Good low light lens?

[Xaqtly]

I'm putting on my photography noob hat for this one - what is a good lens for taking pictures in low light, i.e. no flash in darkened rooms or areas? Assuming we're talking about moving subjects, otherwise I could just do a long exposure. I understand the body has a lot to do with it too, but just for the sake of this conversation, what type of lens would work best? Are we talking about a real low f stop like 1.0? Are there any other factors lens-wise that help in low light, or is it really just that plus real high ISO and a longer exposure?

I'm just asking here because of all the experts we have up in here. :lol:

[SpecMode]

The lower the f-stop, the better. Generally, this makes primes the best low-light lenses, since they tend to be faster than zooms for a given focal length. That, and having a body with a sensor that handles high-ISO shots well.

[PB PM]

Depends, the lower the F-stop the shallower the depth of field, so if you need a lot in focus, having that wont help.

When you say low light, how low? Are we talking about standard house lighting, on the streets at night? Shooting action in the dark (streets) without a flash is not going to happen, unless you crank up the ISO (6400+) even at say F1.8.

[Dorian Gray]

As you know, the f-stop determines the brightness of the image projected on the sensor. The common f-stop scale goes like this:

f/32 - f/22 - f/16 - f/11 - f/8 - f/5.6 - f/4 - f/2.8 - f/2 - f/1.4 - f/1

Each step in this progression (sometimes called a "stop"), from left to right, represents twice the brightness. An f/1.4 lens is twice as bright as an f/2 lens. An f/2 lens is twice as bright as an f/2.8 lens. An f/1.4 lens is 16x brighter than an f/5.6 lens.

So low f-stops let in lots of light, but as PB PM said, the depth of field (the area in sharp focus) falls as the f-stop falls. An f/1.4 lens has very little in focus: if you focus on a dog's eye, its nose may be out of focus. With an f/5.6 lens, both may be in focus at the same time. If you want both in focus, it's therefore not appropriate to use the "faster" lens (a lens with a lower f-stop, which allows "faster" (shorter) shutter speeds).

Worse, since so little is in focus at f/1.4, focus accuracy is critical. That's especially a problem when shooting moving subjects, which don't give you or the camera much time to focus. The camera may focus on the dog's nose, leaving the eyes out of focus.

Optical image stabilisation allows longer shutter speeds than would be otherwise achievable with a handheld camera; but with a moving subject that may be of little or no use, since subject movement may limit the shutter speed to one that can be handheld without image stabilisation anyway.

Another important factor in low light is the focal length. Shorter focal lengths (e.g. 24 mm instead of 50 mm) allow longer shutter speeds to be handheld. They don't magnify one's natural hand shake as much as longer focal lengths.

Taking all of this into account, you might enjoy something like Canon's 17-55 mm f/2.8 IS. It has a range of focal lengths that are suitable for indoor work, image stabilisation for when subjects are fairly stationary, fast autofocus, and a bright maximum aperture of f/2.8: not as fast as many primes, but fast enough to let in plenty of light, without being so fast that focus and depth of field become truly critical concerns.

[GSpotter]

Also consider the cost: With each aperture stop, a lens gets much more expensive. The cheapest fast lenses are the 50mm lenses. The more you get to the extreme focal lengths, the more expensive it gets:
A 50mm/1.8 is typically a rather cheap lens (~100€), 50mm/1.4 is more than 3 times as expensive. And the 50mm 1.2 is about 15 times as expensive!
Same with the 35mm (f/2.0 vs. f/1.4): For one more f/stop, you pay 5 times as much money...

If you want to try out a fast lens, I therefore would suggest to get the 50mm/1.8.

[Mac+]

This is all good advice, but one thing that hasn't been mentioned, and which you'll discover with a faster lens, is the quality of the background blur, or bokeh. With a faster lens and shallow depth of field, the "dreaminess" of the out of focus highlights in the background will become more apparent too. (This is influenced by the amount of blades on the aperture - more provide a better rounded blur, as opposed to a pentagram type shape.)

Also, what are you shooting in low light? Is it primarily portrait shots in a small room or sport shots in an indoor stadium, for example?

A 50mm prime is a great starting point, but if this is not wide enough on a 1.6 crop you might find it frustrating.

[dmegatool]

I got a 50mm and I found it great to play with. I wish I had the 35mm but I'm not frustrated enough to justify the 150$ difference.

[GSpotter]

@Mac+: Good points. Although the amount of blades only matters when you close the aperture. Wide open, you don't see them.

BTW: Here's a comparison of the effects of different apertures on bokeh and depth of field (taken with a 85mm/1.4 lens):
http://farm3.static.flickr.com/2407/2494242974_8e5b1f0a43_o.jpg (http://www.flickr.com/photos/iphotograph/2494242974/)
Only from f/8 on, the depth of field is large enough to get the whole pillar sharp.

[Mac+]

Man, I've get to get myself a digital camera!

GSpotter - Cheers and thanks for posting those examples. Note that you can see the shape of the out of focus highlights even at f/1.4 on the iPhone. My point was if you want nicely rounded oof highlights, consider the aperture blade count.

Christmas lights and night time photography can really accentuate this effect.

Regardless of blade count (and quality of glass as another factor), a 50mm prime will be a nice step up from the kit lens. :)

[dmegatool]

Man, I've get to get myself a digital camera!
D80 in the bazaar ;)

[Mac+]

D80 in the bazaar ;):lol: A good FF Canon camera.

[Xaqtly]

Thanks y'all. I do have an 85mm f/1.8 prime, and as you guys mentioned, the biggest problem at 1.8 is the depth of field if you're trying to keep multiple things in focus. As far as what I'd be shooting, think live show in a dimly lit bar or something, and crowd shots of people dancing.

Seems like a really difficult thing to shoot if you don't use external lighting of some kind. So yeah I mean 6400 ISO and hold it at 1.8, then just experiment with shutter speed and use a tripod. But if there's any motion, I mean we're talking at least 1/20 speed to get anything better than a blurry mess, right?

[PB PM]

For moving subjects, 1/125 or more if possible. At ISO6400 and F1.8 I think you could get at least that, if not better, you might even be able to stop down to F2.8, which is where a F.8 lens will be starting to get sharp anyway.