Saturday, August 20, 2011

What is a pixel, and how does it work?


"In digital imaging a pixel, or pel, (picture element) is a single point in a raster image, or the smallest addressable screen element in a display device; it is the smallest unit of picture that can be represented or controlled."

Most pixels are square, which means they can't be called dots. Some camera image sensors have had both square and rectangular pixels. The Nikon D1x image sensor has both square and rectangular pixels as an example.

The D3's and D300's I use have 12.1 and 12.3 mega pixel (MP) image sensors, respectively. In round numbers that's 12 million picture elements on each image sensor.

There are 2 kinds of image sensor: Charge Coupled Devices (CCD's), which were invented in 1969, the year I graduated from high school (Class of '69 Forever!!! Yep, I'm an old guy), and Complementary Metal–Oxide–Semiconductor, or CMOS - active pixel sensors.

Both types of sensor do essentially the same thing, capturing light and converting it into electrical voltages (signal). CMOS image sensors use less power, and because CMOS uses less power, CMOS generates less heat. Heat is one source of image noise, so less heat, less image noise.
Part of the reason long digital exposures have more noise is because the image sensor gets hotter the longer power is applied to it.

Make a note here: The image sensor in a digital camera (CCD pixels or CMOS pixels), isn't a digital device, it's an analog device. Make another note here: Neither type of image sensor can record color.

OK, so we now have the basics of what a pixel is.

About now you ask, "OK! But how does the voltage (pixel) get changed into a piece of a picture, and how come we can make color photographs if a camera image sensor can't record color?

I'm glad you asked.

First lets handle where the color comes from. The color is mathematically interpolated. For our purposes only part 2  of the definition of interpolate is needed:
in·ter·po·late2.Mathematics . to insert, estimate, or find an intermediate term in (a sequence).

Yep, the color is estimated, but the estimate is pretty accurate because of a filter array that is placed in front of the image sensor, called a Bayer Array:

note that each array segment has 3 colors - red, green, and blue (RGB) and the array is passive. It just sits there in front of the pixels and it uses no power.

Digital images are made using the RGB color model. A single Bayer Array has 2 green squares, because human eyes are most sensitive to green light. The red square covers a single pixel, each green square coves a single pixel, and each blue square covers a single pixel. A 12 MP image sensor has 3,000,000 more of those 4 pixel Bayer arrays (4 times 3,000,000 = 12 MP).

The light falling on any 4 pixels arrayed right together like that, is almost certainly all the same color and the same intensity because those pixels are really, really small.

But not all red light is exactly red. It more often is some subtle shade of red. In the RGB color model different shades of color can be made by adding differing amounts of the three colors in the model.

Pure red is R=255, G=0, Blue=0. Pure green is R=0, G=255, B=0. Pure blue is R=0, G=0, B=255.

Yellow is a mix - R=255, G=255, B=0. Cyan is a mix - R=255, G=0, B=255. Any shades of red, yellow, green, blue, or cyan in between will have some of all 3 RGB colors.

White is a mix of all 3 at maximum value R=255, G=255, B=255.

So though the image sensor can't record colors, by having the Bayer Array in front of the pixels the voltage each pixel generates is in part determined by the color of light falling on the pixel, so the colors in the image can be mathematically interpolated.

The voltages are still analog information though, and the mathematical interpolation can only be performed on digital data. But the voltages the pixels generate are really small, and they need to be amplified. How much the voltages get amplified is determined by the camera's ISO setting.
once amplified the voltages are then input to an Analog To Digital (A/D) converter.

If the camera has been set up to record only Raw image data files, the output of the A/D converter is written to the memory card and the image data is not yet a photo you can see, it's all just 1's and 0's or Raw data. The Raw image data file has to be converted into a photo outside the camera using any of many Raw converters.

If JPEG, TIFF, or Raw + JPEG has been selected for output the JPEG and TIFF files have to be made in the camera.

In the camera, a demosaicing algorithm (a set of rules for solving a problem in a finite number of steps) is applied to the digital data that interpolates the digitized voltages the image sensor/Bayer Array captured, and further processes the image data to complete the JPEG or TIFF file conversion process before the image files are written to the memory card.

Since JPEG is a lossy, compressed, final, ready-to-print, file type, those files require less memory card space. Unfortunately, because so much image data is discarded making a JPEG file they can't be edited very much, if at all.

Thursday, March 17, 2011

Most Popular Lenses

Here are some of the most popular lenses around:

This is one of Canon's least expensive lenses, which is why it is so popular. It's wide open, fast, f/1.8 aperture is handy for utilizing the selective focus technique because of the very shallow depth-of-field (DOF) the lens can produce. The wide aperture is also useful when you don't have a lot of light available. Sharpest focus is obtained by using intermediate apertures, f/4 to about f/11.

Care must be taken with shallow DOF though because the DOF can get very thin, and you have to remember that the plane of focus is always parallel to the plane of the image sensor. If you want both of someone's eyes in focus in a portrait,

Wednesday, March 9, 2011

How Do I Use My Digital SLR's Auto Focus? Part-2

Most digital SLR's will offer some auto focus mode options, like single focus, continuous focus, and that trusty old stand-by - manual focus.

They also offer some focus area modes that use just the one focus point you have selected or some other focus area mode that help keep things in focus in your photos.

Auto Focus Modes

Single Servo Focus
The most commonly used focus mode is Single focus. (Nikon - AF-S, Canon - One Shot AF)
Use single focus when you are making photographs of subjects that aren't moving.
When the shutter is pressed halfway the camera focuses, turns on an in-focus indicator in the viewfinder, and then locks the focus until the shutter is released, or until the shutter button is returned to it's normal position.
That's so

Saturday, January 8, 2011

How Do I use My Auto Focus? Part-1

There is a lot to cover when explaining auto focus, so it will take 2 installments to cover it all.

Auto Focus From The Beginning

The first auto focus SLR for general sale was released in 1978. That camera, the Polaroid SX-70 Sonar OneStep, used an active auto focusing system that sent out sound waves. Today's digital SLR's use a passive system that takes advantage of light coming in through the lens.

So, auto focus sure isn't anything new, having been around for 30+ years now.

How Does My Digital SLR Auto Focus?

It’s called SIR TTL passive phase detection - (Secondary Image Registration - SIR, Through The Lens - TTL).
It’s a pretty simple system actually. It’s used by most dSLR cameras today for that reason.

Wikipedia - GNU Free Documentation License
Here is how it works:
Light enters the lens and is reflected by the main mirror, but the main mirror (1.) is only 50% reflective.

Half of the light goes up through the pentaprism/pentamirror (4.) and to the viewfinder eyepiece (5.).

Thursday, January 6, 2011

How to Get Sharply Focused Images

If the subjects in your pictures aren't sharply focused they won't look very good. They'll be missing one of the qualities that makes a photo an image and not just a snapshot.

Here are some things you can do to get tack sharp images. It's a combination of technique and equipment use, rather than just one thing.

It's the Photographer

You'll hear many people say it's not the equipment, it's the photographer. Well, yes and no. Photography is one of the few pursuits where equipment can make a pretty big difference and we'll look at equipment first.
There are some aspects of photography that if you want to get pro results, you have to use the same equipment the pros use. You can always rent instead of buying if your need is short term, or your photography equipment budget is tight.