digital 101

And you thought f-stops and dynamic range was confusing? As we have learned, color film is composed of three layers of emulsion containing millions of randomly placed microscopic light sensitive grains of silver. One layer is sensitive to red, one to blue and one to green light.

How it works

With digital photography, the image is captured on a sensor that converts light into electrical charges.

These sensors contain a collection of tiny photosites or pixels (short for Picture Element), organized in rows and columns, and covered with red,green or blue filters. The sensors record the number of electrons that strike each pixel during the exposure. A computer in the camera averages the exposure data to give a Red Green and Blue value for each pixel.

Pixels can be as small as 2 or 3 thousandths of a millimeter in size. The more photons of light that strike the sensor, the larger the electrical charge.

On a Nikon D70s, for example, the sensor is 23.7 x 15.5 mm in size with 3008 pixels running horizontally and 2000 vertically - a total of 6 million pixels (ie it is a 6MP camera). The top of the line D2X has the same sensor size, but jams twice as many smaller pixels on the sensor.

This signal is processed through an analogue to digital converter, “digitalizing” the information so it can be further modified or stored. If you shoot JPEGs, the image is sharpened, white-balance is set, and a program is run adjusting contrast and saturation, before the shot is compressed then stored to your flash card.

Mega pixels

The size of a chip is based on the number of pixels it contains. A one mega-pixel camera has one million photosites, an eight megapixel camera has eight million.

In theory, the more pixels, the more detail recorded, the bigger the file size and the larger you can make a print. But all pixels are not created equal.

Your 35mm camera records information in an area 24mm x 36mm. Digital sensors come in a variety of sizes – from 40% to 100% of this size.

Many 3, 4 and 5 megapixel compact digital cameras have small sensors and correspondingly small photosites. The smaller the site, the less signal, the more noise (which is a digital version of film grain) and the less dynamic range (the amount of detail that can be recorded between the pure white of highlights to pure black shadows).

This often means that a 4 megapixel camera with a large CCD will produce better images than an 8 megapixel camera with a smaller CCD.

Sports Illustrated regularly took images from professional 4MP cameras and blew them up to double-page photographs – and they looked great. The same sized print from a consumer point-and-shoot 8 MP camera would probably not look as good.

Buying a digital camera

Before you go shopping for a digital camera, you have to decide what you are going to use it for.

With traditional silver-based imaging, a photographer can go to a photo store, spend a small amount of money and choose from a huge variety of films designed to meet every creative need. Need high speed? Fine-grained low speed? Hyper-saturated colors? Black and white? Even infrared.

With digital cameras, it all depends on the chip. Some chips are great at high speeds – as good as film. Some professional digital SLRs display terrible noise above 160 ISO. Some have silky-smooth skin tones. Some give vibrant colors. Some are capable of big enlargements. Some turn on in an instant, some take several seconds to boot up. Some respond quickly when you trigger the shutter and some have a long shutter lag.

If you just get 4x6 prints, you won’t see much difference between a $300 2 megapixel camera and a $11,000 16 megapixel camera. If you are just e-mailing photos to friends and family, the image size is even less critical.

But if you regularly make 8 x 10 prints, and occasionally blow prints up to 11 x 14, you probably need at least a 6 MP camera with a larger sensor.

But don’t go taking the word of someone in a camera shop. The best place to go if you want to get an idea of what to buy is on line at www.dpreview.com. This is a comprehensive web site that lets you compare cameras and features head-to-head, and is unbiased.dSLR or Compact?

If you have invested a lot of money into a Nikon or Canon autofocus camera system, you can just buy a digital SLR camera body and use your old lenses and flash units.

They make great quality images, and start at $800 for Nikon and $1,000 for a Canon body.

The downside? Most of these cameras are based on film bodies, but they use sensors smaller than 35mm film. It’s like you cropped your photographs, which changes the magnification factor (and focal length) of your lenses. A 50mm lens becomes a 75mm telephoto. A useful 80-200mm zoom becomes a 120-300mm lens. And your wide 20mm lens looks like a 30mm lens – not so wide. This is useful for nature photographers. Not so great for landscape photographers who favor extremely wide angle lenses. Two options are: spend $4,000 for a full-frame digital or buy a wider digital-designed lens, like an 11-18mm zoom (16-27mm equivalent) for about $750.

True digital SLR

Olympus, Panasonic and others have designed camera systems around the smaller sensors. These cameras work like the standard SLRs, with removable lenses. And some are 4/3d design, which yield an 8x10 print – more convenient if you want to put it into a frame than the odd 8x12 format of 35mm cameras. Great idea, unfortunately, to date, the image quality isn’t yet as good as the DSLR cameras offered by Nikon and Canon.

Compact digital “point-and-shoot”

Many pros scoffed at these little cameras, until a photographer won the coveted NPPA “Magazine Photographer of the Year” using a pair of $800 Olympus 5MP pocket cameras.

The shots were good enough for Vanity Fair and Newsweek
magazines.

At the top end, these approx. $700 cameras are pocket point-and-shoots with pro features:

• zoom lenses with ranges that are equivalent to 36-432 mm and faster than many 35mm zooms lenses (f2.8-f3.7 for the $630 Sony DSC-H1)

• full manual controls and a wide selection of auto modes

• built-in flash and a hot-shoe for a bigger flash unit

• large file sizes (you can make a great quality 8x10 prints and on
many, good 11x14 prints)

• RAW file capture (We’ll get to that later)

Downside?

These still haven’t yet reached the quality of DSLRs, but new and improved models are being released every year. Many are prone to shutter-lag. You push the shutter and there can be a perceptible wait for the camera to fire.

Print size

When you expose film, millions of microscopic grains of silver combine to create the image. You can make a great looking 8 x 10 print, and with slow speed film, even 11x14 prints can look good. With digital, the image is measured in dots per inch. The higher the dpi, the finer the image quality and the larger the file size.

Screen resolution on your computer is 72 dots per inch.

With a desktop ink jet printer, the lowest resolution you should print is 240 dpi. Create a great image that you want printed archivally?

Pro digital labs require anywhere from 200 to 400 dpi for large outputs.

A 4x6 picture on your web page can be about 360k.

A 4x6 desktop print is 960 x 1440 pixels – 3.96 MB of information.

An 8x10 desktop print is 1920 x 2400 pixels and 13.2 MB.

An 8 x 10 digital print from a pro lab is 2400 x 3000 pixels at 300 dpi, a 20.6 MB file.

An 11 x 14 is 3300 x 4200 pixels and 39.7 MB.

Interpolation

But you don’t need a 14MP camera to make an 11 x 14 print.

Just as some films can be enlarged, while still looking good, the files of some digital cameras can be resized to make big, good looking prints.

Computer software (like Adobe’s Photoshop) uses algorithms to resample the image – essentially filling in detail between pixels to create a larger file. In some images, especially ones without fine detail, the result won’t be obvious. But try to blow up a family portrait, however, and the interpolation will probably be noticeable in the people’s faces.

The thing to remember is that with larger photographs, you usually stand farther back to view them. For example, billboard images are usually grainy and pixelated, but at a distance, your eyes see the rough image as smooth tones.

As a rule of thumb, unless you like your prints to be wall size, anything more than 6-7MP is probably overkill.

Judging exposure

The most important feature in a digital camera?

It’s not the CCD, that shows you what you have just shot. It’s a feature called the histogram. The CCD is great as a tool to check on composition and to make sure that Granny had her eyes open when you hit the shutter button It isn’t a great way to judge exposure. First, no camera LCDs are calibrated. Second, the look of the image changes, depending on the brightness of the ambient light.

Histogram

The histogram is a graph which allows you to judge where the highlights and shadows are being recorded.

The information on the graph should be slightly to the right side – but not too far right, or you will “clip” your highlights. Important details (the fabric on a bride’s dress or even skin tones) will be recorded as being pure white. Get your print back from the lab, and the face could be a detail less white blob with eyes. Exposing a digital camera is a balancing act – over-expose by a stop and you blow out most of the highlight information available. Underexpose by a stop and your shadows will become posterized – with drastic transitions from dark to lighter tones, rather than a smooth, gentle gradation.

Digital ISO

With film, you have to change rolls to change the ISO speed. One of the cool things with digital is that you can quickly change the speed as often as conditions require it. If you are shooting a scenic garden, and wish to record the most detail, set the camera to its lowest ISO speed. A moment later, you wish to freeze action, and you can easily make the ISO a stop or two faster. The downside? As the ISO increases, image quality usually decreases.

Saving files

Image is captured as an RGB (red blue green) file that is three times as large as the number of mega pixels in the camera. A 1 MP camera makes 3 MB images, a 6MP camera produces 18MB files. Once you have taken the photograph, and the image has been converted from analogue to digital format (a long line of 1s and 0s), it’s time to save the file. Cheap point-and-shoot cameras have a limited built-in onboard memory that can fill up quickly. Better cameras use a variety of small cards, and even CDs to store your pictures. CF, Memory sticks and Smart Media are solid state, reusable cards that come in a variety of sizes. Unlike film, CF cards are pretty durable. Newspaper photographers report that they’ve dropped the cards from trees and put them through the clothes washer without any ill effects.

Don’t try that with film...or with microdrives -- CF-card-sized drives with a tiny built-in computer hard drive.

JPEGS

Most cameras can save photos as JPEG files.

The beauty of a JPEG is that it can take 18MB of data, and quickly compress it to 1 MB, so you can store hundreds of images on a memory card.

JPEGS are compressed LOSSY files, meaning the information you save to the disk will be different than the information in your file when you open the file again on your computer. The more times you save the image, the more degraded the image will become. Also, the more you compress the images, the less fine detail will remain in the photograph. It is usually best to keep the compression at about 8.

PROS

On the plus side, JPEGS are processed quite quickly by your camera. The image is quickly adjusted for contrast and sharpened, before being saved. It allows you to put a great deal of images on a very tiny memory card. They also take up much less space when you decide to archive the photographs.

CONS

Remember that color negative film gives you a seven stop range from your highlights to your shadows.

Shooting JPEGS, you get less than six stops of detail. So exposure is much more critical. It is quite easy to over-expose a shot and blow out all the highlight information -- meaning the face in your portrait will have no details in the skin tones. It is also easy to severely underexpose shadows, resulting in posterization -- dramatic transitions from tone to tone in shadow details.

RAW files

More advanced cameras give you the option of shooting RAW files. This is basically saving the information that was recorded by the image sensor.

A JPEG image is 8-bits of data, meaning that – every pixel records a brightness range from 0 to 255 steps (black to white). This is the same range as used by your computer monitor and desktop printer. But the sensor in more sophisticated cameras can record a larger range of detail -- just like color negative film. Most 35mm style digital SLRS record between 9 and 12 bits of information. At 12 bits that is 4,096 tonal values.

It is usually saved in a compressed LOSSLESS file – meaning you can open the file and get all the data that the sensor recorded. If you want to play with digital darkrooms using programs like PhotoShop, RAW files allow you to retain detail in highlights and shadows that would be lost in JPEG files.

But the files are much bigger. A 1GB CF card can hold 200 compressed JPEG images, but only 50 RAW pictures.

Color space

Some cameras let you choose the “color space”, the range of color your camera will record. Computer monitors are SRGB, a fairly small space that looks great on screen, but is lacking in prints. For reproduction, you should choose Adobe RGB, a wider space, featuring a greater range of colors.

Storage

Photos your grandparents shot on black and white film in the 1930s (or even on glass plates) can be taken to a good lab, and scanned, and a print can be made. No problem. The negatives will be good for decades to come. The prints from grandma’s family album are probably still looking good.

You parents early color prints are probably showing signs of age. Some mediums are more archival than others.

With digital, you have two problems.

First, you have to store the data from the image. CDs and DVDs aren’t yet considered to be durable over the long term. Second, CDs are also a medium that might not exist in a decade – remember 8-track tapes?

Sure, you can keep it on your hard drive, but it too isn’t fail-safe.

It is a problem.

Pros usually back up their images on two hard drives, and burn copies on CD and/or DVD. Belt, suspenders, and another belt. If you are making prints on your own desktop printer, the prints can last anywhere from a few years (with older Epson photo printers) to a century (with the newest Epson printers).

If you use a one-hour lab, the most archival prints are on Fuji Crystal Archive paper. Good for 40 years.

Digital darkroom

The nice thing about digital is that you have the ability to do all the things possible in a lab – make your prints lighter or darker, change the color balance and contrast, even crop the image – all while sitting at the computer.

Most cameras come with some pretty simple software to allow you some artistic freedoms.

Start getting serious about photography, and start spending serious money, and you can so things unimaginable by photographers 15 years ago.

The program used by pros is Adobe’s Photoshop. A consumer version. PhotoShop Elements has many of the same features, at a much more modest price: about $120. Move heads. Change backgrounds. Make pigs fly. All it takes is a lot of time, a lot of RAM, and a fast computer.