A Guide To...Art History

Digital images are made up of square units called pixels.

This jpg image is 222 x 150 pixels-- it contains 33,300 pixels in total, but they are so small you cannot easily see them.

Zoomed in 32 times (to 3200 %) the pixels from the base of the penguin's beak are now distinct and visible.

Pixels per inch.
Used for digital images, display resolution, and scanning output.

The resolution of the image of the penguin is 96 ppi.

The physical dimensions of the image if it were printed at 100 % would be 2.313 x 1.563 in

2.313 in x 96 ppi = 222.048 pixels

1.563 in x 96 ppi = 150.048 pixels

This corresponds to the known dimensions of 222 x 150 pixels.

Make sure you understand the difference in the information provided by resolution (96 ppi) and pixel dimensions (222 x 150 pixels).

*All this information can be found in the Image Size window in Photoshop*

Dots per inch. 
Used when PRINTING.

Look closely at a low quality print, like an image from a newspaper.

With the naked eye or a magnifying glass, you can see the dots of ink used to make the image.

There are relatively few dots per inch, which makes this a low resolution image.

Display resolution describes size, whether it is projected or viewed on screen.

A 16:10 widescreen monitor or projector output can be 1280 x 800 pixels or 1680 x 1050 pixels.

Many older "standard definition" displays and projectors had a 4:3 ratio, and a resolution of 1024 x 768.

Understanding display resolution is important because you can customize the presentation output in PowerPoint (see Page Setup) to correspond to a known projector output in a particular classroom, auditorium, or venue.

If you don't know the ratio, just do the math! 
1280:800 (divide) = 16:10, or 1.6

The combined knowledge of image size (pixel dimensions) and display resolution will help you select and scan images to appropriately meet your needs.

The images we scan come in all sizes, but we often like to level the playing field by incorporating them all into the same presentation.

• A 35 mm slide is roughly 1.5 x 1 inch, as is a 35 mm negative
• Other photographic formats have different size negatives
• Book images can be small or they can be full-page illustrations

How then does one select an output resolution for each scan?

Measure or estimate!

This postcard is about 6 inches in height and I want it to fit within my 16:10 (1280 x 800) PowerPoint slide.

If I scan at 100 dpi/ppi, my image will be approximately 600 pixels tall. 
If I scan at 150 dpi/ppi, my image will be approximately 900 pixels tall.

There is no need to be exact, but having a few more pixels is better than having less,
because stretching your image beyond 100 % can reduce quality.

Resample (verb)
To change the resolution of an image by changing its pixel dimensions. Downsampling is decreasing the number of pixels, and resampling up (or upsampling) is increasing the number. 

Photoshop Help: Resample an Image
Find the option in the "Image Size" window in Photoshop

If, for example, you want to share an image from your digital camera in an email, you may want to resample and adjust the pixel dimensions so that the image fits within the screen, to avoid making your recipients scroll around to piece together the whole image. 

• Check Constrain Proportions and Resample Image
• Change one of the values, either height or width

Quality may be lost when resampling, but weigh it against other factors based on your use and audience.

If you are mainly concerned with printing digital images, then you may never need to resample.  Altering the physical dimensions doesn't change the number of pixels; it just makes the squares smaller or larger accordingly.

JPEG or jpg: A file format generally used to share images over the web.  It is compatible with most applications. JPEG compression is a lossy technique that reduces file size, but also reduces image data.  "LOSSY"

TIFF or tif:  A file format which can be compressed without data loss.  TIFF file sizes tend to be larger than those of JPEGs.  "LOSSLESS"

PSD: Photoshop Document.  This is the native file format for Photoshop.  Save your finished image as a tif or jpg for compatibility outside of Photoshop.

Camera Raw Formats: Professional digital cameras produce these file types in which no in-camera processing was applied.  Even if you don't need to edit your image, to use it once it has been downloaded from the camera, you do need to save a copy of it in another file format.

You may also come across PNG, GIF, and BMP files, among others.  If possible, open the file in Photoshop and "Save As" a jpg or tif, again, for compatibility.

All content in Artstor is delivered to end users as jpgs.

In Photoshop, when saving an image as a jpg, you will be asked to choose a compression amount in the JPEG Options window.

Use the slider or the drop down to adjust the amount of compression.  Under "Preview," the affect on the file size is displayed.  Choose "Maximum" if you are not concerned with file size and want to retain as much data and quality as possible.

Compression:
a technique that reduces file size

Lossless compression:
An image compression technique that prevents image degradation caused by data loss. Lossless techniques usually use lower compression ratios than lossy techniques.

Lossy compression:
An image-compression technique using compression ratios that result in the loss of some image data.


From the Photoshop Elements 10 Help Document glossary