Histogram & Levels Adjustment

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Histogram & Levels Adjustment

Post  Sheilsoft on Mon Apr 09, 2012 9:38 pm

A histogram is a visual depiction of the digital information that makes up your photograph.

This can sometimes be confusing, as different image data can all be displayed in a histogram format.
Take this image for example...

If you press F7 in Paint Shop Pro to view its histogram, you will see all the histogram data for that image...
RGB channels, greyscale, hue, saturation & lightness... all very confusing!

To simplify things, as a photographer, our main technical concern is Exposure... and exposure is all about whether we've captured the correct level of light with our cameras, by using ISO, Shutter Speed & Aperture settings. The histogram information we are therefore interested in is Lightness or Tones.

Before we can understand and adjust our image histograms, we first have to understand, (trumpet fanfare!), Dynamic Range.

If you think of all the colours that make up your photograph, and then converted to black & white, you will have a whole range of tones or 'greys' from very dark greys, to very light greys. The majority of colours (reds, greens & blues) fall into the mid grey band. This range of greys, or 'greyscale', on an accurately exposed image should also contain somewhere, pure black and pure white. Dynamic Range is simply a term to explain an image which contains all tones including black and white, creating a more dynamic contrast. Without the dark or light ends of this range, images will look flat due to lack of contrast.

So if we look at the 'standard' histogram for the above image...

The left hand side of the histogram is the dark tones, with the '0' value being black.
The right hand side of the histogram is the light tones, with the '255' value being white.

As this is a fairly normal shot, most of the image data reads as mid tones, so the histogram displays that as generally peaking in the middle.

The ideal histogram would look like this, with a bell shape profile...

This of course is a theoretical perfect histogram, and all images will have their own unique histogram, but most should have most of the image data in the mid grey (mid lightness) central range, falling off to black (left) and white (right).

It's very useful to load your images into your graphic application (in our case Paint Shop Pro) and look at their histograms. You'll learn a lot about which type of image, subject matter and lighting conditions create what kind of histogram profile.

To view your histograms in Paint Shop Pro, click on Adjust/Brightness & Colour/Levels... from the top menu bar. This brings up the 'Levels' dialog box, which displays the histogram for the current image...

You will also see below the histogram chart, 3 values with 3 diamond shaped markers (adjustable sliders) above them. These three values are 0, 128 and 255 are the default settings for your histogram... 0 black, 128 mid grey and 255 white. Your image's histogram sits within that dynamic range. In this case the histogram stretches from 0 to 255, indicating that it has some black, mostly mid tones and through to some white. In other words, A well exposed image with good contrast! (Note - if the 3 settings are not showing 0, 128, 255... click the 'Reset' button)

Not all images though as they come out of the camera, for various reasons, have histograms that cover the full range... but by using this Levels dialog box, we can adjust their values to correct them!

Here's how...

We've been looking at an image which is perfectly exposed and with normal, average lightness and distribution of colours. This, as seen above, generates as expected an histogram with values ranging from zero to 255, and a fairly centralised mid-range peak. Not all images are like this!

It's quite normal to see images with a histogram placed over to the right, due to the brightness of the tones in the image.
See example below...


Notice that the image is prodominantly light in tone, so all the image data on the histogram is located towards the right side, with little or no dark tones at all.

It's also quite usual to see images that are of darker tones or lighting...


In this case, as you'd expect, the histogram data sits to the left, indicating that the image consists of mainly dark tones, and very little light data visible.

These types of images, although possibly correctly exposed, may also be adjusted to make use of the missing dynamic range. This process can add life to dull images whilst 'normalizing' contrast and saturation. You can do this using two methods...

Method 1 - Using this 'Levels' tool dialog box, simply click the 'Levels' button. The PSP Levels tool will then calculate how much to adjust the sliders on the histogram. It does this by trying to pull in the end sliders to the edges of the histogram data (and centralises the middle slider).

Pros - This is pretty much a one click solution, and you can process lots of images fairly quickly.

Cons - It doesn't always get it right, often squeezing the histogram too much giving unnatural results. If this happens, try Clicking the 'Reset' button, and then use the 'Contrast' button instead. Using the 'Levels' button uses a combination of contrast & colour, and is often too strong... the contrast only method is sometimes less severe.

Method 2 - Manual adjustment (preferred). Using the Left & Right sliders under the histogram, slide them inwards to the point where the histogram data falls down to the baseline. Only adjust a slider if the data does not reach that edge. You will note that the central slider will auto adjust its position to keep it central to both outer sliders. Do not adjust central slider... it maintains a normal dynamic range.

Pros - You get a far more accurate re-positioning of the sliders, and therefore better results.

Cons - It does take marginally longer, but worth the effort.

NOTE - Before each new image. you MUST click the 'Reset' button to reset the sliders to their default position.

By adjusting the outer sliders to fit your image's data, you're effectively squeezing the dynamic range so that black begins where the left slider is, rather than a value of zero, and  white begins where the right slider is, rather than a value of 255. Although a narrower dynamic range, it ensures the image displays the full range of tones.




Remember, these adjustments are only for those images which when viewed have a less than perfect exposure or lack contrast and/or punch. Most times, your camera does a very good job of capturing an accurate exposure, but certain lighting situations or choice of subject can throw up problem images, It's worthwhile therefore to browse through your image catalogue and see what they look like as an histogram... perhaps there are some that can be manually improved!

Howard Very Happy

Nikon D7100, Windows 7 64bit, Lightroom 5.2, Paint Shop Pro X5/X6

Posts : 235
Join date : 2011-09-11
Age : 65
Location : Los Llanos del Peral, Almeria

View user profile http://www.sheilsoft.com

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