L'été dernier je discutais avec un ami, photographe professionnel.
Je lui demandais de m'expliquer la différence entre un senseur "APS-C" et un "Full Frame" si les 2 produisaient une photo de 20 Mpixels.
Pour moi un pixel était un pixel, peut importe sa taille.
Lui m'expliquait qu'un "gros" pixel était mieux qu'un "petit".
Mais... je ne comprenais pas.
Ça m'est resté en tête... Puis je suis tombé sur cet article, TRÈS intéressant où la différence est expliquée:

Taille du capteur et qualité de l'image

Il avait raison !  ;-)  C'est instructif !   Vous souhaitez savoir qui c'est : Marc Dussault

et pour nourrir la discussion concernant les méga pixels :

Est-ce que plus de mega pixels c'est mieux ?


Je me permets de recopier ici l'article pour prévenir le cas, trop fréquent, où l'article est effacé du contenu libre du cyberespace.

Sensor Size and Image Quality

0Email13More... When it comes to digital camera sensors, bigger is DEFINITELY better. Instead of a sophisticated piece of electronics, think of a digital sensor for a moment as a collection of hollow cylinders arranged in a rectangular grid. Also imagine - for the sake of simplicity - that the grid consists of 30 horizontal cylinders and 20 vertical ones for a total of 600 cylinders. The purpose of each one of these cylinders is to gather light so that the light can be converted into an image. First, let's place all of these cylinders in a rectangle that is limited in width to 120 pixels (I'm using pixels as a unit of measurement to avoid any references to inches vs. millimeters). At only 120 pixels wide, each cylinder can only measure 4 pixels across. But if we expand the width of the rectangle that contains the cylinders to 240 pixels, then each cylinder can be twice as wide (8 pixels across). Increasing the surface area for the cylinders without increasing their number means that each cylinder can be physically larger. 4 Pixel Cylinders 8 Pixel Cylinders Digital Sensors and Photosites Instead of cylinders, digital camera sensors contain millions of individual light-gathering elements calledphotosites (where a photosite is like the cylinders in the example above). When a manufacturer creates a camera with a larger sensor, they have two choices: They can keep the same number of megapixels as on a smaller sensor and increase the SIZE of each photosite They can keep the size of the photosites constant, and just cram MORE of them into the larger sensor (thereby increasing the megapixel count) As I discuss on my page on megapixels, more megapixels are really only necessary if you want to make giant prints. Having more megapixels does NOT make your photos look better. But why would a manufacter choose to increase the size of the photosites rather than upping the megapixel count? Once you know the answer, the real benefit of larger sensors becomes apparent: larger photosites DO capture images with higher quality, especially at high ISO settings. Large photosites produce less noise at high ISO settings, while small photosites produce a ton of noise even at relatively low ISO settings. Take a look at these two photos. Compact Camera (Small Sensor) Digital SLR (Large Sensor) The photo on the left was captured with a compact camera with a small sensor at ISO 800. Notice the level of speckling - also called image noise - that is visible. The photo on the right was captured with a digital SLR with a large sensor at ISO 800. Even though the ISO value is the same for both photos, the larger sensor in the DSLR produces less noise. Sensor Sizes - From Compact to DSLR Now that you know that the size of a digital sensor can have an impact on the quality of your photos, let's take a look at the different options that are available to you. There are currently five main sensor sizes in digital cameras: Type Dimensions Description Compact 8.8 x 6.6mm This is the type of sensor that's used in many compact digital cameras. Micro 4/3rds 17.3 x 13mm This sensor is used in mirrorless digital SLRs made by Panasonic and Olympus. 4/3rds 18 x 13.5mm All Olympus digital SLR cameras include this size sensor. APS-C 23.7 x 15.7mm This is the most common sensor size in digital SLR cameras, and is also used in some mirrorless DSLRs. Full Frame 36 x 24mm Used in high-end digital SLRs, this sensor is the same size as a frame of 35mm film. Since just looking at the numbers is not enough to see the difference in size, the following diagram illustrates the differences (note: these are not the actual sizes of the sensors, but the relative proportions are correct). Since 4/3 and micro 4/3 sensors are so close in size, I have not included regular 4/3 in the diagram. Remember: the larger the sensor, the less noise you get at high ISO settings. Put another way, you get BETTER image quality across all ISO values when you use a camera with a larger sensor. Bottom Line The reason that mirrorless DSLR cameras are appealing is because on the outside they have the same size and shape as a compact camera, but on the inside they're hiding sensors that are significantly larger. This means that you don't need to lug around a giant camera if you want to capture higher-quality photos. It's especially nice to know that you can take pictures in relatively dim light without having to compromise image quality too much. Great image quality in a small, light package — it's the best of two worlds.