Understanding Resolution

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General Info

Understanding image dimensions is just as pertinent in Photoshop as it is in Final Cut Pro. However, unlike FCP, Photoshop allows the user to utilize higher output resolutions for the use in high quality printing. In this section, we will help you determine what pixel dimensions are appropriate for your application. We will also help you properly view and adjust pixel dimensions within Photoshop.

Determining a Propper Size

Before we even get to changing the resolution, you need to ask yourself how the image is going to be used. There are three basic categories with regards to image size:

  1. Web-posting
  2. Video
  3. Printing

Web-posting

Images on-screen typically need a maximum of 72 PPI (pixels per inch). If a file has higher resolution than that it simply looks no different on screen. The only real difference will be that the file will be bigger and will therefore be slower to download. All images on this site are at 72 ppi, regardless of their size. Basic image dimensions to use when starting a new image in Photoshop for web-posting should be:

  • 640 x 480 pixels
  • 1024 x768 pixels

These sizes may be larger than you require, although you can always size the image down with the Photoshop crop tool. These file sizes give the user a good idea of how large the image will appear on a web page or in an email.

Video

Similar to web-posting, video is displayed on a screen, therefore the maximum is 72 ppi. However unlike web-posting, video requires certain dimensions- otherwise the images in your video will contain black borders ( think wide screen vhs). Typical video dimensions can be directly translated to Photoshop:

  • NTSC DV 720 x 480
  • NTSC Widescreen 720 x 480
  • HDTV 1280 x 720
  • HDTV 1920 x 1080

Printing

Printing requires a much higher resolution, because the viewer can examine an image much closer. Most photographers do their printing these days with a desktop ink jet printer and the Epson Photo printers are the most popular so I’ll use the’ll be used by way of example. These printers, such as the models 870/1270/2000P are (somewhat misleadingly) listed as 1440 dpi printers. This means that they are capable of laying down that many dots per inch.  To create a color image, they need to use 6 different inks, so any particular pixel reproduced on a print will be composed of some dithered composite of colored dots using some or all of these inks. That’s why you need more dots from your printer than you have pixels in your image.

If you divide 1440 by 6, you end up with 240. This is the true minimum resolution needed to get a high quality photo-realistic prints from a 1440 dpi Epson printer. Many users believe that a 360 ppi output file can produce a somewhat better print. If my original scan is big enough to allow this, I’ll do so, but I don’t bother dressing up a file to more than 240 ppi when making large prints.

Changing Image Resolution in Photoshop

We will start off by importing your image into photoshop:

  • Under the File menu select the Open option and select your image file.
  • If your file has no color profile, Photoshop will ask to assign one. Select the Assign Working Color Profile.

Adjusting Resolution

The first thing we need to do before we start altering image size is check the actual size of the image on the computer screen.

  • Click on the View menu and select the Actual Pixels option. This should enlarge your image to the scale that it would appear on a web page or monitor.

Adjusting Resolution Without Changing File Size
  1. Go to the Image menu and select the Image Size option.  You will see the following menu:

Image:image_size2b.jpg

  • The highlighted portion of the menu allows the user to change the image’s resolution (if you are over or under 72 ppi for video or web-posting) without altering the files size (in bytes). This section basically links the resolution with the file’s dimensions- for instance, if you up the resolution to 300 ppi the dimensions of your file will get smaller.
    • VERY IMPORTANT: To make these changes with out affecting file size you have to make sure that the Resample Image box is unchecked.

Changing File Size

Image:Image_size1.jpg

  • When the Resample Image box is checked the Pixel Dimensions set of options becomes available.

This section allows you to alter the image output size.

  • The Pixels scroll bar allows you to switch between changing your images size in terms of Percent or Pixels.
  • With the Constrained Proportions box checked, your aspect ratio will be kept in tact when you alter image size.

Crop Tool

The Crop tool allows the user to quickly change the size of an image on the fly. It can be found in the Photoshop tool bar under this icon:

Image:crop_tool1.jpg

  • To make a quick crop, select the tool and drag it over your image to fit your desired new file size.

Image:Crop_tool2.jpg

  • Click the Return Key on your keyboard and Photoshop will crop the image.

At the bottom of your image window, Photoshop details on many aspects of your file. Document dimension + size, can be accessed here by clicking on the triangle icon.

Image:crop_tool3.jpg