Intermediate ring

Intermediate rings in different lengths (12, 20, 36 mm) with Canon bayonet

Intermediate rings of the Praktica-B series

Intermediate rings are for cameras with interchangeable lenses , such as, for example, system cameras , between the lens and the camera body used. As with bellows devices, this serves to enlarge the image range in order to obtain a larger image scale for the close-up range.

Optical effect

Spacer rings extend the image distance between the image-side main plane of the lens and the image plane ( film in analog cameras or sensor in digital cameras ) without optical lenses . This means that the object distance to the subject can be reduced, and the image scale is thus increased. Thus, macro photography possible with conventional lenses.

Advantages and disadvantages

Advantages:

Optical coordination with the lens is not required; its optical quality is largely retained.
Mechanically simple and inexpensive construction, only requirements for coaxiality and rigidity are made.
Since the intermediate ring is used at the connection of the camera, it only depends on the connection dimensions and can thus be combined with various lenses. In contrast, close-up lenses , for example, have to match the objective.

Disadvantage:

The exposure value decreases. With an image ratio of 1: 2 it drops by a good f-stop, so it has to be exposed for a little more than twice as long to collect the same amount of light during the recording, and with a reproduction ratio of 1: 1 it even drops by two f-stops . This effect also occurs with many macro lenses. The camera's autofocus sensor may receive too little light as a result to function properly.
It is not possible to focus on objects that are further away with inserted spacer rings.
Specialized macro lenses are better corrected for good imaging at close range.
Since normal lenses are well corrected mostly for longer distances, when exceeding the imaging scale 1: 1 (image is bigger than object) often the reverse position sense of the lens to a good obtain imaging quality. A retro ring with filter thread and bayonet is required for this.

A reduction in the depth of field only occurs in connection with an increase in the image scale. This is no different than with specialized macro lenses.

Instead, teleconverters are required to extend the focal length of a lens, and a close-up lens to shorten the close-up limit .

Intermediate rings exist in different sizes. Their extension is given in millimeters; it describes the additional distance between the lens and the image plane that is created by using the intermediate ring. For lenses with short focal lengths , only correspondingly short spacer rings can be used, as the focal length range between infinity and the closest focusing limit is limited by the focusing (focus ring on the lens). Spacer rings that are too long with short focal lengths of the lens can lead to an image no longer being able to be focused with the specified adjustment range.

Intermediate rings can usually be lined up in a row without any problems for larger image widths and image scales. However, the exposure value decreases with each increase. In any case, it is often advisable to use a tripod for macro shots.

technology

Intermediate rings usually do not contain any optical elements. In the optical sense, these are only light-tight tubes, the inner surface of which absorbs the incident light as completely as possible , so that false light cannot enter the image from either outside or inside .

Depending on the camera model, intermediate rings can establish the electrical and mechanical connections between the lens and the camera housing, so that the aperture and autofocus, for example, remain functional. In doing so, data can also be transferred from the lens to the camera housing, such as the set f-number or, in the case of zoom lenses, the set focal length.

There were also a few teleconverters for focal length extension, the optical elements of which could be unlocked and removed by means of an internal bayonet, and in this way an intermediate ring was available as an alternative.

calculation

The following calculation formulas apply to intermediate rings:

The reproduction scale indicates how large the image on the sensor is compared to the object:

{\ displaystyle m = {\ frac {B} {G}}}

G = size of the object

B = size of the image

m = scale of the figure; m = 1 means that the picture and object are the same size.

b = width of the intermediate ring

f = focal length of the lens

The recording distance d between the image plane (film / sensor) and the object is calculated using the following formula:

{\ displaystyle d = (b + f) \ cdot {\ frac {(1 + m)} {m}} = (b + f) \ cdot (1 + {\ frac {G} {B}})}

Important: If only the maximum shooting distance is to be determined, i.e. the lens is set to infinity, B can be replaced by b and G by f! In the case of lenses with an extension, the image scale can be determined by adding the extension distance to the intermediate ring. Example: It may be possible to focus a special 135 mm lens at 900 mm without an intermediate ring. It already achieves an extension of around 30 mm. If this lens is now placed on the intermediate ring, you can easily calculate the recording distance and the image scale for the intermediate ring with the lens set close by using the above formula by adding this lens extension to the width b of the intermediate ring.

Web link

Commons : Extension tubes - collection of images, videos and audio files

Individual evidence

↑ Macro lens , Wikibook digital imaging methods , accessed on March 31, 2016

[1] Macro lens , Wikibook digital imaging methods , accessed on March 31, 2016