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What makes a lens a "fast" lens?
This term is often used, and can be confusing to new photographers. It’s important to bear in mind that this is an old term and has been in use for many years, specifically, before the digital photography and electronic camera age.
In photography terms like this are often used inappropriately, by mistake, or, simply overlapped with similar terms.
In the context of this subject I will give a quick overview of what it actually means.
Remember: When referring to fast lenses, photographers (including myself) use terms like "wide aperture" or "larger aperture", meaning the same thing. So, in this context "wide or wider", refers to the aperture, not the focal length of the lens.
Sony 85mm f1.4GM is typical of a modern "fast lens"
Simple Answer:
To give a simple answer to the question; In the context of both modern professional and consumer camera lenses, “Fast” means that the lens allows more light to fall on the sensor “faster”, or, in a shorter length of time than a slower lens.
This is achieved by having a larger (wider) aperture. So, in the case of prime lenses; f1.2, f1.4, f1.8 would be considered “Fast” with f1.2 being the fastest in this case. With Zooms f2.8 is considered as “Fast”.
This can be very useful in low light situations.
Newbie Note: The term "fast" only applies if the lens is used at these wider apertures. So if we were comparing 2 lenses and one was an f1.4 and one was an f3.5. Clearly the f1.4 would be faster, but only until we use the lenses at f3.5, then they are the same speed, all the way down the f stop range to f22. So fast lenses are only "fast" at the top end of the apertures. So if we use the Ferrari/Mini metaphor, you don't drive your Ferrari through town a maximum speed. It may be faster than the Mini, but only when you are on a racetrack. In town the Mini is just as good as the Ferrari. Continuing the same Metaphor. The Ferrari is bigger and heavier than the Mini too, and the same can be said of fast lenses. They are generally larger and heavier than slower lenses, that are generally smaller and lighter.
“Fast” means that the lens allows more light to fall on the sensor “faster”, or, in a shorter length of time.
Example Lens Apertures
More Information:
The bullet points below have a couple of aide memoires that you may find useful
- Remember: The smaller the f number, the wider the aperture, or to use the vernacular “the smaller the number, the bigger the hole!”.
I get it... To new photographers this can be confusing, even irritating, however, it's worth learning it by wrote. It will never change, so get on board. It becomes second nature to think like this after a while. - As a general rule of thumb, you can reckon that “fast” lenses are generally larger and heavier than "slower" lenses as they have more glass. This may be a consideration if you are travelling, or hiking a lot with kit.
What “Fast” doesn't mean?:
When talking about lenses for 35mm full frame cameras generally, “fast lens" does not refer to the auto focus, or focus pulling, or the speed of the lens' motors.
On mirrorless/DSLR lenses the shutter is in the camera body, not the lens. unlike medium format cameras that can have the shutter in the lens. These shutters are called leaf shutters. In the case of leaf shutters "fast lens" does not mean the fastest shutter speed on the lens either. The term "Fast" is the same whether referring to cameras with focal plane shutters (in the body) or Leaf Shutters (in the lens).
The term "fast" lens was in common use, long before auto focus was developed. Of course, you may hear people describing the auto focus as fast, and this can be confusing when you are new to the terminology. Sometimes people who are offering help to others simply don't understand this term themselves. Clearly, a replacement lens that has faster auto focus is faster than the previous generation lens, but, in this context, if it has the same aperture, it is not a “faster lens”. It’s best described a lens with a “faster auto focus system”. The speed of the lens remains the same as the widest (largest) aperture remains the same.
As with all aspects of photography there is a balance to be struck when using "fast lenses" and selecting a wide aperture.
When should we use the lens wide open?
It's a common error to think that as you have bought and paid for a fast lens that you should only use it wide open at the widest (largest aperture). This of course is total nonsense. Unfortunately the thinking prevails online. Using lenses at their widest aperture pushes the auto focus to its limits. I would recommend that judicial use of the widest aperture is used at all times. Lenses usually have a "sweet spot" for both focus and acutance it is rarely wide open. However when used on portraits it can give a pleasing look to the face. You can see this below. Shot at f1.4 the shot below has only one eye in focus.
very shallow depth of field - only the leading eye is in focus
Shots like the one above are rarely snapped. They are usually well lit. In this case above there are three light sources. (The sun, a Metz flash with soft box and a reflector.)
Used in combination, these control the contrast and give a pleasing look to the skin. When a wide aperture is used in combination with this type of lighting it can be very effective, and just as important, it becomes technically practical.
Of course, depth of field is subject to other variables, including lens focal length and lens to subject distance, but when you have chosen what to frame and you have the shot composed and lined up, and you are ready to push the exposure button, the only thing that affects the depth of field is the aperture.
shot straight on to allow a wider aperture and increased bokeh in the background
When shouldn't we use the lens wide open?
A common mistake is to shoot wide open when shooting a group. New photographers often do this in an attempt to have the group separated from the background. This often leads to some people in the group looking soft, or in extreme cases being completely out of focus, mainly due to people standing behind others, requiring the depth of field to be deeper. Remember the autofocus will pick a single point and set the point of focus to that point. There are many different focus modes and settings on a modern camera but none of them can change this happening. Focus modes do not control depth of field. They can simply increase the chances of it being focussed at the correct point.
The depth of focus (depth of field) is controlled exclusively by the lens aperture, not the auto focus settings on the camera.
Other subjects like landscape and architecture often look better when the lens is stopped down to a smaller aperture, thus increasing the depth of field.
And finally... Let's discuss Bokeh
What is Bokeh? Bokeh is the quality of the softer (out of focus" areas of an image, specifically the highlight areas, that often look like balls of light, sometimes called bokeh balls. These are created by the use of a wide aperture and by the physical aperture blades themselves. Specialist portrait lenses are designed with more blades to give that circular look to the bokeh effect.
Bokeh, bokeh, bokeh... You'd think it was the only important aspect of a beautiful image.
In recent years there seems to be an obsession with bokeh amongst hobbyist and professionals alike. The phrase came in to common usage during 1997. Before that every photographer was focussing on what was sharp, not the quality of what is soft.
I feel a little restraint with this obsession may go a long way to putting it into perspective. Bokeh is only one characteristic of a good portrait lens. Rendering skins tones and sharpness at a wider aperture are significantly more important features, in my opinion. In over 35 years of shooting no one of consequence has ever remarked on the quality of the bokeh in my images. I'm not even sure that any of my clients or their customers would even know what it means.
Lenses that render beautiful bokeh are almost invariably expensive. No one needs to buy these lenses. They are a nice to have item of kit. If you do not have the budget to buy such a lens my sincere advice is to worry about something else. Other, more cost effective lenses still produce lovely bokeh for a fraction of the cost. Go for one of those lenses and enjoy your photography. If you are lucky enough to make money from your photography buy the best you can get. I have the Sony 85mm f1.4 and it is wonderful, however, other photographers are producing work that is equal to, and/or better than my work using the f1.8 budget version of the lens. One needs to be Socratic about this. After a certain level, the kit becomes moot and the talent become everything. 
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