An overview of Image size and DPI

I've written this mainly to help other artists who may need to produce 300dpi images to create printed versions of their artwork. I hope I've included most of the basics - feel free to skip along to the headings that you are interested in and please leave me any questions in the comments.

What does DPI mean?

DPI means how many dots or pixels will be printed per inch. It’s sometimes referred to as PPI (pixels per inch). Basically, the higher the DPI, then the better quality the prints will be.

However, a file showing a property of 72dpi might be better quality than another file that shows 300dpi – the actual image quality depends on the number of pixels in the image, also known as the pixel dimensions of the image.

i.e

• An image showing 300dpi and pixel dimensions of 600 x 600 pixels will print a 300dpi image at 2 x 2 inches.
• An image showing 72 dpi and pixel dimensions of 6000 x 6000 pixels will print a 300dpi image at 20 x 20 inches.

Conversely, changing an image’s dpi will change it’s print size. So, the same file can be printed quite large at a lower resolution, but the image quality will suffer.

i.e

• A  6000 x 6000 pixel image will print a 300dpi image at 20 x 20 inches.
• A  6000 x 6000 pixel image will print a 200dpi (lower quality) image at 30 x 30 inches.

What DPI do I need?

Web images

For posting images to the web you don’t need a very high dpi to show a good quality image. Image sizes for the web, also need to be kept relatively low so that the image size doesn’t slow down your web page or prevent images from loading at all. The standard size of images for the web is 72dpi.

Printing images

For professional printing purposes, an image being prepared for print should be no less than 300dpi if high image quality is required.
A resolution of around 200dpi can be used, but lowering the DPI produces progressively lower output quality. The ideal DPI is dictated by the required quality and size of the final printed product.

How can I find out the DPI of my image?

For JPGs -

Right click on the photo and click properties.
Right click on the ‘Details’ tab and scroll down to where it shows the image details.
It will display something like this:

Dimensions                   1936 x 2592

Width                            1936 pixels
Height                           2592 pixels
Horizontal resolution      72 dpi
Vertical resolution          72 dpi

To understand what this means in real terms you need to divide the pixels by the DPI to see how many inches the image will be.

Formula to convert pixels to inches: Pixels ÷ DPI = Inches

So, for the example image above, at 72 DPI:

The width is  1936  ÷ 72 =  26.889 inches

The height is  2592  ÷ 72 =  36 inches

However, 72 dpi isn’t high enough quality for printing, so to work out how big your image will print at 300dpi you just need to divide the image dimensions by 300 instead of 72.

So, for the image above, at 300 DPI:

The width is  1936  ÷ 300 =  6.45 inches

The height is  2592  ÷ 300 =  8.64 inches

So if you print this image at 300 dpi then it would print out at 6.45 x 8.64 inches.

Photoshop/Adobe files

If you have Photoshop, you don’t need to get out your calculator.

Just bring up the image menu and click on Image size.
This window will show the pixel and document dimensions and the resolution.
If the image is showing a resolution of 72 dpi you can change this without altering the file, just make sure you untick the box ‘Resample Image’ first.

Then you can change the resolution number to 300 to show how that would affect the printed size.

What are Megapixels (MP)?

A camera’s megapixel (MP) count refers to the number of pixels it can capture by the million. The MP of a standard compact camera is usually based on the production of photo’s in 4:3 proportions. If a camera is branded as ‘4 megapixel’ this means that it will take photos containing approximately 4 million pixels. So, a photo of 4:3 (adjusted at best quality or highest resolution) has 2304 x 1728 pixels = (rounded off) 4.000.000 pixels = 4 MP.

If your 4MP camera is set to take the highest quality photos then you can calculate the maximum print size by dividing the camera’s pixel dimensions by 300 (the ideal print DPI):

2304/300 = 7.68” width

1728/300 = 5.76” height

So 7.68 x 5.76” is the maximum print size that the camera is capable of.

How do I take  an image with 300 DPI?

Taking a photograph

Images taken on a digital camera (or phone) usually display a dpi of 72. These photos will also be printable at 300dpi, you just need to calculate the final printed size based on the pixel dimensions of the photo.

The most important thing to remember when taking a photo with the aim of printing it is that you take as high quality photo as possible. A good quality digital photo should be:

1. taken with a good quality digital camera (good optics and digital sensor)
2. a photo that has not been enlarged either in post-processing or by in-camera digital zoom.
3. properly shot with good lighting and no blur
4. a photo shot within the camera's ideal ISO range (usually a low ISO such as ISO 100)
5. stored in either a lossless format (i.e. TIF) or a very low compressed JPEG (highest camera JPEG quality setting).
6. taken at the highest quality setting their digital camera allows.

If your camera is not capable of taking a photo with a dpi high enough for your required print size, or if you don’t have the experience or equipment necessary to take a high quality photograph you may need to consider using a professional photographer, or having your work scanned.

Scanning

If you're creating a digital image by scanning you have to set the DPI of the scan to meet the required DPI at the dimensions it is to be printed.

Formula for calculating the DPI needed:

Scanning DPI needed = (Required print size* x Print DPI) ÷ Original artwork size*

*The required print size and original artwork size is this formula refer to either the height or width of the image. You should do both calculations and use the higher result for your final scanning DPI setting.

Example:

Required print size = 60x40”
Print dpi = 300
Original artwork size = 10x8”

Pixel dimensions needed:

Width =  60x300 = 18000

Height = 40x300 = 12000

Scan DPI needed:

Width = 18000 / 10 = 1800

Height = 12000 / 8 = 1500

The two DPIs are not the same, as the aspect ratio (proportions) of the two print sizes are different. The rule is to simply pick the largest DPI number, in this case 1800dpi.

Some more simple examples:

To print a photo originally sized 4 by 5 inch the same size - scan it at 300 dpi.

To print a photo originally sized 4 by 5 inches at 8 by 10 - scan it at 600 dpi so that the final print size will be 300 dpi.

N.B Scanning at high resolutions makes great demands on your scanner’s memory, so if you need to scan at very high resolutions you may need to go to a printer with a higher spec scanner.

Can I increase the DPI of my image?

Ideally you should start with an image that’s high enough resolution at the required print size. But, as a last resort you can increase the dpi of an image using photoshop (other programs are probably available). Increasing the DPI of your image whilst keeping the width and height the same (or increasing the width and height whilst keeping the dpi the same) is known as resampling.

Don't resample an image unless you have no other option. The image will not be as high quality as it would be if you took a high enough resolution photograph/scan originally. Never resample the same image more than once as this will degrade the quality.

There are two ways to increase the size of an image in Photoshop, you can either resize the image, or you can resample it. These are not the same thing!

The difference between resizing and resampling: 

• Enlarge by resizing – Increases the size of the image whilst reducing the dpi (untick ‘resample image’ box)
• Enlarge by resampling – Increases the size of the image whilst maintaining (or increasing) the dpi (tick ‘resample image’ box). This will physically increase the number of pixels in the image!

To resize OR resample within Photoshop go to ‘Image’ and click on ‘Image size’. This window will show the pixel and document dimensions and the resolution.

To increase the DPI of your image you just need to tick the ‘Resample Image’ box and increase the resolution from 72 to 300 – your document size settings will remain the same, whilst the DPI is increased. It is as quick and simple as that, but you need to consider that enlarging your image and creating new pixels where they didn’t previously exist is bound to affect your image quality and will not improve it!

Photoshop has a number of ‘interpolation algorithms’ which control how it handles adding pixels when you resample an image to a larger size. There are three main options to choose from:

• Bicubic Sharper – Use this option when resampling your image smaller for best image quality
• Bicubic Smoother – Use this option when resampling your image larger for best image quality
• Bicubic – Not really used much now that the other options are available.

Just remember, increasing resolution in Photoshop WILL NOT increase the quality of the image. It should only be used as a last resort to create images that will print slightly more successfully than lower resolution ones. You will never be able to create a really high quality image from a low dpi file.



This post was put together with lots of help from the following websites and forums, thank you!:

Pentax Forums - How to take images 300dpi

What Printshops really want

Misunderstandings about DPI

Scanning

Difference between image resizing and resampling in Photoshop

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