Image Resolution FAQ

Download Image resolution test file

Download A4 Image resolution test print

What resolution should I use?

Based on our experience we recommend the following image resolutions (based on images at full size):

• A2 or smaller use 300dpi at full size.
• Larger than A2 use 150dpi at full size.
• Larger than A0 use 100dpi at full size.
• Big prints such as those for large banners should use 100dpi or less to avoid overly large and cumbersome file sizes.

Where possible keep all text as vector art. If you do have to convert text to an image (Rasterise the text) then keep the image resolution as high as possible to preserve readability of the text

Why do printers usually specify a resolution of 300dpi?

Historically commercial printers have asked for images at 300dpi because this is the optimal resolution for an image to print using the offset printing process with a halftone at 133 - 150 lines per inch. The optimal resolution is calculated at 1.5 to 2.0 times the lines per inch .This makes the optimum resolution of images 200dpi to 300dpi, with 300dpi being a safer number.

If you don't understand that then don't worry, Just realise that 300dpi is the standard resolution because offset printing is the most common printing method.

Why recommend these resolutions?

For larger prints you don't need the same level of detail so you can use a lower resolution. If images over A0 were at 300dpi then we would end up with files sizes so huge they would be impossible to work on. You can see in the table below how the file sizes increases dramatically as we increase the print size.

A 133MB image contains 34 Million pixels. That's equivalent to a 34 Mega-pixel Digital Camera! With that may pixels - no matter what size you print the image you will have plenty of 'visual detail' . Usually the larger images are viewed further away, this means you can decrease the resolution without any apparent loss of quality for the viewer.

One exception to this rule is display work for exhibitions where big prints are designed to be viewed up close. We have found the image resolution of 150dpi is almost perfectly photographic even at when viewed at reading distance. Dropping the image resolution to 120dpi reduces the file size to nearly half with only a small loss of detail

A CDROM holds 700MB

A typical computers memory is 1000MB to 2000MB (1GB-2GB)

A DVD Disc holds 4200MB

My image is less than 300dpi, Should I increase the resolution?

The simple answer is no. You cant add in detail and information that not there is in first place. When you increase the resolution the computer simply adds in more pixels by averaging out the information in the image. This only makes the file size larger but does not add in any more detail.

Is there any way to turn a low resolution image into a high resolution image?

Unfortunately the only place where you can do that is in the movies, in real life there is no special software that will magically convert a low resolution image to a higher resolution image. If the image is low resolution then there is simply not enough information in the image.

Image resolution vs image size.

A common mistake when selecting the resolution of an image is to forget that dpi (dots per inch) is the ratio of dots to inches. It is critical that you set the image size at the same time you set the resolution. For example if an image was be supplied at 300dpi but A5 in size, when enlarged to A3 (4 times bigger) the actual printed resolution is a low 75dpi.

When creating your images make sure you set the height, width and resolution at the same time. This screen shot shows how to make an image for a banner 2 meters x 1 meters at 150dpi.

Choosing an image size based on 'visual detail'

For very large prints there often comes a point where the files get so large that they become difficult to manage at the recommended resolution.

In this case we suggest the you set the image size based on the visual detail that you want in the image. If the image viewed at full zoom contains enough detail then the final print will likely be more than acceptable. We recommend that when are you reducing the size of the image, keep a close eye on the fine detail in the image, such as hair and eyelashes. If these details are blurry then you can probably safely reduce the size of the image. If they become blocky and pixilated then you have probably reduced the image too much.

Remember bigger prints are viewed from further away and do not need to be as detailed as those that are viewed up close. It is not uncommon to print large billboards with resolutions at low as 15dpi.

Wait, I've always been told to do images at 300dpi.

Try this test at home yourself. Download this PDF file (Image resolution test.pdf) and print it out on your printer. Then stick it to a wall and stand back. You will notice that the further away to stand, the better the lower resolution images look. You can use this as a guide to decide on the best resolution for your print.

Isn't the correct term ppi (pixels per inch) and not dpi (dots per inch)

Yes and no. Technically with images are are dealing with pixels, so yes it should be pixels per inch. But since almost everyone uses the term dpi to mean ppi. We have found using ppi simply causes confusion.

I still don't understand!

No problems. Just send us the highest quality version of your file and we will work out everything here. Call us and we can talk you through the process.