Lens Canon EF 40mm f2.8 STM popularly called a pancake. The reason is obvious - it is unusually thin and light. It weighs only 130g, which is in stark contrast to the lens. Sigma 40mm f1.4 ART, where the focal length is the same, but the weight is 1200g.
Canon EF 40mm f2.8 STM has a maximum aperture of f2.8, which for fixed lens a little... For similar money you can buy Canon EF 50mm f1.8 STM and get a much stronger background blur. As a portrait lens for a crop, the above lens is much more recommended. In general, I do not recommend the EF 40mm f2.8 STM for crop cameras, it will be boring.
Pancake Canon EF 40mm f2.8 STM covers the full frame (and even Fujifilm GFX Medium Format, as my tests showed), so it's worth using it exactly on full frame cameras... This is a standard focal length lens that conveys perspective as we see the world with our own eyes. Strictly speaking, the standard focal length for a full frame is 43mm (36mm by 24mm sensor diagonal). It is the pancake that is closest to this value.
This lens is convenient for shooting indoor reports. The autofocus speed is average here, it does not catch the stars, but it is enough for anything. The aperture is also average, but it is usually enough for most tasks. But the sharpness is excellent throughout the entire field of the frame, starting from the open aperture. I use this lens as a reference lens in my tests. It easily "resolves" as much as 30MP at Canon rand 45mm on Canon R5.
The main feature of this lens is its incredibly compact size, barely perceptible weight and cost. But at the same time it is optically very good. Despite the fact that I have a ton of other cooler optics, this lens takes its special place of honor.
What's so special about the Canon EF 40mm f2.8 STM?
The nice thing about it is that if you want to be light with your full-frame camera, it's easy and simple to do. Do you have a long shoot, where you need to be on your feet for hours? Here is the solution to the problem - Canon EF 40mm f2.8 STM, which can be used to shoot almost anything using the foot zoom (move your feet closer, farther).
This lens refutes the thesis that a full frame is always a heavy and large camera with a large lens. If you know that this is your main focal length, then for the sake of compactness, you don't need to take a crop camera. The Canon EF 40mm f2.8 STM in combination with a Canon mirrorless camera would be a good choice.
What is the difference between 40mm and 35mm and 50mm?
The difference is subtle. About 50mm I can say that this is a different focal length. With 50mm it is sometimes cramped. Additional 10mm, due to which the pancake is wider, make itself felt. But the 40mm f2.8 pancake doesn't blur the background much. You can get a separation from the background, but there will be no direct tangible bokeh with it. In the same time, fifty dollars are famous for their pleasant bokeh. A lens with a focal length of 35mm will be interesting when looking at models with aperture f1.8-2.0 or f1.4. The difference in aperture ratio under some conditions can be decisive. But these lenses are bulkier and more costly.
If you want a similar effect on a Canon crop DSLR, then you should look towards the Canon EF-S 24mm f2.8 STM. Everything is the same, only for crop cameras.
My video review of the Canon EF 40mm f2.8 STM with sample photos: