acceptably sharp". While the physics behind photography is highly exact, the moment it moves from theory to practicality, each shooter is on their own.

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