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<p>[QUOTE=&quot;-jeffB, post: 26600625, member: 27832&quot;]<a href="https://digital-photography-school.com/how-to-use-a-gray-card-to-get-more-accurate-exposures-and-color/" target="_blank" class="externalLink ProxyLink" data-proxy-href="https://digital-photography-school.com/how-to-use-a-gray-card-to-get-more-accurate-exposures-and-color/" rel="nofollow">This seems like a reasonable introduction</a>.</p><p><br /></p><p>Cameras sort of &quot;assume&quot; the scene they&#039;re facing reflects an average of about 18% of the light hitting it, and that all the colors in the scene average out to neutral gray. The camera then tries to adjust exposure and white balance on that basis.</p><p><br /></p><p>If you&#039;re photographing a bright scene (newly-fallen snow), the camera will make it look dark. If you&#039;re photographing a shiny silver coin on a black background, the camera may overexpose the brightest parts of the coin. If you&#039;re photographing a gold coin on a white background, the gold might get <i>under</i>exposed, and the whole shot might take on a blue tint (making the coin look more silver and the background look more blue).</p><p><br /></p><p>Most cameras and phones provide a way to <i>lock</i> exposure and white balance. (I can&#039;t give instructions for that, because it&#039;s different for every device.) So the technique is:</p><p><br /></p><p>1) Set up your camera and lights.</p><p>2) Put the gray card where your coin will go (or even lay it on top of the coin).</p><p>3) Lock your camera&#039;s exposure and white balance while it&#039;s looking at the gray card.</p><p>4) Remove the gray card, replace it with your coin, and take the photo.</p><p><br /></p><p>As long as you don&#039;t move your camera or lights, you can do the first three steps <i>one</i> time, then photograph as many coins as you like. But if you do adjust your lighting, you&#039;ll need to recalibrate.</p><p><br /></p><p>If you use Photoshop or something similar, you can sort of do the same thing in post-processing by manually adjusting the exposure and white balance. But if the exposure or white balance is off too badly, you can&#039;t really fix it in post. It works much better to set the camera correctly from the beginning.[/QUOTE]</p><p><br /></p>

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