Using Levels

I want to be able to adjust tonal range in a photograph such that the lightest pixels in the image become white and the darkest become black. In every other photo-editing program I've used, you can view a histogram and use little sliders and/or spin dials to adjust the white, black, and midtone points to achieve this. For example, move the white slider leftward to where the upper end of the pixels in the histogram begin.

The closest I can come to this in PhotoPlus 7 is to look at levels and adjust these three points using the spin dials. The problem is that you can't see what's happening on the histogram — everything becomes trial and error. First you make the adjustment, then close and reopen levels and see how close you came. Looking at what's happening visually isn't a substitute because what looks black or white will vary from monitor to monitor — it needs to be done by the numbers. What am I missing here?

PhotoShop.

Mike

Hi Bill

Try using the Threshold item in the Image>Adjust menu. This is hopefully what you're after.

Dale.

Hi Dale,

Thanks for your comments, but I looked at the Threshold item and that's not what I'm looking for. This turns an image into only black and white, with no shades of gray. What I want is a full tonal scale, in either B&W or color, depending on the original. But I want the image to cover the FULL tonal range from black to white.

Perhaps an example will help. I scan an old faded photograph. As you would expect, the scan looks faded too (unless an auto-correcting feature is turned on.) If I look at the histogram, I can see why — there are no pixels at black (0) or white (255.) So I go into my photo-editing software (not PhotoPlus) and move the white and black point sliders in until they just touch the two ends of the histogram. Suddenly I have a full color or crisp B&W image. This effect is most dramatic with a faded image, but images that are just slightly off-exposure or taken on an overcast day benefit as well.

You could get a similar effect by playing with the contrast and brightness controls, but that isn't precise. If you and I have our monitors set differently (gamma, brightness), we'll get different results. With a histogram tool, we would get identical results every time. Also, this ensures that we haven't blocked up the shadows or blown out the highlights.

The other response I got to my question was "PhotoShop." While it's true that Photoshop has this feature, it's also true that the free software that came with my scanner and many other scanners have this feature, as well as the $50 software I am currently using. It's a basic and necessary tool.

You are already soooo close on this one, Dale. In "Levels" you have a histogram tool that allows me to adjust the black and white points with spin dials. All I want is two moving pointers that are located at 0 and 255 luminance levels. As you adjust those spin dials, these points would move in to show the adjusted tonal range. For extra credit, you could put a third, slider to allow me to adjust mid-tone brightness without sacrificing either highlights or shadows.

Bill T.

Hi Bill

Thanks for the additional info, very useful. There's a few things you can do... first and foremost, the Histogram dialog, although it doesn't offer adjustment, does feed back the peaks for any Level of the tonal spread. For the Luminance histogram, if you have a peak at 10 and nothing below it (i.e. no pure black) and a peak at 245 with nothing above it (i.e. no pure white) then you can use these as Input values, with 0 and 255 as Output values, in the Levels dialog you've described. So in future the Levels dialog should either feed back the value like the Histogram dialog does, so you can figure out where your photo's darkest and lightest tones are in relation to real black and white, or as you've mentioned, introduce a new slider or other method of stretching the photo's max/min values to equal 0 and 255.

While there isn't an automatic max/min value stretch function right now, it can still be done by using values taken from the Histogram dialog in the Levels dialog. You can also experiment by using the Image>Adjust>Equalisation and Image>Adjust>Stretch functions... these do perform some quite impressive tonal adjustments for images with brightness and contrast problems, and setting a Gamma value of say 2 in the Levels dialog should also differentiate more tones in the image.

Much of the same adjustment can be carried out using Curves. An example, which should darken nearly-black areas and lighten nearly-white areas, without affecting the tonal spread in-between, is attached.

HTH, Dale.

PS - I'm not this brainy really, I had some help!

Dale,

Thanks for your comments. I tried the histogram again and, as you say, I can move the cursor around and see the corresponding luminance value. Then, I went to the levels tool and set the hi/low input values to the end points I'd read from the histogram. That seems to do what I want — a little more difficult than what I described, but workable.

I also tried the Image>Adjust>Stretch function, but I'm not sure what that does. I can't see any change in the image when I make that selection, and it doesn't open up a screen where I can make choices. What else do I need to do?

The gamma function also seems helpful, but the adjustment seems a bit course. Instead of increments of 0.1, I'd like to see increments of 0.01. You can type in any value you want, but it would be easier with a finer spin dial.

Bill T.