Close-up: Larger Than Life
One of the most fundamental concepts to understand in the realm of macro photography is that of "life size." Macro photography deals with taking pictures of small subjects, so it's only natural that we should start our discussion with a look at magnification.
When we photograph something, the image of our subject enters the lens and is inverted at the lens's nodal point. From there, it starts to spread out again, becoming larger (and dimmer) the further it travels (my lens geometry description is a bit oversimplified, but but works for illustration purposes). At the film plane (or sensor), it is recorded for posterity.
But the lens is round and therefore produces a round image, while both film and digital sensors are always rectangular. This means that at least some of the image spills off the edges of what can be recorded and is lost. Current Nikon DSLR's use a sensor that is about 2/3 the size of standard 35mm film, so more of the image would lie beyond their edges and be lost. This feature gives digital its inherent 1.5x cropping factor.
But magnification is a phenomenon produced solely by the lens and is not altered by how much of the resulting image gets captured or by how much gets thrown away. If the size of the image of a subject is the same as the actual size of the subject, the image is said to be "life size." Suppose we had a mythical SLR (foretold in legend, according to some) that took interchangeable backs and we shot the same image on both media, both would have the same magnification. This measure has nothing to do with whether you are using film or digital. Even if we could place a sheet of 4x5 film behind our lens, the magnification thus produced would not change. You would merely capture more or less of the image.
After we get back the film from our mythical interchangeable-back SLR and compare our results to the digital captures, we will need to enlarge the digital results more to achieve the same print size as the film counterparts since they started out smaller to begin with. At this point, the camera and lens are completely out of the picture though (pardon the pun), so any enlarging we do at this point for viewing or printing does not count as magnification. Similarly, if we blow up either of our images to the size of a billboard this doesn't change the magnification either. Only the effect achieved optically in-camera counts.
So, how big is life size? A frame of 35mm film is 24mm x 36mm (about an inch by an inch and a half) so a subject this same size that fills the frame would be said to be photographed life size. On digital, the sensor is only about 2/3 of an inch by an inch so an object an inch by an inch and a half shot life size would hang over the edge and be cropped such that we only saw 2/3 of it. If we could see the entire inch by an inch and a half, the result would be smaller than life size in order to match on the smaller sensor. The magnification of images smaller than life size are usually described as a fractional ratio as in 1:2 for 1/2 life size, or in the case described here, 2:3, or 2/3 life size. Images larger than life size can be described similarly. Really tiny subjects that fill the frame might be 2:1 or twice life size (also written as 2x) or beyond. Shown here are some examples using a US quarter.
The realm of subjects we will be considering in the coming weeks starts at about 1/10 life size. True macro photography is generally regarded as being life size (1:1) or greater magnification. There are many different ways of achieving high magnification, but at least we now have a common frame of reference for describing the results.