topic Re: Does an EF-S lens' focal length have the 1.6 crop factor already calculated? in EF & RF Lenses

Does an EF-S lens' focal length have the 1.6 crop factor already calculated?

bryannemarie — Wed, 08 Jul 2015 15:53:46 GMT

For example, the kits lens that is often 18-55mm purchased with a canon rebel, since it is specifically made for crop-sensored camera, is it a 'true' 18mm focal length, just as an 18mm EF lens would be on a full frame camera (other than smaller field of view)? Or to get the 18mm focal length effect, would you still need to purchase something closer to a 10mm EF or EF-S lens?

Re: Does an EF-S lens' focal length have the 1.6 crop factor already calculated?

JonKline — Tue, 14 Jul 2015 18:41:39 GMT

NO.

If you have an EF and EF-S lens, each 24mm, they will look exactly the same on your crop camera.

Re: Does an EF-S lens' focal length have the 1.6 crop factor already calculated?

TCampbell — Tue, 22 Sep 2015 15:49:14 GMT

All Canon EOS lens focal lengths are TRUE focal lengths... they are not adjusted for crop factor.

The question is completely understandable as crop factor and how focal lengths are affected (really angle-of-view differences) and even what it does to depth-of-field are confusing.

As a generalization, you can identify the "magnifcation" factor of lens by comparing it a "normal" 1x magnification. When the focal length of a lens is equal to the diagonal measure of the imaging sensor then that lens will produce a 1x magnification image in terms of how your eye will view it's distortion.

A "full frame" DSLR (6D, 5D III or 1D X) has an imaging sensor which measures 36mm x 24mm. That works out to a diagonal of 43.27mm (we'll just round to 43mm). Nobody makes a lens with exactly 43mm focal length, but a 50mm lens is considered "normal" on those cameras.

An "APS-C" DSLR (every other current Canon model other than those listed in the last paragraph) has an imaging sensor which measures about 22.5mm x 15mm. That works out to a diagonal of 27.04mm (we'll just round that to 27mm). Again, nobody makes a 27mm lens, but you can get a 28mm lens.

When you consider that the kit zoom for an APS-C provides a range of 18-55 (with 27 being normal and roughly in the middle of the range) then you can see how the lens is offering a zoom range (expressed in magnification factors) of .66x to 2x.

A full-frame camera with, say, a 24-70 lens is offering roughly .55x to 1.6x.

So the angle of view on the 24-70 is slightly wider at the 24mm end (compared to an APS-C camera with an 18mm focal length) but not quite as magnified at the long end. Commonly, however the 24-105mm is used as the kit lens on a full-frame camera and that provides a magnifcation of 2.44x.

The APS-C sensor is commonly referred to as a "crop factor" because that's exactly what it does... it provides the identical result as shooting the image with a full-frame camera, but then "cropping" in on the image (and then probably enlarging that cropped image so it's back to the original size.)

There is a technical difference in the depth of field when you do this. If you want to compare the difference bewteen two cameras with different sensor sizes, you have to multiple the focal ratio of the lens by the crop-factor of the sensor to come up with an effective focal ratio.

This is why point & shoot cameras do not generate much background blur... if you take, say, an iPhone 5... The camera has an f/2.4 focal ratio, but that sensor is so tiny that it's crop factor is 7.21. The focal length is 4.1mm. When you do the math, it's effectively shooting at f/2.4 x 7.21 = 17.3... so that's f/17.3 (at 4.1mm). So it's no surprise that it's simply not capable of generating any background blur. This is a limitation with all small sensor cameras. (BTW, to avoid confusion... this calculation of multiplying the focal ratio by the crop factor is ONLY used for calcuating effective focal ratio for depth-of-field calcuations. It is NOT used for exposure calculations -- in which the focal ratio of your lens is true when it comes to calculating a correct exposure. Only adjust focal ratio for exposure calculations when using a teleconverter on the lens.)

But an APS-C sensor is considered large. Sure, it's not as large as a full-frame sensor but it's still a big sensor. Paired with the right lens, it's not a struggle to get an image with a tack-sharp subject and a beautifully blurred background.

Re: Does an EF-S lens' focal length have the 1.6 crop factor already calculated?

ebiggs1 — Thu, 24 Sep 2015 14:44:05 GMT

Once a lens is made it can not change its focal length. It is what it is forever.

Millimeters are so confusing it is far better to consider the AOV (angle-of-view) of a given lens instead of its mm. AOV never changes no matter what camera it is used on. iphone to 8x10 box camera, the same, same.

You camera is full frame in the sense you get exactly what you see in the view finder. The silly notion that it crops anything was invented by people that like comparing a DSLR to old world 35mm film cameras. That's all.

It is just as easy to learn AOV as it was to learn millimeters. You can think of mm to judge the size of a lens but AOV to decide what it will do on your camera.

AOV chart <---click me

Re: Does an EF-S lens' focal length have the 1.6 crop factor already calculated?

RobertTheFat — Fri, 25 Sep 2015 00:59:57 GMT

@ebiggs1 wrote:
Once a lens is made it can not change its focal length. It is what it is forever.
Millimeters are so confusing it is far better to consider the AOV (angle-of-view) of a given lens instead of its mm. AOV never changes no matter what camera it is used on. iphone to 8x10 box camera, the same, same.

You camera is full frame in the sense you get exactly what you see in the view finder. The silly notion that it crops anything was invented by people that like comparing a DSLR to old world 35mm film cameras. That's all.

It is just as easy to learn AOV as it was to learn millimeters. You can think of mm to judge the size of a lens but AOV to decide what it will do on your camera.

AOV chart <---click me

Even that (adding angle of view computation to your skill set) isn't really necessary. Millimeters can continue to serve you well. All you have to do is recognize that the smaller the sensir size, the more of a telephoto effect you'll get out of a lens of a given focal length and the shorter the focal length has to be to achieve a wide=angle effect. So if you happen to agree that a 17-55mm zoom is a good general-purpose lens on a Rebel or a 7D, you'll want a 24-70mm to get a similar effect on a 6D. A mild telephoto or portrait lens could be 50mm on a Rebel and 70mm on a 6D. And so on. The bottom line is that it's easier to memorize how a given lens behaves on your camera than it is to calculate how it would behave on some other camera when deciding whether to use it.

Re: Does an EF-S lens' focal length have the 1.6 crop factor already calculated?

ebiggs1 — Fri, 25 Sep 2015 13:24:33 GMT

Bob from Boston

And you would have to remember 1.6x, 1.3x and 1.5x and on and on. No such thing with AOV. People don't take to AOV because they aren't used to it. It is something foreign.

It is like the kids I tutor in music, the bass clef kids don't like sharps and the treble clef kids don't like flats. It is simply a state of mind, once it is learned.

Re: Does an EF-S lens' focal length have the 1.6 crop factor already calculated?

Waddizzle — Mon, 05 Oct 2015 15:54:03 GMT

What happens to AOV when you use a 1.4x/2x extender?

Re: Does an EF-S lens' focal length have the 1.6 crop factor already calculated?

ebiggs1 — Mon, 05 Oct 2015 16:36:02 GMT

AOV like any other physical factor of a given lens can not change. AOV remains the same. A tele adapter is more of a true magnifier. It is the "apparent" AOV that changes. A tele adapter does not change the focal length of the lens.

In a crop factor camera vs a FF camera, when you take a picture of a bird, or mountain, etc, for instance the bird will remain the same size in either camera. It is the field of view that is "cropped".

Re: Does an EF-S lens' focal length have the 1.6 crop factor already calculated?

Waddizzle — Mon, 05 Oct 2015 16:58:43 GMT

@ebiggs1 wrote:
.... It is the "apparent" AOV that changes. A tele adapter does not change the focal length of the lens.

In a crop factor camera vs a FF camera, when you take a picture of a bird, or mountain, etc, for instance the bird will remain the same size in either camera. It is the field of view that is "cropped".

Yes, I understand how the image projected by the lens is "handled" differently by a Full-Frame compared to an APS-C sensor. But, it is that "apparent" change in the AOV when using a standard adapter that I was asking about.

Re: Does an EF-S lens' focal length have the 1.6 crop factor already calculated?

ebiggs1 — Mon, 05 Oct 2015 19:16:03 GMT

Well in other words and possibly easier to understand, I guess, 1.4x is not really different than a 1.6 crop camera. Just .2 different.

Re: Does an EF-S lens' focal length have the 1.6 crop factor already calculated?

Waddizzle — Mon, 05 Oct 2015 20:34:40 GMT

Okay, thanks. I'm just a part-time math guru.

FYI, I've found a formula to convert focal length to FOV. The formula that it implements is FOV = 2 arctan (x / (2 f)), where x is the diagonal dimension of the sensor. The FOV is measured across the sensor's diagonal, and is therefore smaller across the horizonal dimension, and even smaller across the vertical dimension.

Re: Does an EF-S lens' focal length have the 1.6 crop factor already calculated?

ebiggs1 — Mon, 05 Oct 2015 20:53:38 GMT

It's not that hard. Just remember 46 is normal. Anything below is WA and anything above is tele.