Thursday, June 17, 2010

The Digital Zone System and RAW Processing

I've had several people ask me how they can improve their outdoor / landscape shots, they want to know how to be able to see the sky and ground at the same time. I thought it more useful to write it up so I can refer people here, plus I'm sure other people want to know who haven't asked. I'm definitely not an expert, but I can explain what I do. If you think of a camera as just something you point at a scene and click the button, this is going to seem like a lot of work. Really, it only takes a few seconds once you get it down.

First, there's some terminology we need to know. Feel free to skip this part if you're familiar with stops and metering.
What is a stop? It's a simple way of knowing how much light you've got. Every time you increase the exposure by a stop, you double the amount of light coming in (decreasing by one stop means half the light). Say I take a 1 second exposure, and I want it to be a little brighter. I'll increase it by a stop, so I double the exposure to 2 seconds. Brighter still? Another doubling to 4 seconds. What if I wanted it to be darker? Decreasing a stop means halving the exposure time to 1/2 second.
You can do the same thing by increasing / decreasing the aperture by stops rather than adjusting the shutter speed, but we usually choose aperture for other reasons, like depth of field.

The 'doubling / halving' thing seems weird at first, but it's actually a really natural way to do it, since that seems to be the way our vision works.

We're going to use full manual mode (M mode on Nikons) so you can set both aperture and shutter speed. Consult your manual if you're not sure how to use this mode.
My Nikon adjusts in 1/3 stop increments. You can change the setting to 1/2 stop increments. Your camera might be different. An easy way to tell is to set the exposure to 1 second, then see how many clicks it takes to get to 2 seconds. If it's 3, then you just need to remember that three clicks = one stop.

For the zone system, set your camera to spot metering. Remember how to switch back to some other metering mode (I like matrix metering) for other times. This is should only take a second or two once you know what you're doing. Again, consult the manual.

Do you know how to use your cameras light meter? When you look in the viewfinder of a Nikon (other brands are probably similar) you see a bar at the bottom with a + (overexposed) on one end and a - (underexposed) on the other end. If you flip the shutter speed back and forth you should see the indicator change. In spot metering mode, you're measuring only the thing the meter is pointing at (is this always the middle point, or is it where you set the autofocus point? Check the manual...). When the indicator is right in the middle, you're exposing properly for whatever object you're pointing the meter at.

Okay finally, we're done with 'prerequisites', let's talk about how to do this!

'Zones' are a division of the tonal range into stops. Zone 5 is a properly exposed midtone. Zone 6 is one stop above that (twice as bright as Zone 5), and Zone 7 is another stop above that (two stops above, or four times brighter than, Zone 5), and so on. Same thing going the other way, Zone 4 is a stop below the midtone, Zone 3 is two stops below, etc.
Digital cameras are better with shadows than highlights, so we're going to set the exposure for the highlights and worry about the shadows later.

Look at what you're going to take a picture of. Visualize the scene. You need to pick the brightest thing in the picture that you still want to have some visible detail. The whitest part of a cloud or running water is usually a good choice. Set this as 'Zone 7'. Easy, just point your meter at the object, and set the shutter speed until the indicator on the meter is right in the middle (Zone 5, remember?) Now, remember how we counted how many clicks are in a stop earlier? Just go up two stops. I counted three clicks per stop on my camera, so I adjust the exposure up by six clicks. That's it.
Well, there's one more thing. Zone 7 is for bright white. What if your brightest object isn't white, or you want it to be a little darker? Put it on Zone 6 instead.

On my camera, when you're looking at the picture on the LCD screen, if you push up it switches to this histogram display. The histogram is the white box in the upper right with the graph / curve in it. See your manual to find out how to do this on your camera.
Don't trust the LCD preview on your camera, use the histogram. The histogram is a graph that shows how bright the pixels are. All you really need to know about it is that the left side is black and the right side is white. The Zone System exposure got you a good starting point, but occasionally you might need to tweak the exposure a little brighter or darker based on what you see here. You're going to have some kind of curve, and ideally you want the curve to be centered and not touch either side. The real world is rarely ideal, though, and with a little experience you'll understand how much it's okay to deviate from the ideal. Again, it's better to be too dark (towards the left) than too bright (towards the right).
If it's just impossible to get the whole curve to fit on the histogram, it's time to think about HDR techniques. We'll save that for another time.

So you've got a picture with a pretty histogram, now it's time to open the RAW image in your favorite photo editing program (I'm going to talk Photoshop because that's what I know).
Wait, you're shooting in RAW and not JPG, right? Shoot in RAW!
So there's a free plugin for Photoshop called Adobe Camera Raw you can get from the Adobe website, and it allows us to open the RAW images. Adobe Camera Raw has a lot of awesome adjustments we'll use to make this image look right.

When I shot this, I set the glowing edges of the clouds to Zone 7, if you're curious.

I've labeled some of the sliders here. See the histogram in the upper right corner? Remember that ideally we want it to be centered and not touching either side. Here, it's smashed into the right side because it's too bright. The slider below give us ways to fix that:

1. White balance. You can change the white balance here. Try it out, go with what looks best. This will save your ass if you accidentally shoot in the wrong white balance.

2. Exposure. This shifts the whole histogram to the left or right.

3. Recovery. This squishes in the right edge of the histogram, pushing it away from the bright side while leaving the dark end alone.

4. Fill Light. Pretty much the opposite of recovery, squishing the left side instead of right.

5. Blacks. This one stretches out the left side instead of squishing it in.

6. Brightness. The opposite of blacks. Stretching the right side instead of the left.

7. Contrast. Stretches out the whole histogram to make it wider.

8. Graduated filter. Can make a bright sky darker without affecting the ground. Play with it and see what happens.

9. Tone Curve tab. Has more adjustments for increasing / decreasing highlights or shadows.

There are some sliders down below (Clarity, Vibrance, Saturation). Be careful with them, it's easy to get carried away. Subtle is usually better.

You can see my adjustments here. If you look at the histogram from before I started, there were clearly two different peaks, one on the left for the dark ground, and one on the right for the bright sky. I started with the Graduated Filter tool to even them out, which joined the two peaks together into one curve. Then I raised the Exposure slider a little because the ground was too dark. That made the sun/sky way too bright (it was already too bright, increasing exposure just made it worse!) so I used the Recovery slider to crunch it back down (be careful with this one, sometimes it makes your photos look flat and nasty if you use too much). Then I used Blacks to stretch the left side towards the edge (you notice in the before picture there was an empty space there). This gives more tonal depth. Finally I did a little tweaking on the sliders at the bottom.
Now you can click 'Open Image' and the picture will be imported into Photoshop for further tweaking.

When you're done, remember to save as an uncompressed TIF, and save the original RAW file too! I promise, when you get better at this, you'll wish you'd done things differently with your earlier pictures, and if you save the RAW file you can go back and try again.