One of the safest ways to buy used lenses is to look at them in person. You need to research defects in the lens before deciding to buy it. Defects in lenses can be classified into 2, namely defects that have no effect on the photo results and another 1 is a defect that affects the photo results. Let's discuss the first part first which has no effect on the photo results.
1. The paint on the lens body is peeling or scratched
This is not related to the inside of the lens, it is only visible on the outer appearance.
2. The zoom or focus rubber is loose or missing
This section is the same as before, only affecting the camera display.
3. Accessories are incomplete
For example, if the lens cap is missing, it's not too much of a problem. you can look for it elsewhere.
4. Drag focus (Feels heavy when trying to focus the camera both manually and autofocus)
It doesn't actually have much of an effect, it's just that if it's too heavy, it might interfere with the performance of the focus motor on the camera (for Nikon AF-D lenses). If you use manual focus, you will feel sore when turning the lens focus. However, some lenses are deliberately made heavy so that the lens focuses more precisely. If it needs servicing, the cost should be quite affordable.
5. Focus is lumpy and rough (not smooth)
Usually because the lens has been dropped or hit so that the circle of the lens barrel is no longer perfect. Usually you can take it to a service center to have it disassembled and the lens barrel returned to its original shape. The cost for servicing should not be too expensive because no parts are replaced.
6. Dent on the front or body of the lens
If the lens is dented on the front, you will have difficulty installing the filter. It can still be repaired but you need to be careful. Usually, if the dent is in the lens body, the focus will not be smooth like point 5.
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7. The lens mount has been modified
Usually the initial lenses have different lens mounts so they need to be modified so that they can be attached to a particular camera body, for example a Nikon preAi lens that wants to be attached to a Nikon D7000 camera. But there is an advantage, you can use it straight away without having to modify it even though the shape of the lens is no longer like the original lens.
8. Lens stabilizer does not work
If you don't need this stabilizer feature, it shouldn't be a problem. But if you often take photos in low light conditions without using a flash, it might be a little difficult for you. But you can practice using manual focus when shooting until you are more stable.
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9. Focus is off or difficult to lock
If the focus is off, then you can only use manual focus. This can be helped by making the aperture a large number so that the opening becomes narrower and the focus space wider.
If your lens likes to miss focus, it may need to be recalibrated so that the focus becomes accurate again.
10. Diaphragm does not click when changing
If you use a manual lens, usually when you turn the diaphragm you hear a clicking sound which functions to determine the level of aperture used. If the clicking sound is no longer there, it will make it difficult for the photographer to estimate the current aperture position.
What's unique is that if you like shooting films, many people actually look for lenses that don't make noise because it can be annoying and also be recorded in the video.