Some subjects are difficult to photograph

When two subjects of different lightness and darkness are in the same photo (Bride and Groom) you will notice the details don’t show up equally.  You can see the texture of the white duck and not in the black duck. [ISO 200, f/8, & 1/40 sec]

Which do you expose for in a wedding photo?  Do you show the detail of the bride’s white dress or the detail in the groom’s black tuxedo? This was the old joke wedding photographers used and the punch line long ago was the bride, because the parents were paying for the photos.

In available light this is still a very difficult shot to pull off. 

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Setup for the first photo

Now the simplist way to correct for this situation is to add some light.

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By adding flash on the right side of the photo to light primarily the black duck I was able to add some detail to the photo. [ISO 200, f/8, & 1/40 sec]

While you might think you only need this to help out in those situations to be sure both subjects look good–it makes a difference even in individual photos.

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You can see that I just added a flash up high at a 45 degree angle to the subject. 
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Black Subject with no flash to fill in the shadows

What the camera sees is the duck without a flash. When the human eye looks at the duck it scans the subject and adjusts to show the different tones so you can see detail like you do with the flash assistance in the next photo. 

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Black subject with the flash to fill in the shadows giving an apparent increased tonal value.
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White Subject available light
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White Subject with flash at 45 degree to the right and 45 degrees above the eyes.
Summary

When you add flash to a subject that has a dynamic range greater than the camera can record you are in essence bringing the values closer to the range the camera can record and therefore mimicking what the eye actually is seeing.

If you want people to talk about how great your photos are, then you need to capture the scene as close as possible to the human eye and to do that you may have to add some light.

BONUS TIDBIT

When you use flash you are putting the subject into the color space that renders the highest dynamic range.