article no 04> VIEWFINDER

SLR viewfinders

When we search for viewfinders in the internet they give us details with various kinds of old viewfinders.Single-lens reflex (SLR) cameras viewed the scene through the taking lens. Early SLRs were plate cameras, with a mechanism to insert a mirror between the lens and the film which reflected the light upwards, where it could be seen at waist level on a ground glass screen. When ready to take the picture, the mirror was pivoted out of the way (without moving the camera). Later SLRs had a mechanism which flipped the mirror out of the way when the shutter button was pressed, followed immediately by the shutter opening. Instead of a waist-level arrangement, a prism was used to allow the camera to be held to the eye. The big advantage of the SLR was that any lens, or other optical device, could be used; the viewfinder always showed exactly the image that would be projected onto the film (or sensor). The live preview feature of digital cameras share this advantage of the SLR, as they also show the image exactly as it will be recorded, with no additional optics or parallax error.

Modern viewfinders

Viewfinders can be optical or electronic. An optical viewfinder is simply a reversed telescope mounted to see what the camera will see. Its drawbacks are many, but it also has advantages; it consumes no power, it does not wash out in sunlight, and it has "full resolution".An electronic viewfinder (EVF) is a CRT,LED or OLED based display device, though only the LCD is commonplace today due to size and weight. In addition to its primary purpose, an electronic viewfinder can be used to replay previously captured material, and as an on screen display to browse through menus.A still camera's optical viewfinder typically has one or more small supplementary LED displays surrounding the view of the scene. On a film camera, these displays show shooting information such as the shutter speed and aperture and, forautofocus cameras, provide an indication that the image is correctly focussed. Digital still cameras will typically also display information such as the current ISO setting and the number of remaining shots which can be taken in a burst. Another display which overlays the view of the scene is often provided. It typically shows the location and state of the camera's provided auto-focus points. This overlay can also provide lines or a grid which assist in picture composition.
It is not uncommon for a camera to have two viewfinders. For example, a digital still camera may have an optical viewfinder and an electronic one. The latter can be used to replay previously captured material, has an on-screen display, and can be switched off to save power.
  Electronic virwfinders

An electronic viewfinder (EVF) is a camera viewfinder where the image captured by the lens is projected electronically onto a miniature display. The image on this display is used to assist in aiming the camera at the scene to be photographed. It differs from a live preview screen in being smaller and shaded from ambient light.The sensor records the view through the lens, the view is processed, and finally projected on a miniature display which is viewable through the eyepiece.. Electronic viewfinders are used in digital still cameras and in video cameras.Old video camera viewfinder cutway, note the miniature CRT
(EVF image given at wikipedia.)

Like the live preview screen, electronic viewfinders can show additional information, such as an image histogram, focal ratio, camera settings, battery charge, and remaining storage space. Some have a focus peaking feature that highlights areas of the frame that are in focus.

They are also in several ways more accurate than optical viewfinders:

•Show the scene from the same viewpoint as the camera lens, without parallax.

•Cope with high zoom-ratio lenses, without needing a bulky reflex mirror.

•Show approximately how the scene will look under the chosen exposure, incl. white           balance, saturation, effects etc.

•Show a low-light scene brighter than it would appear with a OVF, just like the final           image will look like.

•Show 100% coverage of the final image.

•It's possible to review taken images on the EVF. This might be helpful in bright sunlight.

•The EVF can also be used in video mode.
   But Electronic viewfinders have the following limitations.

•There may be a noticeable lag between the changes in the scene and the electronic           viewfinder display.

•A very few EVFs do not have automatic gain control and become virtually blank in low-light conditions.

•An EVF has high power consumption, usually comparable to the main LCD screen. For    saving battery, 

•Electronic viewfinders have been in use with bridge cameras for some years but with  limited resolution and image quality.

Meanwhile, the image quality due to higher resolution and OLED Technology is very high.They are used in most mirror-less modern system cameras. Many professional photographers and advanced amateurs prefer DSLR cameras that have a true optical through-the-lens viewfinder (OVF). From 2006 some DSLR camera models provide both through-the-lens viewing and a "live preview” on the LCD display (as distinct from an electronic viewfinder). These include the Canon EOS 40D, EOS 50D, EOS 60D, EOS 7D and EOS-1D Mark III, and the Nikon D3, D300 and D90…

article no 03> PENTAPRISM AND PENTAMIRROR

Pentaprism and pentamirror

We have discussed about the anatomy of a SLR camera. So we have to know about the optical element called mentaprism.

A pentaprism is a five-sided reflecting prism used to deviate a beam of light by a constant 90°, even if the entry beam is not at 90° to the prism. The beam reflects inside the prism twice, allowing the transmission of an image through a right angle without inverting it (that is, without changing the image's handedness) as an ordinary right-angle prism or mirror would.

The reflections inside the prism are not caused by total internal reflection, since the beams are incident at an angle less than the critical angle (the minimum angle for total internal reflection). Instead, the two faces are coated to provide mirror surfaces. The two opposite transmitting faces are often coated with an anti reflection coating to reduce spurious reflections. The fifth face of the prism is not used optically but truncates what would otherwise be an awkward angle joining the two mirrored faces.

A variant of this prism is the roof pentaprism which is commonly used in the viewfinder of single-lens reflex cameras. The camera lens renders an image that is both vertically and laterally reversed, and the reflex mirror re-inverts it leaving an image laterally reversed. In this case, the image needs to be reflected left-to-right as the prism transmits the image formed on the camera’s focusing screen. This lateral inversion is done by replacing one of the reflective faces of a normal pentaprism with a "roof” section, with two additional surfaces angled towards each other and meeting at 90°, which laterally reverses the image back to normal.

(A perspective drawing showing how a roof pentaprism corrects a laterally reversed SLR image.)

Pentamirror

A pentamirror is an optical device used in the viewfinder systems of various SLR cameras instead of the pentaprism. It is used to reverse again the upside-down and laterally reversed image coming from the reflex mirror.

Instead of the solid block of glass of the prism in pentaprism system, here 3 mirrors are used to perform the same task. This is cheaper and lighter, but generally produces a viewfinder image of lower quality and brightness.

This optical device is often (more precisely) referred to as roof pentamirror  because of the roof like ridge. 

(A roof pentamirror contains 3 mirrors.)

Pentaprism vs pentamirror

As we mentioned before SLRs permit the photographer to see through the lens. Pentaprisms use a prism to redirect the light from the lens to the view finder vs the camera sensor. Pentaprisms are the highest quality viewfinders - they provide the highest resolution and most natural representation of the photo about to be taken. Pentaprisms are higher quality than penta-mirrors and thus are found largely in most professional DSLRs, and also in some entry level and mid range DSLRs (Nikon D90, Nikon D7000, Nikon D7100) they allow more light to pass through permitting a brighter view of the scene being photographed.

Most entry-level DSLRs use penta mirrors which, like pentaprisms permit the image being captured by the lens to be redirected (in this case by a series of mirrors) through the viewfinder. This permits full resolution and a natural immersive view of what you are taking a photograph of. Penta-mirrors provide the same function as a pentaprism by are typically constructed of plastic and cheaper to produce; on average they produce a darker image in the viewfinder.

When we compare these two….,

The  pentamirror is around 1/4 of the weight of the pentamirror. (pentaprism about 43 grams.., pentamirror is about 13 grams ) Pentaprisms are made from high quality chunk of glass and pentamirrors are hollow constructions with some of the walls having reflective surfaces. so it becomes the camera heavier when it has a pentaprism instead of a pentamirror.

if someone ask the question.., “which one is better..?? "

Well ‘better’ is subjective. The image produced by a pentamirror tends to be a little darker — making manual focusing harder under some conditions.

but Autofocus has absolutely nothing to do with the pentamirror in the viewfinder. The autofocus runs through the reflex mirror (the one in the lens mount), and it will behave identically regardless if you have a pentamirror or a pentaprism in the viewfinder.

you can again have a look at the diagram of the optical components of a typical SLR camera shows how the light passes through the lens assembly

However, a pentamirror is lighter and cheaper. This makes the cameras that use them more affordable and lighter to carry. For me.., it’s not useful that much.

If we summarized all above..,

pentamirror =

• cheap to produce.
• slightly darker.
• better for auto focus.

pentaprism =

• bulky heavy and more expensive.
• brighter.
• easier to manual focus.

So which is better? That depends. If you can’t afford a DSLR that has a pentaprism but can afford one that uses a pentamirror, you could argue a pentamirror is better. If you plan to use manual focusing a lot then you may find it easier to focus using a pentaprism especially in dim 

conditions. I personally like to focus manually. Because I need to capture the picture.., always when I half press the shutter release button.., myself is telling me.., that don’t let the camera to take that photograph.., make it by yourself. (letting the camera to take the photograph and by just pressing the button tagging my name on the photograph doesn't make me feel good. :/)

And there is a very important thing to know.., If the design of the pentamirror and its housing is such that moisture and/or dust can enter the chamber,  then the mirrored surfaces may degrade over time and dust could become a permanent annoyance.

So it’s so important to look after our camera always as our eyes. (original Sinhala term 

“as deka wage raekagannawa..”) if you really love photography, your most valuable non- living thing should be your camera or a lens. ( something like 50mm f1.8g)

When I was planning to buy my first DSLR…, I loved both Nikon D5200 and Nikon D7000. When I was comparing these two bodies, I knew that the d7000 has a pentaprism viewfinder when the D5200 has a pentamirror  ( D90 has a pentaprism too.., D90 was a good deal…, but I thought that it's not a very good deal for its price. Cameralk price was about SL Rs98000/- with 18-105mm kit. ) The question that came to my mind is why they didn’t put a pentaprism in to d5200. The answer that I found was this.

“Pentamirrors are cheaper to make as they can be made from thin walls of plastic. Due to the extra cost in manufacture, pentaprisms tend to be found in higher end cameras. Putting a pentaprism in to a very cheap camera would probably add too much to the selling price and reduce its commercial viability.”

But again and again I worried about the features that D7000 had and D5200  doesn't.., then I have got the best answer for all. That was the exact answer i needed.

“ If you don’t have much money a camera with a pentamirror will take much better           photos than not having a camera at all……!!”

SOURCES, 

http://www.youtube.com/watch?v=jRnnPozwenk

http://en.wikipedia.org/wiki/Pentamirror 

http://snapsort.com/learn/viewfinder/type

http://photography.incyder.info/pentaprism-vs-pentamirror-which-is-better/

article no 02> HISTORY

HISTORY

Having a general understanding on the history of SLRs is useful. And also sometimes it'll be very interesting. Anyone who is interested in this topic.., he or she can follow the wiki link below and learn more.  History of thesingle-lens reflex camera

When we talk about this topic, we have to go through some new or untouched terms.., we can clear those later. For now., let’s have a quick look on those old ones.                         

Prior to the development of SLRs, all cameras with viewfinders had two optical light paths: one path through the lens to the film, and another path positioned above (TLR or twin-lens reflex) or to the side (rangefinder). (Let’s talk about these two later.)

Because the viewfinder and the film lens cannot share the same optical path, the viewing lens is aimed to intersect with the film lens at a fixed point somewhere in front of the camera. This is not problematic for pictures taken at a middle or longer distance, but parallax causes framing errors in close-up shots. When there are two separate paths its obvious,  Moreover, focusing the lens of a fast reflex camera when it is opened to wider apertures (such as in low light or while using low-speed film) is not easy.

 TWIN-LENS-REFLEX CAMERA 

A twin-lens reflex camera (TLR) is a type of camera with two objective lenses of the same focal length. One of the lenses is the photographic objective or "taking lens" (the lens that takes the picture), while the other is used for the viewfindersystem, which is usually viewed from above at waist level.

The front of a Kinaflex twin-lens reflex camera. The focus rings of the two lenses are coupled with gears around their circumference in this simple design.

The classic Rolleiflex TLR

In addition to the objective, the viewfinder consists of a 45-degree mirror (the reason for the word reflex in the name), a matte focusing screen at the top of the camera, and a pop-up hood surrounding it. The two objectives are connected, so that the focus shown on the focusing screen will be exactly the same as on the film. However, many inexpensive "pseudo" TLRs are fixed-focus models. Most TLRs use leaf shutters with shutter speeds up to 1/500th sec with a B setting.

For practical purposes, all TLRs are film cameras, most often using 120 film, although there are many examples which used other formats. No general-purpose digital TLR cameras exist, since the heyday of TLR cameras ended long before the era of digital cameras. The main exception is the collector-oriented Rollei Mini-Digi, introduced as a rather expensive "toy" in 2004.

RANGE FINDER CAMERAS

A rangefinder camera is a camera fitted with a rangefinder: a range-finding focusing mechanism allowing the photographer to measure the subject distance and take photographs that are in sharp focus. Most varieties of rangefinder show two images of the same subject, one of which moves when a calibrated wheel is turned; when the two images coincide and fuse into one, the distance can be read off the wheel. Older, non-coupled rangefinder cameras display the focusing distance and require the photographer to transfer the value to the lens focus ring; cameras without built-in rangefinders could have an external rangefinder fitted into the accessory shoe. Earlier cameras of this type had separate viewfinder and rangefinder windows; later the rangefinder was incorporated into the viewfinder. More modern designs have rangefinders coupled to the focusing mechanism, so that the lens is focused correctly when the rangefinder images fuse; compare with the focusing screen in non-autofocus SLRs.

(A Foca camera of 1947 at theMusée des Arts et Métiers in Paris.)

(Leica M7 rangefinder)

Digital imaging technology was applied to rangefinder cameras for the first time in 2004, with the introduction of the Epson R-D1, the first ever digital rangefinder camera. The RD-1 was a collaboration between Epson and Cosina.

The M8 and R-D1 are expensive compared to more common digital SLRs, and lack several features that are common with modern digital cameras, such as no real telephoto lenses available beyond 135mm focal length, very limited macro ability, live preview, movie recording, and face detection.

article no 01> SINGLE- LENSE REFLEX CAMERA. ( SLR camera )

Single lens reflex cameras are very popular among amateurs and as well as professionals. In these kind of cameras, typically uses a mirror and prism system (hence "reflex", from the mirror's reflection) that permits the photographer to view through the lens and see exactly what will be captured, contrary to viewfinder cameras where the image could be significantly different from what will be captured.

 The light from the object reflects from a mirror and is taken through the eyepiece to the eye of the photographer (in contrast to a viewfinder camera where the photographer is looking through a separate lens at the subject)

 

The advantage of the SLR is that the eye sees approximately what the film will record, so that you can freely interchange lenses, use close-up lenses, etc. and still see what you will record on the film(or sensor).

THE ANATOMY OF A SLR

To properly answer "what is a digital SLR?" we have to understand a bit about the mechanics of an SLR camera.

In order to avoid getting overly technical we better have a look on this simplified diagram. key elements are highlighted in colors. Those interested in the minute details can read about SLRs on Wikipedia.

01) Light passes through the lens and strikes a mirror (blue).

02) The mirror reflects the light up to a focusing screen.

03)Light passes through the focusing screen and enters a block of glass called

    a pentaprism (or pentamirror)(purple).

04)The pentaprism reflects the image so that you can see it in the viewfinder.

05)When we take a photo, the mirror flips up and a shutter opens that exposes the digital      sensor (green) to light.

Light, which comes both horizontally and vertically inverted after passing through the lens, is reflected upwards by the reflex mirror, into the pentaprism where it is reflected several times to correct the inversions caused by the lens, and align the images with the viewfinder.

We can have a look at the diagram below to understand the main idea of what we are talking now, and also other parts which have included in the diagram will help us to study the other mechanisms in a DSLR.

This is a great example of what-you-see-is-what-you-get. By using the viewfinder you can precisely compose your image and adjust the focus.

Is the image that you see in the viewfinder 100% accurate? In most cases it isn't. If you read digital SLR camera reviews, you may hear a lot about viewfinder "coverage" and "brightness".

Many digital SLR viewfinders only show you 95% of the image that will be captured by the sensor - this is what "coverage" refers to. For examples.., most of entry level and mid-range cameras such as Nikon d3100, Nikon d 3200, Nikon d 5100, Nikon d 5200.

But almost every pro DSLRs (Nikon Ds) and some mid-range cameras(Nikon D7100)  gives 100% viewfinder coverage.

But the thing is unless we are extremely precise when it comes to our photographs we won't notice the 5% difference.

Digital SLR viewfinders also vary in brightness,(by the way, pentaprism viewfinders are brighter than pentamirrors. We will talk on that later. ;) ) which is another way of saying how clear the image appears. "Bright" viewfinders make it easier to use manual focus, since you can clearly see the details of your subject.

The diagrams below shows that the light is bounced from a mirror through a pentaprism (or pentamirror) to the viewer’s eye while choosing the object for capture.

 Now the question is what happen to the mirror when the image is captured.., Of course.., when the shutter released it lifts the viewing mirror while open the shutter to allow the light to fall on to the film (or sensor).

Generallythere are some common benefits when we use a SLR camera.

  With an SLR camera, the photographer see exactly what the lens sees.

  In most of SLRs, photographer can change the lens on the camera.

  Usually SLRs (Film or Digital) have large image sensors

    (or films) that produce high-quality photos.

  An SLR has a near-zero lag time, and is ideal for action photography.

This is just a simplified article.., If you need further details.., you better search for it. These links below might be useful.

sources, 

http://en.wikipedia.org/wiki/Single-lens_reflex_camera

http://hyperphysics.phy-astr.gsu.edu/hbase/geoopt/slr.html

http://www.digital-slr-guide.com/what-is-a-digital-slr.html

http://alphatracks.com/images/reflex.jpg
http://hyperphysics.phy-astr.gsu.edu/hbase/geoopt/imggo/slr2.gif
http://www.dptips-central.com/image-files/slr_camera.gif
http://www.dallasartsrevue.com/resources/photographingArt/SLR-lens-mirror-penta.jpg

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