>>3258043>>when someone says that they use a specific lens (35mm, 50mm, etc), is that an equivalency?
Everyone is different, everyone's gear is different, and there is no agreed-upon standard. So, when stepping into an online discussion and people talking about focal lengths without context, you always have to ask what camera they're using, if they don't mention it. Some people do speak in terms of equivalencies only, even if they've only ever owned DSLRs with APS-C size sensors; not everybody does, though.
*****However (very important), the lenses themselves are never, EVER marked by the manufacturer with just these so-called "equivalent" focal lengths. Focal length is an immutable property of the lens that doesn't magically change when you mount it to a camera with a smaller sensor.******
That's why it's important to just learn the new system, as if you were learning metric, or else you'll risk forever being hobbled as a photographer. It might help to use words like "ultra wide," "wide," "normal," "short telephoto," "medium telephoto," "super telephoto," etc. instead of using the numbers.>when i'm buying a lens for my camera (nikon d3400), if i buy a 50mm, is it 50mm for full frame, or a 50mm for aps-c
50mm is 50mm. When you put the lens on a D800, for example, it's considered about the same perspective as what the human eye sees (the so-called "normal" perspective). When you put that same lens on a D3400, it's a short telephoto - somewhat tighter of a crop than normal, more "zoomed in," a good lens for head shots of people, or however you want to think of it. >i'd like to get into street photography, and a 50mm prime lens is what i want to get, but i don't know whether to get a 50mm for my sensor, or 50mm equivalency
For so-called street photography on the D3400, you'll want a lens anywhere from about 23mm to 35mm, which covers the normal perspective. I prefer a bit wider for street, so my walk-around lens is a 23mm on APS-C or 35mm on full frame.