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How Depth of Field Functions?

To understand why opening has an impact on the depth of field, we need to rethink the way a lens focuses an image point at a single distance, depending on the distance between the lens and the subject.

Other parts of the topic closer or further away from the objective come to concentrate further.

Far away or closer, forming discs instead of spots of light.

They are called confusing circles. Large overlapping circles of confusion create a blurry picture.

However, provided that the circles are relatively small, they may appear sharpened since our eyes have a limited power of resolution.When visualizing a final print, you evaluate an image with acceptable sharpness, even when the discs are tiny. Are present as opposed to points.

The upper bound of what most people consider to be acute is taken. Diameter of 0.25 mm on the final footprint. Lens makers for 35mm cameras assume that if 25 20cm (10 8inch) magnifications are carried out (magnified film image to this standard, then the largest acceptable circle of confusion on the film is 0.25 divided by 8 = 0.03 mm.

By accepting records up to this size as sharp, subjects slightly closer and further than the subject actually at the point begin to look at the point too. And if the lens opening is reduced all the cones of light, are narrower, so that the pictures of the subjects, still closer and further are put in the sufficiently sharpened focusing area.

The depth of field is expanding.

Again, if you drift away from the topic or switch to a shorter focal length, the focus positions for the images of the parts closest to and furthest from the subject are getting closer. Their circles of confusion become smaller, again improving the depth of field.

Remember that you produce the greatest depth of field when:

G f-number is high (the lens is stopped down) G subject is distant

G focal length is short.

When the subjects exceed about 10 focal lengths of the lens, the depth of field extends further behind the subject than towards the lens. Hence the photographer's comment: «Concentrate on a third party», i.e., concentrate on a part of the scene a third within the required depth of field. With close-up work, however, the depth of field extends even further before and behind the focused topic distance.

Utilize depth-of-field scales on lenses.

You may find that your camera lens carries a depth of field scale, next to its scale of subject distances. The scale gives you a coarse guide to the depth of field boundaries.

is helpful if you are focusing on an area–distance preset when there is no time to visually evaluate focus and depth of field.

The ladders also show how you can earn bonus field depth in shooting remote scene. For instance, if the lens is focused on infinity, focus on the part closest to the subject and read the distance halfway between it and infinity.

This is referred to as the "hyperfocal distance" for the number f you use. Place your attention on this adjustment and the depth of field will extend from half the distance to the horizon.

The depth of field is also used in some low-cost cameras with simple symbols to adjust the focus of the lens. A mountain silhouette usually attaches the lens to its hyperfocal distance; a "group of people" symbol means 3.5 meters and a "single head" means 2 meters.

As long as the lens has a small working opening, these areas overlap within the depth of field. So users have a good chance of getting images at the point as long as they make the right symbol selection.

Remember that depth of field limitations do not occur as abruptly as the numbers suggest – sharpness is progressively deteriorating.

Much also depends on what you consider an acceptable ‘circle of confusion. If you intend to do large magnifications, you will find that the sharpness level is reduced and, so that the depth of field also appears to be reduced, although it may change according to the viewing distance of the print.

Even if your camera gives you an insight into the depth of field effects, as described earlier, you should work well within the bounds of what looks clean, or you may be disappointed by the final impression.

Some photographers may want to get grainy impressions for a deliberate effect.

It is possible to obtain different types of grains in different ways:

  • High ISO films have a greater grain structure than weak ISO films;

  • Large-scale printing from a small negative will create a gritty impression

  • Hard grain can be generated by the method used to process the film.

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