Ask the Teaboy

Q: How do I optimise an image file-size for Printing (i.e. NOT for screen)?

A: Last time we learned how to optimise an image for screen and email. This time we want to look at optimising your image file-size for printing instead.

This is not quite so critical to the well-being of the planet as the last topic, as you won't be emailing these files to all and sundry. In fact, as most image editing software will give you the option to easily scale your images to the size you dictate, the thought of sizing files for printing may seem uncessary to many - and to many it may well be.

But, there are a couple of reasons you may want to read on: 1) if you use online printing services like WebPrints, optimising your image file-sizes can substantially reduce the time and bandwidth needed to upload your files - with the lower bandwidth usage also potentially translating into lower internet connection costs for yourself (also, as we pay to download your files, reduced file sizes mean reduced costs to us, which inevitably gets passed on as cheaper print pricing as a result), 2) optimising size can also mean opportunity to optimise sharpness and other aspects of your image prior to printing (if you're that way inclined, as many of our customers seem to be).

Let's open an image in Photoshop again (still no cats I'm afraid, thankfully so with these Green Finches around...). This particular image is a full 8 megapixel file. We only want to print a standard 6x4" print today, so the 8 megapixels of resolution is a major overkill. Since we're intending to use WebPrints we're going to be bandwidth-friendly citizens (a bit like being environmentally friendly) and shed some pixels - and yet get a better print result while we're at it. If you were paying attention last time (quiet at the back of the class!) this lesson will be a breeze.

After opening an image in Photoshop, and choose Image/Image Size from the menu at the top, you'll be presented with a box as above. When optimising an image for print (unlike when optimising for screen) we must now pay close attention to the 'Document Size' and the 'Resolution' section. Note that this image file has opened up with a resolution of 180ppi. Important to know that 'photo quality' is regarded as about 300ppi (pixels per inch). Any less than that won't be.. er.. photo quality, and any more than that is arguably impossible for the human eye to benefit from (i.e. more detail than we can see).

The resolution of your image will be 'stamped' by your digital camera - for many people it'll be 72ppi, for some 180ppi, etc. As we learned in a previous article, this has no relation to the megapixels of your camera.

Now take a note of what we've done above. By unchecking the the 'Resample Image' check box we can adjust the resolution to 300ppi without resampling yet, and by doing so can now get a clear idea of what we have to work with. By increasing the resolution to 300ppi we can see that our dimensions have conversely reduced - and is now just a tad under 8x12". Since we only want to print a 4x6" print (at photo quality - or 300ppi) we realise we really have more pixels than we need (a bit like taking a week's worth of groceries on an overnight camping trip).

So, lets shed the superfluous data:

We've rechecked the 'Resample Image' box and have adjusted the dimension to that which we require - 4x6". Note that the top of the Image Size dialog box indicates the significant drop in file size (from 23.4mb to 6.18mb as an open file - it'll be even smaller once saved as a JPEG). Then hit 'OK', and we're ready to sharpen before saving.

As mentioned in the last topic, there are many ways to sharpen, and we don't want to delve deeply into this subject here. It's a good idea to view the image at 100% (screen res) when sharpening, to get a clear view of what's happening to your image (choose View/Actual Pixels from the top menu). Like before, a quick way to a good result is to choose Filter/Sharpen/Sharpen from the menu at the top (or 'Sharpen More'). If the effect is too strong, choose Edit/Fade Sharpen and adjust to taste.

Then we just need to save the file. Choose File/Save As from the menu at the top. The Save As dialog box will appear, and you must choose three things 1) where to save the file, 2) what to call the file, and 3) the file type (JPEG).

Then, as above, one more box will appear - where you get to choose the JPEG compression! As you can see from the slider, the further to the right makes a large file (higher quality) and to the left makes a smaller file (more loss of data). From experience we can advise that setting the quality level at more than about 7 or 8 out of 12 makes no visible improvement to the resulting print, but makes the file significantly larger. Dropping to 5 or lower can begin to show artifacts and loss of detail. So, 7 or 8 is a happy medium.

Our resulting JPEG is just over 200kb's as a closed file - it makes a high quality 6x4" print, yet only takes a few seconds to upload for printing. Perfect!

Note: The steps above are a very 'manual' way of adjusting the printing dimensions of a file. If you're doing a lot of files there are quicker ways of doing this - but we share the above as it is important to understand how resolution works, so you can then deal with the other options in a knowledgeable manner.

Look! >>> Other recommended methods:

1. Photoshop Actions: You can create 'Actions' in photoshop to automate all of above (see Photoshop help files for details)
2. Photoshop Crop Tool: On the Photoshop toolbar there is a 'crop tool', where you can set dimensions and resolution (in this case 6 in x 4 in with a resolution of 300ppi). With this tool you can get to your required size and dimension by merely clicking and dragging from one corner to the opposite corner and hitting the enter key.
3. Other Batch Convertors: Another excellent option is to use a program like Irfanview to 'batch convert' a whole bunch of your files in one hit. You can specify sizing dimensions, sharpening, even contrast adjustments etc., to be applied to many files in one go.

--ooOoo--