I decided to see what my jpeg-only digicam can capture in the way of light in stops in real terms, in a real situation After reading: "With 8 bits there are 256 possible values corresponding to decimal numbers from 0 to 255. So with an 8 bit A/D converter you can take a range of analog (sic) numbers and convert them into a digital number with a range from 0-255. Now let's ask the question "what's the dynamic range of an 8 bit A/D converter?". "Well, it's the ratio of the smallest signal it can record to the largest signal. The smallest signal is "1" and the largest signal is "255", so the dynamic range is log(255/1) = 2.4." (ref = http://www.photo.net/learn/drange/ ) so the 65k colours from a 16 bit image therefore have a dynamic range of 4.8 It is then extrapolated ny many that 2.4 / 0.3 (stops) = 8 stops.. 8 bit jpegs have a dynamic range of 8 stops (..and therefore 16 bit images must have a dynamic range of 16 stops?) Using this logic it is clear that jpegs have less dynamic range than other formats. hmmm.. not what I've seen - sure, there may be fewer *steps* in the range of tones, but I haven't seen the output from a tiff/raw capable camera exceeding the jpeg output. from http://www.luminous-landscape.com/tutorials/blended_exposures.shtml "Colour transparency film has a useful dynamic range of about 4-5 stops between the lightest area with any detail and the darkest. ..But, nature can present the landscape photographers with scenes having a dynamic range of 10 stops or more - double that which slide film can handle" it seems 'dynamic range' can mean a multitude of things.. film shooters use the term to mean the range the film can capture from the original scene, digital shooters use the term to mean either the same thing OR the bit depth. Sometimes they even mix the two! So I got out the old Minolta V with the spot meter attachment and the Gretag colour chart and the lowly Sony f717 which is capable of only shooting jpegs and set to work to find what the *actual* dynamic range of the Sony actually is in stops.. I spot metered the white and the black patches at ISO 100, 1/60th and got readings of f8, f45 bang on. The other patches were a little off, but given there are 6 of them, then the white and black points having those readings looks good. I then shot a sequence of images (in the shade *) at f8 across 6 stops as: 1/500th , 1/250th , 1/125th , 1/60th , 1/30th , 1/15th * which is why the values below show a little more blue than a neutral colour should. I then fired up Irfanview and cropped then resized the images, then used ColorCop to measure the RGB values, recorded in the table below. Note I applied a 50% blur to each cell to compensate for the variation in tones between individual pixels as they were not (cannot!) be a uniform colour. 1/500th 1/250th 1/125th 1/60th 1/30th 1/15th 88,94,91 97,108,111 148,152,151 202,210,218 237,240,245 255,255,255 63,64,68 73,82,91 119,125,126 170,183,196 218,223,230 250,251,253 39,42,44 46,52,64 91,95,98 138,150,164 190,202,208 229,234,243 23,23,26 27,31,36 55,63,63 103,115,125 151,161,170 197,210,222 11,12,12 16,14,19 28,29,33 57,69,80 104, 113,125 149, 162,178 4,4,4 5,5,7 10,12,12 20,26,31 42,55,64 82,96,115 Just a note on that final value of 4,4,4 - I decided to do a shot at 1/500th f8 With the lens cap on to record the values for the dark current which came out to be 1,1,1 so 4,4,4 is still significant. What does this tell us? Firstly that my Gretag colour patches are a little out of whack. Second it suggests that my sensor clearly can capture more than TEN stops of light - clearly more than the *limit* of 8 stops a jpeg is theoretically capable of in digital terms. It also indicates the sensitivity curve is a little different from what you'd expect ;) Now I need to do this with a camera that can produce jpegs and raw / tiff file formats to see what we can see regarding whether a tiff can grab a greater dynamic range or whether manufacturer twaddle is deliberately mixing the two meanings for their own amusement. k
