اختبار آلة التصوير للتأكد من دقة الأرقام الدالة على المسافة .كتاب أسس التصوير الضوئي

Testing the camera to ensure the accuracy of the numbers indicating the distance:

If any deficiency in the accuracy of the details appearing in the images is noticed, it is necessary to test the camera to verify the accuracy of its setting of the distance, and that the numbers shown on the lens that indicate the distance exactly match the reality. This test is carried out by installing the camera on a stand, and a stick is fixed in front of it on the ground with a plate with clear numbers or letters, let it be white numbers on a black surface, or black numbers on a white surface. Adjust the distance by means of the camera, whether it is on frosted glass or by means of the numbers shown on the lens. Then imaging takes place with the lens opened to its largest aperture.

This process is repeated at several different distances. It is measured each time between the object and the lens accurately, and the determination of distances is based on the numbers indicating the distance shown in the lens, the distance, and after showing and enlarging the positive images, the accuracy and clarity of writing or letters is tested. In order for the test to be honest, it is necessary to use a slow sensitivity film with small grains, in addition to the necessity of taking into account the correct exposure time, so that the images do not suffer from other errors that are not related to adjusting the distance.

The errors that result in the lack of sharpness of the details in the images, despite the careful adjustment of the distance, are due to one of the reasons shown after:

(1) That the lens is not properly fitted, or that it has a defect resulting from its collision. In fact, it is rare for the defect to be from the lens itself unless it is from. Primitive types, and unless the lens has been bumped or misused or an inexperienced person has tried to disassemble it, defects often result from some other cause. This is noting that the lenses of cameras pass through rigorous tests in their factories before they are released to the market, in order to maintain the reputation of the production.

(2) The lens may not be completely installed in the front of the camera, and this error often occurs in cameras that change their lenses with ones of a different focal length.

(3) The error may result from the absence of the sensitive film or plate at the specified level within the distance of the focal length of the lens, which is the distance at which frosted glass is usually placed in blown cameras.

(4) If the lens is in a reflex camera, the error may occur as a result of a difference between the length of the path of the rays passing from the upper lens to the frosted glass at the top of the machine, which is reflected by the mirror, and the length of the path of rays between the lower lens sensor and the film.

(5) The defect in the reflex camera may be due to the movement of the mirror that reflects the vision lens onto the frosted glass above it.

(6) The defect may result from misuse of the coupled range finder, which may affect a mirror or the glass prism in it.


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