Lens mountfunction

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Two points about a magnifying glass to begin with. First, apparently rather few people understand how to use this complicated scientific instrument. The correct way to use it is to hold it as close to your eye as possible. The second point it that it doesn’t magnify at all. The angular size of the image is exactly the same as the angular size of the object.

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Miranda's lens mount was originally a 44mm screw mount.. They switched to a bayonet mount for the Sensorex.  Some of their lenses have both mounts so they can be used with any Miranda body.

There is no point, however, in bringing the image closer than the near point. If you bring it to the near point, what must the object distance p be? A simple lens calculation shows that \(p = \frac{fD}{f+D}\). The angular size of the image is therefore \(\frac{h(f + D)}{fD}\). Since the angular size of the object when the object is at the near point is \(h/D\), the angular magnification is now \(\frac{D}{f}\) + 1 when the image is at the near point. This, for our \(f\) = 2.5 cm lens, the angular magnification is then 11.

Hence the magnification is equal to \(D/f\). The near point is taken to be 25 cm, so that a lens of focal length 2.5 cm has an angular magnification of 10.

The Edixa m42 mount is oriented slightly off of the Praktica standard.  Lenses with aperture arms and automatic aperture lenses made by other mfg's will not work properly on an Edixa body.  Also, Edixa lenses may not work properly on standard m42 bodies.

Lens mountindex

However, if you put the object at a distance \(f\) (perhaps a few cm) from the eye without using the lens, you simply couldn’t focus your eye on it. Without the lens, the closest that you can put the object to your eye would be \(D\), the distance to the near point - 25 cm for a young eye. The angular size of the object would then be only \(h/D\).

The Argus 21 50mm f3.5 Cintar can be removed and used as an enlarger lens.  It is not the same mount as the later C lenses.

Although the lens does not actually produce an image, it is sometimes said that the lens produces “a virtual image at infinity”. The angular size of this virtual image is \(\alpha\), which is also the angular size of the object, namely \(\alpha = h/f\). Thus the angular size of the image is the same as the angular size of the object, and the lens hasn’t magnified at all!

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Sigma YS mounts are basically T-mounts with an aperture pin.   They are adjustable and must be adjusted to achieve proper alignment of the aperture pin.

The object is in the focal plane of the lens. I draw two rays from the tip of the object. One is parallel to the axis, and, after passing through the lens, it passes through the focus on the other side of the lens. The other goes through the center of the lens. (Since the lens is thin, this ray is not laterally displaced.) Parallel rays emerge from the lens. The eye is immediately to the right of the lens, and it easily brings the parallel rays to a focus on the retina.

Komura used two interchangeable mount systems.   The first was a 47mm threaded adapter similar to a T-mount.  It is called a Unidapt.   Do not confuse this with a rare Tokina adapter which is also 47mm.  The second system is called Uni Auto.  It is a bayonet system similar to Tamron Adaptall.   Both adapters can be very hard to find.  This pic was borrowed from a forum.

NOTE: Don't mistake T-mounts, T2 mounts and YS mounts for M42 mounts.   They are all 42mm but M42 has a different thread pitch.  They are never compatible with each other.

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Tamron Adaptamatic mounts attach to the lens a bit awkwardly and are tightened to the lens by way of a large retaining ring.   Adaptamatic mounts offer limited automation and can be tricky to attach.   Tamron replaced the Adaptamatic with the Adaptall system which offers more control and is much easier to attach.

C-MountLens

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\[\dfrac{\text{angular size of the image (which is}\space h/f )}{\text{angular size of the object when the object is at the near point (which is}\space h/D )}\]

The Sigma mount has two variations.  The first version has two bayonets to support heavy lenses.   That feature was not actually used so they removed the extra bayonet with the introduction of the SD14.  All Sigma AF bodies use the Sigma bayonet mount.  Early Sigma MF bodies used Pentax mounts.

Camera Lens MountAdapter

If you bring the object just a little inside the focal plane, the light emerging on the other side will diverge, as it were from a virtual image that is no longer at infinity. (Figure III.3).

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Tokina lense, made in the 1960's, used an interchangeable mount similar to a T-mount.  The mount was 47mm threaded.  Don't confuse it with a Komura 47mm mount.  These lenses and mounts are rare.  These are also called Hanimex H mounts.

This page titled 3.2: The Magnifying Glass is shared under a CC BY-NC 4.0 license and was authored, remixed, and/or curated by Jeremy Tatum via source content that was edited to the style and standards of the LibreTexts platform.

Vivitar TX mounts look very similar to T4 mounts but they are not fully interchangeable.   T4 mounts can be used on TX lenses but TX mounts can not be used on T4 lenses.   TX mounts have slightly better automation.   T-Mounts thread onto telescopes, mirror lenses and preset lenses.  They do not offer any automation.   T2 mounts are adjustable T-Mounts.

Topman made lenses in the early 1980's.   They used an aperture specific interchangeable mount.  These mounts are rare but not in high demand,.

Petri only made one lens mount, but they also used M42 mounts.  Some Petri lenses have Pentax K mounts but they were made by third party manufacturers such as Cosina.   Petri bayonet and M42 mount lenses were excellent.   The Cosina made lenses are not the same build quality but the optics can be quite good.

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Before examining the magnifying glass, it is probably useful to understand just a little about the workings of the human eye. I am not a biologist, I am very squeamish about any discussion of eyes, so I’ll keep this as basic as possible. When light enters the front surface or cornea of the eye, it is refracted in order to come to a focus on the back surface of the retina. The image on the retina is a real, inverted image, but the brain somehow corrects for that, so that objects look the right way up. While most of the refraction takes place at the cornea, some adjustment in the effective focal length is made possible by a flexible lens, whose power can be adjusted by means of ciliary muscles. The adjustment of this lens enables us to accommodate or bring to a focus objects that are at varying distances from us.

When we use a magnifying glass properly (by holding it very close to the eye) we automatically place it so that the object we are looking at is at the focal point of the lens, and consequently parallel light emerges from the lens before it enters our eye. We don’t think about this. It is just that the ciliary muscles of the eye are most relaxed when they are set to bring to bring parallel light to a focus. It is merely the most comfortable thing to do. Figure III.2 shows a magnifying glass at work. As usual, angles are small and the lens is thin.

For an eye in good condition in a young person, the eye and the ciliary muscles are most relaxed when the eye is set to bring to a focus light from an infinitely-distant object – that is, when the eye is set to receive and bring to a focus light that is parallel before it enters the eye. In order to focus on a nearby object, the ciliary muscles have to make a bit of an effort to increase the power of the lens. They can increase the power of the lens only so far, however, and most people cannot focus on an object that is closer than a certain distance known as the near point. For young people the near point is usually taken to be 10 inches or 25 cm in calculations. The actual real point may differ from person to person; the figure of 25 cm is a “standard” near point. With older people, the near point recedes, so that 25 cm is too close for comfort, and the lens becomes less flexible.