Digital Imaging for Artists By Don Finkeldei.  What you need to know about digital images, file types, printing vs. viewing with a monitor, cameras and how to take a great photo of your artwork.

DIFFERENCES BETWEEN FILM AND DIGITAL IMAGES:

A 35 mm (and larger format cameras) take pictures by exposing a film through the lens.  For the most part, the image has no sub boundaries where color and value change abruptly.  It’s contiguous and homogeneous (excluding the graininess of the film).   A print from film can be enlarged on photographic print paper many times without degrading the quality to the point where graininess of the image is seen.  The quality of the enlarged print depends only on the camera format size, quality of film itself and the size of the print.  For instance, a 35 mm film camera can roughly produce a book quality print of about 8" x 10".

Digital images are recorded as little square “pixels” of a single value/color. It’s not contiguous or homogeneous.  Figure 1 shows how film records color/value over a short microscopic section of an image compared to a digital image of 4 pixels wide.  Notice how film transitions (top part of the image are smooth and how a digital camera renders the same thing as solid color/value squares (pixels).  You normally don’t see the boundaries of the little squares of the digital pixel because they are so small.  However, it is important when printing for book publications.

Resolution of a digital image is related to how many of those little pixels (dots) you can cram into one square inch of an image.  It should be obvious that the more you can cram into one square inch (more and smaller “dots”), the higher the quality of the image and the less your eye perceives the pixel boundaries.   It smooths out the incongruities.

The megapixel size of a camera is probably only important if you wish to print high quality photos.  The size of your camera determines the maximum size of a print you can produce.  Normally, print shops want 300 pixels per inch minimum.  Some want 600 pixels/inch.

 My camera is a 10 megapixel camera.  It’s 3648 pixels wide, by 2736 pixels high.  Multiplying width times height: 3648 x 2736 = 10,000,000,000 (10 megapixels).  The maximum 300 resolution print I can make with my camera is 12.16” x 9.12”.  The math: 3648 / 300 = 12.16” wide.  Do the same for the height.  If I wanted a 600 resolution print, then the max size my camera can yield is 6” x 4.55”.  Of course, I’ll have to crop the image to take out the background.  The actual size will be a lot less.  Normally 30% less on average.  The calculations above are as if I managed to fill my digital camera with the object I wanted to print and didn’t have to crop.  That will never happen in real life!

Below is the maximum print size at 300 pixels/inch available for a few megapixel camera sizes.  (Note: remember, you’ll always have to crop, so actual values will be less.  A good approximation would be to decrease these values by 30 %… but, remember, get as much of the object in the picture as possible so your cropping will be minimal).

2 megapixels: 5 x 4 inches
4 megapixels: 7.5 x 5 inches
6 megapixels: 10 x 7 inches
8 megapixels: 11 x 8 inches (roughly comparable to 35mm film)
10 megapixels: 12 x 9 inches
12 megapixels: 14 x 10 inches

IMAGES FOR EMAIL AND WEBSITE (JPG FORMAT):

You can set your monitor to a few selected standard resolutions -- how many pixels wide and high a computer screen will display.  800 x 600 and 1024 x 768 are typical for a standard non-widescreen monitors.  1920 x 1080 is typical for a widescreen monitor. Resolution of the image doesn’t matter when viewing the image through a monitor. The width and height in pixels are the only thing a monitor deals with.  An image 400 w x 300 h pixels at a resolution of 300 pixels/inch will show on a computer at exactly the same size as the same image 400 x 300 at 72 pixels/inch.  But, if you print the image out, the one at 300 pixels/inch will be 1.3  x 1 inches.  The one at 72 pixels/inch will print out at 5.5 x 4.2 inches. 
Here’s the math:
At 300 resolution: 400 / 300 = 1.3”, 300 / 300 = 1”.
At 72 resolution: 400 / 72 = 5.5”, 300 / 72 = 4.2”

Each image will take up ¼ of a monitor displaying at 800 x 600 resolution regardless of what image resolution  you set it.

Printing is a different story!  Of course, the higher the resolution, the higher the print quality but the trade off is size.  Higher quality (resolution) means smaller size.  Lower quality (resolution) means larger size.

Then why not make website/email images high resolution so that print quality is the same as what you’d need for professional printing?   Because it would be gigantic when viewed on a monitor and  would be way to large to be transmitted via normal internet connection speeds.  For instance, a web page loads into your browser at the download rate of your internet connection.  If you have very large high quality images on your website (or a lot of images), it will take forever to load the page.  Have you experienced that?  Also, you wouldn’t want to send an image in an email that takes 15 minutes to download.  I’m sure you’ve experienced that also.

A special image type called a Jpeg (Joint photographic experts group) was developed for the internet to make images low enough in quality (smaller in kilobyte size) for email and website transmittal via the internet but not to affect the visual presentation of the image too much.  A Jpeg is designed to degrade so it’s compatible with the internet.  Jpeg images make use of something called compression.   A normal digital image has a huge amount of data that’s stored as bytes on your computer inside the jpg file.  Compression throws out some of those bytes of data in a special way so that the visual quality is maintained somewhat, yet the kilobyte size of the image is drastically reduced.  A jpeg image with no compression can be as large as 30,000,000 bytes (30 megabytes.  After 60% compression, it may only be 14,000 bytes (14 kilobytes).  That’s small enough in kilobyte size that transmission time over the internet is tolerable but keeps the image at an acceptable visual quality for viewing on a monitor.

Many people send Jpeg images to print shops for printing.  DON’T.  Why?  Jpeg images are INHERENTLY DEGRADED ON PURPOSE for the reasons described above.  

A special type of image file called a Tiff (Tagged Image Format File) was developed for printing.  It doesn’t compress and has special information that’s included inside the file to make sharp, clear, color prints

IMAGES FOR PRINTING (TIFF FORMAT):

Resolution (pixels/inch) becomes important when readying an image for printing.  The next figure represents a high resolution digital image prepared for 300 pixels per inch.  For example, my 10 megapixel camera takes pictures 3648 pixels wide and 2736 pixels high.  Of course, when photographing a painting, you won’t fill the whole picture with the painting and cropping out the background will have to be done.  You’ll end up with fewer pixels to work with after cropping so keep that in mind.  Fill your frame with as much painting as you can get.  If using a rangefinder type camera, don’t look through the rangefinder.  Use the display, it shows the actual frame.  The range finder does not.

Each square above represents one square inch of print space.  I’ve selected a resolution of 300 px/inch.  How many inches wide will my print be?  It will be 3648 divided by 300 = 12.2 inches wide.  How high will the print be?  It will be 2736 divided by 300 = 9.1 inches high.  Each square will contain 300 pixels high by 300 pixels wide, or 90,000 little pixels in each square inch of the print (300 x 300).  The photo printer will spray one little squirt of ink for each pixel.  If you made the image resolution 72 pix/inch, you’d only get 72 little spray squirts per inch.  Not quite good enough for printed material.  300 pix/inch resolution is.

That’s fine and dandy, but what if I didn’t want a print 9” x 12 inches?  What if I wanted a print 5” wide and still keep it at 300 pixels per inch resolution.  You’d simply set the width in pixels to 300 x 5 = 1,500 pixels wide, keeping the resolution at 300 px/inch.  The height will adjust accordingly if you constrain proportions.  I will explain in the Photoshop section how to do that.

Most printing companies want specific a specific formats, resolution, color space and mode.  The format is almost always Tiff (Tagged Image File Format).  The resolution is generally between 300 and 600 pixels/inch.  Color space is usually Adobe RGB (sRGB is fine, though) and mode is generally CMYK.

Most cameras are set up to use sRGB color space which is a color space proposed by HP and Microsoft because it approximates the color gamut (range of colors) of the most common computer display devices.

Adobe RGB was designed by Adobe Systems to encompass most of the colors achievable on CMYK (see below) printers but will revert to sRGB when viewed from a computer.  Adobe RGB is a better color space for printing because most good printers use CMYK printing. Some cameras can be set up to use Adobe RGB color space, but don’t worry about this too much.  If you only have sRGB (which you probably do), it’s ok.  Not quite the best, but adequate unless you’re a professional photographer.

Mode:  Cmyk or RGB.  These are printing modes.  Cmyk (Cyan, Magenta, yellow and black) is a subtractive color process.  It produces better images and wastes less ink.  RGB is an additive process and most printing companies do not use this as it wastes ink and the images aren’t quite as good.

Below are two magnified images of a small section of a painting.  This demonstrates the difference between an image readied for email/websites (jpg, 72 pixels per inch) and an image readied for printing (tiff, 300 pixels per inch).  The tiff image will print sharply and the jpeg will be fuzzed out due to jpg compression and lower resolution.  A computer won’t show the difference much, but a printer will.

REVIEW: 

Most print shops want tiff format, 300 to 600 pixels/inch resolution, CMYK mode and Adobe RGB color space.

Website/Email images have to be jpeg format, RGB mode and sRGB color space (adobe RGB color space is ok, because it will default to sRGB when viewed on a computer.   

NEXT:

See the article "Photographing artwork with a digital camera".