header image for file sizes article

A word on file size and quality

Web, email, print, resolution, dpi and other matters.

We often receive images of bad quality or low resolution—usually taken from the web—and are asked to print them at photographic quality.

 

The assumption

It is sometimes assumed that a professional organization with high-end printers can take a poor photo and make it look fantastic.  This simply isn’t the case: whilst we have some ability to sharpen and clean up an image, one should remember, put crap in, get crap out.

 

The truth

We at FirstpointPrint have machines which output at 600dpi for images and 1,200dpi for text and vector.  If a blocky, low-resolution photo is fed into it, out comes a lovely, faultless, exact rendition of a blocky, low-resolution photo.

Also, bear in mind that JPEG includes a value for ‘quality’ as well as dpi and pixel resolution.  A high-resolution, large image can still be saved at ‘low quality’, which will reduce the size of the file but result in a poor print.  The quality value is all about the amount of gradient data which is included in the image, that is, how smoothly does one colour or shade blend into its neighbour.  A low-quality jpeg will have blocky steps of shade or colour, rather than a smooth transition, regardless of the print resolution.

Checklist:

  • Get out what you put in
  • Resolution is limited by the source image

 

File size

The size of an image file CAN BE a good clue as to its quality, but it is not to be relied upon.  A poor-quality image can still render as a large file and be none the better for it.  This partly depends on the file format, but there are other considerations as well, which will be addressed shortly.

A reasonable size for an image printed at A4 size would be around 10MB, but a 5MB file should be OK for that size.  If you want to print an A1 poster, it is unlikely that a file of that size will contain sufficient subtlety of data to print well.  20-50MB would be more like it.  Again, these are just rough estimates and are not to be relied on, as the following points should explain.

 

Format

Certain formats allow for more compression than others, resulting in smaller files.  Formats which remove some (probably) unnoticeable detail in favour of smaller files are known as ‘lossy’.  The most common of these is JPEG.  JPEG can be saved at various resolutions but also at various ‘qualities’, which refers to things like details in gradients.  A ‘quality’ of 8-12 will give you suitable printing results and allow you to reduce the size slightly if necessary.

Checklist:

  • JPEG potentially the smallest
  • TIFF larger
  • PSD potentially the largest
  • Avoid PNG, GIF

 

Resolution

This is a commonly misunderstood factor and the term differs in meaning between screen and print graphics.  For web and screen images, ‘resolution’ refers to the number of pixels in each dimension, e.g. 500x750px.  In print, the term refers to the proximity of printed dots and is shown as ‘dpi’ (dots per inch).

Here lies the confusion: images created in Photoshop or any other image package will allow you to specify the resolution and the dpi of the image, but resolution in this sense has nothing to do with print and dpi has nothing to do with screen images.  dpi is disregarded by the web and a high dpi value is simply including unnecessary data (and therefore size) in an image which is displayed in pixels.  Conversely, an image may have a huge resolution in pixels but a low dpi (usually 72dpi for the web) and print correspondingly badly.

Here’s a numerical example: a particular photo of 2,067×1,033px and is to be printed at a physical size of 350x175mm but only has a dpi of 150.  Images should be printed at AT LEAST 300dpi in order to appear fairly sharp.  To condense this photo to 300dpi the physical size drops to 175×87.46mm (half the intended size).  Modern printers (including the ones at FirstpointPrint) print at 600dpi and images appear very sharp, even when studied closely.  To print this photo at 600dpi its physical size plummets to 87.5×43.73mm, a mere quarter of the intended size!  Suddenly your lovely, framed desk photo would barely fit on a postcard if it is to be in focus.

Note that each time the photo is resized to increase the dpi, the pixel resolution stays exactly the same.  This shows that pixels are irrelevant to print and only the physical size and the dpi are important.

Checklist:

  • Pixels are irrelevant to printing
  • dpi is the important figure
  • 300-600dpi for a sharp image

 

The web

The last point comes into play here as well.  We have images sent to us every day which have been dragged from a web page or ‘save as…’ed and emailed to us to be printed.  We have to stress each time that the images may look sharp on screen but will most likely print terribly.  This is because images intended for the web usually have a dpi of 72 (sometimes 96) applied to them.  This figure is mostly arbitrary and merely exists so that a printed web page will at least include any images, albeit it at low resolution.  The low dpi is applied to keep the file size down, as they are intended for the internet and not for printing, and a fast web page relies on images with low file sizes.

The image the client is seeing on screen is generated by the pixel dimensions (screen resolution) of the image.  Pixels are not printed, DOTS are, so the sharp image on screen translates to a fuzzy image on paper and the client wants to know why.  Now you do.

Checklist:

  • Web images usually have low print resolution (often 72dpi)
  • They may look sharp on screen but that is irrelevant

 

Email

One thing to be aware of is that email programmes such as Outlook often resize emailed images in order to reduce the file size and expedite their delivery (as with an efficient website).  This will result in reduced print quality.  This option can often be disabled but if it cannot, try creating a .zip or other archive of the image and sending that instead.  This will lock the image up and protect it from the machinations of the over-zealous, Mr Motivator email programme.  On a Mac, the option is located at the top-right of the email content window; choose ‘actual size’.  On Outlook, this option may vary in location, depending on the version, or it may not exist at all.

screenshot of Mac Mail file size option

Checklist:

  • Email programmes often try to reduce the size of images
  • Use ‘actual size’ on Mac Mail
  • Try zipping the file first