There really are no ramifications. The EF 50 STM is a great lens and you will be very happy with it.
If you are wondering about the difference between EF and EF-S, there really is no difference. EF lenses are designed for cameras with a larger image sensor than 70D but are great for smaller sensors too. The focal length on the lens will be the same no matter what camera it is used on.
Take any EF-S zoom lens you have now and set it to 50mm and that is exactly the same view you will have the EF 50 STM.
(no math required)
The fast f/1.8 aperture if used wide open will have a shallow depth of field and give you great looking blurred backgrounds that most people like. Just keep in mind that it will be more critical to have accurate focus at f/1.8 because the depth of field is so shallow.
f/1.8 also lets in much more light. 2-3 times more light than your average “slow” zoom lens. Instead of shooting indoors at f/5.6 and 1/30 shutter speed, you now have the ability to use f/2 and 1/250 instead. This is great for stopping movement or action.
You also have the option to use a lower ISO. Instead of ISO 3200 you could shoot at ISO 400
Considering purchasing this lens to start a Prime collection, it seems very cheap, what will all the ramifications of using this lens on my 70D EF-s camera be?
Prime lenses are a niche product in today's world. For example, 50 mm is a bit long for general photography on an APS-C camera, of which the 70D is one. But that focal length, and that len's wide aperture (which facilitates a blurred background), make it a good portrait lens. So if you do a lot of portrait photography, you may use it often and be glad you bought it. Otherwise it may just add to the clutter in your camera closet.
I own this lens. Use it as a 50mm on my FF and previously as an 80mm on my APS-C body. Its a decent all around performer and takes extremely sharp photos. Bokeh for the f1.8 is good (not great). Does take great snapshots. Worth the $100 in my opinion.
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"Considering purchasing this lens to start a Prime collection ..."
Do it if it is the first one, it is a good start. Primes have taken a backseat to the great zooms available now. But it is still good to use them as they are truly sharper than most zooms. Plus they are usually faster. The new 50mil is so much better than the old one. I don't recommend anybody buy it.
One problem is 50mm is just hard to use. It just doesn't seem to fit anymore and just a few years ago it was the goto lens. Considered 'normal' ! On your cropper if will be a little long as it is a mild tele for normal use.
It is so nice to have a fast constant aperture like f1.8.
A few decades ago, zoom lenses weren't nearly as good as today's lenses. So photographers used "prime" lenses where the image quality really mattered. Back when I worked for a studio (in the 1980s) I did not know of any pro photographers that used a "zoom" lens for serious work.
But today... most photographers use zoom lenses for serious work because the zoom lenses really are that good.
What the primes will offer you that the zooms typically do not... is very low focal ratios. You'll notice that 50mm lens is available in an f/1.8 focal ratio (and you can get one with an f/1.4 focal ratio... and if you've got the budget, you can even get one with an f/1.2 focal ratio). You typically don't find zoom lenses with focal ratios lower than f/2.8 (and those are expensive). I think Sigma has an f/1.8 zoom... but I'm not aware of any others.
And that brings up the next point... which is why you'd want a low focal ratio. Typically there are two reasons... #1 is that since it collects more light, you can take faster exposures in low light situations. The other reaason is to produce a shallow depth of field and an nicelly blurred background. But the focal ratio is just one contributing factor to depth of field ... focal lenght is another. A low focal ratio on a short focal length lens wont produce much background blur ... for shorter focal length lenses it typically really is just about the ability to shoot in less light.
The other trade-off is that optical weaknesses become more apparent at low focal ratios. Lenses can produce cleaner images if they're stopped down just a bit.
All that aside... the EF 50mm f/1.8 STM is probably the best bargain lens in the Canon lineup. They don't offer a less expensive lens and it's surpisingly good considering it's extremely low price.
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