Now that you're familiar with the essential steps in the workflow process we'll look a little closer at some of the options available at the various stages . From the top Options Bar click on Image > Adjustments.You'll see Auto Levels, Auto Contrast and Auto Colour.
Clicking on any of these will give you a quick fix but this fix is based upon pre-set, average parameters that probably do not correspond to your image and also you will have no idea what the fix applies to your image so you're not learning anything. I never use them.
More precise, manual controls, are Levels, Curves, Colour Balance and Hue and Saturation (Brightness and Contrast we have dealt with already, it's a very basic function, so try not to use it).Instead click Levels and a Histogram pops out like this.
This 'Levels' dialogue box is an excellent control panel for making subtle changes to Brightnes and Contrast and, here also, we can sub-divide the image into it's 3 component parts of Red, Green and Blue for fine tweaking of the Colour Balance. It's one of the most useful Toys in the whole Photoshop box.
The Histogram itself (the black, mountainous shape) displays each pixel's tonal character by a designated number from 0 to 255 (that's 256 numbers) from black to white, running horizontally across the graph. The Vertical scale is the measure of the quantity of pixels in the image with the same tonal quality, regardless of the total amount of pixels in any given image. Put simply, if your Histogram seems to be all on the left side, as the above screen grab is, then the image has a lot of shadow or dark areas in it. If on the other hand most pixels appear on the right (as screen grab below right) then the image has a lot of bright, highlight areas. The screen grab below left depicts an image with lots of mid-tone but, conspicuous by there absence, are bright highlights and dark shadowy areas. It is not essential that every image has a full range of pixels, a very high key image of, say, an Egg on a white background for an Advertising shot will have stacks of pixels on the right side. A black Cat in a Coal Shed, well, you can imagine it!
Notice the three triangular sliders directly below the graph. With the Channel Option box set to to RGB (default) grab the black, left side slider, and drag it - you get a total black-out. Return that then grab the white, right hand slider, and drag it - a complete white-out. Adjusting the middle, grey slider, alters only mid-tone brightness levels. Fine adjustment of all three of these combined provides seemingly infinate control over brightness and contrast levels and, can be quite creative. When you've finished playing return sliders to their original position.
Now, go back to the Channels Option box and with the down arrow > Green. Notice first how the Histogram has altered it's shape slightly. This is because now it only displays the Green content of the image. Grab the white slider again and drag it to a similar position to that shown in the screen grab above and your image will appear like mine, with this kind of Martian Portrait look about it. A total Green-out (sic). Return that and grab the black slider to drag it in the opposite direction. The opposite of Green is Magenta. The words 'a complete Magenta-out' looks stupid in print but you know what I mean. You can do this with all three Channels, Red (opposite Cyan) and Blue (opposite Yellow). While still on a Colour Channel grab and drag the grey mid-tone slider and discover the subtleties of image colour correction.
Next you can see three Input Levels indicated above the graph which appear as default numbers, 0 (the darkest pixel) 1.00 ( the mid-tone setting) and 255 (the brightest highlight or pixel). If you reset these numbers to their original default the image will return to normal as if unchanged.
The Output Levels slider is used to fix contrast. Now, one cannot increase Contrast to less than 0 or more than 255, but this control is excellent for reducing high contrast photos and scans by raising the 0 (black) slider and/or reducing the 255 (white) slider as this screen grab.
If you get lost amidst all the combinations click Cancel and all the changes will be ignored so you can start again.
The Auto button on the right instills the same undesirable and arbitrary fix on the image as Auto Levels etc. etc. No use.
The three pen-like symbols are for pinpointing black, mid-tone grey and white areas of the image. Click on a pen say, white, then click an area of the image that you think is, or should be, white-you see, potentially disasterous.
The Options button will introduce another pop up dialogue box, this like this one on the right, over the Levels box and here one can also apply Auto effects to Enhance Monochrmatic Contrast which is a quick way to change a colour image to Black and White (but not the best way) or Per Channel Contrast Red, Green and Blue again, or Find Light and Dark Colours with some other finer controls.
When you have the image looking as you want it click OK and it's done.
Now you can perhaps begin to appreciate the power of the Adobe Photoshop Software and the amount of control one has just out of this one little Levels box however, if you don't know what's wrong with an image in the first place you could get lost inside it. See new f8 Tutorial on Image Colour Correction
There is no substitute for correctly assessing the faults in an image and applying exactly the right correction to it.
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