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Posterization

Today is rare to find a photo showing fenónemo known as posterization. Understand the why of this situation very soon.

In the vast majority of the photographs transciones between different colors that are present are usually very mild. It is true that if a model wearing emerald earrings, the transition between the color and your skin will be very sharp, but not much detail in areas where fenónemo is appreciable. If we observe, however, abrupt transitions over a large area color photography, especially the sky, the inevitable feeling will be contrived. A sky is not "so." Humans are very good at detecting such anomalies. Another very prominent example, but in another area, is the color of the skin. We are very good at detecting unnatural skin colors. Therefore
posterization is characterized by abrupt color transiones in situations that the viewer of the photograph considered unnatural.
How is posterization? To understand this we must remember what the mechanism for registration of light in a picture just capturing it in the histogram. (See article on histograms).
Each color in a photograph is actually a triplet numbers. Each number represents one of the basic colors: Red, Green and Blue (RGB, in English). So if we for example 8 bits to represent each color, tone vary in the Red channel from the Black (1) the purest possible Red (255). The same with the other channels. With these three values \u200b\u200bwill get varying tones, when combined, from the Pure Black (0,0,0) to white as pure as possible (1,1,1). All other panels of three values \u200b\u200brepresent the remaining tones of the photograph.
But the recording light on the sensor is linear. "What does this?
If we focus on a single channel for greater simplicity in the explanation, we found that 8 bits have 256 different shades.
If we use a sensor with a dynamic range of 6 steps ( See article on Dynamic Range ), we might be tempted to think that those 256 propocionalmente tones are divided between the 6 steps. But not true. The first steps only has 4 tones, the second 8 and so on until the sixth to be 128. Set the ISO setting and shutter speed change step is performed by opening a passage (of course) the diaphragm. So if we open
very (very) little diaphragm (and thus greatly underexposed photograph) in each channel there are only 4 different tones (! EYE from 0 to 3, no 56 to 59, for example). Or that if we err
exposure (causing a "slight" underexposure of 1 step), we are! Lose half of TONAL RANGE!.
All this long explanation is needed to understand the mechanism of posterization, or whatever it is, How is it possible to produce these abrupt transitions in tone?
The explanation is as follows:
Suppose we take a photograph in JPEG format (using 8 bits) and due to an error in the exposure we get very underexposed. We lose two such steps. This means that each channel will have, at most, 32 different tones (in the most clear.) The histogram reflect this situation by presenting the right boundary in an area about 2 / 3 of the maximum possible. Using a graphic editor
modify this "widening" the histogram until it approaches the limit of the right side. What happens then? Well
two shades of red with values, such as 12 and 14, go now to assert, for example, 135 and 201. With the original tones of red had a smooth transition. With the new is an abrupt change. If we combine all these transciones steep channels will very often. If you belong to an area of \u200b\u200bphotography, for example the sky, which are expected smooth transitions, the final result will artificial. This is what happens in the picture accompanying the article.
Why posterization is rare to find today?
Because the digital SLR camera users get Raw files and JPEG files no. They have a bit depth of 12 to 14 bits. With 12 bits, for instance, have tonal values \u200b\u200bof 4096. With 8 bits we have only 256. So there are many more shades to the process of "widening" of the histogram is much less critical, because originally it had a sufficient distance between the numerical values \u200b\u200bof the tones.
Under what circumstances can observe the phenomenon of posterization?
In practice, only two.
a) as a creative resource. That is, done on purpose.
b) If you capture a picture generating a JPEG file. If, moreover, this photograph is clearly underexposed. And if you use a graphics editor to expand the histogram.

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New features in digital SLRs. Are they really necessary? Objectives

The evolution of technical advances in the field of digital photography is absolutely stunning. Perhaps the most explosive after that happens in the field of mobile telephony. But often the question arises whether it is focused on the real needs of the user, or go to the slipstream of the marketing campaigns of companies sector.
If we look at the history of developments in recent years in the field of digital SLRs with an overview we certainly surprising.
During the early years of the first decade of the twenty-first century were fundamental commitment megapixels. Each new model is introduced to the market boasts more pixels than the last. But it was an outrageous race to nowhere. First, because a greater number of pixels only important for large format printing. And most photographers INMESA-fans-print their photographs in A4 format. For this size 10 Mpx is more than enough. Not to mention that many others only display your photos on your computer. And with this, supposedly low-resolution photograph can be shown only partially in common monitors. More megapixels in a constant size sensor only bring more noise to the picture. After the bitter struggle
marketing was diverted to the video recording capabilities. Recently, high-definition formats.
The obvious question is: If there are already specific devices for recording video, so why try to incorporate this feature to cameras? Certainly a video camera with a suitable storage device, such as Recent solid state drives, will this work with greater efficiency than any camera, at a lower cost.
Some claimed improvements in the evolution of the cameras are really questionable. As an example, the following three:

• anyone is willing to trust blindly in the mechanisms of elimination of the sensor motes, if you change targets frequently in dirty environments?. Do
• LiveView systems (Live image) that approximate the use of a digital SLR camera to a compact, meaning that the frame is done directly on the TFT back are really useful for the photographer amateur?.
• EVIL "The new cameras, which eliminate the optical viewfinder and replaced by an electronic viewfinder (sometimes as an accessory) represent a real advantage, apart from size, on a conventional digital SLR?.

And finally, does the IAEA latest trend on the horizon: the new cameras are able to get a screenshot in 3D are the type of device that we really need?. Does anyone believe in their right mind, that the photographs will be viewed on a 3D TV with the concurrence required special glasses?
Personally, my view on this is that camera manufacturers should focus on enhancing those aspects most directly related to quality photo capture. As shown in two examples:
Reduce noise in high ISO values \u200b\u200ballow us to play with this variable with greater confidence and allow us to capture moving objects in non-ideal lighting conditions.
increase the dynamic range of the sensors allow us to get pictures with a similar appearance to those obtained with a moderate use of HDR software, without having to make several shots and then doing the process on the computer.
I am quite sure that it is easy to give more examples of new features that the vast majority do not need and also other improvements current itself would represent a real advantage in what is really important: making photographs capture the best possible conditions.

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Currently, most of the targets used in digital cameras are relex zoom lens. While every photographer knows exactly what a zoom lens, it is desirable to have a little prior explanation of their nature.
A zoom lens is an objective that provides a range of focal lengths, rather than a single focal length. Thus a single objective function can play a lot of fixed focal length lenses. If you have a zoom lens with focal range of 18 to 200 (I assume format APS-C sensor) we can vary the focal length between 18 mm. and 200mm., ie from the limit of wide-angle lenses to telephoto lenses. This variation was achieved by the photographer turning the zoom ring on the lens. Although some lenses allow for a constant maximum aperture opening throughout the focal range, the most common is that the maximum aperture diminishes as we increase the focal length. For example for a zoom feature above, more often than the f3.5 maximum aperture to decrease from 18 mm. to f5.6 at 200m.
How does a zoom lens?
To answer this question, we must first understand that the reasons for any purpose is to allow all the rays of a point P of the focal plane converge at one point P 'of the focal plane (and this for any point P of the plane of focus.) In the article entitled Imaging using a objective, we can find a full discussion on the subject. This must be similarly charged with a zoom lens.
also need to understand, as explained in Article angle of vision. Objectives Angle and Telephoto. Part II, by using a telephoto (long focal length) the viewing angle is small and there is a small part of the scene, which produces a magnification effect or approach. If we use a wide angle lens (focal length small) the viewing angle is greater and there is a large part of the scene, which produces a reduction or withdrawal.
The basic design of a zoom lens can be seen in the figure below.

In its most simplified description involving four different lenses. Following the route from left to right, the first three are NOT mission focus (their goal is not to make all the rays at a point P on the focal plane converge at a point P'del focal plane). This mission is reserved exclusively for a converging lens of fixed focal length is fourth.
addition, we point out three additional facts:
a) The lens, which occupy the first and third are converging, while the lens is in second place is divergent.
b) lenses occupying the first and second position can be moved in the direction of optical axis of the lens. This movement is achieved by rotating the zoom ring. The lens tercerca occupying the position is fixed.
c) The mission of this lens assembly is to converge the light rays from the scene (This is the mission of the fourth lens). Those rays arriving parallel to the first lens, will parallel the third lens. Those arriving at an angle to the first lens, will at the same angle of the third lens.
Their sole mission is to provoke a process of magnification or reduction of the image from the scene. From a dynamic point of view, we can observe the operation in the following figure:

found in the top position for setting a telephoto lens. In it lenses L1 and L2 are very close and the combined effect of the three is to magnify the scene. As a result the sensor will register a small portion of the scene. This corresponds to an approximation.
In the intermediate position the zoom lens behaves like a target "normal", ie the scene is transmitted as is without magnification and reduction (See article on Objectives Normal ). In the lower position
find configuration for wide-angle lens. In it, the lenses L2 and L3 are very similar and the combined effect of the three is to reduce the scene. As a result the sensor record a large portion of the scene. This amounts to a departure. Zoom In
market objectives, the main difference with this simplified descripación is that each lens is actually replaced by a group of lenses, with missions such as the reduction of chromatic aberration of the whole.

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Zoom Optical Viewfinder Nikon stabilized

Today, the defining characteristic of the digital SLR (Single Len Reflex SLR, in English) is the presence of an optical viewfinder.
digital compact cameras since the first models use electronic viewfinders. These viewers are a very small version of TFT observable in the back of these cameras. Their main problem is that its resolution, in many cases is not sufficient for a perfectly sharp observation of the scene. Some models not even incorporated.
A new breed of cameras that are called interchangeable lens and electronic viewfinder (Viewfinder Interchangeable Lens Eletronic EVIL, in English) are different from traditional digital SLR models because they lack precisely optical viewfinder.
The first question we address is the explanation of what is an optical viewfinder. The answer is very simple. It is a transparent element through which channels the light from the scene and has gone through the target. Device is therefore a TTL (Through The Lens, in English) which means that the light has been used previously by the lens.
Its mission is also simple. Used to compose the picture.
The operation of the optical viewfinder can be seen clearly on the left side of the figure heading this article. Light passing through the lens is reflected by a semitransparent mirror. Some light is used by the mechanisms of AF and exposure metering (not reflected in the figure) and some is diverted to a pentaprism that offer the image to the viewer as if we looked at the scene with our naked eyes. When pressing the shutter
happens what can be seen on the right side of the same figure. The mirror rotates 45 degrees up taking as a turning its right end and light in all its intensity, is directed towards the sensor. From the point
of view of practical use optical viewfinder to take into account some considerations:
a) The scene viewed through this device is usually a high percentage (around 95%) of the scene recorded at the sensor end. The professional models raise the percentage to 100%.
In my work as a photographer I have seen this situation almost as an advantage. Because there are situations ended commitment that can be solved by the difference between what is seen through the viewfinder and what you just recorded in the sensor.
b) In the case of digital reflected not produce the parallax problem that occurred once in the chambers analog compact. In the latter, the display did not reflect the light from the target, but was, so to speak, as a separate window to the scene, with a slightly different angle, in some cases (eg, close-up photography) could cause problems.
c) Most camera bodies are associated with the control optical viewfinder diopter change for people with eye problems very mild. It's something that, frankly, I have not tried it on my camera.
d) The importance of optical viewfinder in an SLR camera is not negligible. In fact with the vision of the scene being offered through this device, we find some items. A scene image received from the target overlap or append a few more items:
d.1) points of focus.
accurately indicate where we are focusing on the scene to take the picture.
d.2) Focusing Screen.
This element, very common in the old analog SLR, governed by manual approach, it has become virtually obsolete from the moment the autofocus mechanisms have been done nearly ubiquitous. Its mission was to allow the photographer to make clear the area in which you want to focus.
d.3) Annex to the optical viewfinder (at the bottom, usually) is a small TFT screen that provides information on the parameters of the socket (Shutter Speed, Aperture, Diaphragm, etc).
Well, all these factors depend on the good visibility provided by the optical viewfinder. So that under certain conditions a poor (and small) optical viewfinder us greatly complicate the taking of certain pictures.
e) One last comment to refer to the plastic cover that surrounds the viewer and can cause problems for people with glasses, like myself. In fact, for example when using the Pentax * Ist Ds every 2 or 3 photographs should I clean the above glasses. This does not happen when I use the Nikon D200. And there are times You can lose a good picture for this simple and silly question.