Digital Sensors

Digital Sensors

The digital camera has an image sensor instead of film, capable of converting light into charge Electrical and convert it into electronic signals.

A digital sensor is an array of multi-million pixels. Each pixel is a receptor of photons and the processor of the camera calculates how many photons have been captured by each pixel, for all the pixels of the array. The result of this calculation is converted into levels of intensity or in bit depth (from 0 to 255 for the images in 8-bits).

The calculation at that point still does not include the color information, so it would be able to produce only an image in gray scale. This is the reason for which each pixel is coated with a filter “Bayer” with one of the three primary colors (red or green or blue), so as to pass only the wavelengths corresponding to:

 

The matrix of pixels coated with Bayer filters becomes a sequence of rows R and G + G + B:

In such a matrix (“Bayer matrix”), the number of pixels with green filter [G] is double compared to the red pixels [R] or blue [B] in the proportion 50% green, 25% red, 25% blue:

and this is made to reduce the digital noise and to reflect the fact that the human vision is most sensitive to green that to red or blue. The sensors Sigma Foveon and Sony does not use a Bayer matrix, but try to render the value of the primary colors for each pixel.

The camera processor at this point applies an additional algorithm, called “demosaicing”, by which it extracts from each group of 2×2 pixels (2 green + 1 red + 1 blue) additional information on color, “blending” the net values of the three colors primary and thus interpreting the color of that particular quadrant.

The pixels on the sensor are not adjacent, but separate from one another to accommodate other devices.

The density of pixels on the sensor determines the value in megapixels of the sensor.

 

CCD vs. CMOS

The digital sensors belong to two different technology families: CCD (Charged Coupled Device) and CMOS (Complementary Metal Oxide Semiconductor), both constructed from studies of Savvas Chamberlain.

CCDs are produced by industrial processes more complex than the CMOS and show better sensitivity to low light, while in the presence of high lights tend to create artifacts. However, CCDs generate less noise (noise) compared to CMOS.

The CMOS products are lower cost, since the technology required is more standard. Modern CMOS approximate the performance of the CCD and allow you to be more miniaturized. The energy consumption for operating a CMOS is of 1/100 compared to that necessary to a CCD, because the transistors in the CMOS technology are controlled in tension while in the CCD technology are controlled in current, on the other hand CMOS technology is more slow compared to the CCD.

Currently, it is not possible to assert the superiority of one technology over another in an absolute sense, since each type has strengths and drawbacks that make it ideal in certain photographic situations and less in others. E ‘therefore conceivable that both complementary remain for a long time.

 

Sizes of  digital sensor

Depending on their brand, price range, target market, in the digital SLR cameras very different digital sensors can be mounted. There are three basic types of digital sensors, which are distinguished by their size:

 

● full-frame, a full format, 36.0 x 24.0 mm, equivalent to the classic format of 24x36mm 35mm

● APS-H, 28.7 x 19.1 mm, approx. 63% of full frame (1.3x factor)

● APS-C, 22.5 x 15.0 mm, approx. 38% of full frame (1.6x factor)

The “factor” of multiplication of sensors smaller than full frame is known as crop factor. The size of the sensor determines the size of the recorded image. The same scene is recorded by the three types of sensors in the following way:

So with a reduction in the field of view than the one framed in terms full-frame. While the field of view is less, on the other side of the subject fills most of a sensor smaller than the full-frame sensor. This means that the proportions of the subject with respect to the frame on the various sensors are different.

 

Equivalent focal length

Let us now explore the question trying to make things clear and begin to analyze the following picture:

 

At the top we find the scenery to shoot while below we have outlined our lens with its focal point and the plane (the sensor) where the images are projected.

In red, we identify the location of the outermost rays arriving on a Full Frame sensor. In Blu-ray instead represent analogs that target the smaller APS-C sensor.

The true focal length (50mm) is similar in all cases is two. A simple matter for the geometric field angle ß is less than the alpha and then frames a portion of paesaggioinferiore despite the lens used is the same. So, with the same real focal length we have lens that frame different field angles depending on the type of sensor used.

In the following table we can evaluate the variation of the angle of field depending on the size of the sensor and of the true focal length lens used:To obtain the same angle of view of a lens of 50mm focal length on an APS-C must use a lens with real focal length of 35mm (it is said that the 35mm is a 50mm equivalent when mounted on an APS-C camera).

Analogously to the smaller sensors of compact. The angle of field of a 50mm is obtained with a real focal length of 10mm on the sensor 1/1, 7 “and 8 mm on a sensor by 1/2, 5”.

Since the standard format picture was for a long time, the 35mm is still the custom of the focal point of the objectives in terms of focal length equivalent to that format.

The ratio of the focal lengths of objectives that provide the same angle of view of different sizes as mentioned above is called “crop factor”.

In particular, the crop factor of APS-C is 1.5 for the APS-C Nikon, Sony, Fuji, Konica and Minolta to 1.6 for Canon SLR sensors then to 1.7 per Sigma SLRs.

Thus, for example, a view with a focal length of 35mm mounted on a Nikon APS-C corresponds to a 50mm (35×1, 5 = 50) mounted on an SLR Full Frame. A 300mm telephoto lens on an SLR Full Frame is equivalent (in terms of angle of view and then “perceived rapprochement” of the subject) to a telephoto of 450mm in APS-C (300×1, 450 = 5).

Are summarized in the table the value of the equivalent focal length for different sensors and the related factors of crop.

So in the advertising pages of Mediaworld find that the compact has a zoom lens 35-430mm (Panasonic Lumix DMC-FZ8).

These are values that do not indicate the true focal length lens, but in fact the equivalent to 35mm Full Frame format.