The Curves Tool: Lightroom versus Photoshop
Adobe Photoshop tone curve

Nov 16th

2011

CategoryPosted in Editing & Post-Processing
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The Curves Tool: Lightroom versus Photoshop

Adobe Photoshop Lightroom 3 is a fabulous piece of software for cataloguing, sorting, viewing, editing, presenting and printing images. It does not offer the image editing power of Adobe Photoshop, and nor is it intended to.  Lightroom’s editing capabilities are sufficient for 95% of my requirements – in other words, most of the time.  However, one of the reasons I do find myself launching Photoshop is to use Photoshop’s Curves, as I find that the Curves tool within Lightroom is often insufficient for my needs.  Why?

There are three main reasons; but before we discuss them, here’s a brief review of what a tone curve is…

An Introduction to the Tone Curve

The tone curve can seem intimidating; but in effect, when you construct a tone curve, you are simply telling Lightroom or Photoshop: “For each possible tonal value – or ‘brightness level’ – you find in the image, this is what I want you to do: brighten it, darken it, or leave it unchanged”. (Incidentally, In Photoshop, the tone curve may also be used for modifying colour in an image; but here, we are simply concerned with modifying the tonality – the distribution of light and dark tones – of the image.)  A tone curve represents a function for ‘mapping’ (i.e. converting) every possible tonal value of the image to a new tonal value.  The tonal values before mapping are called ‘input’ values, and are represented by the horizontal axis of the tone curve.  The tonal values after mapping are called ‘output’ values and are represented by the vertical axis of the tone curve.  The input values to be mapped in the RGB colour model run from pure black (luminosity value 0) to pure white (luminosity value 255).  The output values after mapping also run from 0 to 255.

Here is an example of a Tone Curve in Photoshop – the Tone Curve in Lightroom looks very similar:
Tone Curve in Photoshop

So, given that the tone-curve tool in Lightroom’s Develop Module appears similar to that found in Photoshop, what does Photoshop’s tone curve offer us that Lightroom doesn’t?

As I mentioned above, there are three main reasons why I find I can’t always do what I want with Lightroom’s Tone Curve, and need to use Curves in Photoshop.  Here they are:

1. Lightroom Restricts You to a Single Curve

Unless you use a third-party plug-in (which is equivalent to launching Photoshop anyway), Lightroom restricts you to the application of a single curve to your image. Why is this sometimes limiting?  To understand this, we need to consider the nature of a tone curve.

If you make a tone curve steeper in one specific tonal region, you are stretching out the range of tones in that region; which is the same as saying you are increasing contrast in that tonal region, as darker tones will become darker, and lighter tones will become lighter. A fundamental characteristic of a tone curve is that if you make the curve steeper in one specific tonal region (to increase contrast in a specific range of tones), the curve unavoidably become shallower in other regions to compensate.  Put another way: you can never add contrast in one tonal region without also decreasing it in another.  All you can do is redistribute contrast in the image.  This should be clear from the following three screen shots:

Firstly, here is the unmodified tone curve which does nothing to the image – it is a straight line, a so-called linear tone curve:

Linear Tone Curve in Lightroom

Now take a look at a typical curve, the so-called ‘S” curve: The curve has been steepened in the midtones, in order to increase midtone contrast, giving it the characteristic ‘S’ shape:
'S'-Shaped Curve in Lightroom

However, you can see from looking at the curve that the curve is necessarily shallower in the shadows and in the highlights – in other words, the increase in midtone contrast has been achieved at the expense of reducing the contrast in the shadows and the highlights:

Annotated 'S'-Shaped Curve in Lightroom

It should hopefully be obvious from the above that it is impossible to steepen the curve everywhere: we have to decide where we want to increase contrast (steepen the curve), and where we are happy to accept a corresponding reduction in contrast (make the curve shallower).  This may not be a problem; but what if you have an image in which you wish to increase contrast in two (or more) regions of the tone curve, such as the upper midtones and also the shadows?  This is where you come up against the limitation of Lightroom’s single-curve adjustment; you may find it hard to satisfactorily adjust both regions of the tone curve simultaneously with a single curve, owing to the fact that steepening the curve in one region must come at the price of making the curve shallower (i.e. sacrificing contrast) elsewhere; and this will often be unacceptable.  The following screenshot illustrates this point: I have steepened the curve in the shadows and upper midtones/highlights, which results in a loss of contrast (i.e. shallower tone curve) in the midtones:

Steepening the Lightroom Curve in the Shadows and Upper Midtones

In contrast (if you’ll pardon the pun), opening the image in Photoshop and creating two or more masked curve adjustment layers will easily allow you to increase the contrast in different regions of the tonal range simultaneously in different areas of the image, enabling you to have your cake and eat it, too; undeniably, a beautiful thing.  The key to this trick is that Photoshop allows you to apply tone curves as local rather than global adjustments through the power of layer masks.  In other words, you create two (or more) global tone curves in Photoshop, and then mask them, revealing each only in specific areas of an image.  This brings us neatly on to point 2…

2. You Can’t Mask a Curve in Lightroom

If there are two (or more) areas of an image which fall in the same range of tones (e.g. both areas of the image fall in the midtones), and you only want to adjust contrast in one of the areas, creating a curve adjustment for that area in Lightroom will unavoidably impact on the contrast of the area or areas you do not want to adjust.

Consider the following image:

Boat

Let us assume I want to increase the contrast in the blue wooden deck of the boat, but not in the three yellow floats.  This gives me a problem in Lightroom, as although they have contrasting colours, both the wooden deck and the floats have similar luminosity (brightness), and hence occupy the same region of the tone curve.  It is therefore impossible to make a curves adjustment using Lightroom’s tone curve for the wooden deck which does not affect the contrast in the yellow floats.  The reason for this is that the Curves tool in Lightroom is a global adjustment tool: it affects all areas of the image.   Granted, Lightroom does offer the facility for limited local adjustments through the Adjustment Brush, so in theory you could attempt to use the Adjustment Brush to make the required contrast adjustment in the wooden deck only; but although I find the Adjustment Brush works very well for exposure adjustments, when it comes to contrast adjustments, it seems to me to be a fairly blunt instrument: the control offered is nowhere near as powerful or sophisticated as the application of a masked tone curve in Photoshop.

So how would we add contrast to the wooden deck and avoid adding contrast to the yellow floats?

By opening the image in Photoshop and applying a mask to the curves adjustment layer, the effect of the Curve can be revealed only in that area of the image where the adjustment is required (in this case, the wooden deck), and concealed by the mask everywhere else…

Photoshop Curve to be applied to the above image as an Adjustment Layer:

Photoshop Tone Curve for Boat Image

 

Layer Mask used to partially block the effect of the above Tone Curve on the image.  Where the mask is black, the effect of the curve on the image is blocked.  Where the mask is white, the effect of the curve is transmitted to the image.  The mask therefore allows the tone curve to affect the boat deck, and nothing else.

Layer mask

 

The image after applying the above tone curve and layer mask;contrast has been increased in the boat deck only.

Boat with contrast added to boat deck only

Masking is also necessary in the case where you want to apply multiple curve adjustment layers in Photoshop to target different regions of the tonal range simultaneously (as discussed in point 1. above), as you can once again reveal each curve only at the location in the image where you want its effect to be visible.

 

3. Tonal Adjustments with Lightroom Curves Always Affect Colour Saturation

In Lightroom, it is not possible to make a Curves adjustment to modify image tonality without also affecting the colour saturation of the image. Increasing contrast causes colour saturation to increase; decreasing contrast causes colour saturation to decrease*.  (If you are not convinced, try making an extreme contrast adjustment with the Curves tool in Lightroom and watch what happens to the colour saturation of your image.)  The application of steep curves to an image in Lightroom will often produce unacceptable or unwanted changes in image saturation.  In Photoshop, however, it is possible to change the blend mode of a curves adjustment layer to luminosity, which means that only the changes in tonality (contrast) introduced by the curve are visible; the changes to colour saturation are suppressed.  Alternatively, it is possible in Photoshop to work in the LAB colour space instead of RGB, which allows lightness (luminosity and contrast) adjustments to be made without associated changes to colour saturation: changes to lightness in the LAB colour space can be made completely independently of colour.  It may well be that you like the additional colour saturation that a given Lightroom curve gives you; but if you don’t, your only option is to attempt to correct the change to colour saturation by other means (e.g. with the Vibrance and Saturation controls in Lightroom). As a consequence, I often find myself sending an image into Photoshop from Lightroom in order to be able to make changes to image contrast without affecting colour saturation.

*Technical note: This is a characteristic of RGB colour spaces, not a Lightroom limitation. In RGB, the application of a tone curve to modify luminosity (contrast) will also affect colour saturation, since in the RGB colour model (unlike in LAB colour), tonality and colour are inextricably linked. Although raw image files do not have embedded color profiles in Lightroom, the Develop (edit) module in Lightroom uses the color coordinates of Pro Photo RGB together with a linear gamma (tonal response curve), which is the reason why the Curve tool in Lightroom evidences this typical RGB-colour-space behaviour when applied to raw image files.

 

Summary

My intention here is not to criticise the image-editing facilities in Lightroom: as I said at the beginning of this post, I think Lightroom is a wonderful piece of software, and I do most of my image editing in Lightroom. If I can achieve what I want working with raw image files in Lightroom, I prefer to do so rather than launching Photoshop. Although Lightroom uses a tonal response curve to provide information for the histogram and RGB values, it actually manipulates the raw data before it is tone mapped. Working in this linear gamma avoids many of the artifacts that can result in working with a tone-mapped image in Photoshop.  Simply put: if you can do what you want to do working with your raw file in Lightroom, it may well give you a higher quality final image file than if you make the same edits in Photoshop.  Nonetheless, it is important to be aware where the limitations of Lightroom are in order to know when and why to take the image in to Photoshop.  There are many other editing tools which are only available in Photoshop and not in Lightroom; however, it is the Curves tool in particular which is the reason I most frequently need to launch Photoshop to work on an image, and I hope I have managed to explain why.

There are many resources on the Internet which will give you a more detailed overview of the Curves tool in Photoshop.  One I can personally recommend is an excellent (and free!) tutorial from David Nightingale at chromasia.

Or for a briefer look, take a look at this overview of tone curves from www.cambridgeincolour.com.

I hope this comparison of the Curves tool in Lightroom and Photoshop is useful – please feel free to respond with any questions, and I’ll do my best to answer them.

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