More On Curves - Part I

One can never really predict the kinds of questions a post will generate and I sure got a lot of them precipitated by my drive-by of the Aperture 3 curves adjustment tool. Based on emails and comments that post generated I'll walk through a few things in a bit more detail. Consider this the first installment. I probably jammed too many things into too short a space last time.

Today let's talk about contrast as well as a few terms like compression in relation to using curves. For consistency I'll use another random shot from my Mirror-Mirror project. The screenshot at the top represents where we'll end up. For reference here's the SOOC as imported RAW starting point. As usual open images in a new window or download to see full resolution cinema screenshots.

Compared to the radical treatment in the last post this one is far less aggressive. Specifically there is a lot less mid-tone contrast but similar overall contrast. The take-away here is that those two notions are not fixed. You can have the same overall contrast in terms of black to white with radically different things in between. In the next screen shot focus your attention on that first curves control that is expanded.

I turned off the second global curves control so that you could see the effect of just that first one. Don't worry about the two others for now – one is a local adjustment and the other is used for highlight toning which we'll talk about another day. This curves adjustment does one thing. It lowers the overall contrast to the point where all of the tones are now in that visible portion highlighted by the lower left quadrant. You can see outside the realm of the visible due to the curves extended range selected from the lower left drop down menu. Take special note of the relationship of the strait gray line connecting the lower left black point to the upper right white point. See the blue line and how it connects the original black and white points in that lower left visible range quadrant. Look at how steep it is. The new line we made by dragging the white point towards the right is far less steep.

The relative steepness indicates contrast. The new line is less steep than what it was. Less contrast overall. It happens to be strait meaning that there is less contrast everywhere at every tonal range in the image. By lowering the overall contrast to this point every single tone fits into the visible range. In fact there are no absolute blackest blacks nor whitest whites. No 0 tones and no 255 tones. Every single tone is now closer together then they were. Less contrasts moves tones closer together. Obviously more contrast moves them farther apart. This is what I mean specifically when I mentioned that this image treatment has similar overall contrast to the example in the last post. The darkest and lightest tone are about the same distance apart. The difference is all that stuff in the middle and that is a big deal. While we are here note that the original unaltered image has far less detail in the window behind Katya. Let's activate that second curves control and take a look at that…

I didn't touch the end points here – the point representing maximum black or maximum white. I adjusted the middle. If you leave the end points alone and mess about with the middle what's actually happening is that you are raising the contrast in some tonal regions and lowering the contrast in others. You can see that if you compare the slope (technical calculus term) or steepness at any given point on the new curve shape to that original strait line connecting the black point to the white point.

In this illustration there are two points. I put the first one a little above the middle tone range and raised it up. This increased the contrast of everything between black and a bit above the that point. It made all those tones farther apart. See how steep that line is. A bit above that first point you can see the steepness start to decline and become less steep than it was. Lowering the contrast in all the tones above – making them closer together. Compressing those tones. The second point I added is seen down in the shadows. I pulled that down. The result was putting the contrast below that point back to where it was before I added the point in the upper mids. More importantly it further increased the contrast between it and that first point. This ends the discussion on contrast and a few terms you'll hear a lot. The rest are some asides from a shooting point of view and practical working notes.

Other Notes

Having the subject in between the camera and large detail-free whiteness when shooting is a good idea. I do it all the time in high contrast scenes like this where the brightest thing is not the subject. That little bit of detail in the window area makes a lot of visual difference. You'll also see that there is a whole lot more of that detail in the final version compared to the on-import version. Get to know your camera and how far you can push it. I tend to push my cameras to the limits as I do not like the look of bringing the shadows up in post. The best bits/tonal response is absolutely not below the mid-tones and especially not in the shadows. In real world contrasts shooting so that nothing touches that right edge won't necessarily give you the best data or best picture. The alternative… bring lights and reflectors into the equation. For more candid stuff like this that can be a huge detriment to the overall feel and subject reaction. Trust me – these images and the Mirror-Mirror project just won't work with a bunch of gear or assistant holding reflectors.

Contrast is very related to saturation. At least that's the general case. I haven't discussed or shown results manipulating contrast in any special blending modes. I have used the globally normal RGB mode. For now consider other modes special cases. In general increasing contrast will increase color saturation and that is not a bad thing. The reverse is also true. In this rendition the overall mid-tone contrast and color saturation is actually lower than the SOOC default RAW conversion. These treatments are for illustration, I have not decided on a treatment for the final project that some of these will find their way into. Right now I have been shooting for black and white in mind – not color. Here's an extreme ramp-up on that second curve to increase mid-tone contrast for demonstration. Same overall black to white contrast.

Tomorrow – or at least in the next day or two we'll talk about curves and color a bit more. Seems I lost a few folks on that in the last post. As usual feel free to question or comment.

RB

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