ATTACHMENT LENSES.
If your SLR camera has a permanently built-in lens or if you have difficulty using extension tubes, attachment lenses may be used (see the article on lenses).
Because we want to get closer to the object than the scale on the frame allows, we use focusing lenses, which shorten the focal length of the lens. In this way, we can get closer to the photographed object, so as to get it on the negative on a much larger scale.
The effect will be the same, as when using adapter rings, but to get the full sharpness of the image at the edges, we need to use a smaller aperture.
When using add-on lenses, calculate in diopters (dpt) focal length of the combination: lens plus lens.
The focal length of the lens - converging lens system is calculated according to the formula:
Where:
F - focal length of the system,
f - focal length of the lens,
s - focal length of the additional lens.
If, for example, for a 35 mm focal length lens 5 cm we add a converging lens with focal length 25 cm, i.e. with power 4 diopter, then the focal length of the whole system will be: = 4,16 cm
A diopter is a measure of the focusing power of a lens used in optical technology. The number of diopters is expressed as the reciprocal of the focal length in meters and is positive for converging lenses, and negative for distractors. One diopter equals a focal length of one meter; two diopters is the focal length 50 cm, four is the focal length 25 cm itd.
The ratio of diopters to focal lengths can be seen in the table below.
| Focal length in meters | 4 | 2 | 1 | 1/2 | 1/3 | 1/4 | 1/5 |
| Diopter | 1/4 | 1/2 | 1 | 2 | 3 | 4 | 5 |
Counting in diopters is simple. To determine the focal length of a combination of several lenses or an objective and a lens, it is enough to add their diopter values together, and the whole thing can be easily converted in focal lengths.
So, e.g. if focal length lens 50 mm (5 cm) = 20 diopters will be combined with a converging lens of value 5 diopter, we receive 20 + 5 = 25 diopters as a combination value.
And since 1 dpt = 1 meter, to 25 dpt = 1:25 = 0,04 m, That is 4 cm. So the combination will have a focal length 40 mm.
So there, where the construction of the lens does not allow you to get close enough to the subject, and the lens itself is permanently built into the camera or where the use of an extension tube is difficult (np. with auto iris), we use positive (focusing) attachment lenses, by calculating the combination in diopters.
In general, lenses that are too strong should not be used (maximum 2-3 diopters), as this may significantly degrade the image quality.
To extend the focal length of the lens, we use diffusing top lenses (negative). Then, in combination with appropriate intermediate rings, we get a kind of telephoto lens, also requiring the aperture to be reduced to obtain sufficient sharpness. We extended the focal length of the lens while keeping the input diameter unchanged.
We also calculate this combination in diopters, just remembering, to subtract the number of diopters of the attachment lens from the number of diopters of the lens.
Thus, combining a focal length lens 50 mm (5 cm = 20 dpt) with attachment lens negative o – 5 dioptriach, we get 20-5 dpt = 15 dpt, which is converted to focal length, sharing 1 : 15 = - focal length approx. 70 mm.
Because too strong scattering lenses cannot be used (negative), because it causes a significant deterioration of the image, and the gain in increased focal length is negligible even with a pull-on lens 5 diopter, using attachment lenses to extend the focal length of the lens is not very advisable.