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(Explanation of Table No. 46)

The length of extension tubes that lead to the required transfer gauge

88 regarding extreme close up or macrophotography, we have already mentioned that extension tubes are one of the means that allow photographing objects very close to the lens.

Extension tubes are the cylindrical pieces that are used in cases of close-up photography and are placed between the lens and the body of the camera in order to increase the distance between the film and the lens to the extent that suits the distance of the body from the lens, thus facilitating obtaining relatively large pictures of the objects that lie in front of the lens.

The result obtained from the use of extension tubes is identical to the result obtained from the use of the Bellows bellows, whether it is fixed in the camera or additional to it, and all the bases on which the exposure is estimated are identical in both cases.

It is usual to obtain extension tubes of specific agreed lengths, for example, one inch, 1 inch, and 2 inches, as shown in Table No. 46 p. By means of one extension tube, and with moving the dowel forward or backward, the limits of the transfer scale can be increased or decreased within a very narrow range. Table No. 46 p. 99 shows us the limits of the transfer scale that can be obtained when using extension tubes of a certain length. extension of its length.

If the focal length of the original camera lens = 2 inches, the transmission scale limits can range from 0,25 to 0,31.

It is seen that the transmission scale was shown in the table in the form of a percentage of the image’s length (for example 0.25). This indicates that the ratio between the image’s length and the body’s length = 0.25, which is
Exactly equal ratio of 1:4.

It is noted in the table that the minimum transfer scale. That which is when the original lens is set to (e) infinity in the Focusing Scale

As for the maximum transmission scale, it is the one that we get if the lens has been adjusted in dividing the Focusing Scale by the number denoting the closest. It is approximately as follows: distances
feet or 1 . 1 meter for pistols with a focal length of up to 4 inches 26 meters D D 5-7 inches 12 1 4 meters D D D D 8-12 inches

As for lenses whose focal length exceeds the above, they are considered to have very sharp viewing angles for miniature cameras in still photography and all cinema cameras 11 35 mm or less. It is seen in the table that with regard to long extension tubes, he neglected to set the limits of the transmission scale, but set only one number, because of the close proximity between the maximum and minimum limits of the transmission measurement, even though the camera lens was moved forward or backward by turning it around the distance adjustment division.

Example: In a police movie, I was assigned to photograph a fingerprint with a movie camera.

The focal length of the lens = 4 inches, or 10 cm.
The distance of the fingerprint from the lens = 20 inches only.

(a) What is the length of the extension tube that, if placed between the original lens and the camera, could obtain a sharp image of this print?

(b) What is the number or symbol denoting the distance that the lens is set at in the Focusing scale shown on the lens frame?

Answer: To answer question (a), follow the following steps:

1- We must first look for the measure of transfer in this case. From the previous table No. 44 p. 91, we infer that if the focal length of the original lens = 4 inches, and the distance of the object from the lens = 20 inches, then the transmission scale = 4:1

2- From the corresponding table No. 46 p. 99, we find that if the transmission scale is 25 and 0 and the focal length of the original lens is 4 inches, then the length of the extension tubes must be 1 inch, or 2.5 km.

3- We also infer from this table No. 46 p. 99 that the extension tube, which is one inch long, ensures that the transmission scale ranges between 25 and 0 - 37 and 0. In order to answer question (b) above, we say that it was stated in the explanation of this table (on page 97) that if the lens is set to the infinity mark in the distance adjustment division, then it can achieve the minimum previous transmission scale, which is 25 and 0.