Basic Principles of Your Digital Camera

8/3/2019 Basic Principles of Your Digital Camera

1/26

How to master your tool

the digital camera


2/26

Sharpness Tone

Color Light Perspective


3/26

Image Sensors and Pixels Digital photographs are made up of hundreds

of thousands or millions of tiny squares calledpicture elements or just pixels. Each of thesepixels is captured by a single photosite onthe image sensor when you take the photo.

The makeup of a pixel varies depending onwhether it's in the camera, on the screen, oron a printout.


4/26

On an image sensor, each photosite capturesthe brightness of a single pixel. The layout of

the photosites can take the form of a grid orhoneycomb depending on who designed it.


5/26

The quality of a digital image, whether printedor displayed on a screen, depends in part on the

number of pixels used to create the image(sometimes referred to as resolution). Themaximum number that you can capture dependson how many photo sites there are on the image

sensor used to capture the image. (However, some cameras add additional pixels to artificially inflate the

size of the image. You can do the same thing in an image-editingprogram. In most cases this upsizing only makes the image larger

without making it better.) Optical and Interpolated


6/26

This diagram shows the typical sensor sizes compared to 35mm film. The sensor sizes ofdigital SLRs are typically 40% to 100% of the surface of 35mm film. Digital compactcameras have substantially smaller sensors offering a similar number of pixels. As aconsequence, the pixels are much smaller, which is a key reason for the image quality

difference, especially in terms of noise and dynamic range.


7/26

Below is a list of a few digital cameras (asexamples) and their sensor size.


8/26

The marketing race for "more megapixels" would like us to believethat "more is better". Unfortunately, it's not that simple.

The number of pixels is only one of many factors affecting imagequality and more pixels is not always better.

The quality of a pixel value can be described in terms ofgeometrical accuracy, color accuracy, dynamic range, noise, andartifacts.

The quality of a pixel value depends on the number ofphotodetectors that were used to determine it, the quality of thelens and sensor combination, the size of the photodiode(s), thequality of the camera components, the level of sophistication ofthe in-camera imaging processing software, the image file formatused to store it, etc.

Different sensor and camera designs make different compromises.


9/26


10/26

The cone-shaped cells inside our eyes are sensitive to red, green, andbluethe "primary colors". We perceive all other colors as combinationsof these primary colors. In conventional photography, the red, green, andblue components of light expose the corresponding chemical layers ofcolor film.

Similar to an array of buckets collecting rain water, digital camerasensors consist of an array of "pixels" collecting photons, the minuteenergy packets of which light consists. The number of photons collectedin each pixel is converted into an electrical charge by the photodiode.This charge is then converted into a voltage, amplified, and converted toa digital value via the analog to digital converter, so that the camera canprocess the values into the final digital image.

In CCD (Charge-Coupled Device) sensors, the pixel measurements areprocessed sequentially by circuitry surrounding the sensor, while in APS(Active Pixel Sensors) the pixel measurements are processedsimultaneously by circuitry within the sensor pixels and on the sensoritself. Capturing images with CCD and APS sensors is similar to imagegeneration on CRT and LCD monitors respectively.


11/26

The Additive RGB Colors

The cone-shaped cells inside our eyes are sensitive to red, green, and blue. Weperceive all other colors as combinations of these three colors. Computermonitors emit a mix of red, green, and blue light to generate various colors. Forinstance, combining the red and green "additive primaries" will generate yellow.The animation below shows that if adjacent red and green lines (or dots) on amonitor are small enough, their combination will be perceived as yellow.

Combining all additive primaries will generate white.


12/26

The Subtractive CMYk Colors

A print emits light indirectly by reflecting light that falls upon it. For instance, apage printed in yellow absorbs (subtracts) the blue component of white light andreflects the remaining red and green components, thereby creating a similareffect as a monitor emitting red and green light. Printers mix Cyan, Magenta, andYellow ink to create all other colors. Combining these subtractive primaries willgenerate black, but in practice black ink is used, hence the term "CMYk" color

space, with k standing for the last character of black.


13/26

The aboveunmagnified crops ofdigital camera imagesshow high levels of

color noise at highersensitivities. Noise isusually most visible inthe red and bluechannels.

ISO 100 is the"normal" setting formost cameras,although some go aslow as ISO 50. The

sensitivities can beincreased to 200, 400,800, or even 3,200 onhigh-end digital SLRs.When increasing thesensitivity, the outputof the sensor isamplified, so less lightis needed.Unfortunately thatalso amplifies the

undesired noise.


14/26

Color TemperatureMost light sources are not 100% pure white but have a certain"color temperature", expressed in Kelvin. For instance, the middaysunlight will be much closer to white than the more yellow earlymorning or late afternoon sunlight. This diagram gives roughaverages of some typical light sources.


15/26

Color TemperatureMost light sources are not 100% pure white but have a certain"color temperature", expressed in Kelvin. For instance, the middaysunlight will be much closer to white than the more yellow earlymorning or late afternoon sunlight. This diagram gives roughaverages of some typical light sources.


16/26

The shutterspeed determines how long the filmor sensor is exposed to light.


17/26

Shutterspeeds are expressed in fractions of seconds, typically as(approximate) multiples of 1/2, so that each higher shutterspeedhalves the exposure by halving the exposure time: 1/2s, 1/4s, 1/8s,1/15s, 1/30s, 1/60s, 1/125s, 1/250s, 1/500s, 1/1000s, 1/2000s, 1/4000s,

1/8000s, etc. Long exposure shutterspeeds are expressed inseconds, e.g. 8s, 4s, 2s, 1s.


18/26

Aperture refers to the size of the opening inthe lens that determines the amount of light

falling onto the film or sensor. The size of theopening is controlled by an adjustablediaphragm of overlapping blades similar tothe pupils of our eyes.

Aperture affects exposure and depth offield.


19/26

Just like successive shutterspeeds, successive apertures halve the amount ofincoming light. To achieve this, the diaphragm reduces the aperture diameterby a factor 1.4 (square root of 2) so that the aperture surface is halved eachsuccessive step as shown on this diagram.


20/26

We just learned that the next aperture will have a diameter which is 1.4 timessmaller, so the f-stop after f/4 will be f/4 x 1/1.4 or f/5.6. "Stopping down" thelens from f/4 to f/5.6 will halve the amount of incoming light, regardless of thefocal length. You now understand the meaning of the f/numbers found on

lenses:


21/26

Maximum Aperture or Lens SpeedThe "maximum aperture" of a lens is also called its "lens speed". Aperture andshutterspeed are interrelated via exposure. A lens with a large maximumaperture (e.g. f/2) is called a "fast" lens because the large aperture allows you touse high (fast) shutterspeeds and still receive sufficient exposure. Such lenses

are ideal to shoot moving subjects in low light conditions.


22/26

Zoom lenses specify the maximum aperture at both the wide angle and teleends, e.g. 28-100mm f/3.5-5.6. A specification like 28-100mm f/2.8 implies thatthe maximum aperture is f/2.8 throughout the zoom range. Such zoom lensesare more expensive and heavy.


23/26

Depth of field (DOF) is a term which refers to the areas of the photograph both infront and behind the main focus point which remain "sharp" (in focus). Depth of field isaffected by the aperture, subject distance, focal length, and film or sensor format.


24/26

A larger aperture (smaller f-number, e.g. f/2) has a shallow depth of field. Anythingbehind or in front of the main focus point will appear blurred. A smaller aperture(larger f-number, e.g. f/11) has a greater depth of field. Objects within a certain range

behind or in front of the main focus point will also appear sharp.


25/26

The basic rule is, "On a sunny day set aperture tof/16and shutter speed to the [reciprocal of the] ISO filmspeed [or ISO setting] for a subject in direct sunlight.

For example:

On a sunny day and with ISO 100 film / setting in thecamera, one sets the aperture to f/16 and the shutterspeed to 1/100 or 1/125 second (on some cameras 1/125second is the available setting nearest to 1/100 second).

On a sunny day with ISO 200 film / setting and aperture atf/16, set shutter speed to 1/200 or 1/250.

On a sunny day with ISO 400 film / setting and aperture atf/16, set shutter speed to 1/400 or 1/500.


26/26

As with other light readings, shutter speed can be changedas long as the f-number is altered to compensate, e.g.1/250 second at f/11 gives equivalent exposure to 1/125second at f/16.

An elaborated form of the Sunny 16 rule is to set shutterspeed nearest to the reciprocal of the ISO film speed /setting and f-number according to this table:

Aperture Lighting Conditions Shadow Detail

f/22 Snow/Sand Dark with sharp edges f/16 Sunny Distinct f/11 Slight Overcast Soft around edges f/8 Overcast Barely visible f/5.6 Heavy Overcast No shadows f/4 Open Shade/Sunset No shadows

Documents

Basic Principles of Your Digital Camera