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Digital Photo FAQ's & Tips: Image Resolution

This section of the web site is a collection of digital photo Frequently Asked Questions (FAQ's) and Tips.

How many megapixels are required for 4x6", 5x7" and 8x10" prints?

For high quality prints (at 200 ppi), the camera should have the following megapixels (MP):

  • 4x6" prints require at least 1.1 MP
  • 5x7" prints require at least 1.5 MP
  • 8x10" prints require at least 3.9 MP

If you will be doing significant cropping of the photos (digital zooming), more megapixels will be required to make up for discarded pixels.

For additional details, see Megapixels for Digital Photo Printing.

How many megapixels are required for poster size prints?

Because poster size prints are generally viewed from an arm's length or farther, less pixels per inch (ppi) are required to produce a quality image. For good quality posters, 150 ppi is recommended. At this resolution, the camera should have the following megapixels (MP):

  • 11x14" prints require at least 4.1 MP
  • 16x24" prints require at least 9.7 MP
  • 20x30" prints require at least 15.1 MP

The larger sizes may produce good results at 100 ppi, requiring less megapixels. If you will be doing significant cropping of the photos (digital zooming), more megapixels will be required to make up for discarded pixels.

For additional details, see Megapixels for Digital Photo Printing.

Use highest resolution setting

For best results, use the highest possible image resolution before printing or developing your digital photos.

Before taking pictures, make sure that your camera is on the highest resolution setting. Some cameras may call this "large", "super fine", etc., so check your user guide to be certain.

After taking the picture, do not reduce the resolution with photo editing software, unless you have a specific need.

If editing photos to remove red eye, crop, etc., be sure to save the image at maximum quality. This is especially important if saving as a JPEG file (with .jpg on the end). In the case of JPEG, your software may refer to the highest setting as "maximum", "10", or "100". Saving at the highest resolution will give you the sharpest image on paper.

How many photos fit on a memory card?

The answer depends on the following camera factors: resolution (megapixels), image quality setting, recording mode (JPEG, raw), and the amount of image compression. Instead of providing an enormous chart covering the above factors, let me give you an easy method for determining this answer for a specific camera:

  1. Insert a blank memory card of any size into the camera.
  2. Set the camera to the mode you are most likely to use. If not sure, then use Auto or Easy.
  3. Turn on the camera and look for a number on the preview screen—this number indicates how many photos fit on that memory card.

    If the display does not show the number of photos, then take photos until the card is full and keep count. (This will go quicker with a smaller memory card, such as a 64 MB.)
  4. Then, divide the number of photos into the size of the memory card.
  5. For example, if a 64 MB memory card held 25 photos, then the formula is the following: 64 / 25 = 2.5 MB per photo.
  6. Now that you know the size of one photo, you can answer the original question with this formula:

    [memory card size in MB's] / [photo size in MB's] = # of photos

    Note that 1 GB (gigabyte, or billion bytes) is approximately equal to 1,000 MB's (megabytes, or million bytes).

Let's use my 3.2 megapixel camera as an example. In Auto (JPEG) mode the image sizes are 1.7 MB and in P (raw) mode they are 2.5 MB per photo. Here are three memory card sizes and the number of photos they hold:

  • 128 MB card: 75 JPEG photos | 51 raw photos
  • 256 MB card: 150 JPEG photos | 102 raw photos
  • 1,000 MB card: 588 JPEG photos | 400 raw photos