Helicon Filter Help

How to use the histogram

If you want to produce bright, vivid images you cannot do it without the Histogram.

The histogram shows a distribution of pixels. The number of dark pixels is shown on the left side, the number of bright pixels - on the right side.

A general rule is that a histogram of the resulting image should take all the scale without big "hills" on the sides.

On the image below you can see two images (a source on the left and a result on the right) and two histograms (upper for the source, lower for the result):

I have used the Highlights slider to darken the sky and add some contrast to the clouds.

Now let us go through a step by step tutorial about normalizing the histogram.

Step 1. Open the source image

You can open an image with a usual Open Dialog (menu->File->Open) or you can choose the image from the file browser on the Source tab.

If you would like to experiment with the sample image, I would suggest you copy the image from this page and paste it into Helicon Filter.

Step 2. Analyze the histogram

Once you've opened the image, switch to the Brightness tab. You will see the Equalizer with the Histogram on the left bottom corner of the program. The histogram on the top is the original and the bottom one shows it with the resulting changes."

1. If the Histogram is shifted to the left (dark) side, it means that the image is too dark.

Dark image	The histogram has a hill on the left side

2. If the Histogram is shifted to the right (bright) side, it means that the image is too bright.

Bright image	The histogram has is shifted to the right

3. If the Histogram has a big hill on the center, it means that the image is properly exposed but it does not use the whole dynamic range and looks too flat.

Flat (low contrast) image	The histogram has is concentrated in the center

4. If the Histogram is pressed to its sides, it means that the image is over- and/or underexposed. The peak on the right border means that all pixels corresponding to the sky have the same (maximum) brightness. The details (clouds) on the sky are completely lost and cannot be restored by post processing.

The peak on the left corresponds to the pixels in the shadows. Their value is close to 0 and consequently these areas are full with digital noise. If you shoot in RAW format with 16 bit quality and low ISO values to minimize the noise, it is possible to pull the details from the shadows and save the image. If you shoot in JPEG on a compact camera, there is not much hope to get high quality result.

Remember: underexposed image can be saved, overexposed - not!

Flat (low contrast) image	The histogram has is concentrated in the center

The above image is properly exposed but the dynamic range of the scene is too wide for a digital camera so the highlights get overexposed and shadows - underexposed.
The problem can be solved by making two shots with different exposures (one for the sky, second for the shadows).
The two images can be merged by Helicon Filter into one High Dynamic Range (HDR) image with full details on the sky and shadows.

Step 3. Normalize the image

A general rule is to stretch the histogram so it takes all the scale (dynamic range).

This can be done by controls on the brightness tab: Auto contrast, Haze compensation, Local contrast and others.

When changing the brightness, activate Exposure warnings to get alerted if the pixels are pushed over a dynamic range.

1. Normalizing the dark image.

To normalize this dark image I would check the Auto contrast and move the right slider under the checkbox. This will "cut" unused part of the histogram and the program will stretch the rest of it to fill the whole dynamic range.

I would also use the Shadows slider to make the green areas a bit brighter without affecting the bright sky.

Please keep an eye on the right border of the lower histogram. The number of overexposed pixels near the right border should not be too big!

2. Normalizing the bright image

To normalize this bright image I would use the Haze compensation control at first.

Then use the Auto contrast sliders to "cut" unused part of the histogram.

The image definitely needs vignetting correction, which can be done on the Distortions tab later.

Please keep an eye on the right border of lower histogram. The number of overexposed pixels near the right border should not be too big!

Step 4. Save the resulting image.

If you are contented with your result, switch to Save tab and save the result as usual.

If you open the original image later the program will "recall" all parameters that were applied last time. Consequently you will be able to continue editing the original image.

If you have suggestions for changes to Helicon Filter or to the help and tutorials, please contact us at filter@helicon.com.ua