The 50mm f/1.8 STM follows on the footsteps of the extremely popular 50mm f/1.8 USM II lens. The latter, arguably the greatest selling lens ever to roll out of a Canon assembly line. Big shoes the new lens has to fill in and it does that with some degree of success.
The 50mm f/1.8 STM is a budget lens designed primarily for Canon's full-frame DSLR systems. That said this lens will also work on Canon's EF-S mount cameras (APS-C 1.6x crop systems) as well as film cameras.
Why Should I Buy this Lens?
There are two primary reasons you would want to buy this lens as opposed to something like an 18-55mm kit. The first is undoubtedly the wickedly fast f/1.8 maximum aperture. Maximum aperture ensures that the lens can let in a lot of light. How much exactly? A kit lens can shoot at a maximum of f/3.5 when it is set to its widest focal length. Compared to that the 50mm f/1.8 STM can shoot at two stops faster. That means at any given lighting condition and the same shutter speed, the 50mm f/1.8 STM will take in 4 times the amount of light that a kit lens will.
How is that Going to Help?
Two ways really. You can shoot at a faster shutter speed in low light situations and therefore avoid camera shake. Normally, when shooting in low light you either slow down the shutter speed or increase the ISO number.
In the first instance, you risk camera blur which results in blurry images. In the second instance, you risk losing details in the shadows and induce noise.
A fast lens allows you to avoid both eventualities.
The advantage other being the STM auto-focusing motor
STM stands for Stepper Motor. This technology works much smoother than the original USM auto-focusing motor powered lens. There are too distinct choices here. One that is geared towards using a lens that locks focus fast, regardless of the associated jerky movement. These lenses are unsuitable for video work.
The other type of lens is the smooth focusing one. Like the 50mm f/1.8 STM. The fact that these lenses focus very smoothly means they are perfect for shooting videos.
Still, on the subject of focusing, the 50mm f/1.8 STM has an electronically coupled focusing ring. Meaning, the focusing ring is electronically connected with the focusing motor. That also means if the camera isn’t turned on you won't be able to focus manually.
There is a semblance of a manual focusing override. But you have to press and hold the shutter release half way to be able to focus manually.
I said there are two primary reasons. Actually, there are three. The third being the price tag. You can't hope to get a better piece of the optical tool at just $125 (or 100 £ depending on where you are).
Build quality is, to say the least – you get what you pay for. The lens mount is metal which is great. But there is an overgenerous amount of plastic used in the construction. This lens will not take too much abuse. There is no weather sealing either.
This lens comes with the older Super Spectra Coating (SSC) that is the precursor to the modern Subwavelength Coating that comes in more expensive lenses. Rounded 7 aperture blades make up the lens diaphragm which creates a nice bokeh.
Image quality is the same as the performance of the older 50mm f/1.8 USM II. It's no wonder as the same optics have been used in the new lens.
Canon currently sells three versions of the 50mm prime. This being the only one that has the Stepper Motor auto-focusing technology. Though this is the 'slowest' lens of the three. But being less than one-tenth the price of the most expensive 50mm that Canon makes, it is certainly an option you cannot overlook.
Having said that the other two lenses that Canon sells are f/1.4 and f/1.2 respectively. With the last of the two being an L lens, which obviously means better build quality. None of the lenses come with image stabilization.
So, there are two ways of looking at it. If you are looking for a cheap 50mm lens that focuses smoother for video work, then go for the 50mm f/1.8 STM. Otherwise, if you have plenty of money to burn and need absolutely the best go for the 50mm f/1.2 L.