1. CASTLEFORD CAMERA CLUB
Introduction to Histograms

2. Histograms

3. What is a Histogram?
How is a luminance histogram produced? First, each pixel is converted so that it represents a luminosity based on a weighted average of the three colors at that pixel. This weighting assumes that green represents 59% of the perceived luminosity, while the red and blue channels account for just 30% and 11%, respectively. Move your mouse over "convert to luminosity" under the example image above to see what this calculation looks like when performed for each pixel. Once all pixels have been converted into luminosity, a luminance histogram is produced by counting how many pixels are at each brightness — identical to how a histogram is produced for a single color.
A histogram is a two dimensional representation of your image in a graph form, showing you how many pixels of a particular brightness level there are in the photo.

4. How is a luminance histogram produced?
First, each pixel is converted so that it represents a luminosity based on a weighted average of the three colours at that pixel.
This weighting assumes that green represents 59% of the perceived luminosity, while the red and blue channels account for just 30% and 11%, respectively.
Once all pixels have been converted into luminosity, a luminance histogram is produced by counting how many pixels are at each brightness — identical to how a histogram is produced for a single colour.

5. Types of Histogram
Luminance ( Brightness) Colour (RGB)

6. Luminance Histogram
Instead of sorting the pixels by colour, the histogram sorts the pixels into 256 levels of brightness from black (value 0) to white (value 255) with 254 gray levels in between.
The levels of "brightness" are then stacked accordingly. The height of each "stack" or vertical "bar" tells you how many pixels there are for that particular brightness. "0" and "255" are the darkest and brightest values, corresponding to black and white respectively.

7. Luminance Histogram

8. Luminance Histogram

9. Why are Histograms Important
Histograms are important because even though today’s cameras have excellent metering systems there are lighting conditions that can “fool” the cameras internal light meter resulting in an incorrectly exposed image.

10. Why are Histograms Important
Why can’t I just look at the LCD screen and tell if the photo is exposed correctly?
Relying on your LCD display alone can fool you. For example if you have your LCD display set to the maximum brightness a photo might look over-exposed when it really is not.
There are times when viewing the LCD in sunlight that the screen looks washed out or is hard to see due to the bright sun.
Finally you have the fact that even the largest LCD’s on cameras are really too small to accurately judge a photo’s exposure and colour.

11. Luminance Histogram Clipping :-
0ccurs when the photograph is incorrectly exposed, which can lead to blown-out highlights (where no detail can be seen) or clipped shadows, where detail is lost in darker areas of the photo. Since you want detail to remain in all parts of the photograph, this is one of the most important uses of the histogram.

12. Histogram In Action

13. Histogram In Action

14. Histogram In Action

15. Histogram In Action

16. What are Colour (RGB) Histograms ?
A colour histogram is a simply a histogram that shows the colour level (saturation) for each individual RGB colour channel. Also known as “Three Colour Histograms”

17. RGB Histogram

18. Why Use Colour (RGB) Histograms ?
The advantage of a colour histogram is that you can see if individual colours have been clipped or are over or under exposed.
This is important because it is possible for a luminance histogram to show little if any clipping while a three colour histogram would show clipping on one colour channel. This would result in a loss of texture or detail for that colour similar to what you get with an over-exposed image.

20. What can I learn from a colour histogram? Read more:
Colour histograms can help you determine if the white balance of your image is correct. If the spike for each colour is located at the same place on the histogram for each colour channel then the photo is balanced or neutral.  This can be a good way of doing a quick check of your white balance

21. What can I learn from a colour histogram? Read more:
It is important to realise that it does not work on all images.  For example an image that contains a lot of one colour such as a blue sky will usually have the spike for that colour offset even if the white balance is correct.

22. To help you establish how far to go in the image brightening direction, most SLR cameras have a setting called “highlight warning”.  It will make any overexposed highlights “flash” or blink when you preview your images on your camera screen.  Many people call this, “the blinkies”.
To do this on a Nikon, preview an image and press the Up or Down buttons (near the OK button) until you see the highlights flashing or outlined.  This is the “highlight mode”.  If you choose this setting, the camera will remember your setting for the next image you preview.   You may need to activate this feature “highlight warnings” in your settings menu first.
To do this with a Canon press “Display” or “Info” button (depends on your model), until they show up on the screen when previewing images.   You also may need to turn on this feature in the menu settings.  Check your camera’s manual if you aren’t sure where to find it.

23. Summary
Both over exposed and under exposed images mean we are generally losing valuable detail and a slight adjustment to our exposure can make for a much better image.
Learning to read and use a histogram will help you become a better photographer

24. By using the tools your camera provides for you, it is easier to see how to adjust your image exposure.  There is a lot more to know about the histogram, and you can use it when you process your images in Photoshop or Lightroom as well.  Keep in mind that if you shoot JPG format, nailing the exposure in camera is even more critical.  If you shoot RAW format you have some leeway to make adjustments later