Hey Joe, whataya know?
There are a lot of tips, tricks and hints about photography. Some of them are so right they are truisms (a tripod will increase the sharpness of your photos). Some are so wrong their persistence defies my ability to understand (the camera never lies).
Which still leaves a some stuff which falls in the category of, It ain’t what you don’t know; it’s what you know that ain’t so that gets you in trouble. There are a lot of those. Most are, usually, harmless. The problems arise when the edge cases small failures actually come into play.
Take depth of field. The “rule” is that longer lenses have shallower depth of field. It ain’t so. Depth of Field is a function of ratio. At a given number of focal lengths the depth of field will be “x”. I’ll simplify the numbers. Take a 50mm lens. Assume that a focal distance of 10 x FL = 12”. So at 500mm from the front of the lens there will be a 12” Depth of Field. For a 100mm lens the same DoF won’t be gotten until 1000mm.
The apparent effect is, at a given distance the shorter lens has a greater DoF. Since it’s also got a wider field of view, it also seems to have more things in the details, which fools the mind into thinking the wider lens has more resolving power. If you take the same picture, from the same relative distances, and compare them, the DoF will be the same.
Related is the idea that a smaller aperture = greater sharpness. It’s mostly so. The smallest detail which will be resolved sharply is the same size as the diaphragm opening. If the iris is open to 6mm, items of that size will be sharp. The problem comes in when one tries to enlarge the image. There are a couple of things which affect the way the light behaves. Refraction is the way the glass bends the light to bring everything into focus.
Diffraction is the way light bends when it passes by something solid. Pinhole cameras use nothing but diffraction to focus the light. It’s not the sharpest image, but one can do it (one can test this by curling one’s index finger until only a small point of light comes through. Things will be sharper. It’s most dramatic if one is nearsighted. I can read this page, through my finger, at distances where nothing but a gray blur is visible without glasses).
But it’s blurry. If the lens is stopped down too far, diffraction causes the edges of lines to blur some. At smaller print sizes this isn’t a problem. There is a point at which the further resolving power of stopping down starts to fuzz edges. For most lenses this is about one-stop smaller than the middle of the range. It will vary, depending on the resolving power of the lens. Macro (and copy) lenses, will do better than most.
If you want to find out when the iris stops improving sharpness for a lens, get a page of print. Set it up at a reasonable distance, and focus on it. Take a series of shots, stopping down as you go. Blow the image up to a large size, and then compare all the exposures at that size. When the letter edges start to diffuse, that’s one stop smaller than the sharpest for your lens. Check out all the stops, see how much it degrades from the best aperture, to the last. Mostly this isn’t a big deal, but if you need maximal sharpness, you need to test the lenses. It’s not the camera which the iris affects, but the lens.
Some things are new. Digital imaging means some of the things we used to do, we don’t need to keep doing. A lot of filtering can be done in edit, instead of in camera. This is, by and large, to the good. Filters introduce more chances for things to go wrong. There are two surfaces to get dirty, or scratched. They might introduce optical problems. Errors in grinding (or casting if it’s optical resins) can create flaws in the image.
Which means we don’t need color correction at capture, and so we lose one more chance to have things go south. If you shoot raw you can fix mistakes (like tungsten for daylight) with a mouse-click. Polarizing, on the other hand, isn’t something which can be done with the computer. Some of the effects can (the increase in saturation) but things like reflection neutralization need to be done before the image is taken. Neutral Density filters are also things to keep in the bag. Best are really long graduated filters, and a holder system (a la Cokin), to let you rotate and slide them.
I will say one other thing about color filters: they are still useful when doing Black and White. You can choose to play with it after the fact, but doing it when the picture is taken is more effective. It’s a technique to use when you plan to render the image in monochrome, because neutralizing the color is hard (it can be done, but some of the data will be lost). Luckily, digital makes it easy to shoot the image twice. It’s time, not money, you have to spend