Digital Camera Focus Depth of field and Aperture, focal length, focus distance

If we examine the planes close to the sharp focus of a point, we will see that in them images are formed of the points that are found at a very similar distance from the lens to the focal distance. The confusion circles are minimal and the images appear imperceptibly out of focus.

We have seen that the position of the focal plane in regard to the cone's vertex (sharp image of the point) influences on the size of the confusion circle, but this also depends on the amplitude of the angle (solid angle) which forms a cone. The diameter of the beam is determined by the diaphragm's aperture . The smaller the aperture, the narrower the cone. A narrow cone produces, at the same distance, a much smaller confusion circle than a wider cone. The objects that appear out of focus at a determined distance and aperture become more focused with a smaller aperture because the size of the confusion circle has been reduced.

Depth of Field and Focal Length

The reason why the depth of field diminishes with the increase of the focal length depends directly on the relation 1/ p + 1/ q = 1/ f . We can use this formula to get to know how q varies (distance were the image is formed from a point situated at a p distance from a lens of equal focal length as f ). When doing so, we can conclude that the sharp focal planes of the different distances are closer to each other the lower the focal length is. If this is true, it's also true that the confusion circle of an unfocused point will be smaller, the smaller the focal length is.

Depth of Field and Focus Distance

The depth of field of a lens at a determined aperture is less at shorter distances and larger at greater distances. The sharp focal planes are grouped close to the lens for distances larger than 100 times the focal length, but are separated each time more as the focal distance decreases. When the focal distance ( p ) is equal to the focal length ( f ), the image can't be picked up at any distance because it is formed at infinity.

Focus Depth

The focus depth -frequently confused with depth of field-, refers to the tolerance of the focus in regard to the correct placement of the film on the sharp focus plane. When there is a maladjustment between the distance we should place the film at and it's effective position, the resulting lack of focus depends on various factors. The focus depth is the space in front and behind the sharp focus plane, were there is an acceptable image. A lack of focus, in strict sense, will be invisible if the confusion circle is small enough. We can represent the depth of focus as the space around the vertex of the two cones were the confusion circle has a diameter inferior to the given (.25 mm).

Therefore the focus depth will be greater the narrower the converging ray's cone is.

• The smaller the diaphragm's aperture:
• The greater the focal length of the lens, on the contrary to what happens with the depth of field.

At an equal aperture, a lens of a short focal length gives place to a more opened cone (the lens is closer to the film) than one of a longer focal length.

• The smaller the focal distance: again, the contrary to what happens with the depth of field occurs.

This is explained because for focusing at short distances a greater separation between lens and film is needed, which, the same as with aperture, produces a stretching of the cone.