An intro to levels in photoshop?
Jason <><
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#1. "RE: An intro to levels in photoshop?" | In response to Reply # 0
danshep Charter MemberWed 18-Sep-02 09:13 PMIn answer to your question, No.
Now that THAT's over, here is a book I got, which was recommended to me, which covers levels, curves, and a ton of other stuff.
Photoshop Restoration & Retouching by Katrin Eismann
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#2. "RE: An intro to levels in photoshop?" | In response to Reply # 0
A detailed explanation would be better from a good book, but here's my version of the short story:
The histogram is a graph of the pixels in your image that have a particular brightness value. Black values are at the left, middle grey is in the middle (natch), and white is to the right. The higher the graph of the curve, the more pixels that correspond to the value along the dark to light scale.
At the left side of the levels is the black point control. Sliding the pointer to the right will set the black point to encompass more of the darkest pixels in the image. Note that as you slide it, the midpoint grey stays at the same ratio position between the black point and the white point. This means that as you set a new black point sliding to the left, the rest of the image gets darker.
The white point is on the right. As you slide it to the left, you set more of the pixels to white. The middle grey again will retain the same ratio between the white and black points and the rest of the image will lighten as you slide the white point left.
The mid point slider lets you change the spot along the histogram where middle grey is. This adjustment also affects the overall lightness/darkness of the image.
If you adjust the composite levels, the color balance for the image will remain the same. If you adjust each color channel individually, you will alter the color balance of the image. Describing this in detail is more than I'm up for. Play with the controls for one channel and you'll get a sense of what the interactions are.
Curves can be used to do identical manipulations of the tonal points as with levels, but you can fine tune the distribution by shaping the tonal curve. I'm not up for describing this tool in detail, but it is very much worth learning. Unfortunately, Photoshop doesn't display a histogram in the curves control panel.
Beware of Auto Levels or Auto Curves. This operation basically sets the black point and white point to the ends of the histogram curve for each channel. That means that color balance is affected. Try using an Auto Levels on photo with a strong orange sunset. The Auto Levels will emphasize the weak blue channel and the result will be a dramatic color shift. In an "average" image, the colors will change but won't be all that obvious. If you know that the color balance is correct, never use these auto functions and beware of using them on images with dominant colors on a large portion of the image.
Whew.
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#3. "RE: An intro to levels in photoshop?" | In response to Reply # 2
jnscbl Basic MemberThu 19-Sep-02 09:23 AMSimply put, I said simply put, sliding the two end points corresponds to adjusting CONTRAST, and sliding the center point corresponds to adjusting BRIGHTNESS. Again, simply put, sliding the two endpoints up to the exact beginning of the curve on each end of the histogram, just in the single RGB channel, is about what AUTOCONTRAST does. If you go into each red, green, and blue channel, and do the same thing, you get about what AUTOLEVELS does. I'm not speaking as a tutor, just another user who has learned to find my ass in the dark. I have Elements, not Big Daddy, so I don't have curves, but have used it a little. It seems to be a joystick control, with the four "pots" being Input Level, low and high, and Output Level, low and high. This is just idle speculation on my part. Where I see it being used, it's primarily to fix an underexposed B&W negative, without blowing the highlights. If you have Elements (or anyone else reading this) just play with those controls, which are in the same box as Levels.
--scott
"Less is not more. Enough is enough. Less is less."
David Vestal
--scott
"I am always doing that which I cannot do, in order that I may learn how to do it."
Pablo Picasso
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#13. "RE: An intro to levels in photoshop?" | In response to Reply # 3
You're correct that Autocontrast doesn't adjust levels independently.
But autocontrast is just like any other automation. If you like what you get, great, but it will average out the contrast of whatever image apply it to. That's destructive if you have an image that needs lower or higher contrast than normal. If all you do is choose between autolevels and autocontrast, you really don't need to know what the histogram is telling you.
You can moderate the contrast effect manually by setting the midpoint so that contrast is boosted in the shadows or the highlights without punching up both ends of the scale. I find this much more effective for most of my images. Often I want to deepen shadows without screwing up the tonal range in highlights as you'd get by simply increasing contrast.
I don't know what you mean when you call curves a "joystick control", but I suppose you're trying to keep it simple. You can adjust the endpoints of the curves as you describe, but you can also add points to the curves and shape them to create a wide array of effects. There's an autocurves that does pretty much the same thing as autolevels. The attached image shows what curve shaping can do. Simple or simplistic, it all depends on your purposes. Less is less.
Attachment#1 (jpg file)
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#4. "RE: An intro to levels in photoshop?" | In response to Reply # 13
when we got our first DCS camera the dealer tried to offer some pointers ... was really too excited to pay much attention ... but he did go over levels as one of the main points ... ..... ... our main use of levels remains the task of adjusting the overall tonal range of the image ... basically to 'stretch' a flat tonal range out to use the full range of the printer ... if you have tones built up to the left or right you need to look elsewhere ... if all your tones are on the scale then levels can be used to stretch them out ... first try sliding the white and black poing arrows in until they just meet the first tones ... click the preview checkmark on and off to see if levels has offered an improvement ... if you like the results hit OK ... one further thing you can do in levels is to adjust the middle slider to the right or left... this basically alters the contrast of the image ... don't take this to far as the effect seems more subtle on the monitor than in the print ... look at the center box above the graph ... it always starts out at 1.00 ... don't go much further than 1.15 or .85 unless looking for special effects ... again switch the preview check on and off ... ..... ... a bit more advanced use of the levels box: go into the red channel and adjust the white and black sliders ... then the green channel ... then the blue channel ... adjust each seperately ... this will often adjust any colour inbalances ... but only adjust the center slider when in the RGB channel ... ..... ... if you are working with an 8bit image the levels adjustment can be done as an adjustment layer ... layers/new adjustment layer/levels ... then you can adjust the layers settings when done ...
hope that helps
david place
DLPimages-
#5. "RE: An intro to levels in photoshop?" | In response to Reply # 4
zucchini Registered since 04th Mar 2002Thu 19-Sep-02 04:19 PMBJ, Dplace, and Scott,
Thanks for the detailed info, that's a bunch to wade through. So when most people are post-processing their images, are levels and curves the primary tools used? I gotta learn how to use this stuff...
Thanks!
Jason
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"You miss one-hundred percent of the shots you don't take.
Jason <><
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http://me.jasonkuo.com-
#6. "RE: An intro to levels in photoshop?" | In response to Reply # 5
levels is still our primary tool for adjusting images ... we use Kodak DCS760&660 cameras with nikon lenses ... so for us there is some prior processing in the Kodak PhotoDesk program where we adjust white balance and any exposure error ... this in the RAW file before going to tiff and PhotoShop ... ..... ... levels is very intuitive for photographers ... does have it's limitations ... by the time you reach the limitations you should be starting to learn curves ... we find curves less intuitive ... a much steeper learning curve ... ..... ... then there is hue and saturation ... basically makes flat images look like velvia ... but is very easy to over do it ... take it easy with saturation ... always work on a copy ... and in levels so you can reduce the effect if necessary ...
dplace
DLPimages
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#7. "RE: An intro to levels in photoshop?" | In response to Reply # 6
jnscbl Basic MemberThu 19-Sep-02 06:04 PMBJ,
I returned to the original question and realized that since this was a scanner question, maybe you should go into a little more detail about getting the histogram right at the scanning stage. Most of my post-processing expertise relates to digital camera output, which doesn't require anywhere near the degree of skill, knowledge, and effort that scanned film does.
--scott
"Less is not more. Enough is enough. Less is less."
David Vestal
--scott
"I am always doing that which I cannot do, in order that I may learn how to do it."
Pablo Picasso
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#10. "RE: An intro to levels in photoshop?" | In response to Reply # 8
try www.scantips.com - you get a good refence starting point for scanning issues.
Lyle
Check my weekly display at www.stavast.org !
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#9. "RE: An intro to levels in photoshop?" | In response to Reply # 7
You're right that scanning can give you some challenges that are often best solved by tweaking levels or curves in Photoshop. Shadows are difficult to get detail without making them a little flat and muddy, the highlights usually are okay with my front end software and scanner. Getting a decent histogram at the scan stage is very software and somewhat scanner specific. It is a challenge for me with the stuff I use, which is why I have to fine tune pretty much every scan with Photoshop. I think you're right that digital cameras produce more consistent images for the most part.
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#11. "RE: An intro to levels in photoshop?" | In response to Reply # 5
Obregon Charter MemberFri 20-Sep-02 07:49 AMI'd suggest that before curves another essential tool is Hue/Saturation. The tweaking of colors that you can do with this tool will really let you create the picture that you see in your minds eye. I always add a levels layer and a hue/saturation layer but I probaly only use curves about 15% of the time. Curves is a valuable tool, but in most cases the benefit that you get from adjusting Hue/Saturation does so much more for the picture. But of course, I wouldn't do without any of them. And of course, cropping, sharpening, cloning....
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#12. "RE: An intro to levels in photoshop?" | In response to Reply # 11
Saturation is very useful, but hue is downright dangerous if you don't use it for the right purpose.
Using hue to shift the master image color is a bad idea for color correction. If you start sliding the main hue control you shift all of the colors in an image (changing all channels simulataneously). You can select and modifly only ranges of colors like reds, greens, neutrals, etc. This is sometimes very useful, but it's not the same as color correcting the image with unbalanced channels. Go too far with selective color editing and you'll get posterized colors and odd transitions with respect to the rest of the image.
The selective color tool gives you essentially the same capabilities for shifting groups of colors, but it gives you control of the components of each color as well so it's a more precise way of editing so that you can do something like shift the sky color to a more printable blue or clean up greens in foliage.
With most photo images, correcting color balance is a matter of adjusting the relative curves of the color channels to a pleasing, neutral balance. If an image has, say, too much red, the red curve needs to be balanced for the whole image, not just in specific colors. Curves, levels, or even the variations tools are the appropriate tools for that kind of image tuning.
I often adjust saturation in photo images, and hue controls are tools I use to create artificial color balances or colorized monotones. You can't correct color balance using hue as you can using curves.
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G
Can someone explain to me what exactly is going on with a levels change? Does it matter if I do RGB or do each individual color? It seems cool, but I just don't know what I'm really doing.
Thanks!
Jason
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"You miss one-hundred percent of the shots you don't take.