What is the difference between EF and EF-S lenses and what is STM?

[ScottyP · Accepted solution]

Hi.

EF lenses work on both types of Canon DSLR; full frame and crop sensor cameras.  Some are affordable and some are very expensive.

EF-s lenses only work on crop sensor cameras, and are consumer grade, affordable lenses.  

STM is just a quieter focus motor in a lens. People who do a lot of video like that, because you are less likely to hear the sound of the lens focusing recorded in your videos.  Ultrasonic (USM) focus is the other good focus motor, and it is pretty quiet too, and it is faster for sports, etc...  I would not worry much (or at all) about getting STM over USM unless you are shooting a lot of video and you are pretty particular about it.

Full Frame cameras.  Full frame cameras have a sensor equivalent to the old 35mm film size. The lenses will all give you the field of view stated on the lens, so 50mm will really be 50mm, 35mm is 35, 200 is 200, etc.  Full frame cameras include mostly pro and advanced enthusiast models such as 1Dx, 5D3, but there is also the more affordable 6D (which I use).

Crop cameras.  Crop sensor cameras have a smaller sensor.  The sensor is smaller than the image cast upon it, so the image splashes out around of the edges of the sensor.  This means the image is basically cropped down and you lose the edges, which is like zooming in on every shot, once the image is displayed at full size. The "zoom factor" is 1.6x the normal focal length of the lens, so a 100mm lens will give you an image that appears to be 160mm, a 200mm lens will give a field of view like a 320 lens, etc...  Crop cameras include the entry level cameras like Rebel T5, and the next step up like Rebel T5i, T6i, etc., and then the next step up, the 70D, and finally the high-end 7DII.

L lenses.  EF lenses are further divided into regular lenses and "L" lenses. The "L" lenses are higher quality both in optics and in construction, and some of them are to some degree weather resistant (not waterproof!). And they are much more expensive.  They have red rings on the ends.

[TCampbell]

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@ScottyP wrote:

Seriously, though, no one on the street, not even most photographers or even scientists would look at a blue color palette at the paint counter and think "warm colors".

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Most people (photographers, etc.) do associate the yellow/orange/red with being "warmer" colors.  But in science... blue is technically warmer.  

See:  https://en.wikipedia.org/wiki/Black-body_radiation

Anything that isn't at absolute zero emits radiation in the form of heat.  This isn't just things in the visible spectrum... it goes much farther.  People are black-body radiation sources (we emit heat) ... which is why we show up so easily on thermal imaging cameras.  Although those wavelengths are down in the infrared spectrum and our eyes can't see in infrared.  

The warmer it is, the more energetic the photons are as they are emitted.  Eventually if they are warm enough... the light shows up in the visible spectrum (but on the 'red' side of the spectrum).  As the temperature gets hotter and hotter ... the curve starts to favor the 'blue' side of the spectrum (the hottest stars are blue).  

[TCampbell · Accepted solution]

It may help to understand "why" there are both EF vs. EF-S lenses and it has to do with making cameras economically more affordable so that more consumers can buy them.

EF lenses are the original EOS lenses designed for 35mm film cameras and full-frame digital cameras (a "full frame" digital camera has a sensor which is the same size as a single frame of 35mm film.)  It's about 36mm wide by 24mm tall.  Incidentally, it's called "35mm" because that's the width of the film spool... including the perforated edges that ride on the film sprockets in the camera.  In other words the exposed area may be only 24mm tall... but there's 12 more millimeters for the perforated edges.

All lenses are, of course, round... not rectangular.  But you get a rectangular image out of the camera.  This is because the lens is projecting an image of your scene into the camera and that projects a round "spot" or image circle.  The sensor is in the middle of that circle, but since the sensor is rectangular, it only detects the light which happens to land directly on the sensor surface.  Anything that spills off the sides of the sensor isn't captured.  What's important is that the image circle being projected into the camera has to be at least as large as the diagonal measure of the sensor itself... that's 43.3mm from corner to corner.  The image circle is actually a little larger than that.

But full-frame digital sensors are expensive.  Canon realized that they could use a slightly smaller sensor and produce cameras that are more affordable.  These smaller sensor measure about 23mm wide by about 15mm tall (these are not exact dimensions... but close.)   BTW, this happens to be the size of a single frame of APS-C film.  APS-C stands for "Advanced Photo System - Classic" size.  These were handy little "drop in" film cartridges designed to be very convenient for consumer use.  When digital cameras use sensors in that same size, they are referred to as "APS-C" size sensors (even though we aren't actually using APS-C film.)

Effectively this means even more image spills off the sides and less image lands on the sensor.  It has the effect of taking an image with a full-frame sensor, but then "cropping" the image so you only save the area in the middle.  For this reason they call it a "crop frame" sensor and the crop factor happens to be about 1.6x.  There are other crop-factor size cameras as well, but for the DSLR, the APS-C size is the most popular by far.  You can get APS-C size sensor DSLR cameras from Canon... and just about every other brand of DSLR on the market.

What does this have to do with your question on lenses?  Everything!

Canon realized that since quite a bit of image is simply spilling off the sides of the sensor and isn't captured, they could produce lenses that project smaller image circles.  Doing this would require lenses that have smaller "glass" elements.  Some optical problems are reduced because the glass elements are smaller and don't need to be as thick. That also means they don't need as many corrective elements in the lens.  The lens gets smaller, lighter and substantially less expensive to manufactur and yet... you get (here's the best part) the SAME quality images (you don't sacrifice quality to do this.)  This was yet another way to make DSLR cameras more affordable to the consumer market.

This smaller, lighter, and more affordable lenses are called... EF-S lenses.

There is a trade off... remember that these lenses project a smaller image circle and are desgined specifically for use on DSLR camera bodies that have APS-C sensors.  That means you cannot use them on DSLR bodies that have "full frame" sensors.

Also... since a primary motivation of producin these lenses is cost reduction (so they can be sold at prices that more consumers can afford) there are some other areas where they reduce costs as well.  For example... high end lenses may have more aperture blades for a more rounded aperture opening.  They may have a better "build" quality.  They may be "weather sealed".   There's a market where this is exactly what the consumer wants (and it's a big market.)  Pros and advanced amateurs may be more demanding in these areas and refuse to sacrifice to save money.  You can still mount an EF lens on an APS-C camera.  Canon's very best quality lenses get the "L" designation.  The "L" is always appended to the foacl ratio value. (e.g. EF 70-200mm f/2.8L IS USM II".  Canon colors the "L" red and they paint a red stripe around the end of the lens.  But in order for a lens to qualify as an "L" grade lens it has to be usable on any EOS camera... not just the APS-C cameras.  For this reason you can't find an EF-S "L" lens.

Every EOS camera can use EF lenses.  No exceptions.

Every EOS camera that has an APS-C sensor (with the exception of the 10D) can use EF-S lenses.  They can also use EF lenses.

As for motor types, there are three major motor types..

1)  Conventional motors (which are the slowest and also the noisiest)

2)  USM motors (UltraSonic motors -- which tend to be very snappy and also very quiet)

3)  STM motors (Stepper Motors -- which are much faster than conventional motors, but not quite as fast as USM motors.  They were, however, designed to be particularly quiet so that the noise of the focusing motor would be very difficult to detect when recording video.)

See:  http://www.usa.canon.com/cusa/consumer/standard_display/Lens_Advantage_Perf

[RobertTheFat]

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@TCampbell wrote:

But full-frame digital sensors are expensive.  Canon realized that they could use a slightly smaller sensor and produce cameras that are more affordable.  These smaller sensor measure about 23mm wide by about 15mm tall (these are not exact dimensions... but close.)   BTW, this happens to be the size of a single frame of APS-C film.  APS-C stands for "Advanced Photo System - Classic" size.  ...

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So Ken Rockwell's contradictory etymology is bogus?

[TCampbell]

Though Ken Rockwell is often entertaining (and often designed to provoke reactions), he's talking about APS "film".  The "APS-C"-sized digital sensor (which isn't film) has nothing to do with film and the only thing in common is the dimensions of the "frame".

[TTMartin]

While STM is a focus motor it also indicates the newest consumer grade lenses from Canon. In addition to adding the new focus motor Canon updated the optics in STM lenses. Modern design and manufacturing gives STM lenses image quality that rivals first generation pro level L lenses from just a few years ago. So when you purchase a Canon camera with an STM kit lens there is no reason to immeadiately go out and spend more money on a new lens. That is a big advantage for Canon users now.