[This journal is B&W specific]
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First you ought to ask yourself, what is contrast?
Contrast is the difference between the tones of grey that your image has.
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For a black and white image to have good contrast, you need to have the whitest whites and the darkest blacks in your image, as well as a good mix of greys in between. If you can't point to the solid black in your image, as well as the pre white, you don't have good enough contrast.
http://static.photo.net/attachments/bboard/00F/00FzkU-29347284.jpg
That is an example of a very low contrast image. As you can see, the image is mostly grey. There are no blacks or whites,