How do camera lenses work? How can a good lens improve a picture so much more than a bad lens if the body is the same? Isn’t it the body that’s doing most of the work?
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The glass lenses inside bend light to make things clearer, allowing you to focus your camera like you focus your eye. Macro lenses bend the light to magnify small things, telescoping lenses bend the light to help focus on things far away. Good lenses are made of high quality glass and are shaped to be ideal for focusing for their particular use, while providing a sharpness and clarity that cheap lenses can’t match usually.
Better lenses, better pictures. The body is only there to hold and adjust the distance between lenses to allow for proper focus.
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Aperture: the lens will have an Iris, how big the Iris opens, the bigger it opens the more light it passes through. (More on this later)
Field of view/zoom: different lenses have different zoom levels, depending on the optics (actual glass lenses) more or less light will enter at different zoom levels. (More on this later)
Lens quality: light passes through lenses, crappy lenses will not focus the light correctly, you can have the best body in the world but if you lens cannot focus the light properly it won’t work as well.
Now take a good prime lens. Zoom is fixed, lens quality is often the best you will find (less moving parts means the manufacturer can focus on making better lenses). The aperture will often be wider, I have a prime lens with Max f_stop (related to aperture, focal length, and lens size) of f/1.2, the f-stop can be modified with aperture making it “slower” by closing the Iris a bit more (reducing aperture)
Now take a crappy zoom lens, zoom is variable lets say 18-55mm. The quality of focus specially on the edges will degrade a bit since the optics might not focus all wavelengths equally. The max f-stop will vary with zoom. On 18mm probably you will have f/3.6 while on 55mm you will have f/5.6
If you take a more expensive zoom lens the quality of focus on edges will be often better specially with lenses with apochromatic elements. The f-stop might be f/4 all the way from 18mm to 55mm
Edit. For f-stop the higher the number the darker the image but you get more things in focus (depth of Field).
The lense is there to focus and direct light into the sensor, think of it as trying to look through glasses that are the wrong prescription, it’s a similar concept.
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