Wikipedia:Reference desk/Archives/Science/2013 September 21

= September 21 =

Camera lens for fast-paced sports
A friend has a choice of two lenses for her Nikon camera: Nikon 70-200mm f/4 or 70-200mm f/2.8. Which should she buy (price is not the issue) and why? She will be taking photos of professional hockey games. Bielle (talk) 20:21, 21 September 2013 (UTC)


 * Homework? The "which" and "why" will be well explained in any basics of photography book; she should buy the book before investing in the glass. 88.112.41.6 (talk) 20:34, 21 September 2013 (UTC)


 * No, not homework. Bielle (talk) 20:46, 21 September 2013 (UTC)


 * For taking pictures of fast-moving objects, f/2.8 would prob'ly be a better choice, because it will gather more light and thus allow shorter exposure times (which, in turn, will reduce motion blurring). The tradeoff, however, is a shallower depth of field. 24.23.196.85 (talk) 21:04, 21 September 2013 (UTC)


 * To avoid confusion, in case anyone should interpret 24's answer as indicating that there is a tradeoff in choosing the f/2.8 lens, there isn't (and I'm sure that 24 didn't intend to say there was). F/4 and f/2.8 indicate the maximum apertures of the two lenses. See F-number for a somewhat technical explanation of what these numbers mean. The lower the number, the more light is allowed to pass through the diaphragm. Going from f/4 to f/2.8 doubles the amount of light, allowing you to halve the exposure time, with the advantages in sports photography that 24 explained. It is correct that the depth of field is shallower with the f/2.8 setting. But if you use both lenses at f/4, the depth of field will be the same. Another factor to consider with a telephoto lens, is that you'll easily get a blurred picture because of not holding the camera steadily enough. Is she planning on using a tripod? If not, an f/4 lens with built-in image stabilization might be a better choice than an f/2.8 lens without image stabilization. You said "Nikon lenses", but you are probably aware that there are other manufacturers such as Sigma and Tamron who produce lenses for Nikon cameras. I'd definitely recommend doing some research and reading reviews of the lenses under consideration before buying. --NorwegianBluetalk 23:00, 21 September 2013 (UTC)


 * It also has to be mentioned that in practice you can crank up the ISO value to get the exposure time you need, so what the f/2.8 will allow you to do is to work with lower ISO values which means that you'll have less noise. You do have noise reduction options, but that will affect image sharpness.


 * Perhaps a moot issue because of motion blurring mentioned above by the IP and NorwegianBlue and also the limitations of the lock on autofocus (you won't be able to use the best autofocus points when using the "lock on mode" to keep a moving target in focus), but in theory a perfect lens with higher aperture has a higher absolute sharpness (the part that is perfectly in focus should be sharper). This is because the angular resolution is of the order of the wavelength divided by the aperture. But I think that even for the best commercially available lenses, the quality of the lens will lead to the maximum image sharpness being quite a bit less than the theoretical limit. The higher quality f/2.8 lens at that diaphragm of f/2.8 will have a similar quality as that other lens at f/4, so I don't think that the maximum image sharpness will be better for that f/2.8 lens. But you would expect that when both lenses are used at the same diaphragm that the higher quality f/2.8 lens should have a better sharpness. Count Iblis (talk) 00:25, 22 September 2013 (UTC)


 * I shoot the Tamron 70-300mm f/4-f/5.6 lens on my Nikon camera. I recommend it over the 70-200mm and even over the Nikon 70-300mm, and I have done side-by-side comparisons with all of the above.  Why would I choose the lower aperture and the off-brand lens?
 * Truthfully, the super-huge apertures degrade image quality more than they improve it. They defocus - because they have shallower depth of field; but they also defocus because it's harder to build perfect glass at that size.  Off-angle lighting and geometric distortion are easily noticeable to me on the f/2.8 70-200mm lens.  Chromatic aberration is quite a bit worse than the 70-300mm model.
 * Besides, you bought a Nikon for its superb image noise performance, right? The sensor and the digital processing on Nikon cameras outperform all the competition - you can verify by checking side-by-side image comparisons at a website like Digital Photography Review.  In conventional wisdom, you shoot at lower ISO and wider aperture to reduce noise; but I challenge you you notice noise on a Nikon D90 at ISO400; and I challenge you to do it double-blind at ISO1600 vs. ISO400.  So, the supposed rule-of-thumb is less relevant.  I've shot plenty of outdoor sports games - night football, even where the lighting is worse than a hockey game - at f/5.6 at 300mm, and never had to slow my shots or worry about noise.
 * So, why would I recommend the Tamron variant? Most of the time, I would strongly recommend the on-brand lens, because most of the time, the glass and mechanicall quality makes a difference.  But on the 70-300mm f/4-f/5.6, I happened to compare the Tamron and Nikkor side-by-side, and found the Tamron vibration reduction mechanism to be quicker and steadier than its on-brand competition, and its image quality was indistinguishable.  The extra zoom, vs. a 200mm lens, is nothing to scoff at: you get 50% better control of the framing or your shot, in exhange for a little extra noise that nobody else will ever notice.  You get detail and optical sharpness, which you can later trade off in post-production for even lower noise than your f/2.8 can provide.
 * Ultimately, you should probably rent these lenses and try both. The f/2.8 Nikkor (and many others mentioned) are quite expensive, and if you aren't sure you want it, you should spend the $50 or so to rent it for a weekend from a high quality photography store.  Nimur (talk) 13:58, 22 September 2013 (UTC)

Do batteries contain a clear, harmless liquid?
Today I was changing an AA battery, and it was stuck rather hard. I felt it give a little, but some colourless liquid suddenly squirted onto my hands. Terrified that I'd gotten acid on my hands, I immediately (within about 3-5 seconds) I ran to the sink and ran my hands under cold water for about 30 seconds. When I realized I wasn't feeling any pain or burning I stopped, and removed the battery while wearing gloves. What could that liquid have been? Pokajanje &#124; Talk 20:26, 21 September 2013 (UTC)


 * We don't give medical advice. You should see a doctor if there is evidence of a burn, but the liquid is more likely to be an alkaline than an acid.  You did the right thing by running your hands under cold water.  I've done exactly the same without any harmful effects.    D b f i r s   20:55, 21 September 2013 (UTC)
 * This is not medical advice. I'm asking what the liquid was. Pokajanje &#124; Talk  21:01, 21 September 2013 (UTC)
 * The electrolyte in a standard alkaline battery is potassium hydroxide. For KOH's material safety data sheet, see here.  Red Act (talk) 21:05, 21 September 2013 (UTC)
 * The liquid was a suspension of zinc in potassium hydroxide. 24.23.196.85 (talk) 21:07, 21 September 2013 (UTC)


 * If it was a cheap battery, then the liquid was probably Zinc chloride which is an irritant and can harm skin when in concentrated form (the leaks are often diluted).   D b f i r s   21:09, 21 September 2013 (UTC)


 * Our article on zinc-carbon battery says that they generally contain a "layer of NH4Cl or ZnCl2 aqueous paste". I don't think there's a guarantee about what it is though.  I doubt it was potassium hydroxide because when you get that on your fingers and wash they seem really slippery. Wnt (talk) 14:13, 22 September 2013 (UTC)
 * Your fingers feel slippery with potassium hydroxide because it saponifies the oils on your skin (I realise you probably know that Wnt, but I just wanted to add it for the OP and others). For some reason I have always really liked that fact. Equisetum (talk &#124; contributions) 17:29, 22 September 2013 (UTC)