# New to SLR term what is lens  EF-S 18-135mm f



## ashuthinks (Aug 2, 2012)

Hi
i'm new to SLR please let me know all basic terms

what is lens  EF-S 18-135mm f means?

i have canon eos 550d


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## pgriz (Aug 2, 2012)

EF is the lens mount for Canon EOS cameras.
EF-S is the lens mount for crop sensor Canon EOS cameras (5xx, 6xx series, Rebels, 20d, 60d, 7d, etc.)
18-135 mm is a focal length of the zoom lens, ranging in this example from 18mm to 135mm.
f - usually denotes the maximum aperture available.  A lens having f/3.5-5.6 is a variable aperture lens which has f/3.5 maximum aperture at the wide setting, and 5.6 at the telephoto setting.  Constant aperture zoom lenses would have something like f/2.8 or f/4, which means that the same maximum aperture regardless of focal length.


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## TCampbell (Aug 3, 2012)

The "S" on the end of "EF-S" stands for "Short back-focus".  The rear-most element on the lens is a little closer to the sensor when compared to an EF lens.  

Though your photos are rectangular, the lens is actually round.  The image that the lens projects into the camera's sensor body is a circular spot.  That circular image needs to be at least as big as the diagonal measurement of the sensor.    Canon makes some cameras that have sensors as large as a single frame of 35mm film.  That size is referred to as "full frame".  But the vast majority of DSLR sensors are smaller than this... they are referred to as "crop-frame" sensors and the most common size (by far) is the "APS-C" size.  

A Canon "EF" lens is designed to work with all Canon EOS cameras regardless of model (and this includes EOS film cameras ... which they no longer produce.)  

Since the vast majority of DSLR cameras use the APS-C size crop-frame sensor, Canon realized that they can reduce the lens size by creating a lens which only projects an image large enough to cover the crop-frame sensor AND they can do this without sacrificing quality.  BTW, Canon isn't alone... Nikon and everyone else does the same thing. Nikon calls them "FX" lenses and "DX" lenses (FX is their full-frame and DX is their crop-frame) whereas Canon uses the term "EF" and "EF-S".  

With your 550D, you can use any Canon EOS lens regardless of whether it's an "EF" or "EF-S" lens (and there also are some special purposes lenses... such as the MP-E and the TS-E lenses which don't have an "EF" or "EF-S" prefix but these work as well.)  If someday in the future you chose to buy a new Canon DSLR and that camera happened to use a full-frame sensor (generally only the high-end cameras have these) then you would not be able to use the EF-S lenses with those cameras.  But I don't advocate that you buy lenses based on what you "might do" someday... I'd only do that if you were "sure" you plan to do that.

As for the "f":  The "f" will always be followed by a slash (/) and a number... and sometimes two numbers separated by a dash (-).  That number is the lowest focal ratio of the lens.  

A focal ratio is simply the focal length of the lens divided by the diameter of clear aperture.  For example... if you had a 50mm lens and the clear aperture (the opening through which light can pass) was 25mm in diameter, then that lens would be an f/2.0 lens because 50 ÷ 25 = 2.  

The bigger the opening, the more light the lens can gather.  If the lens can gather more light then the shutter doesn't need to be open as long.  This makes low focal ratio lenses very attractive when shooting in poor lighting conditions -- especially when you NEED a fast shutter speed because your subject is moving.

If there are two numbers (e.g. f/3.5-5.6 in the "EF-S 18-135mm f/3.5-5.6") then it means that you a zoom lens where the focal ratio changes as the focal length changes.  The first number is the focal ratio when you're at the 18mm end of the zoom range and the second number is the focal ratio at the long end of the zoom ... at 135mm.

The beauty of focal ratios is that they are consistent regardless of lens.  For example... if you set the camera to f/4 then you'll get the same amount of light with that lens at f/4 as you'd get with ANY OTHER LENS which is ALSO set to f/4.  It if you set a 50mm lens to f/4 (which would mean the clear aperture is 12.5mm wide) then you get exactly the same amount of light as if you set a 100mm lens to f/4 (which would mean that the clear aperture is 25mm wide).  This makes it VERY convenient for metering the light.  If the light meter tells you that there's enough light to take a 1/250th second long exposure at f/4 then it doesn't matter which lens you have attached to your camera (as long as that lens is capable of using that particular setting... some lenses are capable of much lower focal ratios than others.)

Though a larger opening allows the lens to collect more light, it also has an effect on the focused area in an image.  When you focus, there's actually a range of distances at which objects will appear to be acceptably focused.  That range is called the "depth of field" (of DoF).  At very low focal ratios, the DoF will be narrow -- meaning subjects need to be very close to the focused distance to appear in proper focus.  But at very high focal ratios the DoF gets much broader.   While almost any lens can reduce the size of the aperture to create a broad DoF, not as many lenses allow for very wide apertures which create very shallow DoF.  A shallow DoF can be highly desirable when you want to help focus attention on your subject by creating a photograph which has a tack-sharp subject... and yet it has a beautifully blurred background.

As you shop for lenses, you'll find that lenses that offer lower focal ratios are more expensive than similar lenses which don't offer low focal ratios.


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## PlanetStarbucks (Aug 3, 2012)

If you're considering buying it, it's a good all around lens.  I have one and it's good to have when you don't know exactly what you'll be shooting.  It can zoom pretty well, and go fairly wide.  There are better lenses for wide and telephoto, but the 18-135 is a good one that can do many things well.


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## Helen B (Aug 6, 2012)

TCampbell said:


> The "S" on the end of "EF-S" stands for "Short back-focus".  The rear-most element on the lens is a little closer to the sensor when compared to an EF lens.



That never seemed completely logical to me - it need not be a general rule. Fortunately it doesn't seem logical to Canon either, because they are now saying that the S stands for 'small image circle', which is a much better description.


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