Magnification, Lenses and Depth of Field
The first challenge in photographing small things is to get the subject large enough in the photograph. The relationship between subject size and the size of the image on the camera's sensor is called magnification. Magnification is the number you get by dividing the image size by the subject size. We call it magnification, even if the image is smaller than the subject. In that case, the magnification is simply a fraction. If you photograph a large beetle, maybe two inches long, and the image on the sensor is one inch long, the magnification is simply 1/2. If you photograph an ant, maybe 1/4 of an inch long, and the image on the sensor is 1 inch long, your magnification is now 1 divided by 1/4, or 4. Most lenses won't give you that much magnification. In this section, we'll discuss how lenses magnify an object, and how to achieve more magnification than a particular lens provides "out of the box."
The key concept in understanding magnification is that a lens acts somewhat like a lever for light rays. More precisely, the lens acts as the pivot point of the lever. The two arms of the lever are the light rays coming from subject to lens, and the rays traveling from lens to sensor. <DIAGRAM OR PHOTO> The ratio of the two arm lengths determines
Lenses and magnification are subjects that can either be thought of in simple terms, or as you get deeper into the subject with almost infinite complexity. We're going to start with the simple end of spectrum, because it is the foundation for the rest, and, depending on your needs, may be enough. Later we'll look at some of the complexities that can come into play.
The most basic lens is a single piece of curved transparent material, usually glass, that can be positioned in front of a sensor in order to project an image onto it. An ordinary magnifying glass is an example of such a lens. A lens like this, or any lens for that matter, has a property called focal length. That's how far the lens should be placed from the sensor to focus on an object that's very far away. You can see this with a magnifying glass by going outside and making an image of the sun on a piece of white paper. You'll see that as you move the glass forward and back, there will be a single distance at which the image is at its smallest and sharpest. That distance is the focal length. In order to focus on objects closer to the lens, the lens will need to move away from the paper (or sensor). The distance between lens and paper where the subject is properly focused is called extension. You can play with this yourself in a dark or dimly lit room using the same magnifying glass, a flashlight as subject, and a piece of paper. (Other subjects can work, but usually the images of subjects other than lights are quite dim.) You'll notice that as you move the subject closer to the lens, the image becomes larger, and the lens-to-paper distance (extension) starts to grow rapidly. This isn't a one-for-one relationship. In fact, if your subject is at one focal length from the lens, the paper will need to be infinitely far back. There's an important rule at play. The subject moved closer, image size increased, and extension increased.
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Photographers are all familiar with lens focal length. Longer focal lengths give a closer look at a subject, while shorter lengths take in more of the area surrounding the subject. That's still true in macro photography, but it's not the whole story. There's a relationship between focal length, lens-to-subject distance, and a distance called extension. Extension is how far the lens has to move away from the camera's sensor in order to focus properly.
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Most camera lenses have limited close-focusing ability. To photograph insects you'll need to get a lot closer than a typical lens will bring you, and as you go to smaller and smaller subjects, the need for special optics becomes more and more acute. There are lots of methods for getting close, depending on camera type, size of subject, and acceptable cost. First, we'll look at some general principles, then go into more specifics about particular kinds of equipment.
We'll be talking a lot about magnification, and what it means photographically. Magnification is the relationship between the size of a subject and the size of the image of the subject on the camera's sensor. If the image is 2 times the size of the subject, the magnification would be 2 times or 2x. In ordinary photography, the image size is much smaller than the subject, so it seems strange to call that relationship magnification. But we can treat it in the same way. For instance, if you photograph a person who's six feet tall (72 inches) and the image is one inch tall, then the magnification is just a fraction, 1/72 times. If you're photographing a mountain that's 10,000 feet tall, or 120,000 inches tall, and the sensor image is one inch tall, then the magnification is 1/120,000 times - a pretty small number. On the other hand, if you are photographing a beetle that's 1/4 inch long, and its image is 1 inch long, the magnification is 4 times (4x). We'll be interested in magnifications ranging from about 1/10 to 5 or so. Magnification higher than that starts to get into a range where microscopes are needed, though the dividing line is not completely clear-cut.
Photographers are all familiar with the concept of lens focal length. This can either be a single length, in "prime" lenses, or a range of lengths in "zoom" lenses. Ordinarily, focal length is the thing that determines how large a subject will appear on the sensor. For any particular lens-to-subject distance, longer focal lengths mean a larger image of the subject. For subjects at normal working distances - anywhere from several feet to many miles, that's how it works. That relationship is not quite true for close-up subjects. There are some important factors that only become important at close-up distances. The most noticeable of these is the amount of lens extension. Extension is just the distance the lens needs to move from its rest position at infinity focus in order to focus on anything closer than infinity. For now, let's ignore zooming, and stick with a single focal length. When the lens is positioned at exactly one focal length in front of the sensor, objects that are very far away (infinity) are in focus. To focus on closer objects, the lens needs to be moved out away from the sensor. This is often referred to as extension. Unless the subject is very close, the extension amounts are small. But the closer the subject gets, the moreextension is needed. At 1x magnification, the lens needs to be two full focal lengths from the sensor. For 2x magnification, the lens moves out to three focal lengths from the sensor. You've probably seen a rule emerging. For any given magnification level, the lens needs to be moved forward by that many focal lengths from its position at infinity, for a total distance of magnification plus one, times focal length. The plus one comes from the position of the lens at infinity focus, then you just add one for each whole number of magnification. Most lenses aren't made with that much extension range built in. Specialty macro lenses, however, do typically allow for one full focal length of extra extension, giving 1x magnification, the image on the sensor is the same size as the subject.
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Magnification is the relationship between the size of a subject and the size of the subject’s image on the camera sensor. It's usually expressed as the number you get by dividing the image size on the sensor by the size of the subject. Most of the time, the image will be smaller than the subject (think of photographing a mountain - the subject is much much bigger than any sensor). When the image is smaller than the subject, magnification will be a fraction such as 1/5 (or its decimal equivalent 0.2) for an image that's one fifth the size of the subject. It may seem peculiar to use the word "magnification" to mean making something smaller, but that's just what a fractional magnification is.
Magnification is obviously a very important factor in selecting a lens for an interchangeable lens camera, or selecting a fixed lens camera. To get a detailed image, you'll want to fill a significant portion of the sensor with the image, so knowing subject and sensor size will give you a good idea of how much magnification you need. As a rule of thumb, I'd suggest aiming for a magnification that yields an image extending across half the sensor or even more. Almost all cameras and macro lenses will let you use less magnification if you like, (with one notable exception, the Canon MP-E 65mm macro lens), so if your smallest subject can be magnified enough, you're in good shape.
Notice that we’ve talked about the size of the image on the sensor. That’s important, because sensors come in lots of different sizes. The large sensors in full frame DSLRs are 36 millimeters wide by 24 millimeters tall. (That’s just a hair under an inch and a half wide by an inch tall.) So a one-inch butterfly at 1:1 magnification will fit on the sensor with half an inch to spare. A lot of DSLRs use smaller sensors, with slightly varying sizes, but generally around ⅔ the size of the full frame sensors. Current Canon and Nikon DSLRs use sizes of 22.3 x 14.9 and 23.6 x 15.6 respectively. With either of these sensors, the one-inch (25.4 millimeter) butterfly will not quite fit completely on the sensor at 1:1 magnification. With these smaller sensors, less magnification is needed to get an image filling the frame. In the last couple of years, “mirrorless” camera systems like the Micro Four Thirds systems from Panasonic and Olympus, the Nikon 1 series, the Canon EOS M, and others have introduced sensors in a wide variety of sizes, all smaller than full frame.
As we’ve seen, with a smaller sensor, less magnification is needed for any given subject size. Small sensors are less expensive to manufacture, and therefore the cameras using them are also less expensive. Smaller sensors have another less obvious advantage, they lead indirectly to increased depth of field. To understand this fully involves a detailed discussion of depth of field.
Depth of Field
If a lens is focused for a subject at some distance, say one foot, then only objects at that distance will be imaged with perfect sharpness. Perfect isn't quite the right word, because even the very best lenses have inherent limitations that limit sharpness, but at any rate, that one distance is where sharpness is at its very best. Anything that isn't at that exact distance will be imaged with less sharpness, with the degree of blur increasing as the distance from the optimum increases. Depth of field is the range of subject distances within which the subject still appears "acceptably" sharp in the image. "Acceptable" is a highly subjective term. it depends both on how critical you are as a photographer, and the intended use of the final image. Sharpness that might be perfectly fine for a small image on a web page, could be totally unacceptable in a 16 by 20 inch print. We're going to work through some of the optics and math involved in calculating depth of field. If you're not interested in the theory, you may want to skip all that and go straight to the results, including some depth of field tables. That's fine, in the end, the results are all that matter for photography.