![]() A 50mm macro lens can double as a "normal" lens on a full frame camera. Macro lenses can, of course, be used for non-macro work, and again the focal length may be a consideration. However longer focal lengths mean larger, heavier and more expensive lenses. It can also make working with live subjects (e.g. A larger working distance may mean that it's easier to get light onto the subject (since the camera is less in the way). From the macro point of view, increasing focal length means increasing working distance. Each has it's advantages and disadvantages. Macro lenses typically come in 3 focal lengths, "normal" - 50mm, 90 or 100mm and telephoto - something like 180mm. So for a full frame camera that means you can fill the frame with an object as small as 4.8mm x 7.2mm (3mm x 4.5mm for an APS-C camera). "5x" or "5:1" means that the lens will cover an area 1/5th the linear size of the film or sensor.So for a full frame camera that means you can fill the frame with an area as small as 48mm x 72mm (30mm x 45mm for an APS-C camera) ![]() "Half life size", "0.5x" and "1:2" all mean that the lens will cover an area twice as wide and high as that of the sensor.So for a full frame 35mm camera, that means you can fill the frame an area as small as 24mm x 36mm (15mm x 22.5mm for an APS-C camera) "Life Size", "1x" and "1:1" all mean that the lens will cover an area equal to the size of the film or sensor.Magnification can be specified in three equivalent ways, descriptive, magnification and ratio. Even 3rd party macro lenses are usually quite good. It's generally true that macro lenses are very well corrected and so yield very sharp images. For example the $85 Canon EF 50/1.8 II has 6 elements, while the $250 Canon 50/2.5 macro (which focuses to 1/2 life size) has 9 elements as shown below. Obviously this is a more complex design than a lens with fixed elements and more elements may be required, so the cost is higher. What it does is allow the design to be optimized for both close and distant focus. This may actually cause a change in effective focal length, but that doesn't really matter much. As you focus the lens, the elements of the lens move around in relation to each other in order for compensate aberrations at different focus distances. So how do you get a lens that performs well both focused at infinity and focused at life size 1x magnification? Well, you use what are called floating elements. Moving the lens further from the sensor/film than it was designed to be introduces additional optical aberration. You can make a 50/1.8 lens focus closer by using extension tubes of course and I've seen it said that since extension tubes contain no optics they can't degrade image quality, but that's not really the case. Field curvature, distortion and spherical aberration increase. Optical aberrations increase as you focus on nearer and nearer objects by moving the lens further and further away from the sensor/film. For a simple 50mm lens, if you put it 100mm from the sensor/film plane, it will give a 1x magnified image of an object that's 100mm from the lens. If you move it far enough, you can get 1x magnification. However that's not really what most photographers would mean by a "macro" lens.Ī true macro lens is optically corrected to give a flat field, minimum distortion and high sharpness when focused at its closest distance, and at that distance provide a magnification of at least 0.5x (1/2 life size), though most true macro lenses will provide 1x (life size) magnification.Ī simple lens (such as a typical 50/1.8) focuses by linear extension, which is just a fancy way of saying that you move the whole optical assembly closer and further from the sensor/film in order to focus. To a lens maker it appears sometimes to be a lens with "macro" printed on it! I've seen "macro" applied to any lens that will close focus to a magnification of about 0.25x (1/4 life size).
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