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Which type of lens are used in Twin Lens Reflex (TLR) and SLR cameras? TLR cameras uses two lenses, one for the viewfinder and one to expose the film; but in SLR only one lens is used for the viewfinder and to take a photo.

I have a doubt which types of lens are used in both types of camera.

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    What do you mean by "type of lens" in this context? – Please Read Profile Mar 15 '18 at 11:17
  • And why have you tagged this with photoshop-elements? What does that have to do with it? – Please Read Profile Mar 15 '18 at 11:19
  • Common abbreviation style is really SLR or DSLR. Are you curious about any particular implementations? Or examples? Because this currently doesn't seem answerable – AthomSfere Mar 15 '18 at 12:00
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    Your question is unclear. What is your doubt? – Mike Sowsun Mar 15 '18 at 12:37
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Your question as written isn’t clear, so I’ll provide a little history on how the SLR evolved. Maybe this will answer your question or provide you with some information so you can reword your question.

Early cameras required you to focus and compose by viewing through the lens. Then the film was inserted behind the lens blocking any further viewing. This was done with a stationary camera on a tripod.

The Twin Lens camera was developed in the late 1870’s and allowed the camera to have one lens for the film image, and one lens for viewing. This allowed the photographer to hold the camera in their hands giving much more mobility and freedom.

Later smaller “Rangefinder” cameras were developed, but they still relied on one lens for viewing and one lens for the image.

Single Lens Relex camera’s were developed in the 1930’s and combined the film image lens and viewing image lens into one lens. This was complicated and expensive but offered lots of advantages with only a few disadvantages. The image going to the film plane is normally reflected up to a viewfinder. At the moment the photo is made, the mirror must swing out of the way and allow a direct path to the image plane.

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Each type of camera requires different lenses to meet the requirements of each individual camera, but they all basically allow a visual image to be projected. This image will be projected on a film plane (or digital sensor), or in a viewfinder.

The great feature of the SLR is that the viewfinder always shows you exactly what will be in the final image. Other cameras using 2 lenses will always have some errors in composition due to parallax error.

  • Great answer Mike. I'd add that one of the complications with TLR, in addition to the parallax you mention, is linking adjustments (such as focusing) is mechanically complex. Any slop or imprecision in the linking will also cause some differences between what you see, and what you get. And of course, the lens cost is twice that of using a single lens. – scottbb Mar 15 '18 at 13:47
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    I would not be so harsh about TLRs - the focusing done by moving a common front standard is not too difficult (especially when compared to rangefinders). The cost of two lens need not be double - the viewing lens can be of much simpler, i.e. cheaper, construction then the taking lens (a combination of triplet and a Tessar of the same focal length used to be common). And you get some benefits compared to SLRs - such as when composing image using with a heavy filter - jla-analog.net/flexaret-filter-adapter – Jindra Lacko Mar 15 '18 at 14:48
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The camera lens is a projection lens. Such a design gathers the image-forming light waves that emanate from the subject. The larger the working diameter of the lens, the more light that will be gathered, the brighter the resulting image. The lens is constructed using a transparent material like glass or plastic. The shape resembles a lentil seed, hence the name “lens”. The classic shape is convex, meaning the lens bulges outward, making it thicker in the middle. This shape forces the Light rays from the subject to strike the lens at different angles. As these rays transverse the lens, the direction of their path is changed. This path change is called “refraction”.

The refracted light rays are caused to bend inward. We can trace their revised path; it resembles a cone shape. If we position a ground glass viewing screen or film or image sensor at the apex of this cone of light, a focused image is revealed.

If the lens design is a solitary (one element) convex lens, the resulting image will be degraded. Now we are talking about the seven aberrations (lens defects) that plague. Over the years, we have learned to mitigate each. This is accomplished by constructing a complex lens. This is a multi-element design. Each element has a different shape; some are convex, some are concave, some are cemented together, and some are air spaced. Such a complex array is necessary to force the image forming rays to come to a precise focus at the apex of the cone.

Additionally, twin lens designs and single lens designs each have idiosyncrasies. Now the optician chooses shapes and materials that best work for the task at hand. Suppose a wide-angle lens is tasked to be fitted to a single lens reflex body. A wide-angle must have a short cone of light (short focal length). This is a problem, because a reflex design imposes a moving mirror between the lens and film/sensor. Now the optician must somehow elongate the cone and still retain the wide-angle effect. Retro-focus to the rescue! The optician fits a backwards telephoto. Ever look through a telescope backwards? The view is wide-angle. This design elongates the image forming rays allowing space for a mirror and shutter.

The optician has many tricks up his/her sleeve. A telephoto is a long focal length lens. If the barrel remains long, using the camera will be awkward. The optician shifts the focal length measuring point (rear nodal) forward. This design shortens the barrel length.

To answer your question: Basically all cameras lenses and constructed around a simple singe element convex lens design. Because such a design is wanting as to image quality, the optician add lenses of various shapes to mitigate aberrations and to custom fit the lens to the task at hand.

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