35mm f/2 vs. 40mm f/2.8 vs. 50mm f/1.8 D5500?

The difference is in the field of view. Among the three, the 35mm sees the widest while the 50mm is the narrowest. For APS-C sensors, 35mm is standard/normal which is approximately the same field of view as human eyes. At 50mm the view will be narrower but the subject will be larger and seem closer to you. The 40mm is in between.

Those f/numbers are maximum aperture sizes and they largely depend on the design of the barrel. Most 40mm lenses are flattened (pancake design) for portability which limits the aperture size to f/2.8. With 35mm and 50mm, there are several variations to suit different budgets. The lower the f/number, the more costly it gets.

If you are after a general purpose prime lens for your camera, get the 35mm. If you are going to shoot portraits, the 50mm f/1.8 is the best among those three. For street shooting, where a good mix of different types of photos are expected, the 40mm pancake is great.

Geek and Steve P have answered your query; seems like you are easy to offend... or imagine offense when none is intended; so to save you hurt feelings, I won't go into details other than to say, read their responses again. Actually, you need to mature; there's is absolutely nothing condescending in any of the responses you take offense from. Grow up and continue to LEARN without fussing with those helping you by providing answers to your questions; please stop acting overly sensitive for your own good. No one likes insolence or impetuousness or rudeness by those asking questions.

Ben,

There is nothing condescending in Geek's or Steve's answers. The same cannot be said of your comments. Both of these people are well accomplished photographers. Steve has international showings.

Listen. Learn. You choose a lens based on what you intend to shoot. As Steve points out, the 35mm is a basic or normal perspective lens and the 50mm a portrait lens on a crop factor camera. On a full frame the 35mm is a slightly wide angle, and the 50 is a normal. The 40 is a compromise in between. If you're looking for a good all around lens, the 50 is your best bet. They call it the Nifty Fifty for a reason.

I have both a full frame and a crop factor cameras which take the same lenses. Aside from the perspective (focal length) differences due to the crop factor, the image quality is the same. They are just as sharp, and within the confines of the focal length, render the depth of field correctly. My question is why buy lenses from Yongnuo? Sigma and Tamron make excellent second tier lenses, and you know what you're buying with them. The reviews I've read went from ok to meh on the yongnuo's. So save a little longer, buy something you'll have for life.

Geek was not giving you a "condescending" answer. He was answering as best as could be done to your very amateurish question. What do you mean which lens is "better" for a crop sensor camera? That is an absurd question. The "better" lens is the one that SUITS YOUR NEEDS. What do you want to shoot? Landscapes? Then use the 35mm. Portraits? Then use the 50mm. The 40mm gives you something of a compromise between the two, but all of your effective focal lengths are going to be affected by the 1.5 crop factor.

As said, the manufacturer made the 40mm as a 2.8 for their own reasons. It could be so the lens would be smaller than the 50mm 1.8. The smaller and lighter 40mm 2.8 could be another factor in choosing it over the 50mm. Again, ... what is important to YOU? Do you value / need smaller size and weight over the larger aperture? Do you do a lot of low light photography?

Don't expect us to be able to read your mind. YOU have to know what is most important in your photography and choose lenses accordingly. We cannot just magically know all this.

"What are the key differences that seperate these lenses?" - Focal length and aperature.

"Which are better for crop frame?" - any of them will work.

" Why is 40mm 2.8?" - Because that is how Yongnuo made it. Ask Yongnuo.