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What makes a lens fast?

Discussion in 'Lens Matters' started by Markdb, Feb 20, 2012.

  1. Markdb

    Markdb Active Member

    Might be a daft question but I'm new to dSLR's so forgive me. I've seen lens's referred to as fast but what does that mean?

    The reason I'm asking is that I took my camera to one of my daughters dance comps yesterday and I only have the standard canon 18-55mm kit lens that came with the camera. Not used to taking pics of people, and I tried various settings but still found it difficult to get decent photos. The lighting wasn't brilliant in the place and I was also not that close to the dancers. I'm thinking of buying a lens that would be up to the job but I've no idea of what to look for. Need something that can zoom in quite a bit and freeze the motion. I can post up some shots of what I came home with if it helps?

  2. NosamLuap

    NosamLuap Rebmem Roines

    A fast lens is one with a wide aperture. The aperture on your kit lens is probably f/5.6 when zoomed in to 55mm. a "fast" 50mm lens might have a maximum aperture of f/1.8 or f/1.4. F/1.8 is just over three stops faster than f/5.6, which means for a given shutter speed it will let in over three times as much light (or, more usefully, it will let you use a shutter speed that's three times faster to avoid camera shake)

    A tripod or monopod could also help. Also, subject to the nose handling of your camera you may be able to increase the ISO further, which makes the sensor more sensitive to light.

    If this is gibberish to you, then I can recommend the book "understanding exposure" by Bryan Peterson- it's a very good introduction to the technical side of photography and will help you understand what it is about your images that isn't working.
  3. AlecM

    AlecM MiniMe

    Mine manages quite well, despite my Depardieu tendencies...!

    Paul's absolutely right in that lens 'speed' is determined by how much light it lets through - he's said it all, actually!
    The one thing missing is the use of flash to boost the amount of light you have to work with - HOWEVER, no doubt the dance people are similar to my daughter's rhythmic gym group in that they take a very dim view of the use of flash.

    (see what I did there....!?!?:D)
  4. TimF

    TimF With as stony a stare as ever Lord Reith could hav

    As Cartier-Bresson apparently said, using flash during a theatre performance is like firing off a pistol in the stalls!

    In addition to what Paul has said so well there are 50mm lenses out there with speeds as high as f/0.95 which is over five stops faster than your kit zoom at its long end. It is of course worth mentioning that an ultra-wide aperture can have a big effect on depth of field. As in most things it's a question of trade-offs.
    Last edited: Feb 20, 2012
  5. NosamLuap

    NosamLuap Rebmem Roines

    Doh! My phone thinks it knows what I want to type better than I do!
  6. Learning

    Learning Ethelred the Ill-Named

    I agree with Cartier-Bresson. I would go further. The use of flash at a gig, in a theatre, in a pub or restaurant should be a flogging offence; hanging would be going a bit too far.
  7. NosamLuap

    NosamLuap Rebmem Roines

    Hmmm - the rest I agree with, but a pub? I would happily use a flash in a pub (I'd happily use flash in many restaurants to be honest, but only a relaxed one...)
  8. Learning

    Learning Ethelred the Ill-Named

    Do you count McDonald's, Pizza Hut, and Burger King as restaurants? I wouldn't mind you using flash there. I wouldn't do so myself because of the same reason I wouldn't mind you doing so.
    Back to the subject. At modest focal lengths anything wider or equal to f1.4 is fast. By 400mm anything wider or equal to f4.0 is fast. Those f0.95 primes are very fast and are of limited use, very good for purpose but a waste of money for anything else.
    Last edited: Feb 20, 2012
  9. pilliwinks

    pilliwinks Well-Known Member

    As a pedant, and bearing in mind that the original poster is new to photography, I'll just offer a small correction. Going from f/5.6 to f/2 does let in more light, but each extra stop doubles or halves (depending on which way you're going) so 3 stops means 8 times as much light, and a factor or 8 (not 3) to be applied to the exposure time. It's still only three click stops round the shutter speed dial, but if you camera doesn't have one, that number doesn't help. The 8 times does.
  10. NosamLuap

    NosamLuap Rebmem Roines

    Lol! No, I don't. I was thinking of my local tapas bar or Mexican restaurant.

    I tend to leave the camera at home when we're at Michelin star joints ;-)
  11. NosamLuap

    NosamLuap Rebmem Roines

    Whoops - sorry, you're right. To busy feeding baby as I typed...

    (and being an even bigger pedant - I think on most Nikons at least, the shutter speed dial "clicks" in 1/3rd or (optionally) 1/2 stop increments. ;) )
  12. GeoffR

    GeoffR Well-Known Member

    Well yes, there are lenses as fast as f0.95 but they aren't cheap! A Leica 50 f0.95 well set you back the trifling sum of £8,000, give or take a bit. A Cosina Nokton 17.5 f0.95, in 4/3 fit, is a snip at £1,000 and the Noktor 50 f0.95 is a gift at around £700.

    Canon produced one but I can't find a price on that one, the fastest Nikon I could find any information on was only f1.2, Olympus made a similar lens for the OM system a 55 f1.2, neither is exactly cheap and none are AF.

    For all practical purposes the fastest readily available lenses are f1.4, 50mm optics aside, even these aren't cheap a Nikon or Canon 35 f1.4 is comfortably over £1,000. A 50 f1.4 is pretty affordable by comparison at around the £300 mark for either Canon or Nikon.
  13. Markdb

    Markdb Active Member

    Thanks for all the replies, makes perfect sense.

    You're right, the event organisers were pulling their faces when people were using flash/ lol

    So I'm looking for a prime lens with a max aperture of f/1.8. Just had a look on the net and Canon do a 50mm f/1.8 which seems pretty cheap, (EF 50mm f/1.8II), would this be acceptable for what I need?

  14. IanJTurner

    IanJTurner Well-Known Member

    I've got one - and I'll tell anyone who'll listen how great it is! But you mention you wanted a bit more zoom than your 18-55, so I'm not sure it's what you're after.
  15. PhilW

    PhilW Well-Known Member

    It's a fabulous lens - every canon user should have one :)
  16. Snorri

    Snorri Well-Known Member

    Canon EF50mm f/1.8 II
    Although this one is not a zoom lens it might be worth considering. It is very sharp and retentively fast and the price is only about £80-90.
    Cant go wrong really...
  17. Markdb

    Markdb Active Member

    Thanks for that, it seems very cheap for a canon lens at around £80 compared to others I've been looking at.
  18. Snorri

    Snorri Well-Known Member

    It is, a real bargain. Well if you put it next to the 50mm 1.4 you will see the difference in built quality it is very light plastic and the missing USB motor. But that said you will not see that much difference in optical quality but you will definitely see it in the price.
  19. Bawbee

    Bawbee Well-Known Member

    One of the 'fastest' lenses that I ever owned was a Canon EF 300mm f/2.8L - aye, when it dropped off my tripod and fell about 5' and dropped down ~12 concrete steps; even though it was in slow-motion at the time :eek:
  20. Snorri

    Snorri Well-Known Member

    Correct me if I am wrong but, Markdb you were thinking of macro for you tank some time ago? If that was the case you could get even better use of the Canon EF50mm f/1.8 II by putting extension tube on it and use it for macro work as well.

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