Ok so i have been asked to get back to the basics of photography and will be starting off with the F-Stop. Il leave the topic on for a week so that people can get to read up on it and ask questions that they might have.Were will one find a F stop on a lens?
Simple here ; What is the meaning of F-Stops
F-stops are a bit confusing because the numbers appear as decimals. Lets take for instance the following line :
F1.4 | F2.0 | F2.8 | F4 | F5.6 | F8 | F11 | F16 | F22 and so forth...
F-1.4 lets in the most light through the lens while F-22 setting lets in the least.
Just by looking at the numbers above going from from F1.4 to a F2.0 doesn't seem that one is taking the light and halving it and What's more, F2 is a larger number than F1.4 and sounds like it should have more light, not less. Neither does f/4 to f/2.8 sound like doubling the amount of light. In fact, each of the numbers in this sequence is a halving/doubling of the amount of light from its immediate neighbours, just like the shutter speed settings are. For example:
The reason that both the halving/doubling and the smaller numbers mean more light things make sense is that the f/stop is a ratio. The ratio is between the diameter of the aperture in the lens and the focal length of the lens. The focal length is generally measured in millimeters, so we'll stick with those as our unit of measure. On a 50mm lens, f/2 is saying that the diameter of the aperture is 25mm. The ratio is: 50/25 = 2. That seems pretty straightforward.
So what makes a lens fast?
Lenses are referred to by their maximum aperture (that's the biggest hole, the smaller number).
Lets take a lens for example a "x 50mm " lenses and we will just change the F-Stop
50mm F1.4 | 50mm F2.0 | 50mm F2.8 | 50mm F3.5
All four of these lenses have a f/4all the way up to f/16. These are distinguished by the maximum amount of light they could let in. The 50mm F3.5 when set to it's maximum aperture of F3.5 lets in one third less light that the 50 F2.8 and the 50 F2.8 at its maximum aperture, lets in only half the light of the 50 F2.0 at its maximum aperture and so forth. Lenses which have wide maximum apertures and let in lots of light are called fast lenses. Lenses which let in comparatively less light at their maximum apertures are called slow lenses. The 50 F1.4 is a very fast and the 50 F3.5 would be kind of slow. Would you always use a fast lens ?
The answer is No. The reason being is Weight and the actual cost of these fast lenses. To get those larger diameter apertures means you need larger pieces of glass which means larger lenses. These lenses are alot heavier and and therefore become harder to auto focus.
The size is also really obvious in the long lenses. The weight balloons and the cost skyrockets. Lets look at the canon 400 fixed lenses
The canon 400 F2.8 : Dimensions 6.4 x 13.7" and weight is 5.35kgs costing R+-70k
The canon 400 F5.6 : Diemesions 3.5 x 10.1" and the weight is 1.25 kgs costing R+- 11k
You also see the size in the zoom lenses. Most modern consumer zooms are handy, light and slow. They are also have a variable maximum f/stop across their zoom range. The professional level lenses tend to be fixed F- stop.
Even on shorter lenses the difference is noticeable 50 f/1.2 is much heavier than 50 f/1.8. The viewfinder is bright in the f1.2 and that last fraction of a stop can be handy sometimes, but the camera weighs a lot on the neckstrap and you start to question its value if you're shooting at f/11 anyway.
My Zoom lens says it's 24-85mm F2.8 to 4 What's that mean?
This relates to the size versus lens speed issue. Lets take a lens 24-85 f/2.8-4 zoom lens, as an example. It's not exactly small and light, but it tries. If you think about it, the size of the elements needed to get f/2.8 at 24mm is much smaller than the size needed to get f/2.8 at 85mm. To keep lens sizes and costs down, the lens manufacturer accepts that the lens wil be faster at the wide end of its focal length range and slower at the long end. As a result, you can open up to f/2.8 at the 24mm end of the range, but only to f/4 at the 85mm end. This is particularly noticeable in small lenses that do, say, 70-300mm where at the 300mm end the maximum f/stop is f/5.6 or worse. It also shows up in the compact point and shoot superzooms, which can have unspeakably slow long focal lengths. You'd better have a sunny day!What is stopping down?
When you stop down a lens, you are going to a larger number/smaller aperture and therefore less light. Going from f/8 to f/11 is stopping down. The opposite is opening up; going from f/11 to f/8 is moving towards the smaller number/larger aperture and therefore more light.How am I supposed to remember that f/stop sequence?Small F
LIGHT | BIG
F - Small light
So basically here is a picture to sum it all up ! .
Hope this helps you guys!! IF there is something i have left out please post it so we call all read it and learn from it also!
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