Setting Black & White Points – Should You Care?

The short answer is – Yes, very much

The longer answer involves more questions –

1 – Do you know what black & white points are?

2 – How they affect your image?

3 – How you set them?

Illustrated answers  & more follow….

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Click to enlarge

D300_130516_083600__DSC3779 acr sep glow

Yet Another Big Oak Image, 3

I never get tired of this tree

This image, as compared with the example that follows,

Illustrates what I look for in an infrared image –

Lots of blacks & whites that are truly black & white

Otherwise, without contrast, things tend to blend together

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Click any following image for a larger view

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1 – Do you know what black & white points are?
They define which areas of an image should be black and which areas should be white

They are an image’s

Darkest and lightest points

Usually just short of pure white or black

These may not exist in your image as captured, but

In most images they’re needed and

Can be achieved in post-processing

FYI – The white point is where your printer stops printing

And allows the paper’s white show through

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2 – How do black & white points affect your image?
They control the tonal range of your image which

Maximizes the image contrast

This is true for both B&W and color images

Most images look best when using the full range of dark to bright

Not all though – consider a moody low-contrast fog scene

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3 – How do you set black & white points?
First ask yourself –

Is there a part of the image which should be completely black or white?

Secondly –

Does the  histogram show this?

5-17-2013 10-35-28 AM
To achieve the proper histogram appearance

and thus proper black & white points

The Levels & Curves feature of most post-processing  programs can do the job

Here is an excellent tutorial

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I’ll illustrate the general idea of setting black & white points

Using a feature of Capture NX2

a. Starting with this captured (infrared) image (provided by a friend)

Q – We ask whether the black & white is represented?

A – Image contrast is too low for my taste (it is a matter of taste)

Confirmed by the histogram

5-17-2013 1-01-08 PM

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b. NX’s histogram allows the problem to be handled directly

1. It allows B&W point thresholds to be found

2. Done by enabling the double threshold (green check)

3. And dragging threshold sliders

until the image’s darkest and lights points are revealed

4. Below I’ve dragged the dark threshold until black shows

Red circle, middle right

5. Lower left notations show this is at a tonality of 64 (0-255)

5-17-2013 1-02-38 PM

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c. Similar to the above, I found the white point by

1. Dragging the right slider from 255 to 203

Notice the new histogram below which now encompasses

The entire 0-255 Tonal range

Also notice the actual point that was selected for

The white point (red circle near center)

Black point can be seen in the roof of house at right

The strength of these points can be adjusted to

Change pure black or white to something less, like

Maybe 5 and 250 (or whatever) instead of 0 and 255

5-17-2013 1-05-53 PM

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d. Here’s the before|after

Q – Why the color shift?

A – The “black” point I selected wasn’t a neutral color

It should be (especially for a color image)

Not doing so will introduce a color cast

Obviously some red leaked onto the “black roof”

It makes no difference in this IR image since

The B&W processing step will take care of it

Fatal in a color image though

5-17-2013 1-01-08 PM

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e. The final step is to do normal post-processing

But – you must do the point setting first

Here is the view in Silver Efex Pro

Once the IR conversion to B&W is complete

5-17-2013 1-25-27 PM

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For the complete story enlarge this next image

It’s the upper left & lower right images we’re comparing

5-17-2013 1-41-01 PM

A future post will describe the Color Efex Pro step

Giving digital infrared the more surreal IR film look

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0 thoughts on “Setting Black & White Points – Should You Care?”

  1. Usually I just drag the little triangles below the histogram towards the center until they are about to touch the area where the histogram starts rising. When over-/under-exposure warnings are turned on, you will easily see if you went too far. That avoids color-shifts since I am not sampling individual pixels.
    If you want to go, excuse me, anal, you may want to convert to Lab color space, work in L and convert back. Lab color space is anyway a very intuitive and powerful tool in your collection. Eg see here: http://www.broadhurst-family.co.uk/lefteye/MainPages/Lab.htm

  2. That’s a good question. If the image has something that’s truly neutral (white, black, any shade of gray – in general anything where R=G=B) that’s what you pick. Many programs allow you to sample a point for its RGB component values so look for an area where the three are equal or very nearly so. If you use the pure histogram approach shown in the tutorial, it’s not really a problem since you never have to “pick”.
    Unfortunately, often the darkest or brightest areas are not neutral. Look for a resulting color cast and take care of that as best you can.
    Now correcting white balance is something different and is easy (with RAW). Choose anything neutral & if there isn’t anything in the scene, take a reference shot with a gray card (or your handkerchief) to use later in post-processing to calibrate your WB adjustment.
    Sorry I don’t have a better one-size-fits-all answer. Anyone else?

  3. I can see why you never get tired of the big oak. I wish I had one! 🙂
    Thanks for posting this. I really am new to digital post processing and this is a great help. Thanks for the link too. Just a quick question…How do I prevent I selecting a non-neutral black/white point in a color image?

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