Why is fast lens important?

I understand the reason behind having a fast lens, such as 2.8 will let in more light then 4.6. But what I am trying to understand is, everyone seems to go nuts about needing a faster lens, but don't people use a lower fstop? If so, then why would it matter to have a faster lens (higher fstop).

To have a greater DOF, you need a lower fstop, so why is it so important to have a 2.8 lens?
I know photographers who have a 4.5 lens, and they are not happy with it, they want a faster lens. How often does a photographer really shoot at 2.8?

I have two reasons.

1. Sometimes I deliberately want a shallow depth of field to isolate the subject from the background. It can turn a busy background in to a smooth, creamy out of focus blur that doesn't distract from the main subject.

2. Sometimes the ambient light level is so low that I need to use a large aperture, even though I might wish for more depth of field. I encounter this most when shooting school theatre, and sports in poorly lit gyms. I need action stopping shutter speeds that can only be achieved with high ISO and large aperture under those conditions.

Thanks Cod. I understand your two reasons. It's just overall it seems like for many photographers the lens they have never seem fast enough and they want more.

On average how much does a photographer really shoot at 2.8? (or shooting at their highest aperture) I know every photographer is different, depending on what they shoot.

If you don't shoot in low light (or use a tripod), and if you don't need large aperture as a DOF control, then you probably don't need a f/2.8 zoom. If I were only shooting landscapes at very small apertures I'd rather haul a smaller f/4.5-5.6 zoom than a big f/2.8 zoom.

I think it is true that many of us purchase fancy equipment we really don't need. That's just part of the fun. On the other hand I think that a large max aperture is a feature that most people will learn to use. When a friend asks me for advice about buying a DSLR I usually encourage them to buy whatever body (I think most are pretty good these days), but budget some money to trade out the kit lens for a f/2.8 lens. I think they will notice the fast lens, and find it more useful than more megapixels, faster fps, more AF points, etc....

I do shoot at f/4 or larger apertures (smaller f/#) for the majority of my wedding, portrait, and live music photography. Even most of my family snaps are probably shot in that range. I do use it for shallow DOF, but there are other ways to control that. Most important to me is that it lets me shoot in 1/2 to 1/4 the amount of light as a f/4-5.6 lens.

Most consumer zooms maintain constant aperture size as you adjust focal length. Since the formula for f/stop is focal length/aperture=f/# this means as the focal length increases, but the aperture remains the same size, the f/stop increases. You get f/4 at 28mm, but f/5.6 at 75mm. F/2.8 zooms maintain a constant f/stop as you adjust focal length. The size of the aperture changes along with the focal length so that the f/stop remains the same. If I set the lens to f/2.8 at 28mm it's still f/2.8 when I zoom out to 75mm.

It's popular advice that lenses tend to be sharper stopped down a bit. This isn't true for every lens, but in general it's accurate about the lenses I own. I used to say I was buying f/2.8 lenses to shoot at f/4, and if I had a f/4.5 lens I'd be using it closer to f/8. I don't have much experience with consumer zooms so I can't comment how sharp they are at max aperture, but the f/2.8 lenses I've used from Canon, Sigma, and Tamron are very sharp at f/2.8. They contend well with Canon prime lenses set at f/2.8, and by the advice above the primes ought to have the edge. Pixel peeping there is an improvement in sharpness even stopping down 1/3rd stop. By f/4 it's pretty much as good as it gets. The thing is that in prints and web display the difference is almost impossible to see. So I've learned that I should have no fear shooting at max aperture.

I've spent the last 6 or 7 years shooting only with primes and f/2.8 zooms. My new compact camera's regular lens is f/3.5 - 5.6, and it's driving me crazy! Being stuck with f/5.6 in a dim nightclub is tough when I'm used to f/2.8.

cod wrote: I have two reasons.

1. Sometimes I deliberately want a shallow depth of field to isolate the subject from the background. It can turn a busy background in to a smooth, creamy out of focus blur that doesn't distract from the main subject.

2. Sometimes the ambient light level is so low that I need to use a large aperture, even though I might wish for more depth of field. I encounter this most when shooting school theatre, and sports in poorly lit gyms. I need action stopping shutter speeds that can only be achieved with high ISO and large aperture under those conditions.

Add shooting wildlife at dawn, dusk, or in overcast weather and you've covered it.

Henry's right also. Kind of like jumping out of your Grand Cherokee into your Rav4 and wondering why the seats don't heat up on a frosty morn

One more thought. Larger aperture lenses tend to be the more "pro" lenses, meaning better build quality, weather sealing, and faster focusing, in addition to larger apertures. I used to use a good "consumer" 70-210 /4-5.6 lens. When I upgraded to a 70-200mm f/2.8 I was hoping to see an improvement in focus speed and accuracy in addition to having a larger aperture and I certainly did. Focus is much faster and the new lens tracks moving subjects better.

I shoot at f/2.8 all the time.

Scotty wrote: I shoot at f/2.8 all the time.

Why? on all your lenses? I understand having more light, but don't you ever want more of a DOF?

520snic wrote:

Scotty wrote: I shoot at f/2.8 all the time.

Why? on all your lenses? I understand having more light, but don't you ever want more of a DOF?

Of course, don't you ever want thinner depth of field?

Here's an example on when I wanted smooth bokeh, from thin DOF.

Here's one that helped me from shooting on a tripod, but able to shoot faster, without bumping out my ISO. Also helped with the vision of the shot.

Scotty wrote: Here's one that helped me from shooting on a tripod, but able to shoot faster, without bumping out my ISO. Also helped with the vision of the shot.

Great shot

Sometimes the larger aperture means the difference between getting a shot & not getting one...Plus the other reasons listed above. You will hear people say that the larger apertures result in "softer images"...But remember that sharpness isn't everything, there are times when it isn't what you want.

I guess the OP isn't the target market for a fast lens.... what do you shoot anyway? Landscapes only?

Another question to ask is why would you want to shoot with a massive DOF all the time? Don't you want blurred backgrounds sometimes?!

Sometimes you want the DOF control. Then a 2.8 or faster lens will let you have this control. You can always shoot a 2.8 lens at f/8 if you want. You can't shoot a f/4 lens at 2.8.

As for the question... yes I shoot at 2.8 all the time. I do events and I need the speed and I like the subject isolation.

Subject isolation is a major reason to shoot at fast apertures. After all, you can get a deep dof with any ol' camera.

A fast lens isn't important. The pictures that you wish to make are important. If a fast lens is the tool that will help you make the pictures that you want, then by all means use one. If not... fast glass may not be necessary.

Photographers who only make traditional landscape pictures may never need fast glass. I used a 4x5 view camera for close to 20 years and only on a rare occasion made an exposure wide open and I was using lenses with maximum apertures of f/5.6, f/9.0, and f/11! Most exposures with the 4x5 were made at f/16, f/22, or f/32.

One of my most important mentors, Galen Rowell, routinely used those "amateur" zooms like the Nikkor 24-50mm f/3.3--4.5, and even the cheap little 28-80 f/4.5-5.6 kit lens. He made some great pictures that enlarge well to 32"x48" prints with those lenses, because after testing them rigorously he discovered that they were very good indeed when used between f/8 and f/22. The reason he sought them out in the first place was because they were very compact and light in weight, and he took them rock climbing, trail running, and mountaineering. Of course, he also owned fast glass like an 80-200 f/2.8, a 35mm f/1.4, 85mm f/1.4, and 300mm f/2.8, but he VERY RARELY took any of those on self-propelled trips into the mountains.

It's just a matter of selecting the right tool for your photography.