Check out above picture, using 30mm f/1.4 shoot wide open at 1/40 sec with ISO speed of 250. I was able to freeze the motion and shoot the picture hand held. I just love fast lens! Just as much I love fast bike!
Lens speed is related with the maximum aperture opening or minimum f number of a particular lens. With wider aperture fast lens delivers more light intensity thus allowing faster shutter speed. Hence the name fast lens. Lens with aperture f number of 2.8 and lower is considered a fast lens.
Fast lens come in both zoom and fix or prime lens. But zoom lens, lens that can change its focal length, have “speed limit” at f/2.8. I don’t think there are zoom lens that have aperture of bellow f/2.8. That’s why I love fix or prime lens, it can go bellow – way bellow – the speed limit of f/2.8. My all time favorite fast lens are the Sigma 30mm f/1.4 and the Canon EF 50mm f/1.4. Some prime lens can go low even further, like the Canon “L” series EF 85mm f/1.2 or the ultra expensive Leica 50mm f/0.95 Noctilux-M ASPH. Last time I checked you can buy 3 brand new Ninja 250R with the price of that Leica lens.
What are the benefits of fast lens? For me are the following:
1. Low light shooting
As in the top picture, with fast lens low light shooting is much-much easier. In low light you can use flash, but flash some time make the picture unnatural, especially if you want to capture the ambiance lighting. With fast lens coupled with today high ISO capability of current digital camera I can almost shoot in any lighting condition available.
Check the above picture. It was an evening at the hospital where my wife was working, I shoot it with ISO 400 hand held wide open at f/1.4. No flash. Sharp and capture the moment with out the need of tripod and long exposure. The above was taken with the Sigma 30mm f/1.4.
2. Object isolation
With wide aperture you will have shallow depth of filed, meaning that using a fast lens you can isolate the object from the rest easily. With the object in focus, the rest will be blur. In some condition the blur will be exaggerated so use it wisely.
Above with Sigma 30mm f/1.4 and bellow with Canon EF 50mm f/1.4
3. Nice back ground blur
With fast lens you can have a nice background for the object. One of the reason why fast lens is a preferred lens for a portrait shoot. Some of fast lens also have beautiful “bokeh” or the quality of the out of focus area of the picture. Google “bokeh” if you want to know more. The Sigma 30mm for me personally have better bokeh then the Canon EF50mm.
Above portrait of my son with Canon EF 50mm/f1.4. Loves the background of the above picture. Bellow with the Sigma 30mm/f.4. Pretty cool bokeh.
So if you want the above quality, fast prime lens is the way to go. But prime lens is not as convenience as zoom lens. For framing zoom lens is much-much easier, if you want the object to be closer you zoom in and zoom out if you want wider angle. For prime or fix lens you are zooming by moving closer or away from the object. More physical.. :)
But nothing beat fast prime lens in term of picture quality even though I need to move around more for taking picture.