The role of global edits in post-processing
A “wholistic” approach to imaging 🙂
About global edits
Global edits are the enhancements we make to the whole photograph.
Almost any edit can be done globally, but the most important ones are the ones we make at the beginning of our workflow – most commonly colour (white balance) and tonal range (exposure, brightness, contrast). These are the focus of this article.
If you are dealing with Raw files, global enhancements are performed in your raw conversion program. The real power of raw processing lies in global editing capability.
If you have image files (eg jpegs), the global enhancements are done in Photoshop.
Once the global edits are made, then we move on to local edits, where we target selected areas or specific colours in our image.
The common mistakes
There seems to be two errors that novices make when approaching their images. Either they expect too much from their global edits, and hope that no other editing will be required; or they pay too little attention to global edits, and jump straight in to their local editing instead.
It’s very important to understand both the benefits and the limitations of global editing.
My approach
I guess you could say I expect an 80/80 result from my global edits – that is, I aim to get at least 80% of the pixels at least 80% of the way to where I want them.
The remaining 20/20 is then done selectively – some extra contrast here, some shadows lightened there, a skintone corrected over there.
Of course, while the first 80/80 only takes a few seconds, the other 20/20 always take much longer. But if my global corrections aren’t done well at the beginning, I’m adding significantly to my local editing workload, and that’s a PITA.
The benefits of global editing:
It’s fast
You can achieve as much as possible, as quickly as possible. In fact, if you’re preparing proofs, the quick global edits should be enough. You can worry about the fiddly local edits when the customer orders prints.
It makes things easy
The better your initial global adjustments are, the less stuff you need to do locally later. Amen.
The limitations of global editing
It’s … well, global
Global editing shouldn’t be used to correct one part of an image, to the detriment of the remainder.
Too often I’ve seen people adjust the white balance of an entire photo to try to achieve "perfect" skintones. Not only is this quite difficult, it frequently makes the rest of the photo look strange.
Good global edits are essential, but they don’t negate the need for local editing. Well-executed local edits are the difference between a nice photo and a great one.
Why is it so important to do global edits first?
For accuracy
The human eye and brain are feeble, and easily fooled. They can’t properly judge colour and tone without something to compare to.
When you’re editing a selected area, your brain is judging the efficacy of the adjustment by comparing it to the surrounding pixels. If the surrounding pixels are flawed, the comparison is flawed, and you end up with dodgy results.
Therefore, it’s critically important to have your entire image "in the ballpark" before trying to make local adjustments.
There’s no turning back
As soon as you make even one tiny local adjustment, you can’t make any further global adjustments. Your local adjustment will be exaggerated or changed by a subsequent global adjustment, thereby rendering it useless.
In this very simple example, the photo is a little underexposed:
If I immediately select the cat and lighten her, she looks nice, but the rest of the photo is still a bit dim:
If I subsequently try a global Curves adjustment to correct the problem, the cat starts to glow outrageously, because of the double-correction:
If I’d done the global correction first, the cat would have been almost perfect, and perhaps require only a subtle local adjustment later.
The three purposes of global editing
Ok, so what are these global edits I’ve been banging on about?
Well, broadly speaking, global editing has three important purposes: overall contrast, overall midtone brightness, and overall colour balance.
1. Global contrast
Contrast is all about getting your shadows and highlights right. In most photos, it’s good to have nice strong shadows and nice bright highlights, to make full use of the 0-255 tonal range.
Of course, contrast must be approached with care, to avoid unwanted channel clipping. Pay close attention to your histogram when adjusting contrast.
2. Global midtone brightness
Midtone brightness speaks for itself. It’s important to have nice shadows and highlights, but let’s face it, most of the image is a midtone of some sort, and it has to look right. You don’t want your image to appear too dark or too light.
3. Global colour balance
Overall colour balance is best known as "White Balance", that goal which is both familiar and frustrating for many photographers. White balance must be achieved in your raw program – don’t wait until the photo is in Photoshop.
Optional global edits – VERY optional
The three global enhancements that I just described are the important ones, and it’s no coincidence that the first half-dozen sliders in ACR control these factors.
But of course there’s more sliders in ACR, and more global enhancements available in Photoshop. I only recommend using these mildly, if at all, at the beginning of your workflow. They’re more suitable for cosmetic tweaks near the end – sort of "icing on the cake", if you like.
Let’s discuss them …
Midtone Contrast Enhancement (MCE)
You can increase the contrast in the midtones of an image by darkening the near-shadows, and lightening the near-highlights. It doesn’t affect the extremities of the tonal range of your image, so it’s really a form of "false contrast".
In Photoshop, this is most commonly done using an S-curve, and in ACR, we use the poorly-named "Contrast" slider. (I’d love to see it more accurately named the "Midtone Contrast" slider, to avoid confusion about its role.)
The important thing to realise about MCE is that while it increases contrast in the middle tones, it correspondingly reduces contrast in the light and dark tones. Hence, it shouldn’t be used without due care. I like to save it for an extra "kick" at the end of my processing, if needed.
Local Contrast Enhancement (LCE)
This is commonly called "Defog", and is performed by the "Clarity" slider in ACR, or by the Unsharp Mask or High Pass Filters in Photoshop.
As with MCE, LCE only manipulates contrast within the tonal range, and is not an adequate substitute for true global contrast. If you achieve full tonal range (black blacks and white whites) in your initial global editing, you’ll find the need for LCE diminishes significantly.
Once again, I find this a handy last resort for some images, but I never use it early. Be aware that LCE can induce channel clipping if used strongly.
Saturation (and Vibrance)
I’m a big fan of saturation – I love bright colours in my images as much as I love great tonal range. But saturation is definitely icing – you must bake the cake to perfection before icing it.
Saturation, by its nature, pushes colours towards their clipping limit (255). If you do this too early, you leave yourself with no room for tonal adjustments as needed.
I can’t stress this enough – don’t saturate early!
The addition of Vibrance to ACR and Photoshop is handy because it adds saturation with less risk of clipping. It might be "low fat" icing, but it’s still icing. Bake your cake first.
Conclusion
Good global enhancements don’t build great images by themselves, but they provide the solid foundations on which great images are built.