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Depth of field

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Depth of field (DOF) is the distance in front of and behind the subject which appears to be in focus. For any given lens setting, there is only one distance at which a subject is precisely in focus, but focus falls off gradually on either side of that distance, so there is a region in which the blurring is tolerable. This region is greater behind the point of focus than it is in front, as the angle of the light rays change more rapidly; they approach being parallel with increasing distance.

Several factors determine whether the objective error in focus becomes noticeable. Subject matter, movement, the distance of the subject from the camera, and the way in which the image is displayed all have an influence. However, the most important factor is the actual degree of error in relation to the area of film exposed.

Light from a point source at the correct distance will produce the image of a point on the film. A point farther away or nearer will produce the image of a disk whose border is known as "circle of confusion." The diameter of these circles increases with distance from the point of focus and so can be used as the measure of error or blurring of the image.

For a 35 mm motion picture, the image area on the camera negative is roughly 0.87 by 0.63 in (22 by 16 mm). The limit of tolerable error is usually set at 0.002 in (0.05 mm) diameter. For 16 mm film, where the image area is smaller, the tolerance is stricter, .001 in (0.025 mm). Standard depth of field tables are constructed on this basis, although generally 35 mm productions set it at 0.001 in (0.025 mm). Note that the acceptable circle of confusion values for these formats are different because of the relative amount of magnification each format will need in order to be projected on a full-sized movie screen.

(A table for 35 mm still photography would be somewhat different since more of the film is used for each image and the amount of enlargement is usually much less.)

Another factor to be considered is that the film format's size will affect the relative depth of field. The larger the area of the film is, the longer a lens will need to be to capture the same framing as a smaller film format. In motion pictures, for example, a frame with a 12 degree horizontal field of view will require a 50 mm lens on 16 mm film, a 100 mm lens on 35 mm film, and a 250 mm lens on 65 mm film. Conversely, using the same focal length lens with each of these formats will yield a progressively wider image as the film format gets larger: a 50 mm lens has a horizontal field of view of 12 degrees on 16 mm film, 23.6 degrees on 35 mm film, and 55.6 degrees on 65 mm film. What this all means is that as the larger formats require longer lenses than the smaller ones, they will accordingly have a smaller depth of field. Therefore, compensations in exposure, framing, or subject distance need to be made in order to make one format look like it was filmed like another.

Hyperfocal distance Edit

The hyperfocal distance is the nearest distance at which the far end of the depth of field stretches to infinity. Focusing the camera at the hyperfocal distance results in the largest possible depth of field. Focusing beyond the hyperfocal distance does not add depth of field to the far end (which is already at infinity), but it does subtract from the focus area in front of the hyperfocal point. Therefore there is less total depth in focus. Likewise, focusing ahead of the hyperfocal distance results in a gain of focus area ahead of the point, but loses the focus area behind the focus point including the subjects near infinity.

Calculating depth of field Edit

For a given film format, depth of field is calculated from three factors: the focal length of the lens, the effective diameter of the aperture, and the camera-to-subject distance. While it is commonly said that lenses of short focal length have greater depth of field than long lenses, this rule of thumb is not strictly true because it takes into account only one of the three factors. In fact, for a given subject framing and aperture, lenses of all focal lengths have exactly the same depth of field. This is because subject framing is dependent on two of the factors (focal length and subject distance), while aperture is the third. Once the three factors are set in a fixed proportion, the depth of field will be the same.

An example makes this easier to understand. Take a photographer using a 400 mm lens to shoot a subject (for example, a bird) 10 metres away. Assuming an aperture of f/2.8, the depth of field of this shot would be 10 cm. Should the photographer now switch to a 50 mm f/2.8 lens, the depth of field at 10 metres is now 7.62 metres. However, once the photographer has moved to 1.25 metres from the bird, being the distance required such that the bird fills as much of the frame as it did with the 400 mm lens at 10 metres, the depth of field is exactly the same as before, 10 cm.

Casual photographers may be surprised to find that depth of field is not strictly a function of lens length. The common saying that short lenses have greater depth of field than long lenses actually relates to how lenses of each type tend to be used, long lenses are often for distant subjects, whereas wide angle lenses are often used up close.

Artistic considerations Edit

Depth of field can be anywhere from a fraction of an inch to virtually infinite. For instance a shot of a woman's face in closeup may have shallow depth of field (with someone just behind her visible but out of focus); a shot of rolling hills would be likely to have great depth of field, with the objects both in the foreground and in the background in focus.

Aperture effects Edit

The aperture controls the effective diameter of the lens opening. Reducing the aperture size increases the depth of field; however, it also reduces the amount of light transmitted, placing a practical limit on the extent to which the aperture size may be reduced. Photography lenses almost invariably work best at medium apertures.

For any given lens and aperture, the depth of field is maximized by focusing the lens at the hyperfocal distance. The hyperfocal distance is the point of focus chosen to create a depth of field from infinity to a near point that is half of the hyperfocal distance. For example if a lens is focused at infinity and the closest point of acceptable sharp focus is 10 m away (the hyperfocal distance), the depth of field will extend from 10 m to infinity. If now the lens is focused on a point 10 m away (at the hyperfocal distance), the depth of field will still extend to infinity, but the nearest point of acceptable sharp focus will be 5 m, maximizing the depth of field. If the lens is focused on a point closer than the hyperfocal distance, the depth of field will no longer extend to infinity, greatly reducing the depth of field.

Depth of field versus film format size Edit

As the equations above show, depth of field is also related to the circle of confusion criterion, which is typically chosen as a fraction such as 1/1000 or 1/1500 of the film format size. Larger imaging devices (such as 8x10 inch photographic plates) can tolerate a larger circle of confusion, while smaller imaging devices such as point-and-shoot digital cameras need a smaller circle of confusion. For equal field of view and f-number, depth of field is inversely proportional to the film format size.

In practical terms this means that smaller cameras have deeper depth of field than larger cameras. This can be an advantage or disadvantage, depending on the desired effect. A large format camera is better for photographs where the foreground and background are blurred, while a small camera maximizes depth of field, so that objects behind or in front of the focus plane are still in good focus. This difference between formats goes away if the cameras are compared with equal aperture diameters rather than equal f-numbers; but the smaller camera can not usually use a large aperture diameter, so can not achieve a very limited depth of field.

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