So what do you really need to know before you buy a lens?
1) Pay attention to the sensor size (film size) of your camera.
The size of the FILM inside your camera completely changes how a lens will look.
Now, INSTEAD of film, cameras have digital sensors. So you need to know if you have large sensor or small sensor.
“Large film” in a digital camera is called a full-frame sensor.
“Small film” in a digital camera is called a cropped sensor.
2) Pay attention to the aperture of the lens you buy.
Aperture is like the size of your pupil getting larger or smaller depending on how much light is available.
f22 is a pinhole opening (super tiny hole)
f1.4 is a wide opening (like an inch and a half wide hole)
If you’re in a darker area, you will want to open up the hole in the lens really wide to let more light in! Remember the wider the aperture opens up, the lower the f-stop. So f2 is a really large hole but f10 is a small hole.
Most cheap lenses you will buy will be not be able to open that wide. A typical crappy kit lens will start at f4, which means it can’t go below f4 ever.
If you buy an f4 lens, you miss out on f3.5, f3.2, f2.8, f2.5, f2.2, f2, f1.8, f1.6, f1.4, and f1.2 which are increasingly wider holes.
The 50mm f1.8 is an exception to this, it can open up quite wide (f1.8) for about $110.
Aperture also affects how much beautiful background blur you can get with the lens. If you buy a lens that starts at f4, you miss out on being able to take shots with great background blur. You can still get some blur, but the lower the f-stop goes, the more intense the blur can become. With cheaper lenses that start around f3.5 or f4, you miss out on this.
ESPECIALLY needed with 100mm and up, because we’re not meant to hold the camera stable when we’re that zoomed in.
Unless you’re shooting outside, you’ll really need that stabilization
Look for IS lenses or VR lenses.
4) Chromatic Aberration
Green or purple lines surrounding background objects — some lenses deal with this better than others. The cheaper canon 85mm has notoriously bad chromatic aberration. Essentially the more you pay for a lens, the better it’ll deal with chromatic aberration. Worse comes to worse just use Lightroom to help fix that in post, but if you can buy a better lens, do it!
Some lenses are built sharper than others. But honestly, you may not want the sharpest lens available. There’s an awesome lens made by Sigma (sigma art 50mm) that is now being chosen by some professionals over the Canon 50mm f1.2L lens, but again some others want the creaminess-type look of the Canon lens over the tack-sharp nature of the Sigma. It just depends on what you care more about.
6) Never buy a lens that has haze, fungus, or scratches on it.
Tip: Don’t focus on the 5-star or 1-star reviews of a lens. Read the 3-star reviews. That way you’ll hear about the specific good and bad things about the lens.
Link: How to buy on eBay used