What Happens When you Push the Button


Understanding what actually occurs when you depress that shutter button

NOTE: In the interests of simplicity, I have used lay terms in this explanation as I am trying to provide a simplified explanation for a lay person to understand.

Most photographers know that the majority of modern prosumer DSLR and mirrorless cameras offer various options for controlling the various shooting modes of a camera.

AUTO: Some camera brands use a green coloured “AUTO” icon, while other brands use a small “green camera” icon: When set to this shooting mode, the camera automatically controls everything required to capture an average image.

For most amateurs this is quite adequate, but what does the camera really do about aperture, and shutter speed when the shooting mode is set to “AUTO”?

By means of various algorithms the camera will measure the light, then apply an average aperture calculation, to achieve an averagely metered scene, employing an average shutter speed, at an average ISO sensitivity, all interacting to produce an average image!

AV: Aperture Priority, where you choose the aperture, then let the camera set an appropriate shutter speed to achieve a reasonable image ….

TV: Shutter Priority, where you choose the shutter speed, and the camera sets the aperture to achieve an acceptable result….

Many professionals and amateurs use these semi-automatic modes, and for the most part, are happy settle for this kind of “OK” result. Is it really “OK”, or is it still only a compromise!

MANUAL: You set the aperture and shutter speed and give yourself the best possible opportunity to maximise the potential of an image.

How is an image captured: Light passes through the aperture (Hole) *The hole remains closed until the shutter button is depressed.

When the shutter button is depressed, it opens the aperture which is comprised of a group of circular blades which draw back on a spring mechanism. The size of that predetermined hole is known as the aperture.

Two things control the amount of light which passes through the lens — The diameter of the aperture, and the speed at which it opens and closes. This is known as the shutter speed.


After passing through the aperture, the light which lands on the sensor, which is comprised of a fine grid of individual photo sites / pixels, each of which performs a function of measuring light.Cameras are identified by the pixel rating of their sensor eg: A 16 Megapixel camera has effectively 16,000,000 little photo sites, and each of those photo sites performs in an identical manner, but all independently of one another.

Once measured, the light at each separate photo site / pixel and then converted to a mathematical algorithm by the ADC (aka: Analog Digital Converter) Much the same way as a light meter reads the intensity of light, each individual photo site / pixel on that sensor, is in reality a light meter which can read the intensity of light from a level of zero up to 256 levels of light intensity, at it’s very own point where it is situated on that grid.

The grid position of each photo site / pixel is also allocated an individual photo site-specific algorithm representing the co-ordinates of its own grid position

The light also passes through three filters — RED, GREEN and BLUE. These colour measurements are also converted into mathematical algorithms which are then, all combined together with the original light measurements, grid positions and one or two other algorithms.

All these algorithms are then combined into a large group of photo site / pixel-specific algorithms and stored in the camera’s storage medium as one mathematical file or image.

COLOURS — RGB Filters: Most of us who use post-processing software, will be aware that most of our cameras can be set to record a colour gamut (aka: Colour range of either sRGB or AdobeRGB) using three primary colours which constitute the basic colour spectrum which we see as colour in general terms.

Each photo site also records the its own colour blend of those three primary colours as three separate algorithms. One for the amount of RED, one for the amount of GREEN and one for the amount of BLUE measured at each photo site.

The ADC measures, converts and records the algorithms of each individual photo site (aka: pixel) on the grid of the sensor using a computer binary code.

The ADC reads each vertical line of photo sites from top to bottom, starting from the extreme left margin and systematically working across the face of the sensor, dropping each line of information into a temporary buffer, until the entire face of the sensor has been covered. The accumulated algorithms are then dumped into a file inside the storage medium (SD / CF Card etc) in the camera.

NOTE: All computer generated mathematical algorithms are simply a vast group numbers represented by a computer language known as binary code, which a coding system using the binary digits 0 and 1 to represent a letter or numerical digit. *It is important to understand this as it will make it simple to understand the difference between of RAW and JPEG files which I will cover in another article describing how a JPEG file is compressed and why a JPEG file is so difficult to work with post-processing software.

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