What is the mm product of APS-C cameras?

[Anonymous]

I have a rebel t6i.  The kit lens it came with was the 18-55MM STM.  It is my understanding that 18MM is ultra wide angle, correct?  So if my lens, with these specs, were compatible with a full frame camera it would be an ultra wide angle lens, correct?  It is my understanding, however, that it is not compatible with full frame cameras.  It is also my understanding that the ultimate product of the pictures I take with my T6i are not ultra wide angle because of the crop factor of the APS-C sensor, is this correct?  If so, when I have the lens set to 18MM what am I actually seeing?  Is there MM you can put to the final product with the crop factor?  Finally, if I understand all this correctly, why don't they just name the lens with the MM it puts out with an APS-C sensor if it's not compatible with full frame anyway?

[RobertTheFat]

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@Anonymous wrote:

I have a rebel t6i.  The kit lens it came with was the 18-55MM STM.  It is my understanding that 18MM is ultra wide angle, correct?  So if my lens, with these specs, were compatible with a full frame camera it would be an ultra wide angle lens, correct?  It is my understanding, however, that it is not compatible with full frame cameras.  It is also my understanding that the ultimate product of the pictures I take with my T6i are not ultra wide angle because of the crop factor of the APS-C sensor, is this correct?  If so, when I have the lens set to 18MM what am I actually seeing?  Is there MM you can put to the final product with the crop factor?  Finally, if I understand all this correctly, why don't they just name the lens with the MM it puts out with an APS-C sensor if it's not compatible with full frame anyway?

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Because the focal length of a lens is what it is; it's not just a naming convention.

People tend to make this whole thing more complicated than it is. If you're using only an APS-C camera, you don't have to worry about what focal length a lens would have to be to give the same result on a full-frame camera. Just start getting used to the idea that on an APS-C camera a "normal" lens is about 32mm (although the usual lengths available are 30 and 35), 17 to 18mm is a wide angle, 50 is a mild telephoto or portrait lens, 100 is a pretty strong telephoto, etc. Once you get used to it, it's a lot less confusing than worrying about conversion factors to a camera you don't ever use.

[Waddizzle]

The focal length of a lens describes a physical RATIO of internal components, which describes how the lens is made.

A camera lens projects a circular image onto a section of 35mm film, or an image sensor in a DSLR.  A section of film would be exposed through a window, so that all of the film would not be exposed at once. 

A full frame sensor is the same size as the 35mm film window..  This is what makes older lenses that were used with film cameras work with today's DSLR cameras.  The problem is that making an image sensor that large is somewaht expensive.  An APS-C sensor is smaller than a full frame sensor, so only the center of the image circle falls upon the sensor.

Because the APS-C sensor only "sees" just the center portion of the image, the net result is that the final captured image will seem as if it has been cropped, magnified, or zoomed in.  This apparent zoom is a fixed ratio relative to a full frame sensor, and is often referred to as crop factor.  Different camera brands have slightly different crop factors, depending upon the exact size of their APS-C image sensor relative to a full frame sensor. 

Canon APS-C sensors have a 1.6 crop factor.  Multiple the crop factor by the focal length fo the lens to obtain a "35mm film equivalent" focal length when the lens is used on an APS-C sensor body camera.

Some lenses are specifically designed to project a smaller image circle, one just large enough to cover an APS-C sensor, but not a full frame sensor.  A lens that projects an APS-C size image circle can be smaller, and less expensive to manufacture. Canon identifies these type of lenses as "EF-S".  Canon's full frame lenses are identified as "EF".

Even though the smaller EF-S lens projects a smaller image circle, the RATIO of the size of the internal components are the same as a full frame EF lens. 

For example, a 50mm full frame lens will project an image circle on an APS-C sensor.  A 50mm APS-C lens will project an image circle on an APS-C sensor that is the same size as the full frame sensor.  The difference between the lenses is that the full frame lens projects a much larger image circle.

[kvbarkley]

The focal length (mm) is an optical property of the lens. It has nothing to do with your camera. Your glasses have a focal length. The reason they don't label them with the 35mm equivalent (Full Frame) is that 35mm is just a convention, the most popular film size when digital took over. If APS had eliminated 35 mm before the digital transition, crop factor lenses would not be crop factor!

To your point, yes, 18 mm is not very wide for crop frame. That is why canon came out with the 10-22.

[ebiggs1]

The mm of a lens is what it is measured at.  it can not change once the lens has been created. It doesn't matter what camera it is used on.  Be it a P&S Powershot, a Rebel, a 35mm film or a medium format, etc.  It is what it is.

A far better way to tell how a lens will work on any given camera is AOV, or angle of view.  It is constant and never changes no matter what camera body the lens is on.

The AOV for a 18mm lens on a Rebel is approx. 100 74 degrees but the very same lens used on a FF will be only 74 100 degrees. ...  Nothing is cropped. Nothing is enlarged. Nothing is changed.

If you want your Rebel to produce a photo with the same AOV that the 18mm lens did when used on the FF, you will need to match the AOV. 100 degrees.  Easy, right. All lenses come with this spec or you can use a table to learn it. 100 degrees for your Rebel, you need a 10mm lens. If you had a Nikon you will need a 12mm lens.

I wish they had not come up with the term 'crop factor'.  It is so misunderstood shown even by some posts here. The main thing to remember is "Nothing is cropped. Nothing is enlarged. Nothing is changed." Learn to check the AOV spec and all will be good.

The 'S" in Canon's EF-S stands for short focus. That is why an 'S' lens can't fill the larger sized FF sensor. Not because the 18mm lens is different.

[RobertTheFat]

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@ebiggs1 wrote:

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A far better way to tell how a lens will work on any given camera is AOV, or angle of view.  It is constant and never changes no matter what camera body the lens is on.

The AOV for a 18mm lens on a Rebel is approx. 100 degrees but the very same lens used on a FF will be only 74 degrees. ... 

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That's about as clear a contradiction as the English language allows. I assume that it's not what you meant to say.

[ebiggs1]

Yes, Robert it is inversed.  Trying to do too many things at one time here!

"The AOV for a 18mm lens on a Rebel is approx. 100 74 degrees but the very same lens used on a FF will be only 74 100 degrees. ... "

[Waddizzle]

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@ebiggs1 wrote:

Yes, Robert it is inversed.  Trying to do too many things at one time here!

"The AOV for a 18mm lens on a Rebel is approx. 100 74 degrees but the very same lens used on a FF will be only 74 100 degrees. ... "

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A swing, and a miss.  You need the coffee this week.

[ebiggs1]

Thank you.  I will put on another pot. 

[Waddizzle]

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@RobertTheFat wrote:

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@ebiggs1 wrote:

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A far better way to tell how a lens will work on any given camera is AOV, or angle of view.  It is constant and never changes no matter what camera body the lens is on.

The AOV for a 18mm lens on a Rebel is approx. 100 degrees but the very same lens used on a FF will be only 74 degrees. ... 

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That's about as clear a contradiction as the English language allows. I assume that it's not what you meant to say.

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I hope this clears it up.