<p>The above explanations are probably what you're looking for but let me take you deeper (bwaahaa haa haa) and introduce you to an engineering term called dynamic range, more properly instantaneous dynamic range (IDR). IDR can be thought of as the ratio of the brightest thing that can be captured (by film or sensor in a camera) to the dimmest thing that can be captured (and still seen). Some numbers here will help demonstrate my point. Let's say a camera has and IDR of 10 "stops". Stops is a photography term used to denote a double or halving of "brightness". Let's assign a numerical value of 1 "unit" to the dimmest thing that can be captured by the camera and seen above whatever noise level there is. Let's not worry about exactly what a "unit" is for the moment. 10 doublings of 1 mean that the brightest thing that could be captured (w/o overexposure) would be ... hmmm, 2, 4, 8, 16, 32, 64, 128, 256, 512, 1024 .... would have a value of 1024 "units". So what happens when a star has 2 units of brightness and the Sun has 4096 units and you want a picture with both of them in it at the same time ? </p><p>Well it depends. Cameras have exposure controls that allow you the user to set where the IDR of 10 stops is positioned. Again some numbers will help. Let's say you do what normal cameras generally try to do and that is to avoid overexposure. Overexposure is what happens when something brighter than the camera can capture is let into the camera. Any very bright object that would be want to be > 1024 "units" will only get a value on 1024 in the camera. Now if you block some of the light entering the camera the brightness of everything will be reduced. Let's block some of the light in our example. Let's block enough that the Sun's brightness of 4096 is reduced to 1024 units. Sort of like putting on sunglasses. What happened to our star ? It's brightness also got reduced by the same factor of 4 so it's now gone from 2 to 0.5. Alas it's below the value of 1 unit that can be seen and so doesn't show up. That's basically what's happening in any photo (film or digital) or video when the stars can't be seen. For some reason (usually a brighter object in the scene) the IDR of the camera has been positioned (via the exposure controls) so that the starlight is too dim to be recorded. </p><p>Now just to confuse you some more, we could fiddle with the exposure controls so as to reduce the brightness of every thing by only a factor of 2. Now the star goes from 2 units to 1, and can be seen. The Sun goes from 4096 to 2048 but since the IDR of our camera's film or sensor is limited, it'll be recorded as "only" 1024 units. The Sun gets overexposed but we can see the star. If you have a digital camera that has manual exposure controls you can play this game (though perhaps not with the Sun and stars) and see these type of results yourself. </p><p>There, now you're completely bored. Can you tell I'm an engineer with a photography hobby ? <img src="http://sitelife.space.com/ver1.0/content/scripts/tinymce/plugins/emotions/images/smiley-laughing.gif" border="0" alt="Laughing" title="Laughing" /></p><p> </p> <div class="Discussion_UserSignature"> <p>-----------------------------------------------------</p><p><font color="#ff0000">Ask not what your Forum Software can do do on you,</font></p><p><font color="#ff0000">Ask it to, please for the love of all that's Holy, <strong>STOP</strong> !</font></p> </div>
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