Depth of Field with Nikon CP5700

The small image size of most digital cameras, except those professional models that are made to take standard 35mm lenses, sometimes causes consternation among experienced film photographers. The reason is that the depth of field is greatly extended, removing one bit of control we’ve used in the past to separate subject from background. Anyone who has done portraits at f2.5 using a 105mm lens knows how the beautifully soft backgrounds can enhance the result. The depth of field at ten feet is about five inches, just enough to keep your subject’s face sharp.

Try that with the Nikon CP5700: Focused at ten feet at f2.8, the 27mm zoom setting (equivalent to the 105) yields a depth of field of about two feet. You’d need a lot more distance to the background to make it noticeably soft.

It’s not all bad news, of course. When you want a lot of depth of field, such as placing a figure in the foreground of a grand landscape and keeping some zoom—let’s say 20mm (equiv. to 80mm)—to preserve perspective in facial features, focus at the hyperfocal distance of 20.6 feet at f8. Sharp from 10 feet to infinity. Your 35mm camera with the equivalent 80mm lens must be stopped down to f32 to get sharp focus from 11 feet to infinity.

All this, of course, depends upon what you call "in focus." According to J. Andrzej Wrotniak at: http://www.wrotniak.net/photo/dof/, sharp focus is defined this way:

"A (hypothetical) lens without any optical flaws, placed at a given distance from the image plane (film or optical sensor surface) will create point-like images . . . for point subjects at [only] one . . . distance (as measured along the lens axis).

"For a point subject at any other distance, its image will be a circular spot, referred to as circle of confusion."

Notice that this holds true for any lens, regardless of quality and regardless of the focal length of the lens. It specifies only the distance from optical center of the lens to the image plane.

A short-focal-length lens at precisely twice its focal length from the image plane will produce the same size image as a lens of twice that focal length set at precisely its focal length from the image plane. The subject for the short lens, however, will be just twice its focal length away in front of the lens.

The chart of steps in Manual Focus brings up a question: If the focal distance is stepped in increments of 1 cm at the nearest points, is the depth of field great enough to cover the steps? From calculations found on the Web, I estimate that at any distance greater than 15 cm (13 clicks from the bottom), the depth of field at f2.8 is at least as great as half the distance to the next step. Closer than that, and you may need to adjust the distance from the lens to your subject to get "perfect focus."

On the other hand, if you can increase the light on your subject to get an f-stop of 7.7 (maximum), any distance greater than 11 cm (9 clicks) will provide depth of field to allow continuous focus. The chart below shows the near and far distances of sharp focus for Macro shooting (within 30 cm, or 12 inches). The following chart puts those figures into graphical form.

You can see that the depth of field increases rapidly at subject distances greater than that. The distance between clicks also changes quickly. If you are using the Confirm Focus feature of the camera, you may find that the "sharpened edges" that are displayed when the camera is in focus may extend over several clicks of the Manual Focus control.

"Clicks" are from nearest point of manual focus (3 cm). When shifting to manual focus, the first click of control dial does not change current setting. Shaded area is available only in Macro Mode, in which zoom control must be moved until distance scale is displayed in white (between 8.9mm and 11.8mm).

Values are taken from formula: d = s/[1 ± ac(s-f)/f²]

with plus in the denominator used for the near, and minus for the far value.

s — the subject distance (measured from the lens entrance pupil)

f — lens focal length

a — aperture (or F-stop), e.g., 2.8

c — the diameter of the acceptable circle of confusion. (0.008mm)

 

per http://www.wrotniak.net/photo/dof/

What is interesting to me about the above charts is that the hyperfocal distance, which is the distance you can focus on and have everything in focus from infinity to half the hyperfocal distance, is at 160 centimeters at f2.8 and minimum zoom, giving you sharp focus from 80 cm (31.5 inches) to infinity. This is achieved in Manual Focus by going all the way to the farthest focus point (64 clicks to the right) and then backing off just 13 clicks. If you stop down to f8, the closest point in focus is reduced to 40 cm (15.74 inches), without changing the distance you've focused on manually. This does not agree with the standard hyperfocal distance calculation formulas, which give the hyperfocal distance with a 9mm lens at f2.8 at 12.4 ft, and at f8 at 4.4 ft. The near points calculate to 74 inches and 26 inches, respectively.

Check these figures before republishing!

In planning close-up photography, remember that you do not have the full range of zoom available to you in Macro Mode (3 cm to about 31 cm). Therefore, you must frame your shots by moving either camera or subject. So it’s helpful to have either a macro rail to mount your camera on, or a sliding platform to place your subject on. In the field, for example when shooting flowers, it might help to know beforehand how far from the flower you’ll have to be to get it all into the frame, so you can set up your tripod with the least amount of trial and error.

The following chart shows field sizes at different distances (in centimeters and inches) with the zoom set at 11.6mm—about the longest setting in Manual Focus Mode—for each click setting of the control.