Image Resolution

Image resolution is very important when using any sort of digital imaging program. It is what decides the print dimensions and quality of your image.

 

Printed dimensions and quality are measured by dots per inch (dpi). 300 dpi will produce high quality for any size image. Once printed it is referred to as ppi or Pixels per inch.

 

Many home printers can produce quality images at 150dpi but most companies will ask for a 300dpi image.

 

Printing is done using four inks, Cyan, Magenta, Yellow and Black. There are four separate dot patterns for each color. Dots per inch refers to the number of dots in a square inch.

 

At 300 dpi all of the dots in each square inch are next to each other with no space between them. An image at 150 dpi has 150 dots per square inch. With that in mind you can begin to see how a lower resolution produces a lower quality or how an increased image size becomes a lesser quality image.

 

Suppose you took a 300 dpi image at 4in. x 4in. and increased the size to 8in. x 8in. You only have the beginning 300 dpi per inch at the original size. That means that you have taken those dots per inch and stretched them to cover the 8 x 8 area and have significantly lowered the dpi of the image to 150dpi. Because of this  it is better to decrease size than increase- as decreasing an image size will actually increase dpi, as I said before you have the original 300dpi to work with. So basically changing an image size changes the dpi. So if you decreased a 300dpi 4in x 4in image to 2in. x 2in. you have just changed the image to 600dpi because those dots have just moved closer together rather than farther apart as I the first example.

 

But…

 

Printing at higher than 300 dpi will just take longer to print as the dots begin to overlap and it is only needed for the dots to be next to each other. Unless you are planning to upsize an image there is no need to go higher than 300dpi.

 

Photoshop has a great image resize feature and will allow you to adjust or add dpi so that upsizing an image will retain quality. There are also a variety of Photoshop add ons that will help with this.

 

Some printers will tell you that all they need is 200dpi. Other times newspapers or magazines may go as low as 100 or 85 dpi. Lowering dpi for images is safe as you are simply eliminating dots that already exist. But raising dpi can be dicey as Photoshop or add ons will need to add dots per inch to give you the desired dpi. If the upsizing is not drastic Photoshop does a pretty good job. But if you need to significantly increase the dpi and size than ultimately image quality is still lost even though you still have a 300dpi image because Photoshop is taking the information in the image and simply doing the best that it can in deciding which dots to add. And as good as that may be, it will never be as high quality an image as if those dots came from the actual image rather than placed there by Photoshop.

 

300dpi will produce a higher file size but because of everything mentioned, if file size is not a factor than you are better off having the largest size image at 300 dpi and sizing down as needed.

 

Megapixels:

 

A megapixel is a measurement like square feet. There are 1 million pixels in a megapixel.

 

To find the largest photo quality that you can print with your camera divide the pixel resolution by 300 (meaning 300dpi)

 

For instance:

 

For a 6 megapixel camera you would divide

3 008 / 300 = 10.0266667

And then:

2 000 / 300 = 6.66666667

(These numbers come from the chart below)

So you know that you can safely take a 10×6 photo quality image at 300dpi with a 6mp camera.

 

As mentioned before your home printer can produce a very nice image at 200-150dpi. So you can consider that as well.

 

So if printing on your home printer you can cheat a little. Just divide the pixel resolution by 200 instead.

 

For instance:

 

For a 6 megapixel camera you would divide:

 

3 008 / 200 = 15.04

And then:

 

2 000 / 200 = 10

You know that you can safely create a 15×10 print or smaller image on your printer from a 6mp camera.

 

 

Megapixels vs. Maximum Print Size Chart

Megapixels	Pixel Resolution	Print Size @ 300dpi	Print size @ 200dpi	Print size @ 150dpi
3	2048 x 1536	6.82″ x 5.12″	10.24″ x 7.68″	13.65″ x 10.24″
4	2464 x 1632	8.21″ x 5.44″	12.32″ x 8.16″	16.42″ x 10.88″
6	3008 x 2000	10.02″ x 6.67″	15.04″ x 10.00″	20.05″ x 13.34″
8	3264 x 2448	10.88″ x 8.16″	16.32″ x 12.24″	21.76″ x 16.32″
10	3872 x 2592	12.91″ x 8.64″	19.36″ x 12.96″	25.81″ x 17.28″
12	4290 x 2800	14.30″ x 9.34″	21.45″ x 14.00″	28.60″ x 18.67″
16	4920 x 3264	16.40″ x 10.88″	24.60″ x 16.32″	32.80″ x 21.76″

 

 

Less than 150 dpi and your pictures will look pixilated and fuzzy.

 

The settings will vary from camera to camera. Check your manual for pixel resolution and settings. These are not the only factors in producing high quality images with your camera. You must also consider lighting, brand, lenses, etc.