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Ever wonder why professional sports photographers have huge lenses costing thousands of dollars. One of the things they are paying for is a lens with a wide diameter that can let in more light. This has some advantages as discussed below. It is also more expensive to make such a lens, however, and it becomes more expensive as the focal length increases. Below, I discuss wide aperture normal lenses. A normal lens is one with an angle of view that is similar to perspective the human eye would see. Lenses with a wider angle of view are called wide angle. They take in more of a scene. Those with a narrower angle of view are call telephoto. They take in less of a scene, but bring things closer.
So far I have three normal lenses with a maximum aperture of f1.2 - a 58mm Minolta, a Konica, and a 58mm Canon FL. The Minolta was acquired for about $50 ($100 for it and a Rollei 35 camera), the Konica for $35 with a camera, bag and other lens, and the Canon FL for $20. They are all in very good cosmetic and working condition.
To understand what's special about these lenses, let's get technical first. The maximum lens opening is often measured as an f-stop or f-number which is related to both the focal length and the diameter of the lens. The f-number equals the focal length divided by the diameter of the lens. For example, a 58mm focal length lens with a 29mm diameter has an f-number of f2 (58/29 = 2). Common maximum f-numbers for normal lenses are f2, f1.8, f1.7, f1.4, and f1.2. A lens with a 58mm focal length and an f1.2 f-number has a diameter of 48.3mm, which results in large and expensive optics. (Fondly remembering Algebra I which I taught for 7 years, since f-number = focal length/diameter, diameter = focal length/f-number, or here diameter = 58/1.3 = 48.3mm.) Many cameras in the 1970s frequently came with an f1.7 or f1.8 normal lens. For somewhat more, you could get an f1.4 lens. Relatively rare, however, was an f1.2 normal lens. Lenses like this on today's cameras are even rarer today since it is hard to make autofocus lenses with that large of diameter. By the way, the lower the f-number, the larger the diameter of the lens.
To summarize the last paragraph in non-technical terms, lower f-number like f1.2 instead of f2 or f2.8, etc., means wider diameter, more fancy glass and higher cost. What benefits does that give you photographically? First, it means the lens lets in more light. This means you can shoot at a higher shutter speed which is why large aperture lenses are often call "fast" lenses. Alternately, at a given shutter speed, you can photograph in dimmer light. Second, when used with a single lens reflex camera where the image you view is the image the film will see, you will have a brighter viewfinder all other things being equal. This will make it easier to focus accurately. Third, the larger the aperture (i.e. the lower the f-number), the more limited the depth of field. In other words, things in front of where you are focusing, and things in back of where you are focusing, will be out of focus. Often, you want this in, for example, portrait photography where you only want the person's face in focus and the background out of focus. Does it make that much difference? In my opinion, usually not. For my Sony A350 I have an older 50mm f1.7 Minolta normal autofocus lens which is great. An f1.4 or f1.2 lens would not be worth the extra expense to me. That being said, an f1.2 lens is a beautiful work of engineering to behold! Additionally, such lenses are usually very well made and have great resolution. In other words, a 58mm f1.2 lens is fantastic!
With the exception of Pentax, and to a lesser extent Nikon, you cannot fit an old manual focus lens on a newer autofocus camera. The lens mounts are usually different. There are adapters you can purchase relatively cheaply. Some adapters merely allow it to physically fit the newer camera mount. The adapter adds some distance, however, and hence acts like a macro tube. That's cool in that it allows closer focusing, but not so cool in that the lens can no longer focus at infinity. To avoid this, some adapters add an optical element which allows focusing at infinity. You are adding a cheap optical element to a very nice lens and hence most likely degrading the optical quality, however. A third alternative many people are now doing is to take off the old mount, file down the base as may be necessary to maintain focus at infinity, and then add a modern mount. With any of these options you are not limited to using the Minolta lens (or whatever brand) on a Minolta or Sony camera (or whatever brand). Rather there are adapters for most conversions - for example, I could get an adapter to put an old Minolta manual lens on a Canon EOS camera, digital or film. With any of these methods, obviously the lens does not autofocus. Also, you loose automatic functions. Apparently some people go so far to add some electronics to preserve some of the automatic functions. Some manufacturers still make very fast normal lenses. For example, Canon has an autofocus EOS mount 50mm f1.2 lens. So why go to all the trouble to make a conversion from an old lens? Answer: The new Canon 50mm f1.2 lens costs over $1,600.
Another solution to using old manual lenses on modern autofocus SLRs is to make the conversion to an Olympus mount. Olympus Digital SLRs have a smaller sensor and lenses with adapters still apparently focus at infinity. Also, Nikon and Pentax cameras will still fit older Nikon and Pentax mount manual lenses. Apparently, the backwards compatibility is especially good with Pentax cameras and lenses. Indeed, with a Pentax screw mount to KA mount adapter you can even use older Pentax screw mount lenses. Like Minolta/Sony, Pentax and Olympus also have image stabilization inside the camera, not inside the special and more costly image stabilized lenses that Nikon and Canon have. Therefore, all the lenses you can fit on a Sony, Pentax or Olympus DSLR become image stabilized. Pretty cool.
So far we have discussed wide aperture normal lenses. As the focal length increases, the diameter must also increase to keep a constant f-stop and hence a constant quantity of light hitting the film or sensor. This makes sense if you consider that a telephoto lens has a narrower angle of view and hence takes in less light. To make up for this, the lens diameter must be larger. It also makes sense from the definition of an f-number. f-number = focal length/diameter. Therefore, to keep the f-number constant, the diameter must increase by the same multiple the focal length is increasing. Let's say we have a 50mm lens with a maximum f-number of f4. Let's call the f-number fn, the focal length f and the diameter d. Therefore, fn = f/d, or for our example, 4 = 50/d. Solving for d, we get d = 50/4, or d = 12.5mm. Now let's say we have a 200mm lens also with a maximum f-number of 4. Again, fn = f/d. Solving for d we get d = f/fn or d = 200/4 = 50mm. Therefore, as the focal length increases 4 times, the diameter must also increase 4 times. Making wide diameter lenses like this is difficult and expensive. Here are some examples from glancing at new Canon lenses at B & H Photos. A 200mm f2 lens might be $5,000. For the same $5,000 you might get only an f4 400mm lens. A 600mm f4 lens would be nearly $8,000. An 800mm f5.6 lens is a whopping $11,000.
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