A great question and one that is asked by almost all photographers at some early point in their photography career.
F/2.8 has become a standard for almost all professional lenses,
OK first question, what is F/2.8.
First thing to notice is that it is an F with a Devided sign / then 2.8
the f stands for focal length. It should not be written as f2.8.
This video will help explain what the "F" I am talking about!
The best reply to the video I got was this one
The amount of light that comes in is directly related to the area of the aperture disk. So if you want to let in half as much light as with an f/1 aperture, you have to divide the area of the disk by 2, which means that the diameter has to be divided by the square root of 2=1.414... rounded to 1.4. You can think about that as 'steps' (which might have been misspelled to 'stops' at some point in time, but I'm not sure about that one) where the amount of light is divided by 2 each time.
cedricdodo 4 months ago
more details after the break
Tough maths if like me you suck at maths.
Now when a lens says it is f/2.8 and it is a zoom lens, it may call it self a constant aperture lens. This means that as you zoom the aperture size will not change. This makes making settings on your camera nice and easy if you are changing your focal distance.
Cheaper lenses such as your 18-55 kit lens will say something along the lines of f/3.5-5.6 which means at the shortest focal distance it will have a bigger apertuer size (f/3.5) but when you zoom into 55mm the aperture will have effectively become smaller f/5.6
You can obviously change the aperture to a smaller size at any time, but remember a smaller aperture is a larger number ie f8 or f16 because remember you need to have the "/" symbol.
OK so what is the difference between f/3.5 and f/5.6?
About a stop.
Ok smart guy what is a stop?
A stop can be measured in a couple of different ways, it is usually the doubling (or halving) of a setting.
for example in shutterspeed terms one stop is going from 50th of a second to 100th of a second and a second stop would be 100th to a 200th of a second photo.
or another way to look at it is by saying iso 800 to make it one stop brighter i would have to go to iso 1600, and one stop more than that would be iso 3200.
Aperture works from f2.8 then the first stop is f/4
f/2.8- f22 is 6 stops
six stops is 64 tims less light
or here is a good one
how do you get over 4billion (BILLION!) times more light into your camera?
change the settings from 1/8000th, iso 100, f/22
30second exposure, iso 6400 f1.4
4 billion sounds so much cooler than saying "a total of 32 stops difference"
The next thing is that f/2.8 can give you a very nice blurry background (called Bokeh)
The larger the aperture the more shallow the depth of field,
in simple terms
big hole = really mushy background
The image above is with a Nikon 50mm f/1.4 at f/1.4 (remember f/1.4 is now a massive 2 stops bigger comapred to a f/2.8) this makes anything out of focus get really out of focus.
A macro lens lets you get as close as possible to a subject and in doing that gives you a super duper shallow depth of field which can be great for taking pictures of bugs to really help separate them from their background.
The shorter the focal length (ie 10-20mm) the depth of field is increased (more is in focus) than compared to longer focal lengths.
If you have a wide angle lens such as the Tokina 11-16mm, even at f2.8 because it is such a short focal length, a lot more will be in focus compared to say a 100mm f2.8
Tokina and was at f/4, making most of the subject in focus but only the furthest background ever so slightly blurry.
For landscape pictures if you are using a wide angle lens you will not need to stop down more than f/4 to get everything infocus unless you have something very close to the camera that you want tac sharp. the only reason to go below f/4 is if your lens becomes sharper at the smaller apertures
ok ok so you get the physics of it all but again the question still stands, whats the big deal about f/2.8???
Sharpness: with any lens there is what is called a sweet spot for the lens, and that is to do with how it performs on sharpness tests and it is usually 2-3 stops down ( aperture stops) from the widest that the lens performs its best (sharpness, but also in other terms such as Chromatic aberration, vignetting, and contrast)
So if you are able to get your lens to be super mega duper sharp at f4 or f5.6 that is a lot better than f8 or f11.
Why not f2.7 or f/2.5 or f1.7
well it seems like there is no reason for that and there are lenses that do have those apertures.
Nikon do a bunch at f2 (nikon 35mm f2 and also a 200mm f2 as does canon)
Minolta did a 50mm f1.7
Voigtlander did a 75mm f/2.5
Pentax have a 35mm f2.4
and the list could go on but I cant be bothered.
But the big thing here is that the big name brands like Sonicanikony have worked hard at making the glass that they buy for their lenses to be as good as they can be and they have landed on a simple maths term of the length of the lens / the aperture size making it 4time as small as the length, this makes the glas a little easier to make and have less issues such as distortion and vignetting they may have got from trying to be brighter at the same price. (thats not to say they have'nt gone brighter as you will see a Nikon 200mm f/2 but the price is shitloads! around 5k compared to a 70-200 f2.8 which is around 1.5k big difference!)
The big companies have made it that an f/2.8 is a standard of excellence. They have put in the effort and the finance to make the f/2.8 lenses to be as sharp as they can be and to perform as well as they can on the whole host of image quality criteria