1. What if focal length
Focal length is the distance from the center of the lens to the imaging point
The aperture is a fraction that describes the ratio of aperture (entrance pupil) diameter to focal length.
3. How to differentiate between lenses:
Camera name itself says, like: Nikon 18-55mm f/3.5-5.6 AF-P. Maximum aperture is f/3.5 but it shifts gradually from f/3.5 at the wide end to just f/5.6 at the longer focal lengths. It can be identified by column as well, like:Nikon 50mm 1:1.4G(Max aperture f/1.4).
Nows lets get into the details:
1. What is focal length:
Focal length is the distance from the center of the lens to the imaging point (focal plane) where the light for the image is collected. When a lens is described as a "50mm lens," it is referring to its focal length. Different focal lengths create different levels of magnification and change the viewing angle of the resulting photograph. As the focal length value decreases, the lens becomes a wide-angle lens, and as the value increases it becomes a telephoto lens.
Changing the focal length changes the angle of view. The shorter the focal length (e.g. 18 mm), the wider the angle of view and the greater the area captured.
2. Focal length and aperture relation:
The aperture is a fraction that describes the ratio of aperture (entrance pupil) diameter to focal length. That’s all it is.
Aperture is literally “length over diameter.”
Notice the position of “f” traditionally shown when describing aperture.
An f/1.0 lens would have a diameter equal to the focal length.
At f/2.0, the aperture diameter is half of the focal length.
For f/8.0, the diameter is one eighth the focal length. Eight times as long as lens width. “f” over 8.0.
With the same aperture and a different lens length, the ratio stays the same.
A 50mm lens at f/4.0 is 12.5mm effective diameter.
A 200mm lens at the same aperture of f/4.0 would be 50mm diameter.
3. How to differentiate between lenses OR How to check the aperture of a lens by it's name and how to select the best lens for you:
Minimum and Maximum Aperture of Lenses
Every lens has a limit on how large or how small the aperture can get. If you take a look at the specifications of your lens, it should say what the maximum and minimum apertures are. For almost everyone, the maximum aperture will be more important, because it tells you how much light the lens can gather at its maximum (basically, how dark of an environment you can take photos).
A lens that has a maximum aperture of f/1.4 or f/1.8 is considered to be a “fast” lens, because it can pass through more light than, for example, a lens with a “slow” maximum aperture of f/4.0. That’s why lenses with large apertures usually cost more.
In contrast, the minimum aperture is not that important, because almost all modern lenses can provide at least f/16 at the minimum. You will rarely need anything smaller than that for day-to-day photography.
With some zoom lenses, the maximum aperture will change as you zoom in and out. For example, with the Nikon 18-55mm f/3.5-5.6 AF-P lens, the largest aperture shifts gradually from f/3.5 at the wide end to just f/5.6 at the longer focal lengths. More expensive zooms tend to maintain a constant maximum aperture throughout their zoom range, like the Nikon 24-70mm f/2.8. Prime lenses also tend to have larger maximum apertures than zoom lenses, which is one of their major benefits.
The maximum aperture of a lens is so important that it’s included in the name of the lens itself. Sometimes, it will be written with a colon rather than a slash, but it means the same thing (like the Nikon 50mm 1:1.4G below).
But larger aperture is always not good. They can cause "Diffraction".
Diffraction is actually quite simple. When you use a tiny aperture like f/32, you literally squeeze the light that passes through your lens. It ends up interfering with itself, growing blurrier, and resulting in photos that are noticeably less sharp.
When do you start to see diffraction? It depends upon a number of factors, including the size of your camera sensor and the size of your final print. Personally, on my Nikon full-frame camera, I see hints of diffraction at f/8, but it’s not enough to bother me. I actually use even smaller apertures like f/11 and f/16 all the time. However, I try to avoid f/22 or anything beyond it, since I lose too much detail at that point.
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