The Flow Slider/Selection Box

The Flow Slider/Selection Box

The Flow Slider/Selection Box.

A lot of people are confused when it comes to the difference between "Opacity" and "Flow", and it's not hard to see why. However, they really are two completely different things, although Flow is somewhat related to Opacity.

What "Flow" does is control how much paint your brush puts down when you paint over an area with your mouse, which technically is exactly what Opacity does, hence the confusion. The difference though between Opacity and Flow is that with Opacity, as long as you don't lift your finger off the mouse and then click again to continue painting, your brush will add exactly the amount of "paint" you've specified in the Opacity value, and no more. No matter how many times you paint over the same area, as long as you don't lift your finger off the mouse and then click again to continue painting, your brush will never add more paint.

With Flow, that's not the case. If you have a value of 10% specified for Flow, the first time you paint over an area, you'll lay down 10% of your paint. But here's the difference. The next time you move your mouse over the same area, you'll add another 10% on top of the original 10%, even if you haven't lifted your finger up off the mouse. Paint over the same area again, and yet another 10% will be applied. That's the difference between Opacity and Flow. Opacity keeps the paint % value the same no matter how many times you paint over the same area (with the mouse button still down), while Flow increases the amount of paint in an area by the % value every time you pass the mouse over the same area.

If you're not confused yet (and hopefully you're not but if you are, the easiest way to see the difference between Opacity and Flow is to experiment and try them out on your own), here's one final thing you need to know about Flow. Flow is limited by the value you've set in Opacity. What I mean by that is, Flow can never, no matter how many times you paint over the same area, never ever add more paint than the value you've specified for Opacity. In other words, if you've set an Opacity of 50% and set Flow to 100%, Flow will only add 50% of the paint to the canvas because it's limited to 50% by the Opacity value. It can't go beyond that. You can paint over the same area until the cows come home but if you're Opacity value is set to something less than 100%, Flow can only add paint up to the % value you've specified for Opacity.

If you're now completely confused about the difference between Opacity and Flow, like I said, the easiest way to see the difference is to simply grab a brush, pick a color, change the value in Opacity and start painting. Then change the value again and paint again. Once you're comfortable with what the Opacity value is doing, set Opacity to 100% and start changing the Flow value while you paint. It won't take long before the difference between the two becomes clear.

Again, there's several ways to change the value in the Flow option box, which are all exactly the same as they are for the Opacity option, so I won't bother repeating them here. Simply check them out in the description of the Opacity option above if you haven't already.

http://www.photoshopessentials.com/basics/brushes/photoshop-brush-options-bar.php