4: Sensor

Color

Humans are only sensitive to three different colors (wavelengths, but let’s not get too complicated) of light. These colors are (very) approximately red, green, and blue.

Wait (I can hear you say), I see more than three colors! Not really—when you perceive a color such as yellow, what you are really seeing is a mix of red and green. By mixing red, green, and blue in varying amounts of intensity (brightnesses) you perceive many different colors.

A camera’s sensor works in much the same way as our eyes. Each microscopic pixel has a color filter covering it. This filter is either Red, Green, or Blue (RGB), making that pixel sensitive to only that color. If it has a blue filter covering it, only blue light is let in to activate the pixel. The same for red and green. This is how the sensor interprets a full range of color.

Since every pixel in the camera sees only one of three colors, the number of pixels that accurately depict color is only a third of the total number of pixels. The camera’s processor guesses at (interpolates) what color each pixel should be based on its neighbors’ color and tone.

Most color is made or viewed using three colors, and it is a process that has been in use for well over a hundred years.

For example, three colors make all colors that can be seen on the display with which you are viewing this book. The screen is actually made up of very small areas of only red, green, and blue. Unlike the sensor, each set of the three RGB colors makes up one ‘pixel’ on the screen. 

So, while a pixel in a camera image is one color made from components of RGB, the screen has the set of three RGB colors contained in each ‘pixel’. 

Pixels and Bits

Since those tiny pixels in the camera image are the basic building-blocks of an image, they deserve a closer look in relation to to color. Remember that unlike the screen, each pixel in a photographic image is simply a square of a single color. 

In a black and white (grayscale) image, each of the pixels can be any one of 256 levels, or shades of gray. This 256 level image is called an 8 bit image.

Color images have one of these 8 bit channels for each of the three colors (Red, Green, and Blue). These are called 24 bit RGB images. Each pixel in 24 bit images can be one of about 17 million colors.

You might see some images that are described as having more than 24 bits (8 bits per channel). Each pixel is still a single color, but there is added information such as tones that exceed 256 levels or an overlay of transparency information.

Pixel Flaws

Previously it was stated that the more pixels a sensor has, the better. This is not strictly correct because in the real world, every pixel is far from perfect. We have already seen that each pixel can only decode one of the three colors by itself, so must interpolate some color information. 

When working together, individual pixels in camera sensors have many flaws. One of these flaws can be readily seen: In Photoshop or some other image program, look at a photograph with a large color area like the sky. Now enlarge it enough to see the individual pixels. You will notice that what should be all the same color and tone of pixels is populated with many different colors and tones. The better the sensor, the less variation in tone and color you will see, but even with a sensor in an expensive camera you will see quite a bit of variation.

This sensor defect is called noise, and in some situations it can very noticeable, such as with small-sensor cameras and photographs taken in low-light situations. There are a many other sensors defects (including in the route from the sensor to the processor), but it is enough to know that all pixels have faults, and as a general rule, inexpensive cameras have inferior pixels and more faults in sharpness, noise, color, and tonality.

Sensor Size

A general rule of thumb is that larger sensors (in physical size) have better pixel quality and therefore better image quality. This is regardless of the number of megapixels. Unfortunately, a larger sensor also means a larger, heavier, and more expensive camera.

The largest common sensor size is called a full-frame sensor because it takes it’s size from the 35 mm film used in film cameras. The width of this sensor is 36 mm (1.4 inches). This size of sensor is capable of very high quality, but the sensor itself can cost well over a thousand dollars, and cameras that use this size are large.

The next size smaller in common sensor sizes is called an APS-C, which also takes its size from a film camera format. This sensor’s width is 23 mm (0.9 inches). It is the sensor size used in most cameras discussed at the beginning of this book, and in many ways it is the sweet spot in sensor size. Although it has less than half of the area of a full-frame sensor, it is still big enough to make very high quality images. It is a fraction of the cost of the bigger sensor, and allows the camera to be much smaller and less expensive. Only slightly smaller is the Four Thirds System sensor which is also a very common size.

Sensor sizes smaller than the one inch sensors found in smaller mirrorless cameras come with much lower potential image quality. With improvements in cellphone camera sensors (which are necessarily extremely small), it would even be safe to say that small point & shoot cameras are rarely worth buying unless you want the telephoto feature they have. These small cameras have few usable controls and their image quality is only slightly better than cellphones.

White Balance

Different light sources emit different colors. For instance, the light from the sun is yellowish-orange compared to the light from the sky, and the light from the sky gets much more blue after the sun goes down. Indoor lighting is much more yellowish than daylight, unless it is fluorescent light, which is also greenish.

Your eyes (actually brain) pretty much always adapt to the color of these light sources, so you don’t even notice that they are different colors. But you can see them by fooling your brain a bit: After the sun goes down, but while it is still light out, turn on your inside lights, then walk out of your dwelling and look back into the windows. See how yellowish it looks in there? Walk back in and the yellow will disappear.

The camera, like our eyes, adjusts to different colors of light if you leave it set to Automatic White Balance. But our eyes do a much better job than the camera. Take a photograph of green grass and your camera might think you are taking a photograph in an office building and take out a large portion of the green. Or take a picture indoors and your camera might think you are just taking a picture of a bunch of yellowish things, so it won’t take the yellow out. 

If you are saving images as JPEG, this color cast cannot be removed effectively during editing.

You have two choices to solve this white balance problem. The first is to tell the camera directly what type of light is illuminating the scene. You will find white balance settings for many lighting situations on your camera, but there are three main categories. 

DAYLIGHT is the color balance for things lit by the sun. There are pickier settings on your camera, but any one of the daylight (sunlight, shade, cloudy) will get you into a range where you can make final adjustments in an image editing program.

INCANDESCENT (sometimes called tungsten) is the color of indoor lights that get very hot to the touch. This is also the color of many of the newer LED lights that are labeled as warm white.

The different colors of the light (from daylight colors to incandescent light) can be described as a color temperature, which is designated by Kelvin. 5200K (Kelvin) is the approximate color of mid-day day-light, and 2800K is the approximate color of that light bulb that gets hot or the LED. This is important because you will see these K (Kelvin) numbers going by both on your camera and in image processing applications (where it will be in the form of a slider labeled temperature).

FLUORESCENT is the description for lights that are fairly cool to the touch and are shaped as tubes or as coiled tubes. The color of these lights cannot be described only by Kelvin, and the additional setting and correction for this color is labeled as tint in image editing applications.

Setting the color for the light source in your camera is the first solution to this problem of the color of lights. The second solution is a lot easier and better in most circumstances. It is to use RAW format.

RAW Format

In the first chapter RAW format was briefly explained and you were directed to save your files in this format. Following is a more complete explanation of why RAW is better than JPEG.

Remember that the sensor outputs numbers according to the amount of light hitting the pixels. RAW format saves those levels from the sensor fairly directly, without much input from the camera’s computer—instead, it uses the computer you download them onto to process the images. There are significant  advantages to this way of processing images:

WHITE BALANCE: With a RAW image, the white balance is not applied to the image in the camera as it is with JPEG. Instead the white balance your camera is set to is only used as a reference, and it can be changed in editing. 

So in the previous examples of white balance, the photograph of the smoky house is balanced two different ways from the RAW file. If instead it was saved as a JPEG and the color temperature was set to 2800K (either by auto white balance or setting it for tungsten), the best color that could be had would look something like the top image to the right. No amount of adjustments can correct the color.

If this was the only advantage of RAW format it would be a terrific way to save images. But there are more advantages...

EXPOSURE CORRECTION: At the beginning of this chapter it was noted that an image is made up of 256 tones or levels. With RAW images, more levels are actually saved. So, a color RAW image is more than 24 bit. How much more depends on the camera. What this means is that if your exposure is a little off, it can be easily corrected, as the sensor is actually saving tones above white (255) and below black (0). Getting the correct exposure is still important, but not nearly as important as with a JPEG format image.

More levels being saved also means that an image can have more adjustments applied during image editing. In a Raw processing program such as Adobe Photoshop Camera Raw, contrast can be increased, colors changed, and tonality fine tuned. The quality of the image will not be diminished as with a JPEG.

OTHER SETTINGS: Many other parameters of the image are also set in the camera with JPEG format, such as sharpness and type of color rendition. With RAW files these parameters can instead be set or changed any time after the image is taken. 

PERMANENCE: You cannot change the tonal information in a RAW format file. Although this might not seem like an advantage, it is. When you make a change to a RAW file, it doesn’t change the image information in the file, but instead lists instructions on how the changes should be made. These changes are applied whenever you view the image.

For example, if you open a RAW image, then make it too bright, it will not actually change the image part of the file, but instead includes instructions to make this image too bright. This means that you can always go back to every bit of information contained in the original image. 

These instructions for the RAW changes may be saved in several places, including in a portion of the file reserved for instructions or in the image editing application itself. Often they are saved in a separate file, called a sidecar file. This file should always be moved whenever you move your image file.

These are the reasons people interested in taking good photographs almost always save their images as RAW format. 

Any RAW format file can also be converted to a Digital Negative (.DNG). This is a format Adobe developed as a standard RAW format. The Digital Negative format has a good chance to become a standard across the industry. Already some manufacturers are using it as their cameras’ RAW file format.

Light Source

Any light falling on your subject is actually a combination of many different wave-lengths (colors). Not all lights are created equal, however, and some light sources are deficient in some colors. If you can find a orangish streetlight, stand under it in a colorful outfit. You will find that your outfit will be totally without color. Your clothes just can’t reflect a color that is not there to begin with.

Generally, lights with bad color are found in big-box stores, arenas, and other large areas that need inexpensive lighting. Try to avoid photographing with these.

ISO

How sensitive the sensor is to light is governed by the number you set for the ISO Speed (often called just ISO so as not to be confused with shutter speed). So, to take photographs in low light situations you can set the ISO to a higher number to allow you to take photographs at a shutter speed that will prevent blur. 

Most things in photography come at a cost, however, and being able to make the sensor more sensitive to light also degrades the image. The most noticeable effect is an increase in noise—that random dot pattern covering the entire image that was discussed earlier. At higher ISO settings there is also an unpleasant shift in colors and sometimes horizontal bands across the image.

Newer and larger-sensor cameras perform better at higher ISO settings, but following are some general observations that apply to most cameras. Your camera will also have intermediary ISO settings.

ISO 100: This is the lowest ISO on most cameras. It needs the most amount of light and has the highest quality.

ISO 200: This number makes the sensor twice as sensitive to light as ISO 100. In most cameras, this is almost indistinguishable in quality from ISO 100.

In photography, twice or half the amount of light is referred to as a stop. This will get more important later in this book, but you should remember that definition now.

ISO 400: This is a good all-around setting for most cameras. There is some increase in noise, but you get 2 stops more sensitivity to light as you do with ISO 100 (4 times the sensitivity).

ISO 800–1600: 3–4 stops more sensitive to light than ISO 100. Are you getting the way ISO numbers double with each stop of sensitivity? You can really see the noise at these speeds. In other words, a strong curse or two is in order when I forget to reset my ISO from 1600 if I am taking photographs where I need quality but don’t need the sensitivity.

ISO 3200–[your camera’s maximum]: These very high ISO speeds show glaring faults, with not only plenty of noise, but also banding and color shifts. But if that is the only way to get the photograph, then go for it. Life is not always ideal.

Changing the ISO setting while shooting is straightforward. If you can’t hold the camera still enough or the subject is moving fast enough to cause blur, you probably want to set your ISO higher (to a higher number). Otherwise, just leave it at 400 (perhaps 100 if you are out on a sunny day). 

If you need to see the amount of noise or blur using the camera review, enlarge the review image (see your camera instructions) since the small image will not readily show details such as these.

Automatic ISO

When camera designers set up the parameters for automatic ISO, they err on the side of getting a photograph without motion blur. What this means is that ISO goes up very soon in auto ISO mode, making for very noisy images. Not good. I have had plenty of students who complain of very noisy images on automatic ISO—this is one reason why I had you disable auto ISO at the beginning of this book.

That being said, there may be times when you want to use auto ISO and let the camera pick the speed for you. Perhaps you are in a situation where the light varies widely from one second to the next, when you just don’t want to think about it, or when you loan a camera to your grandmother. 

Some cameras (especially newer ones) have automatic ISO settings that let you specify how it works. These settings make automatic ISO much more controllable and practical.

You can set the highest ISO the camera will choose so that you can avoid those ISO speeds which look like shite to you. You can also pick the slowest shutter speed (sometimes called the minimum shutter speed and sometimes called the maximum exposure time!) so that the ISO will not increase unnecessarily.

Now Go Shoot

Like the last chapter, a hefty portion of this one has background information that serves to increase your understanding of the photographic process. What is hap-pening behind the curtain is important whenever you are trying to get better in any pursuit.

But the chapter also contains some things you can use right away. The color of different types of light and what to expect from this. The importance of using RAW format. Changing your ISO to make the trade-off between image quality and preventing blur. 

Keep these things in mind as you shoot photographs. Some day there may be cameras that do all of this thinking for you—that can read your mind about how to interpret a subject or the colors of that subject. But I doubt it.

EXPERIMENT

Take four or more photographs of exactly the same thing, but change the ISO setting on your camera. 

You might want to have plenty of light for this, as lower ISOs will automatically set your shutter speed slower. This might cause a blurry image which will prevent a valid comparison. You could instead use a tripod or some other camera support to ensure sharpness.

Rename your files something like ISO 100, ISO 200, ISO 1600. You don’t have to remember which is which since all image editing programs will report the ISO used along with other metadata.

In an image editing program compare these at 100 percent (1:1) magnification and note the differences. Which is the highest ISO that has the most quality? How bad is the quality of the highest ISO? 

Now view the image in the fit to window magnification. How is the quality at this magnification? Could the images be used small on a web site or email? Different cameras will present different answers to these questions.

Sensor Questions

What device in your camera converts light levels to numbers?How many pixels are in a modern sensor (very approximately)?What three colors is the sensor sensitive to?

How many colors can one image pixel be at once?

How many shades of gray are possible in an 8 bit image?How many ‘bits’ are there in an RGB (color) image?

What is one shortcoming (defect) of pixels on a sensor?What is an advantage of a smaller sensor? A disadvantage?How did the full-frame sensor size originate?

Why can you usually not see the different colors of light? Why might your camera get the color of a scene wrong? What type of light is hot to the touch? What type is cool?

What measuring system defines the color temperature of light?

What is an advantage to shooting in RAW relative to white balance?

What is an advantage to shooting in RAW relative to exposure?In what type of file are changes to RAW images often stored?What is the Adobe standard format for RAW files?

Why are some light sources bad? 

What defects do high ISOs introduce?

What ISO gives the best quality?

What term describes twice or half the amount of light?Which ISO best for very low light?