Tuesday, August 04, 2009

Image Resizing

Image resizing. Have you ever gotten one of those emails that contains 5 FULL SIZE pictures from someone’s brand new DSLR? You know, the ones that are 50MBs in file size and would probably be fine if you’d received it at home, but it just so happens that you’re not, and so instead of waiting 3 minutes for it to download it actually crashes your phone for a week. Yeah. One of those emails. This post is about how to NOT be one of those people…

Good image resizing is critical if you want to display your images well across a broad range of venues. This post is going to look at resizing things for the web/email.

There are two key issues… quantity and quality. Let’s discuss quantity first. It’s fairly safe to generalize that every digital camera takes pictures that have more pixels than your monitor can display at any one time. That’s OK… our computers are smart, and they automatically scale images so we can see the whole thing at once. For most web use, we can save a significant amount of time (upload & download) and space if we simply cut out the excess quantity.
The largest images I put up on the web are 1024 pixels wide. That’s the extreme. Most stuff is significantly smaller than that. Don’t ever post images that are 1024 pixels high. Most people’s screens don’t have that many pixels and it just takes longer to download. So… here’s my rules of thumb:
Facebook: 640 pixels wide or high
Flickr: Up to 1024 pixels wide; 800 pixels high
Email: 800 pixels wide or high

I don’t really use any other social networking sites, but note this: they take whatever you upload and resize it to what you actually see on the site. Don’t send them huge files. It takes forever to upload, and the final size is the same. The closer you can get to what they do, the less reprocessing is going to happen.

OK, let’s talk quality. Everything we’re dealing with here is a JPG image. (We can talk about GIF and PNG somewhere else.) Shoot in RAW? Great. Me too. The internet doesn’t understand RAW. It like JPG. Compression is the big (or small) reason why JPG is such a popular file size. We can compress images without losing tons of quality. (Think MP3 files for audio… much smaller than CD quality and not a whole lot different, right?) This is where the magic comes in; if you apply the right amount of compression to your resized JPG image you can reduce the number of pixels and get the file size to a minimum, thus saving the world. Here’s the skinny:
Photoshop/Photoshop Elements: do a “Save As” and select JPG level 8
Photoshop/Photoshop Elements: use the “Optimize for web” and use 75-85
Picasa: Export as a JPG, set your pixel dimensions, use custom quality 75-85
iPhoto: it’s evil… Mac users tell me this too… if you have to use it, look for something in the 75-80% quality range
Photoshop Lightroom: Export your image, set your pixel dimensions, use JPG quality 75-85

Note: This really does diminish the quality of your image! If you take your original, high-resolution image resize it, save it as a new file, and use a compression of 75 you probably won’t see ANY artifacting or “fuzzies.” If you save that new file and compress it to 75 again, you will…

DON’T SAVE OVER YOUR ORIGINALS. DON’T COMPRESS YOUR IMAGES MORE THAN ONCE.

Cheers!
R

A couple walks into the late evening sunset
Image cropped and resized using my Facebook defaults.

1 comment:

Peter J. Serven said...

Nice article Rowan! And yes, iPhoto is very evil. :)