How to focus the SLR?

First of all, we must understand a basic concept:

A SLR lens is equivalent to a convex lens.

No matter how complicated the lens structure is and how many lenses it consists of, it can be regarded as a convex lens as a whole.

Then there is the problem of convex lens imaging:

In the picture, AB is an object and A'B' is an image. The dotted line positions in the figure are 1 double focal length position and 2 focal length position respectively, and the double arrow line in the figure represents the convex lens position.

Usually, the object distance of a SLR is greater than a focal length, so the images are all inverted real images and imaged on the other side of the lens (see Figures A, B and C).

The so-called focusing is to make the position of the image accurately coincide with the photosensitive device (such as film, CCD or CMOS), so that the photosensitive device can clearly record the image.

Regardless of the fixed-focus zoom lens, the focal length will not change when focusing (the zoom lens can change the focal length during composition to change the viewing angle to achieve the effect of shooting different objects near and far, but once the composition is completed, the focal length will usually not change). At this time, the only focusing method is to move the convex lens position of the lens to change the position of the image and make it coincide with the sensor plane.

Of course, since the SLR lens is only equivalent to a convex lens as a whole, it is not necessary to move all the lenses of the whole lens back and forth when focusing, and the same focusing effect can be achieved by moving several lenses/groups:

The picture shows the lens part in square brackets in the optical structure diagram of Sigma 50 1.4A will move back and forth when focusing (the position diagram of the lens group after moving back and forth has been indicated, which does not mean that the lens is cut in half). The overall effect of this design is actually equivalent to the convex lens of the whole lens moving back and forth, so that focusing can be realized.

Answer over

I hope my answer can help you!