|MadSci Network: Chemistry|
Thanks for your question, David - it’s an interesting one. On the one hand it could be very simple to answer - I could just give you some numbers and leave you to “gauge the difference in resolutions” as you put it, or I could do the difficult bit - i.e. help you with the “gauging” bit. I’ll do the latter, but this won’t be short! Before getting into all the complexities, here’s a graph that compares the resolutions of different image capturing devices by plotting their positions on a curve of comparative image quality as a function of the pixel density. The chart shows that to approach photo quality resolution with a digital camera, you need a sensor with very high pixel density and high dynamic range. For a 4X print, it takes about a million pixels to deliver the kind of image quality you get from a 110 negative. To approach 35mm quality, you need at least 6 million pixels. It helps a lot in all of this, to understand the limitations of our own vision, so let us start with brightness, and think about how much we as humans can cope with. We can distinguish about 100 levels of brightness and very early in our neural pathway, connecting eye to brain, we split the signals into colour and brightness (or in jargon - chrominance and luminance). We have three chrominance receptors, roughly sensitive to blue, red and green light. Our sense of luminance is derived about 60% from our green receptors, 30% from the red and only 10% from the blue. We can see luminance detail about 0.2mm wide in an image about 350mm or 14 inches away (normal viewing distance). We cannot see small details in chrominance. Now for some stuff on photography. A typical colour negative film (such as consumer 35 mm film) has silver-halide crystals to sense the light. These are (depending on the speed of the film - faster = bigger), in the range of 0.5 to 3 micro-metres in diameter. (1 micro-metre, is one millionth of a metre). However there is a lot more to the “graininess” (or as we measure it in the business - “granularity”) of a colour film than simply the size of the light sensitive elements. The image you finally see is formed of tiny dye-clouds, centred on the position of a silver-halide crystal that was exposed and developed. These dye clouds are formed from many very tiny (sub-micron) oil droplets which were coated with the silver halide, and each contains a chemical which reacts with oxidised developer to form dye. Dye clouds vary in size and can be generated from a number of silver halide crystals, depending on how much exposure there was in an area; they can overlap with each other, and of course the packing density of the silver-halide grains themselves has an impact on the granularity. So, the size of image elements is not easily describable in general terms for silver-halide colour films. Another complicating factor in this comparison is that the grains are randomly (as opposed to regularly) distributed in any given area of the film. Typical consumer electronic or digital cameras are a little easier - at present, these seem to be settling down in the one to one and a half million pixels per image. This gives you a start on how much information is captured. To sense an image each pixel needs to know about colour and about brightness, so there is a certain amount of information to collect about each pixel. How much information is collected is in itself variable from camera to camera. Typically 256 levels of luminance will be detected (note this is over twice the number of levels we can distinguish, although that comparison isn’t quite right – our perceived levels are spaced logarithmically) as well as spatial sampling data and chrominance data. All of this is recorded at some pre-determined level of complexity or “bit- depth”. More bits mean more discrete levels can be recorded - but the image file generated gets bigger. Now to reproduce all of this information to something we can recognise requires a bit (no pun intended) of translation. To do a good job translating the transmittance of a good slide film takes as many as 10,000 discrete levels, and would be done in an expensive device or scanner and would generate a very large image file. The comparison can be made on different levels, depending on what you want. For a typical snap and 6” x 4” for example, a consumer digital camera can yield results which most people can hardly distinguish from a conventional print. It’s when you want bigger images that the differences start to show up. Whereas with modern film, enlargements continue to look good to quite large sizes, you would need much more information to make really good enlargements from a typical consumer digital camera. In fact you probably need uncompressed file sizes in excess of 30MB to come close to being competitive with film when making enlargements. This just reflects the rather large amount of information that traditional film can capture in an image. So, it depends on whether you want or need all that information. One way of getting digital image files is via a scanner, and just for comparison purposes the table below shows how much information is typical for a number of scanning applications: Application Number of pixels bits per pixel bytes per image 35mm colour film 6 million 24 18 million 4” x 5” film 24 million 24 72 million 4” x 6” print 2 million 24 6 million X-ray film 20 million 12 30 million So, I think you can see that film remains the lowest-cost for highest quality method of capturing images, but it all depends on what you want to be able to do with the images. Where money isn’t the main issue (professional applications, for example) - but speed of transmission of images is, there are very expensive, high quality digital cameras available. I doubt if they’d make a lot of sense for holiday snaps, though. By the way, - I think the digital world opens up so many possibilities for us to use our precious images, and I’m very excited by it all. For me, and I speak only personally, I find scanning ideal as I can use old images just as easily as new, and today scanners are cheap and easily available, if you have a decent PC. (You probably have one yourself, you certainly have access to one as you are reading this!)
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