Saturday, July 02, 2005

Tone or custom curves: an explanation

UPDATE 28 March 2008: Although I wrote this article almost three years ago I have notice that is visited almost every day. So a quick update. I now shoot with Canon cameras although my wife and fellow pro still uses Nikon. Both manufacturers make great cameras.

I use Lightroom to develop my RAW images and shoot using the most neutral setting. I'm happy with the results and I don't use custom curves when shooting anymore. Still the information below may be useful and the principals of science apply as they always will.


I often use tone, or as they are also known, custom curves on my digital slr. Other photographers have asked me to explain what they are. So here's my brief overview.

I do most of my digital photography nowadays with a Nikon D70 digital slr. It's light, doesn't cost the earth and does all I need it to. The quality, if you know how to extract it is superb and can happily be used for everything from magazine covers to superb large prints. This explanation of tone curves is based on my experience with the D70.

Back to basics

Each film has a characteristic curve. Kodak explains a characteristic curve: "It shows the relationship between the exposure of a photographic material and the image density produced after processing."

So what about digital

The same principle applies to digital photography. Here, unlike film, a large part of the processing is done in camera, even when you are shooting raw. You cannot duplicate a digital tone curve, as far as I am aware, during post processing because it relates to how light captured is handled directly after it hits the sensor, ie digital film.

Nikon explains it as follows: "As photographs are saved to the memory card, they are processed to adjust the distribution of tones in the image, enhancing contrast. Tone compensation is performed by means of tone curves that define the distribution of tones in the original image and the compensated result."

Simply put tone curves can:
  • increase contrast
  • reduce contrast
  • alter the way colours behave under different lighting situations
  • brighten mid- tones
  • extract more detail out of highlight areas
  • brighten shadows
Custom curves are especially used to achieve the last three bullet points. Nikon provides excellent built in curves but the D70 and D100 when used with the normal tone curve, sometimes look like they're underexposing the mid-tone areas. Now you could try compensating by increasing exposure but this would be at the cost of highlight detail. A good tone curve can boost your mid-tone areas, lighten up shadow and keep more detail in your highlights - and that's why I like them!

But a word of warning. Getting them right is not easy. The adjustments have to very subtle or you'll spoil the colours and tonal balance. The curve has to be kept as linear as possible in the mid-tone areas.

Custom curves are by no means always the best solution. In high contrast scenes you'll probably want to try the Nikon's low contrast curve and in misty scenes you'll no doubt want to boost contrast with high contrast curve. The D70 also has built in curves for landscapes and portraits. No doubt other DSLRs will have similar options. But the benefits of using custom curves will be the same.

The key thing to remember is that unlike film, digital photography requires some of the normal darkroom lab work to take place inside the camera.

Nikon's D70 requires the custom curve to be loaded into the camera while it's tethered by usb cable to a PC/Mac running Nikon Capture 4 software . At the moment you can only load one custom curve at a time. As with all things the custom curve business is, if you'll pardon the pun, a rather steep learning curve. Just something you've got to keep trying out in different circumstances.

Please let me know if you've found this article useful.

Paul Indigo
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