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FORUMS Canon Cameras, Lenses & Accessories Canon EF and EF-S Lenses 
Thread started 17 Dec 2014 (Wednesday) 20:13
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I Have A Stupid Question

 
Chief_10Beers
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Post edited over 4 years ago by Chief_10Beers. (2 edits in all)
     
Dec 17, 2014 20:13 |  #1

I do have the concept of a EF Lens Efective Focal length on a ASP-C Camera. So why does Canon List the EF-S Lenses Focal length as fitted on a Full frame camera even though said Lens will not fit on a Full Frame Camera. Why did they not give the actual Efective Focal length on said Lens that will only be used on a ASP-C Camera?...............


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Dec 17, 2014 20:22 |  #2

Because they give the actual focal length of the lens. Same reason why your P&S has a 4-14mm lens or so...because that's the actual focal length.

Putting a lens on a crop camera doesn't do anything to the focal length. That is still the same...it just crops the FOV.


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Dec 17, 2014 20:25 |  #3

The focal length of an EF lens and an EF-S lens that are marked the same IS the same. No lenses are marked with "effective" focal lengths (primarily because there is no such thing).

The "crop factor" number and the term "35mm equivalent focal length" were invented simply to help someone who was familiar with focal lengths on a 35mm film camera who is migrating to an APS-C format digital SLR. The focal length of a lens does not change if you attach it to different format cameras.

An "effective focal length", if there was one, would require that one camera format is the standard by which all other camera formats are measured. There is simply no one camera format that is the standard.


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Chief_10Beers
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Dec 17, 2014 22:45 |  #4

I understand all that, let me put it in a different way, What determins a focal length in a Lens? Is it the Lens it self? Or in a lens based on the 35mm film camera?
Going back to my original question, a EF-S 17-55mm f/2.8 is 17-55 on a full frame camera but the effective range is 27-88mm on a crop that is designed for, makes no sense at all to me. Why not badge the Lens as a EF-S 27-88mm f/2.8?

I also Understand the Backward compatibility of the EF Lenses to the crop Cameras, that makes sense to me as they are Based on the 35mm format...............


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Dec 17, 2014 22:55 as a reply to  @ Chief_10Beers's post |  #5

The lens itself determines the focal length..it has no bearing on the 35mm format.

It's not badged as a 27-88mm F2.8 because its not ;)


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Post edited over 4 years ago by Wilt.
     
Dec 17, 2014 23:04 |  #6

The FOCAL LENGTH of any (non-zoom) lens NEVER CHANGES due to format size!

What does change is the amount of the image circle which is captured within a frame of a certain dimension.


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Dec 18, 2014 00:47 |  #7

Chief_10Beers wrote in post #17338867 (external link)
I understand all that, let me put it in a different way, What determins a focal length in a Lens?

All of this stuff:
http://en.wikipedia.or​g/wiki/Focal_length (external link)

IMAGE: http://upload.wikimedia.org/math/6/9/c/69c2f3340719b7506c05b8cec36ca211.png

IMAGE: http://upload.wikimedia.org/wikipedia/commons/thumb/8/8b/Focal-length.svg/180px-Focal-length.svg.png

IMAGE: http://upload.wikimedia.org/wikipedia/commons/thumb/f/fc/Thick_Lens_Diagram.svg/400px-Thick_Lens_Diagram.svg.png

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Post edited over 4 years ago by SkipD. (6 edits in all)
     
Dec 18, 2014 06:16 |  #8

Chief_10Beers wrote in post #17338867 (external link)
I understand all that, let me put it in a different way, What determins a focal length in a Lens? Is it the Lens it self? Or in a lens based on the 35mm film camera?

The focal length of a lens is determined by the optics of the lens - PERIOD. The focal length is not dependent on the box that you connect the lens to.

What is important in a lens' design (relative to using it on a particular camera) is that it creates the image at the correct distance from the lens' mount and, of course, that it has the appropriate mount (or can use an adapter) to fit the particular camera. If you could adapt a 300mm lens designed for an 8" x 10" view camera to fit on your Canon SLR, it would provide the same image size as a Canon 300mm lens would.

Chief_10Beers wrote in post #17338867 (external link)
Going back to my original question, a EF-S 17-55mm f/2.8 is 17-55 on a full frame camera but the effective range is 27-88mm on a crop that is designed for, makes no sense at all to me. Why not badge the Lens as a EF-S 27-88mm f/2.8?

I also Understand the Backward compatibility of the EF Lenses to the crop Cameras, that makes sense to me as they are Based on the 35mm format.

I repeat - FORGET the "effective" term when thinking about the "crop factor". The proper term is "35mm equivalent focal length". Why is this an important difference? It's because the "crop factor" is simply used to compare the field (or angle) of view of a particular focal length on two different format cameras - a so-called "full-frame" camera with a 36mm x 24mm film frame or sensor size and a so-called "APS-C" camera with a sensor size of approximately 22.3 x 14.9mm.

The lens' focal length DOES NOT CHANGE when you apply the lens to the different format cameras. "Format", by the way, refers to the dimensions of the film frame or digital sensor in a camera.


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Post edited over 4 years ago by msowsun. (5 edits in all)
     
Dec 18, 2014 06:46 |  #9

Keep in mind that in addition to APS-C and "Full Frame" there are other formats. A Large Format camera with a 50mm lens can have an equivalent focal length of only 25mm. A 50mm lens on a Large Format is no longer a "normal" focal length. It is now an Ultra-Wide.

http://en.wikipedia.or​g/wiki/Image_sensor_fo​rmat (external link)

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Dec 18, 2014 07:10 |  #10

OK, I gets it, a 50 is a 50, a 100 is a 100...............


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Dec 18, 2014 08:41 |  #11

Chief_10Beers wrote in post #17339348 (external link)
OK, I gets it, a 50 is a 50, a 100 is a 100...............

One problem with Canon's advertisement of their EF family of lenses is that even though the EF-mount lenses fit at least three different format cameras, they label the focal lengths as if the only format was the 35mm film format. For example, if you look at the description of the Canon EF 35mm f/1.4L lens in their list, it is described as a "wide angle" lens. If that lens is used on an APS-C format camera, it would be considered a "normal" focal length (slightly longer than the definition of normal, actually).


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Dec 18, 2014 11:15 |  #12

Chief_10Beers wrote in post #17338642 (external link)
I do have the concept of a EF Lens Efective Focal length on a ASP-C Camera. So why does Canon List the EF-S Lenses Focal length as fitted on a Full frame camera even though said Lens will not fit on a Full Frame Camera. Why did they not give the actual Efective Focal length on said Lens that will only be used on a ASP-C Camera?...............


There is no "effective focal length". It is a measurement in mm and it does not change when you put it on a different sensor.
The field of view or angle of view changes.
Why should the EF-S and crop bodies reference themselves to and lie about numbers to compare with to some other sensor?


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Dec 18, 2014 15:58 |  #13

Chief_10Beers wrote in post #17338867 (external link)
Going back to my original question, a EF-S 17-55mm f/2.8 is 17-55 on a full frame camera but the effective range is 27-88mm on a crop that is designed for, makes no sense at all to me. Why not badge the Lens as a EF-S 27-88mm f/2.8?

Because badging EF-S lenses with a modified focal length, rather than the actual focal length, would just confuse the hell out of people. It just doesn't make sense to do that.

As it stands, an EF 50mm and an EF-S 50mm are identical focal lengths and will provide identical images on a crop camera. So if you relabel the EF-S as an 80mm, as you suggest, then you would have two lenses giving identical field of view but one is 50mm and one is 80mm. With many people, it would end up as a set of lenses labelled using two different systems.

Consider the EF-S 55-250. Doing as you suggest would rename that as an EF-S 88-400, but imagine someone considering a zoom and looking at that lens and also the EF 70-300. The numbers would now suggest that the EF-S 88-400 is longer at both ends than the EF 70-300, so give better reach for birds etc. However, the reality is that the 70-300 is actually the longer lens (by 50mm at the long end) and will give the better reach.

It is so much easier to understand when two lenses with equal field of view, are labelled the same. Why on earth would you want to confuse things, by labelling some of them with anything other than the actual focal length.

It is pointless thinking about how your lens would be on another camera, why do you need to think "this lens is 200mm, so that makes it a 320mm if it was on another camera that I don't have". The only reason would be if you were used to 35mm film cameras (for example) and wanted to compare what focal length you would need on your new digital crop body to give you the same field of view you like on your old gear.




  
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Post edited over 4 years ago by Nethawked. (2 edits in all)
     
Dec 18, 2014 21:14 |  #14

It's not a stupid question, I'd bet many have asked the same at one point.

A rectilinear lens will always have its focal length defined by the distance between the center of the lens and infinity focus. This is physics, unchanging and unrelenting. Marketing differently would be a mistruth. The field of view, which is what you're interested in, is measured in different dimensions and is a characteristic of both sensor and lens.

Here's a good read on the subject.

http://en.m.wikipedia.​org/wiki/Angle_of_view (external link)




  
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Dec 18, 2014 21:34 |  #15

Take a magnifying glass, turn it to the sun, and burn some wood with the concentrated rays. The rays converge to a point (OK, nearly a point).

The distance between the burning point and the magnifying lens is the focal length.

It's the same with camera lenses. Take the camera lens, turn it to the sun, and use it to burn some wood with sunlight. The distance from the burning point to the lens is the focal length. (More precisely, it's from the burning point to a place in the lens called the rear nodal plane. The rear nodal plane effectively takes the place of all those glass elements in the lens.)

So that's the focal length, and it can be determined without even attaching it to a camera.


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