Resizing Smoke & Mirrors Part 1

Many of the tools, functions, and filters in Photoshop or Photoshop Elements are strait forward. You see the result as soon as you complete the item. Some however are more insidious by nature. One such is the Image Size dialog box.

Yes this is the dialog box I am talking about. Just what does it all mean? Rather than going from top to bottom, we will start with the simple stuff. At the bottom are three check boxes, Scale Styles, Constrain Proportions and Resample Image (we will get to this one in a little bit). When Scale Styles is checked and you resize the image, any of the elements of styles will be scaled at a proportional level. Meaning if you double the size of the image you will also double the size of the stroke in a style layer.

Constrain Proportions is simply keeping the aspect ratio (relation of hight and width) the same when resizing. The two circled chain links in the screen capture above indicate the fields are locked. There is rarely a reason to not constrain proportions unless you want a squished face. If you make a Pixel Dimensional change it will be reflected in the other three fields, yes even the Document Size fields.

Moving on to Pixel Dimensions and Document Size. Here can be the smoke and mirrors part. Lets start with Pixel Dimensions indicated by the yellow arrow. Simply put, Photoshop is telling us exactly how many pixels the image is in Width and Height. If this image had just been imported from the camera, we could do the math and determine the sensor size was 8.7 megapixels. The 25.0M figure is a little misleading as this is the flattened file size. The reason for this difference is that each pixel contains data that represents a light value, so depending on the bit depth of the image sensor this value is different than the pixel size.

Before we move on, the red (or slightly orange) arrow above highlights the Document Size. This is the physical size of the print. If you want to print a 8x10 in portrait, then these figures would be... You guessed it 8.0 and 10.0. There is a linear relation between the Pixel Dimensions and the Document Size. An easy way to keep these two straight is to think of Pixel Dimension as resolution for display (think CGA of 1024x768) and Document Size for print dimensions. Now onto actual document resolution (not display). This is the field under the Document Size named, oddly enough... Resolution.

The green arrow above points out the actual resolution of the document. What this means is how many pixels per physical inch will be in the document. Most printers will provide the best quality at 240 or 300 pixels/inch while displays max out at roughly 72 pixels/inch.

Now there is a relationship between these three settings similar to aperture, shutter speed, and ISO on a camera. When one changes, one of the others change. If any of the three settings in the Document Size changes, the Pixel Dimensions will change also. When one of the Pixel Dimensions fields are changed, the Document Size will change with the exception of Resolution. This is why when you change the Pixel Dimension before adjusting the Resolution, you have to go back and reset the Pixel Dimension.

That should be enough for you to chew on today. In Part 2, I will go over the last item in the dialog, that of Resampling Image.

Cya then... Doug