Fast vs. slow lens?

<p>What does it mean if a lens is a fast lens, or a slow lens? I was recently told that the Nikon 16-85 mm f/3.5-5.6 is not a fast lens. What does this mean?</p>

<p>The "f" value indicates the lens maximum aperture. Think of the lens as a pipe, and f-stop indicates the pipe diameter, or how much light the lens can "flow". The f value is ratio between focal length and diameter. For example, a 50mm focal length lens with front element max aperture of 50mm is f/1.0. Similar example, but with 25mm max aperture, the lens would be f/2.0</p>

<p>Fast refers to lens with a large physical aperture (small f number like f2.8, f2.0, f1.4, etc.). Slow refers to lens with smaller physical apetures (larger f numbers like f5.6). I know of 3 reasons for fast lens. The first allows for a faster shutter speed (needed for things like shooting sports). The second relates to controlling the necessary power output of your flash/strobe (smaller f stops mean less light is needed from the flash/strobe which yields less heat in the flash/strobe and faster recycle times). The third allows for more artistic control over the composition (make the background blurry while the subject is in sharp focus). There may be more but I think this covers the simple stuff.<br>

--Wade</p>

<p>This makes sense, but I'm still confused about where the "fast" and "slow" come into the picture and what they mean exactly? A bigger aperature=bigger diameter which is obviously a faster lens, but is it faster in that you can get away with shorter shutter speeds or faster in that it can focus faster? What exactly does "fast" mean? Why is a "slow" lens a bad thing?</p>

<p>Faster as in you can use it at larger apertures meaning you can set the shutter faster. Higher ISO film is also referred to as "fast" for this reason.</p>

<p>Fast and slow lenses only refer to the relative amount of light, maximum, that they let into the camera for recording. The more light the lens lets in, the faster the lens is considered. The need for a fast lens varies depending on what you shoot and under what conditions. Some camera features only work completely with a lens faster than say f2.8. I know on mine, it does affect the focusing as certain focus points act differently and more are available with lenses at this or wider (faster) apertures.</p>

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<p>"What exactly does "fast" mean?"</p>

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<p>Yup, shorter (therefore faster) shutter speeds is where the term comes from.</p>

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<p>"Why is a "slow" lens a bad thing?"</p>

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<p>What a lot of folks seem to miss is that a big aperture isn't a magic fix for shutter speed. A lens' aperture also controls depth-of-field, so there's no free lunch. Bigger aperture = shallower DoF, and while fast lenses can get great subject separation that slow lenses just can't (i.e. better blurry backgrounds), DoF can get too shallow, sometimes to the point where a subject's eye might be in focus, but not the other eye, the nose, and the ears. Almost no lenses perform their best optically at their maximum aperture, but a fast lens stopped down to, say, f/4 is almost always sharper than an f/4 lens shot wide open. So for faster shutter speeds in low light, creamy blurry backgrounds, and better sharpness at large-ish apertures, you need a fast lens - a slow lens just won't cut it.</p>

<p>OTOH, many slow lenses get pretty sharp by f/8 (some of them impressively so), and besides being cheaper, they're usually physically smaller and definitely lighter than fast lenses. That's can be an important consideration for hikers/backpackers/trekkers and mountain runners like the late <a href="http://www.mountainlight.com/gallery.html">Galen Rowell</a> , who made some pretty great images with 'slow' lenses.</p>

<p>Is a slow lens bad? Depends on what you want to do with it. If you want to take pictures of small children running around, indoors with low light... yes. Auto focus generally dosen't work well in low light and slow lenses make it ever worse. Longer shutter speeds also tend to lead to blurred images due to camera shake and subject movement. If all you want to do is take pictures of landscapes and other static things where using a tripod would not be an issue, you can get away with slower lenses. Generally speaking, slower lenses are more limited in what they can do.</p>

<p>a lower f/stop allows for faster shutter speeds, therefore a lens with a low f/stop (maximum aperture) is a fast lens.</p>

<p>A sometimes overlooked asset of the fast lens, is it's ability to make focusing in low light easier. Having a bigger aperture (F stop), it allows more light to both the film/sensor. And the eye via the viewfinder.</p>

<p>In analogous fashion, except for the lowly 50mm, a 'fast' lens also rapidly drains one's wallet. (It let's more light in and more money out)<br>

;o)<br>

On a serious note tho, most 'fast' lenses are the more expensive due to the glass and also are much better built. (The price point matches the customer and their expectations) As quite remarkable & excellent high ISO performance and AF system improvements seem to really be coming of age with DSLRs, the averge person may be quite happy with a max aperture of 3.5 or 4. More affordable, smaller and lighter seem to be where much of the camera world is headed.<br>

Jim</p>

<p>The 35/1.8 DX is a fast lens. The 16-85/3.6 - 5.6 needs much more light to take a shot. However in most of the times you need a sharpness in a "depth of field", than you will also go to 4 or higher.<br>

16-85 is a great lens.<br>

http://joergvetter.oyla.de/cgi-bin/hpm_homepage.cgi</p>

<p>Funny, Jim, but so true.</p>

<p>All of this makes perfect sense. Thanks for all the responses.<br>

I was looking at some of the zoom telephoto lenses like the Sigma 120-400mm and the Sigma 150-1500. I don't remember the specific f/stop value but I know that neither of them was lower than 4. Why would this lens still be good for sports and freezing action (as I was told at the store) with that aperature?? </p>

 

<p>That's Sigma 150-500 mm (not 1500!)</p>

<p>Maryanne, today's digitals can handle higher ISO's very well, better than really fast film. For instance, I shoot a lot of environmental portraits indoors using iso1600 and with the way the camera handles it and appropriate back end software handling of noise, these images could be blown up to 60 inches wide without issue. A film shot at 1600 would be very grainy at that size and the grain itself would become integral to how the image was seen.</p>

<p>Also, many who sell these lenses think of sports as an outdoor activity in the middle of the day and most of the time these lenses don't even have to be set at all that high of an iso to get usable results in those conditions.</p>

<p>(edit: it might be worth posting questions regarding the lenses you are considering, not all lenses are created with great optical performance and the f-stop the least of your worries--I say this not knowing anything about those you mentioned!)</p>

<p>Maryanne,</p>

<p>Don't believe everything you are told in the store ! :)</p>

<p>On a more serious note. Fast is also a relative term. A fast 50mm is something below f2.0. You can get 50mm f1.8 for $100 new. On the other hand, a 500mm f2.0 would be a HUGE lens and cost more than a car. That would be considered EXTREMELY fast ! In a more reasonable size, a "fast" 200mm would be an f2.8. The 80-200mm f2.8 zoom lenses cost over a thousand dollars. Check out the online camera stores for the details. As the lens gets longer, the f stop that is considered fast goes up too.</p>

<p>Getting back to the sports use. In Professional situations, the stadium, basketball court, tennis court, etc are pretty well lit. Lots of big lights. What the Pros worry more about is reach. So, if you watch the guys on the sidelines, a lot of their lenses are LONG. They need that closeup of the action without being on the field.</p>

<p>For most of the rest of us, the lens range stops at about 300mm, unless you are big into shooting birds, and then the lenses get longer and lots more expensive.</p>

 

<p>An f4 lens would be fine for daylight sports, maybe night games at big college stadiums, but doubt it would work as well for high schools or in gyms unless you shoot at ISO's of 6400 or above.</p>