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Photography 101

Photography 101: Understanding the histogram, Levels and Curves (part 2)

This two-parter is an accompaniment to the Photography 101 posts on the basic concepts of photography. It’s not a post that will help you take better pictures; rather, it will help you understand the pictures that you have taken, and make more meaningful edits. It’s fun and easy to mess with different filters in apps, but if you ever wanted to understand the mechanics of a photo and cut down on the ‘I wonder what will happen if I use this filter’ time you spend editing photos, this is a good place to start. Part 1 was the theory; this is the practical.

So, you remember in Part 1 how I disappointed you greatly by pointing out how you’d spent hundreds of pounds/dollars/sexual favours on procuring one of the most sophisticated bar chart computers in history. The bar chart that your camera creates – the histogram – is extremely useful in guiding how you’ll edit your photos.

What’s your favourite tool?

In this post I’m going to use Photoshop, but you don’t need something that powerful. On mobile I prefer Snapseed, which is also available as a desktop app for Windows but I haven’t tried it. Whatever editing tool you use, the features we’re about to look at will always be called Levels and Curves. Unless they’re called something else.

Ugh, filters

I won’t be discussing filters in this post. The only filter I ever use is a neutral density filter and although I may discuss it more fully in a future post, it’s not likely. If you want to use filters in Instagram or something else, go nuts. I won’t judge you (I will) (hard). My preference is for getting the picture you intended to get, not taking 200 pictures and then finding one that works with a filter you like. Getting it right in camera is just how I work; no shade if you want to use filters (total shade). For me, editing is just about cropping and getting the tonal spread right, but You Do YouTM.

It’s all about the midtones

You remember how the histogram works. The higher the bars on the left, the more shadows in your photo; the higher the bars on the right, the more highlights. It’s possible to get a histogram with high bars only in the middle, which would look like this:

A picture that had a histogram like this would have very few shadows or highlights. That doesn’t make it a bad picture, it just tells you that the tonal range is quite narrow. Landscapes and seascapes often look like this – there might be lots of shades of blue, green, yellow and brown (which register as midtones) but no shadows or highlights. Here’s a good example of a pic that looks like that.

This is a picture of a beach in Cornwall. You see how the colours in this picture are a little bit muted and basically flatter than the beach itself, with no real contrast and nothing to catch the eye? Photographers have a technical term for photos like this: we call them ‘shit’.

The Photoshop histogram

Let’s have a look at how that looks in Photoshop with the histogram (if you’re using Photoshop, this is the luminosity histogram).

And in close-up:

Let’s start apply some corrections to this photo using levels. We can never fix the fact that the photographic subject is as tedious as Jacob Rees-Mogg’s spank bank but we can at least make what’s there look a little more lively.

The marker on the left of the histogram (the dark arrow below the baseline) indicates the point at which tonal values in the photo become pure black. You see from the histogram that there is no pure black in our photo; but if we were to move the dark arrow to the right, we’ll change values in the photo accordingly. Similarly, the white arrow on the right moves where the highlight values start.

I’m pretty sure that will make no sense; so why don’t we look at what happens when we move these arrows about?

Black 0: this is the original state of the photo before we start moving things about. You see that the value under the black arrow is 0 and the arrow itself is all the way to the left.
Black 50: we have moved the black arrow to the right so that it now has the value 50. Essentially, this means that the darkest 50 tonal values in the original just got progressively darker. See how the whole picture is now darker?
White 230: I’ve reset the black arrow to 0 and moved the white arrow on the right side in from 255 to 230. Again that’s affected the whole image; it’s noticeably lighter than the original.
Black 50 white 230: now I combine what happened in the previous pictures, with the black arrow at 50 AND the white arrow at 230. The overall image has much more punch, even if it is still as dull as shit.
Midpoint 0.5: leaving black and white where they were, I adjust the grey arrow down to 0.5. This has changed the midpoint of the tonal range, making the overall image darker.
Midpoint 1.2: this time the midpoint arrow is moved to 1.2, lightening the picture whilst still retaining some contrast.
Final: you can’t make a silk purse out of a sow’s ear, but these would be the final settings that I personally would choose. There’s depth in the sandy colour, a wide range of blues in the ocean, drama in the clouds and menace in the dark cliffs. It’s a shite picture and no editing is going to fix that, but at least it has punch.
Comparison: the final and the original side by side. All done through levels.

Doing it with Curves

Curves is a similar concept but instead of adjusting points, you adjust a straight line that represents changes to the tonal values. That’s why it’s called Curves, see, because you’re adjusting a straight line, obviously.

Imagine editing the histogram in Levels but instead of having three arrows to adjust (black, grey, white), you had 255 arrows and could adjust every tonal value individually! Wouldn’t that be a ballache. Instead, that’s why we have Curves to allow for fine-tuning. Let’s return to our picture’o’shite:

Original: here’s our picture with a straight curve, or as photographers call it, a line.

Lighter: to make an adjustment, grab the line. This makes a point you can move. You can make as many points as you like – that’s why it’s more powerful than Levels. Here I created one point in the middle and dragged it outwards. You see how the rest of it has curved out? Moving that one point has adjusted all the tonal levels in the photo, just as with Levels.
Darker: I moved the point I created below the original line, which has made the whole image darker.
Shallow S: the way to quickly add contrast with control is to add a couple of points, dragging the first down and the second up, to create a shallow S shape.
Deep S: here’s what happens when you adjust the S shape to make a much deeper S. See how the very top and bottom of the S are now flat? This is exactly the same as when you move the black and white arrows inwards on the Levels histogram. Effectively, we’ve replicated the final image from above, but with Curves.
Inverted: remember when you were a kid and the third word you looked up in the dictionary was rude? Or the third thing you typed on your new calculator was BOOBIES? You’re going to do the same with Curves. And here’s what will happen when you completely invert the line. Yes, this is how most early 70s Bowie videos were made.
M-shaped curve: here, adding three points to take the total to five, we’ve made the curve resemble that global symbol of rainforest destruction and career stagnation, the MacDonald’s golden arches.
W-shaped curve: obviously these wackier images are produced by abusing the curve. The point is to show you that the amount of control that you have with Curves is far beyond that that you get with Levels, so it’s worth spending a little time playing with.

So what have we learned? More importantly, why have we learned it?

What a jolly good question. Often you won’t be able to control your lighting as much as you like. You might have too much light and not enough shadow to create a low key photo, for example. And that’s where these two tools come into their own – you can add back highlights or shadows with judicious use. You can never totally rescue a picture, unless you use RAW images, which is a whole other topic, but you can at least get closer to the effect you wanted.

Obligatory bum shot

Let’s look at that theory in practice using Curves.

Original: the light is actually nicely controlled and the curve is flat.
Lighter: by using two points anchoring the curve above the line, I can make the whole thing much brighter.
Darker: similarly, anchoring the curve below the line makes the whole image darker and here only the highlights are preserved.
Contrast: if I wanted to get a bit more contrasty and there’s no reason I ever would because this is Very Bad Indeed, I can use lots of anchor points to get a very defined S. This have given me pure black shadows, and blown-out highlights.

Hopefully, those last few images show how it’s possible to revitalise a flat image and introduce a bit of drama and move towards the image you had in your mind.

9 replies on “Photography 101: Understanding the histogram, Levels and Curves (part 2)”

Ahh curves are the bane of my photographic existence..I do get how they work but I was never able to use them effectively. I always wondered why there are so many tools in Darktable that essentially do the same thing….maybe each of us has a different way of translating a picture into a tool.

Liked by 1 person

This is really helpful – thank you. I do sometimes play around with the light when I am editing but have never really understood what I was doing. I use Snapseed so it is useful to understand a bit more about the effects 😊

Liked by 1 person

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