العدسات الإضافية كعدسات منفرجة الزاوية .. كتاب آلة التصوير

Additional lenses are obtuse angle lenses

We learned earlier that the positive additional lens shortens the focal length of the original lens. If it is easy to move the lens back to shorten its distance from the film (as in bellows cameras), then it is easy to make out of the two lenses together an obtuse angle lens. Thus, we find that the relative area occupied by the image of the body on the film has decreased (Fig. 101).

The new focal length after adding the additional lens:

It is easy to calculate the new focal length after adding the additional lens if we apply the following equation:

New focal length =
The focal length of the add-on lens x the focal length of the original lens / the focal length of the add-on lens + the focal length of the original lens

If we denote the new focal length of the sum of the two lenses (with the symbol ``

The focal length of the additional lens is denoted as `

The focal length of the original lens is denoted by p

The previous equation can be put in the following abbreviated form (1):

p`` = p` x p / p` + p

If the focal length of the original lens is 10 cm, and the focal length of the additional lens is 20 cm, then the new focal length = 20 x 10 over 20 + 10 = 200 over 30 = 6.67 cm. Since these additional lenses are often not corrected against the optical defects known in simple lenses, it is very likely that the image will suffer from some defects and optical distortion, especially near the edges of the image. This defect can be overcome by narrowing the aperture of the diaphragm.

The difference between the two cases of using the additional positive lens for close-up photography and using it as an obtuse lens:

The reader may be confused between the two functions that the additional positive lens performs, as we mentioned above:

First: The additional positive lens is used when we want to photograph objects close to the Close lens, and then the additional lens increases the relative area occupied by the image on the film (Fig. 98, page 161).

Second: The additional positive lens shortens the focal length, which makes the two lenses together (the original and the additional) an obtuse lens, thus reducing the relative area occupied by the image on the film (Fig. 101 p. 167).

It may seem at first glance that there is a contradiction between the function of the additional positive lens in each of the two cases, as it sometimes increases the relative area of ​​the image, and at other times it reduces the relative area occupied by the image.

(a) If the additional lens was placed in front of an original lens in a camera whose design allowed for shortening the distance between the lens and the film from the usual limit (as in the case of using bellows cameras), then the sum of the two lenses together is represented as an obtuse angle lens when photographing distant objects, and thus less The relative area occupied by the image on the film or photosensitive plate than it would have occupied without an additional positive lens. What we mean when we say “by the usual limit” is that most of the usual photographic machines (except for blower machines) are, by design, governed by a minimum distance between the lens and the film, and this minimum is the focal length of the original lens, which is the distance the lens is from the film when the lens is adjusted. On a body located at infinity.

(b) It is also possible that this same additional positive lens, and with the previous camera whose design allows shortening the distance between the lens and the film from the usual limit, the lens increases the relative area occupied by the image on the film or the sensitive plate than what we obtain without it, This is if the object is very close to the lens Close up, which is the case that requires lengthening the bellows.

(c) If the additional wave lens was placed in front of an original lens in a camera, the design of which does not allow the distance between the lens and the film to be shortened from the usual limit and this focal length of the lens, then this lens is only suitable as an auxiliary lens for the original close up lens only. . Then the relative area occupied by the image when adding the additional positive lens will be greater than what it would be if the imaging was done without placing it in front of the original lens, and in this case the lens will not be suitable for photographing distant objects, but the maximum distance that is allowed between the object and the lens is The same focal length of the additional lens, otherwise a sharp image is formed at all.

In sum, what determines the function of the additional positive lens: that is, whether it will increase the relative area of ​​the image or will it reduce the relative area of ​​it, is that we ask:

Does the design of the camera permit a shortening of the distance between the lens and the film (as in bellows machines)? Or does its design not allow it? If the design allows for that, then it is easy to make the two lenses together (the original and the additional) an obtuse lens if the distance between the object and the lens is a great distance (which is the case that calls for a short length of the bellows), or make it a lens for close-up photography if the object is close to it. (This is the case that calls for lengthening the bellows), but if the camera does not allow this, then the two lenses are then represented as a lens for close-up photography only (i.e. the reproduction lens as it is sometimes called), and then the maximum distance that must be available between the lens and the abutted body will not exceed The focal length of the additional lens (in order for the image to be sharp).

Separating additional lenses

If an additional diverging lens is placed in front of the original photographic lens, the focal length of the two lenses will increase, and this is contrary to what we have previously mentioned regarding positive additional lenses.

It is understood from the foregoing that the image enlargement ratio increases to double if the previous diverging lens is added in front of the original positive lens.

And since the placement of this additional diverging lens entails a lengthening of the focal length of the original lens, therefore it was necessary that these diverging lenses should not be placed in front of the camera lens unless they are of a type that allows increasing the distance between the lens and the sensitive film (as is the case in bellows cameras). double or triple).

And where the additional separated lens is no more than a single lens and suffers. There are many optical defects, so it is better to narrow the aperture of the diaphragm whenever possible, otherwise these defects will appear clearly, especially if the percentage of magnification exceeds the reasonable limit (more than twice, for example).