|Re: Any good way to take pictures of calculators?|
Message #12 Posted by Dieter on 17 Sept 2011, 12:35 p.m.,
in response to message #10 by Dave Shaffer (Arizona)
I do not think white balance is crucial. All digital SLRs I know of support a raw image format. In this case the white balance can be set and fine-tuned in the raw image processor - without the slightest disadvantage compared to setting WB on the camera beforehand. It may work even better.
The more crucial point is exposure. White backgrounds may lead to underexposure if the user relies on auto exposure. However, within certain limits even this can be corrected afterwards when the camera is set to raw.
Diffraction, on the other hand, is a common subject in photography groups. ;-) As you mentioned, the critical f-stop depends from the image format, i.e. the size of the image sensor in a digital camera.
For those who are not familiar with all this: it's quite simple. As the lens is stopped down, depth of field increases. In other worde, f/16 will render a wider area in the image in sharp focus than f/8. That's fine for this kind of photography, where more sharp details of the subject mean more information.
However, there is a downside. As the lens is stopped down, another effect become more and more dominant - diffraction. In simple words: at small apertures the picture becomes more and more blurred. First you barely notice it, then some fine detail is lost, and finally the picture is evenly sharp from the front to the background, but the level of sharpness is so low that it can hardly be considered sharp anywhere.
So, there must be an optimum somewhere. Up to a certain point depth of field increases, then the whole picture gets less sharp until it's no longer acceptable. This point, the critical aperture, depends from the image size. As a general rule of thumb you can say that the maximum f-number should be roughly the (diagonal) imsage size in millimeters, or, if possible, preferably only half that.
So, if you're using a regular 35 mm camera (film or digital "full frame") where the image size 24x36 or 43,3 mm diagonally, the absolute limit is something like f/45. But if possible don't stop down further than f/22. Most digital SLRs use smaller sensors with a diagonal size of 27 or 28 mm, so it is best to keep the aperture below f/16. Typical compact (point and shott) cameras finally have extremely small sensors, usually something like 7 mm, which means that you should not stop down further than f/4. But since most of their lenses are not good enough to be limited by diffraction, f/5,6 or in some cases even f/8 do not make much difference. That's exactly the reason why virtually all compact cameras with manual aperture setting do not allow the user to stop down further than f/5,6 or f/8.
The interesting point finally is this: Yes, you should keep the aperture wider open the smaller your camera's image sensor is. But, and that's important, although the aperture now is wider, you will not lose any depth of field. Due to certain physical reasons, f/16 in a typical digital SLR and f/4 in a common point and shoot camera do not only show the same amount of diffraction, they also (ceteris paribus) will produce the same depth of field. In other words: you do not gain or lose anything, regardless which format you use. The same result (in terms of both DOF and diffraction) is only obtained at different aperture settings.
That's also the reason why 8x10" field cameras have no problem with f/64 or more. Since their image format is roughly 8x as wide and high as that of a "usual" camera, f/64 results in the same amout of diffraction (and also the same depth of field) as f/8 with the latter.