What is a Fast Lens?
‘I keep hearing about ‘fast’ lenses and how great they are, but I’m not sure what they are and why I need one. Is it something to do with how fast its shutter speed can go or how fast it focuses? Can you shed some light on it?’ - Theresa
The speed of a lens and how ‘fast’ it is refers to the maximum aperture of the lens. The larger the maximum aperture the faster the lens is.
When a lens is talked about it generally is described with its focal length (for example it could e a 50mm lens or a 300mm lens) as well as its maximum aperture (usually it will be a number with an ‘f’ in front of it - for example f/1.8 or f/4 or f/5.6). The smaller the number is the bigger the maximum aperture is.
Remember that aperture is the size of the hole in the lens that lets light in when you hit the shutter. So the bigger the maximum aperture - the more light that your lens will allow in.
The reason that a lens with a big maximum aperture is referred to as fast is that it lets more light in and therefore you can use faster shutter speeds even when there might not be much light around.
Why would you want a fast lens?
Fast lenses can be advantageous over slower ones in certain shooting conditions and types of photography. They really come into their own where there is either low light (for example if you need to shoot indoors but can’t use a flash) or where you need to use a fast shutter speed (for example in sports or even wildlife photography). They are especially useful when you need both a fast shutter speed in low light (ie indoor sports).
In general - a fast lens is any lens with a maximum aperture of f/4 or more (ie f/2.8, f/1.8, f/1.4 etc). The lens pictured above is a Canon 50mm f/1.2 lens - very fast but also quite expensive.
Keep in mind also that because you’re using larger apertures that this has an impact upon the depth of field that you get in your shots. Larger apertures lead to shallower depth of field which can be a great thing if you’re wanting to make your subject really stand out from its background (more on this in our introduction to aperture tutorial).
Fast lenses can be really useful to have but unfortunately they can also be quite expensive to buy. However if you’d like a more economical fast lens you might like to check out 50mm lenses. For example both Nikon and Canon have some lovely fast lenses in this focal length.
Here are a few:
Canon
- Canon EF 50mm f/1.2 L USM Lens for Canon Digital SLR Cameras
- Canon EF 50mm f1.4 USM Medium Telephoto Lens for Canon SLR Cameras
- Canon EF 50mm f/1.8 II Camera Lens
Nikon
- Nikon 50mm f/1.2 Nikkor AI-S Manual Focus Lens for Nikon Digital SLR Cameras
- Nikon 50mm f/1.4D AF Nikkor Lens for Nikon Digital SLR Cameras
- Nikon 50mm f/1.8D AF Nikkor Lens for Nikon Digital SLR Cameras
Update: As Silverhalide mentions in comments below - a fast lens has the disadvantage of being larger in size. I’ll let them explain in their own words:
“A distinct disadvantage of fast lenses is their size — a 50mm f/1.8 needs a front element of at least 28mm (50/1.8); a 300mm f/1.8 needs a front element of 167mm (6-1/2″). A 6.5″ lens is a big, heavy piece of glass. This is the reason that fast lenses are so expensive. Lens manufacturers can use higher (optical) density glass or other tricks (Canon’s DO lenses) to reduce the size of the lens elements behind the first one, but the first element needs to be that big to capture the light.”
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April 18th, 2007 at 6:47 am
“is that it lets light in faster”
Wow. Fast lenses change the laws of physics? How’d they do that? special glass?
No. Fast lenses let more light in. More light means a shorter (faster) shutter speed is needed, thus “a fast lens”.
Another advantage of fast lenses that isn’t mentioned is depth of field. The larger the aperture, the shallower the depth of field. Shallow depth of field is useful for isolating a subject from the background (think portraits, athletes, birds — the subject is in sharp focus while the backgound is nicely blurred out of focus and free of distractions).
A distinct disadvantage of fast lenses is their size — a 50mm f/1.8 needs a front element of at least 28mm (50/1.8); a 300mm f/1.8 needs a front element of 167mm (6-1/2″). A 6.5″ lens is a big, heavy piece of glass. This is the reason that fast lenses are so expensive. Lens manufacturers can use higher (optical) density glass or other tricks (Canon’s DO lenses) to reduce the size of the lens elements behind the first one, but the first element needs to be that big to capture the light.
April 18th, 2007 at 8:42 am
Expensive, yes - but don’t forget there’s a ton of older, manual focus lenses that offer fast apertures, if you’re willing to meter and focus manually on your dSLR.
These older lenses can be a little soft when it comes to detail - but hold their own against the latest digital glass when it comes to low light conditions.
Not a bad trade-off for something a fraction of the cost of new lenses!
Lenses I use:
Nikkor 50mm f1.4 (£45)
Nikkor 105mm f2.5 (£60)
April 18th, 2007 at 9:59 am
They can be pricey but they are worth it. Once you get used to a fast lens (i.e. fixed aperture), it’s hard to go back to variable aperture lenses.
Lenses I use:
Canon 85mm f1.8
Canon 17-40 f4 L
I noticed that I am taking a lot more portrait shots after I got the 85mm lens!
April 18th, 2007 at 11:23 am
I’ve always thought the term “fast lens” to be quite a misnomer. I think this post does a lot to help clear that up.
April 18th, 2007 at 12:19 pm
Like Puplet said, older fast lenses are plentiful if you look in the right places. I’ve got my eye on a few f/1.4’s on eBay that might go for around $50. Once you hit the 1.2 mark, expect to pay the big bucks — no matter how old the lens is.
April 18th, 2007 at 2:12 pm
Can you tell me some lenses for Pentax?
April 18th, 2007 at 8:46 pm
‘very fast but also quite expensive’
how about:
very fast but also ‘very’ expensive . . .
April 18th, 2007 at 11:01 pm
If you haven’t got a fast lens and shoot indoors or in low light situations go get a lens now. You will be amazed at the difference. If you want to dip your toe into the water and have a Nikon camera check out the Nikkor 50mm f/1.8. You can find them for just over $100 brand new.
Nikkor 50mm f/1.8
Nikkor 85mm f/1.4
April 19th, 2007 at 12:20 am
Wire — there’s lots of great fast lenses for Pentax, and generally of very high quality. What are you looking for?
April 19th, 2007 at 12:26 am
i treated my self to the Canon EF 50mm f/1.2 L USM Lens for my 5D a few weeks ago and its hardly been off the body.
its a great lens to use, and gives fantastic results.
it is an expensive addition to my kit, but well worth the money in my oppinion.
April 19th, 2007 at 12:31 am
For those looking for a fast Canon lens that wont break your wallet, I must recommend the Canon prime 35mm f/2 wide lens. Its under 300$ at most places (I found it for about 250$ on Amazon) and it has been nothing but excellent. It takes super sharp images and with a f/2 max aperture I get some really nice DOF and nice looking bokeh. I am pretty new to photography but this lens has caused me to stick the kit lens in storage and keep this one on my cam at all times.
April 19th, 2007 at 12:39 am
I highly recommend renting an expensive lens before you buy. I was seriously considering an investment in the Canon 2.8 400mm and decided to rent it for a week to try it out. That’s a huge chunk of change, and I was glad I rented it first.
I took it out for a day-hike in the woods for wildlife photography and found it to be a beautiful lens. The downside (and what changed my mind about buying it) is that the thing weighs a ton. My arm is still sore. So by renting it first I saved some money. Maybe one day I’ll decide to buy it after all, but not now.
I’ll plug the place I rented it from because they had great service: http://www.lensrentals.com/
April 19th, 2007 at 7:59 am
I’m lookig for a big aperture, not very expensive, but useful.
April 19th, 2007 at 1:26 pm
I own a Canon 50mm f/1.8 and it’s my primary lens. This lens almost never leaves my Canon 10D. It’s an all rounder and worth every penny I invested. I prefer natural lighting over using flash so a fast lens does the trick. With it’s ability to handle DOF, it’s really good for potraits, wildlife and sports photography.
April 20th, 2007 at 1:14 am
I just got the Sony/Minolta (Sonolta? Minony?) 50mm f/1.4 prime lens for my (also new) Sony Alpha 100 (replacing a KM Maxxum 5D). Check back in a week or two…
April 20th, 2007 at 10:50 pm
I have loved photography for a long and have just recently gotten a little more serious with it, I think I have a good eye but I am still have a hard time when it comes to all these different lenses, I have a Canon Rebel Rebel XTi with the Canon 17-85MM I do a lot of sport photos (my son races motorcycles) and I would really like a little better lens for this, but I also like to take nature photos and want to get into some portrait photography as well.
Are there any good informative sites, etc to find more info on just lens?
April 21st, 2007 at 10:07 pm
I recently hired a fast telephoto for my Nikon D100 as I was shooting an indoor conference with a lot of action going on. I already own a 50mm f1:4 which is awesome but requires me to be a lot closer than I need to with the tele.
I don’t know how I’m going to manage without the tele now… or how I ever managed to do it before with my old lenses… It made a massive difference.
I’m shooting a lot of events so the hire place will be seeing a bit more of me until I’ve enough dosh to get a fast tele of my own…
April 22nd, 2007 at 4:04 pm
Wire, the Pentax shooter,
Start at the B&H site, look at the fast ones, then go to the used part of the B&H site. Check KEH, and your local pro store’s site. Look for a while before you leap.
I do not shoot Pentax, but this is what I do. Research, and research some more.
April 26th, 2007 at 3:56 am
Personally, I think ‘fast’ is a misnomer.
It’s a ‘bright’ lens.
May 3rd, 2007 at 6:58 am
I use my Canon f1.4 50mm on the 30D pretty much all the time.. lovely lens, good fun DOF… Thanks, Steve for the point on the weight of the 400mm… i’m really wanting the 100mm-400mm.. will go rent one for a shoot and see how my bicep is afterwards!
Sime.
May 30th, 2007 at 6:20 am
I have a triumphirate of Canon f/1.8 lenses for use on my 1.6x crop-factor Canon 20D:
28mm f/1.8 (45mm equivalent, or ‘normal’ view)
50mm f/1.8 (80mm equivalent, or ‘portrait’ view)
85mm f/1.8 (136mm equivalent, also useful for portrait work)
These are all useful for available light work, and of course, for those situations where you want a shallow depth of field, but I also find them useful for flash work, as well.
Consider that you gain an effective 2-3 stops of flash range over the more typical zoom lenses. This equates to stretching your flash range out quite a bit, as well as helping to avoid that ‘dark cave’ look where the background brightness falls off sharply. Coupled with the Canon DSLR’s ability to take practically noise-free shots at ISO 400, 800, or even 1600 with some of the latest bodies, you have a really flexible setup for natural light or flash situations.
Try shooting in Manual mode, or at least Av (Aperture preferred) and experiment with the camera and flash settings to see how you can balance the background room light with the lighted foreground subject.
When you want to understand exactly how the Canon EOS flash system interacts with your flash and camera settings, check out:
http://photonotes.org/articles/eos-flash/
July 19th, 2007 at 4:31 am
I just bought a 20mm f1.8 Sigma lens for my Canon 20D, I already knew this but Ill say it anyway, I shoot mostly nightlife photography, and with a 3.5-5.6 (zoom) you would probably need to bump your ISO setting to at least 400-800, or in extreme cases 1600, just to get a shot.
1.8 does a good job of bluring out the background, while letting an extensive ammount of light in, so you can shoot at a faster shutter speed, and acheive the same level of brightness, etc, as you would on a 3.5 or 5.6 F. Plus its sharper too because theres less blur, and less grain.
-Hope that helps someone out there.
-Eric