Exposure

Exposure correction is usually the first task to perform when improving digital images.

Besides the Automatic Corrections provided in the menu Auto, you can access the manual corrections from the menu Exposure.

Smart Flash

To reveal dark details lost in the shadow, use a Smart Flash correction. This is especially useful when you cannot solve the problem by any other mean. In fact, even curves cannot help in this case.

Reduce Hilights

To reveal details lost in the high tonal range, use a Reduce Highlights correction.

In the above example you may notice - especially in the central part of the picture - that many details have been recovered, and the resulting picture overall does look better.

Exposure > Smart Contrast

Allows to change the contrast in a non linear way.

The advantage of the "unlinear approach" is clear when you have a picture with a well stretched histogram: if you still want to increase the contrast, using the standard approach (linear), you will just clip the high and low tonal ranges, which is not good because you're giving away pixel information and creating over-dark and over-bright saturated areas.

The Gamma / Levels is also suitable to change contrast non linearly.

Unlinear Contrast is also the best choice to smoothly decrease contrast.

Exposure > Exp. Compensation / Midtones Compression

With this correction it is possible to act on the picture's dynamic range, by increasing its exposure and compressing the midtones range, giving the picture a more alive appearence.

Exposure > RGB Curves

This is one of the most effective tool for correcting Exposure as well as Color. The Curves let you adjust the entire tonal range of an image. But instead of making adjustments using only three variables (highlights, shadows, midtones), with Curves you can adjust any point along a 0-255 scale while keeping other values (points) constant. You can also use Curves to make precise adjustments to individual color channels in an image.

The horizontal axis of the graph represents the original intensity values of the pixels (Input levels); the vertical axis represents the new color values (Output levels). In the default diagonal line, all pixels have identical Input and Output values.

Click any points on the curve that you want to remain fixed. For example, if you want to adjust the midtones while minimizing the effect on the highlights and shadows, click the quarter and three-quarter points on the curve.

You can add more control points to the curve, locking those values. To remove a control point, right click with the mouse on it. You cannot delete the endpoints of the curve. To adjust the curve drag the points until the image looks the way you want it.

Exposure > Gamma/ Levels

What is Gamma:

Since the human eye has a nonlinear response curve (it is more sensitive to variations in low light than to an equal variation in bright light), it is important to track this nonlinearity with the display of different gray levels.

The curve that relates the amount of light emitted from the monitor to the 256 different gray levels in the digital image is called the gamma curve. There are several different standard gamma curves in common use. They all follow a power law of the following form: b = vγ where b is the brightness of the light emitted from the screen, v is the gray level value in the image and γ is the monitor gamma.

The standard value for γ that most closely approximates the response of the human eye is 2.22. This is the value used by the television industry for displaying video images.

The prepress industry has standardized on a value of 1.8 which more closely corresponds to the characteristics of printing presses.

Macintosh computers often use the value 1.4 for some unknown reason, while most uncorrected PC monitors have a gamma of about 2.3. Displays with lower value of gamma are brighter than displays with higher values.

If an image was designed to be displayed on a monitor with a gamma of 2.2 and the monitor's actual gamma is set to 1.8, the image will appear too light and washed out. Conversely, if an image was adjusted to look good using a computer with a monitor gamma of 1.4 or 1.8 and viewed at 2.2, it will look too dark. For this reason, it is very important to set your computer's monitor to the same gamma setting as will be used to print it or to convert the pixels from monitor gamma to printer gamma. If gamma is ignored, there is no way to guarantee that prints will not be too light or too dark.

Changing Gamma in Focus Photoeditor:

If you need to permanently convert a picture from your monitor's gamma to Printer or Macintosh Gamma then you can use the automatic conversions found in the menu Correct > Quick Gamma Correction > Optimize Gamma for > choose device

If you want to manually adjust the gamma, from main menu choose Correct > Gamma / Levels.

Levels:

Besides the Gamma value you will be able to adjust at the same time also the Shadow and the Lights levels.

Fine-tuning the Levels of shadow and light is very useful for example if we know the picture is going to be printed and we need a perfect quality. Increasing the shadow level will produce more vivid and contrasted pictures on a printer.

For optimal results the histogram of the picture should be properly stretched before using this tool.

Exposure > Histogram Stretch

What is an histogram:

This is an example of histogram representing the luminance channel of a picture:

An histogram is a diagram of the intensities of the pixels.

To obtain an histogram we scan each pixel and read its intensity, then we report on a diagram the number of pixels with intensity of 0(min) , the number of pixels with intensity of 1 etc.. until intensity=255 (max).

On vertical axis is the Number of pixels and on horizontal axis the intensities between 0 and 255.

Full color pictures have 3 color channels: R, G, B. Combining these channels into one we obtain a channel called luminance that is like the average of the 3. So we can display till to 4 histograms: Red, Green, Blue, Luminance.

One of the most common and powerful histogram operation is called: Contrast Stretch, that is a stretch of the luminance channel over the full histogram range.

For example in the diagram above we can see that in the range 220-255 there are not many pixels. Stretching the histogram between 0 (Min) and 220 (Max) will increase the contrast of the picture. A good stretch does not cut away any significant information.

After a stretch you will see that the histogram will have discontinuities: this is a normal consequence of this operation. If you see that the diagram already has these discontinuities then most likely the picture was already stretched and you don't need to stretch it again.

While the stretch operation on the luminance has the effect of increasing contrast, stretching the RGB channels individually can remove color casts. To stretch the histogram you have to choose a Min and Max value of the range; a good choice is to pick as min-max range the one where most of histogram values are located.

Exposure > Shadows-Midtones-Hilights

This regulation can be used to increase or decrease the contrast of a picture, by controlling the intensities of 3 tonal ranges. When you want to increase contrast, you should decrease shadows and increase the hilights. Decreasing the contrast is the opposite operation.

Corrections applied are linear.

Exposure > Brightness Contrast

This is a tool to adjust at the same time the level of brightness and the level of contrast. It's a very basic regulation, though effective if used correctly.