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Sally Jane Photographic Art

Adobe Photoshop Tutorial


Photoshop Navigation Pallet - Histogram

histogramThe histogram is a pallet which by default is located in the navigation pallet at the top right hand corner of your Photoshop desktop.  If it is not there click on Window in the menu bar and select histogram.  If there is no image open this pallet will be blank but once you open an image it will show a bar chart that will look something like this although the shape of the black area will vary from one image to the next.

You can get a larger view of this histogram by clicking on the little arrow at the top right hand corner of this pallet. That will bring up a list of options so click on Expanded view and it will enlarge the pallet and provide more information.

expanded histogram

You will see there is a little warning triangle on the graph that turns yellow as you pass your mouse over it (in Photoshop but not on this image).  This informs you that the histogram is using the cached information so is only approximate and needs updating.  By clicking on this icon you will get the up-to-date accurate histogram reading of the actual image displayed and ignoring the cached data, the icon will disappear.  It will re-appear as soon as you edit the image. By default, whenever you open an image the initial histogram will have this warning triangle present even though you have not made any changes. You can update the histogram just the same by clicking on the little circular refresh icon just about the warning triangle as well. Why there are two icons that do the same thing is anybodies guess.

So what does a histogram tell you?
This is a reading of how many pixels are showing each of the brightness values available to this image.  In a typical 8 bit image there will be 256 brightness values that are possible to be displayed.  0 (on the left side of the graph) represents black and 256 (on the far right side of the graph) represents white.  The rest are all gray values.  The more pixels that are displaying a particular brightness value the higher the graph will read at that point.  By looking at this histogram it tells you, without even seeing the image to which it refers, that there are more darker shades than lighter ones because the bulk of the graph is off towards the left and if you look very closely there is a spike right on the 0 end.  This is because this image has a thin black border around it.  Normally a spike at either end would not be ideal because it would indicate blowing out areas of either white where the light has created hot spots or black where detail has been lost in shadow.

OK so what is a bit?
A bit is a single piece of binary data.  Because it is binary it has two states, on or off.  In pixel terms it is easier to refer to them as black or white.  So 1 bit of data can show either black or white.  If each pixel can show 8 bits of data this allows it to display one of 256 different states.  2 to the power of 8 (28) or 2x2=4, x2=8, x2=16, x2=32, x2=64, x2=128, x2=256.  OK so that ends the maths lesson…promise.  The rest you can just take for granted J. So each pixel can represent one of 256 different brightness values and these are then plotted on the graph which is the histogram. 

It is possible to work with images in Photoshop in 16bit or 32bit.  Now the histogram has to display far more information because in a 16 bit image each pixel is now capable of displaying one of 65536 different brightness values.  Your monitor screen cannot show this but this information is still important when editing an image and I’ll explain this later.

In the expanded view you will see there is a drop down box which by default will showhistogram channels selector RGB for a colour image.  That tells you that this histogram is combining all the colour data in its reading.  You can, by clicking on this drop down box, select a histogram that just shows you one colour channel or all of them separately on one graph in which case each graph is shown in its individual colours. If the image you are viewing is a grayscale image this box will be grayed out and just say Gray.  If however the image is just desaturated it will still show you all the colour channels.

If you have layers in your image you can select to see a histogram based on just the layer you have active by selecting this option from the drop down box underneath the graph entitled Source. The rest of the information displayed is stats. I've not yet found a use for this stats info but I'm sure there are people out there that find this immensely interesting. Ummm.

Ok so now you have an understanding of this tool I will discuss how to use the Histogram with 8bit or 16bit images.



Histogram Video

Recommended Further Reading for Photoshop


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