What single number do we look at to see how good a camera is? The megapixel count; the bigger, the better. Yet what I mostly do with photos is post them on the web or email them to people; then I have to reduce their size to get them to fit thru the Internet’s plumbing. I wanted to briefly share how I shrink .jpeg photos.
I use two methods, usually in combination; increasing the data compression and reducing the pixel count. Each method has drawbacks. Too much data compression turns gradients of color into “bathtub rings.” And too small a pixel count gives a picture a Lego-brick look. By combining both techniques, you can avoid these extremes.
Data compression in Photoshop 11 happens when you write the file; File > Save as. After setting the file name and location, you get a quality slider control. For each position, Photoshop shows an estimate of the file size that would result.
In this example, even by using the poorest quality setting I wasn’t able to squeeze under my 100K target.
I use two methods for reducing the pixel count. One is called “resampling.” To do it, Image > Resize > Image size. You can work with Pixel Dimensions or Document Size; I use the first one. On the bottom edge, notice the choices for how Photoshop will resample the photo, and the advantages of each method.
You can work in pixels or percent of current size. Pixels is the more precise choice. I leave the proportion constraint checked and reduce one dimension by an even divisor; in this example, 4, yielding a new width of 576. The other dimension adjusts automatically. This way, I know that every new pixel will totally encompass a set of old pixels (in this case, 16 of them). I think doing this calculation gives a less noisy result. Typing an arbitrary pixel count would let new pixel borders slop over old pixel borders.
In this example, by dividing the pixel count by 4 I was able to File > Save as, specify medium quality data compression, and still clear 100K.
Another way of reducing the pixel count is to crop the photo. Some photos could use cropping anyway, to get rid of distractions or improve the composition.
In this example, I cropped out the top and bottom of the photo (tho I didn’t much like the effect), reducing its height to 1991 pixels. It wasn’t quite enough; even at maximum data compression, the file size would have been 106.4K.
If I discover that I’m still overshooting, I undo and start over from the original file. (Don’t overwrite it!) Multiple shrinks combine to make more noise. (If you’re into psychoanalysis, you might also keep this maxim in mind.)