Tim
I read with respect your comments but I am not sure you read all of mine...
I NEVER said that the physical focal length of a lens changes, That would be a physical impossibilty.
If I may quote from the Article I referred to:
"Equivalence, at its most simple, is a way of comparing different formats (sensor sizes) on a common basis. This is already the way most lenses are talked about: it's quite common to say that a compact camera includes a '28-120mm lens' but the key and (often unspoken) word in that description is 'equivalent.' It's a simple way of describing the range of fields-of-view that the lens offers, cancelling out the effect of sensor size by using a common reference point" (the bold and Italics are mine).
My whole discussion was:
First: the two lenses that the OP referred to use virtually the same equivalent focal length. The fact that one is identified as an APS-C lens and the other is not is irrelevent, because the manufacturer uses the Equivalence value of the focal length in their lens markings and descriptions. i.e. they both have their focal lengths described in terms of what would be used on a 35mm FF camera. So it would be inaccurate to mulitiply one by the crop factor and not the other as they are both using the same frame of reference (excuse the pun 
) Essentially the behaviour of the lenses will be very similar on his crop sensor.
As regards lens labelling: the numbers on lenses provided by manufacturers are Equivalent Values: Any FF lens DOES have a physical focal length of what the label says. Lenses specifically designed for smaller formats do not have the same physical focal lenght, but they are documents as having the same Equivalence values. For example: the lens on a SX60HS has a physical focal length of between 3.8 - 241mm, it is patently NOT the values of 21 - 1365mm which is what is listed on the lens or the documentation: why? Because camera manufacturers use Equivalence FL values to describe the AoV (and by implication the perceived magnification) that the lens will have on a FF sensor. They do this to provide the result of combining the different sensor size with the optics of the lens in the camera. It's not just marketing hype was was dismissively suggested, it is a convention used by (to my knowledge) all camera and lens manufacturers. That is ONE reason why EF-S lenses are physically smaller than their EF counterparts and similarly micro 4/3 lenses are physically smaller still. Generally, people don't care about the physical focal length of the lens they care about what the camera will capture.
Second: IF one actually wanted a specific Field of View (e.g. something that truly produced a FoV of a 24mm lens) of on a crop sensor then one should realize that the frame of refrence for all lenses is for that of a full frame sensor. Without altering the physics of the lens in any way (because it can't) the sensor will "see" less of the image projected by the lens, essentially altering the resultant Field of View captured and it does so by the crop factor of the lens. This is absolutely and clearly demonstrated by the reference material I submitted to make my point.
As I commented in the scenario of the classic 50mm lens. Stick a lens labelled 50 mm on a FF and it gives you a "normal" FoV, stick the same 50mm lens on an APS-C camera and the image received generated by the sensor is akin to that from an 80mm lens. No, the lens did not change in any way, but what the sensor received DID.
That's all very well, but what if I wanted a "normal" FoV on my crop sensor body, because of the difference in what the sensor sees, then I woud choose a lens that has a smaller focal lengh and wider FoV and I would calculate that by dividing the 50mm Equivalence value by 1.6, coming out with a requirement for a lens with an equivalence focal length value of around 33mm (give or take depending on what's available). The images that that lens and sensor combination would generate woud have a similar FoV to that from the FF camera with a 50mm lens.
In the end a lens cannot operate on its own, it needs a sensor to capture what the lens delivers: one cannot isolate a lens on its own in terms of the image you will get. While a specific lens' physical properties are, without anyt debate, a constant (so a 50mm lens is a 50mm lens), when combined with a sensor that is not a FF one, the field of view is not the same as for the FF one.
As I have said already, my point of view is born out by both the article and the videos I submitted. Here is another if you like:
http://www.robsphotography.co.nz/crop-factor-advantage.html
I'll insert some comments inline... but I'll start off by saying I don't think I disagreed with anything you said. But as I read your reply here, I'm thinking there there is one point where we disagree.
BTW, I did read your full message. I did not read the embedded links.
@Tronhard wrote:Tim
I read with respect your comments but I am not sure you read all of mine...
I NEVER said that the physical focal length of a lens changes, That would be a physical impossibilty.
I'm wondering if you thought my reply was somehow directed at you. It was not. Nothing in my message was meant to say you stated the physical focal length changed, etc.
If I may quote from the Article I referred to:
"Equivalence, at its most simple, is a way of comparing different formats (sensor sizes) on a common basis. This is already the way most lenses are talked about: it's quite common to say that a compact camera includes a '28-120mm lens' but the key and (often unspoken) word in that description is 'equivalent.' It's a simple way of describing the range of fields-of-view that the lens offers, cancelling out the effect of sensor size by using a common reference point" (the bold and Italics are mine).
This is all true. But one area of caution... the OP is asking about lenses specifcally on Canon EOS DSLR camera bodies. If you start bringing in other cameras where they do use focal length "equivalence" it can confuse the issue. Canon does not use focal length equivalence for EOS DSLR lenses such as the EF & EF-S lenses.
While Canon doesn't do this for these particular lenses, they do for some other cameras & lenses and of course loads of other manufacturers do for their cameras & lenses.
Your point is completely valid as a generalization (when applied broadly, it often tends to be true), just not when applied to these two specific lenses being discussed.
My whole discussion was:
First: the two lenses that the OP referred to use virtually the same equivalent focal length. The fact that one is identified as an APS-C lens and the other is not is irrelevent, because the manufacturer uses the Equivalence value of the focal length in their lens markings and descriptions. i.e. they both have their focal lengths described in terms of what would be used on a 35mm FF camera. So it would be inaccurate to mulitiply one by the crop factor and not the other as they are both using the same frame of reference (excuse the pun
Ok, if I properly understand what you meant to write in this paragraph, then this sounds like a point of confusion.
You wrote: "because the manufacturer uses the Equivalence value of the focal length in their lens markings and descriptions".
The OP is asking about a Canon EF 28mm lens and a Canon EF-S 24mm lens. The manufacturer here is Canon and these are both Canon EOS DSLR lenses.
While 35mm equivalance is often used elswhere, it is not used when referring to EF & EF-S lenses.
As regards lens labelling: the numbers on lenses provided by manufacturers are Equivalent Values: Any FF lens DOES have a physical focal length of what the label says. Lenses specifically designed for smaller formats do not have the same physical focal lenght, but they are documents as having the same Equivalence values. For example: the lens on a SX60HS has a physical focal length of between 3.8 - 241mm, it is patently NOT the values of 21 - 1365mm which is what is listed on the lens or the documentation: why? Because camera manufacturers use Equivalence FL values to describe the AoV (and by implication the perceived magnification) that the lens will have on a FF sensor. They do this to provide the result of combining the different sensor size with the optics of the lens in the camera. It's not just marketing hype was was dismissively suggested, it is a convention used by (to my knowledge) all camera and lens manufacturers. That is ONE reason why EF-S lenses are physically smaller than their EF counterparts and similarly micro 4/3 lenses are physically smaller still. Generally, people don't care about the physical focal length of the lens they care about what the camera will capture.
I addressed this above. The caution here is that this is true much of the time... but isn't true all of the time. In other words, not *all* lenses made for focal lengths other than 35mm are listed as "35mm equivalent" values. That will depend on the convention of the camera & lens manufacturer.
Canon does not use equivalent on EF-S lenses. They use true focal length.
You'll find many lenses designed in EF-S are meant to approximiate what you'd get using the full-frame version ... but Canon marks them with true focal lengths.
I'll give an example:
The Canon EF 100mm macro lenses have a 100mm focal length. But for the APS-C bodies, Canon makes the EF-S 60mm macro lens. 60mm is the true focal length of that lens (it's not a 35mm "equivalent"). If you multiply the focal length by the 1.6x constant (the crop-factor for Canon's APS-C sensor) you get 96mm. In other words, using a 60mm macro on an APS-C camera will give roughly the same angle of view that you'd get if you used a 100mm macro on a full-frame camera.
Similarly you can notice things like the EF 24-70 vs. the EF-S 18-55 (or 17-55) where these are both standard zooms that offer a gentle wide-angle through gentle narrow-angle field of view. But for the EF-S camera the entire focal length range is shifted down because when you apply the crop-factor, the angles of view work out to be fairly similar (though not identical ... you get the same utility value out of them.)
Second: IF one actually wanted a specific Field of View (e.g. something that truly produced a FoV of a 24mm lens) of on a crop sensor then one should realize that the frame of refrence for all lenses is for that of a full frame sensor. Without altering the physics of the lens in any way (because it can't) the sensor will "see" less of the image projected by the lens, essentially altering the resultant Field of View captured and it does so by the crop factor of the lens. This is absolutely and clearly demonstrated by the reference material I submitted to make my point.
As I commented in the scenario of the classic 50mm lens. Stick a lens labelled 50 mm on a FF and it gives you a "normal" FoV, stick the same 50mm lens on an APS-C camera and the image received generated by the sensor is akin to that from an 80mm lens. No, the lens did not change in any way, but what the sensor received DID.
I completely agree with all of these points. I don't think I wrote anything that contradicted this.
That's all very well, but what if I wanted a "normal" FoV on my crop sensor body, because of the difference in what the sensor sees, then I woud choose a lens that has a smaller focal lengh and wider FoV and I would calculate that by dividing the 50mm Equivalence value by 1.6, coming out with a requirement for a lens with an equivalence focal length value of around 33mm (give or take depending on what's available). The images that that lens and sensor combination would generate woud have a similar FoV to that from the FF camera with a 50mm lens.
Also true.
BTW, 50mm isn't a perfectly precisely a normal angle of view. It's very fractionally narrower than normal... just not enough to be obvious.
If the focal length of a lens matches the diagonal measure of the sensor, you'll end up with a normal angle of view.
E.g. for a full frame camera it happens at about 44mm and 50mm is close enough. For a Canon APS-C sensor it's about 27mm (and 28mm is certainly close enough).
In the end a lens cannot operate on its own, it needs a sensor to capture what the lens delivers: one cannot isolate a lens on its own in terms of the image you will get. While a specific lens' physical properties are, without anyt debate, a constant (so a 50mm lens is a 50mm lens), when combined with a sensor that is not a FF one, the field of view is not the same as for the FF one.
As I have said already, my point of view is born out by both the article and the videos I submitted. Here is another if you like:
http://www.robsphotography.co.nz/crop-factor-advantage.html
I think everything you say here is accurate and the only issue where things got tripped up is, if I understood you correctly, you were thinking that EF-S lenses were using "equivalent" focal lengths.
If ever in doubt, it's possible to measure this.
If you look here: https://www.tawbaware.com/maxlyons/calc.htm
You'll find a "Dimensional Field of View" calculator.
If you have a full-frame camera and a 50mm lens, then you could:
- Measure out some distance from the sensor plane (say... 10')
- Measure a subject at that distance (for example another measuring tape placed to measure the width of the frame)
- Use the calculator at that website to determine what focal length it would need to be to get that measurement.
This would be a way to find out if a lens' advertised focal length is true vs. equivalent.
BTW, camera companies like to round off values and there is the issue of focus-breathing ... which means a 50mm lens isn't necessary precisely 50mm and as you adjust focus, that true focal length will change slightly. A decent lens tends to keep things within 5% of what it claims to be. Some lenses are "heavy breathers" and focal length can change substantially (20-30% or more).
Hi Tim
Thank you for you excellent responses. I think my use of language has, on occasion, been less clear than I had intended - right now I'm more than a bit stressed dealing with my partner's terminal cancer, rehabbing my knee replacement and having our second house move in six months. Stress does that to one... 
You're right about the lenses showing the focal length - I actually was looking at the side of the SX60HS where it gave the equivalence values in ginormous letters, while the actual physical values were in tiny letters around the front and hidden by my protective filter -- my bad.
One thing this dialogue has done is made me consider how we really express, both in metrics and semantics the impact of the relationship between lens focal length, sensor size and FoV. It's not that the science is unclear, but often how that is translated into marketing, reviews (many reviewers use equivalence values without saying so), and general discussion. As one reviewer put it, "Actually explaining the changes in FoV between sensors by expressing them as Equivalent focal lengths doesn't do anyone any favours, even if it is a convenient or indicative metric".
In trying to help the OP, I was obviously saying, like you, that both lenses he was considering were very similar, and if he was going to use the crop factor multiplier on one then he needed to do it for the other. What I was also trying to figure out, and didn't know from his post, was if he was actually after a FoV equivalent to that of a FF 24mm (or whatever lens), hence my reverse calculation. Still, he seems to have come to a conclusion that has worked for him and in the end that's good.
Now, onto the much more important question of what lens I shall buy next... 
@kvbarkley wrote:I see it as a relic. If APS had taken over from 35mm film before the transisiton to digital, we would be saying that FF cameras have a 0.625 factor!
The only people who care are those that are used to a particular angle of view in 35 mm and want the same one in APS-C sized sensors.
Otherwise it just adds confusion.
I agree that it seems to have come from the dominance of the 35mm film format, just as the APS came from Kodak's Advanced Photo System film format, but was not so popular and, I believe, died out in the mid 1990's.
Still, almost any metric we consider is, to some extent, arbitrary. I have always wondered why, when the rest of the planet is using the metric system (much more condusive to digital computers), the USA has stuck, at a consumer level) with the imperial system. Especially given that at the time the metric system was developed the USA was breaking away from the UK and had strong ties to France. I realize that the cost was considered too high when it was finally mooted in the last century, but I will say that behind the customer interface much of industry has gone metric - especially in things like cars and of course science. I understood that issue of conversion between metric and imperial was the root cause of the error that was built into the original Hubble telescope and caused its optics issue.
The transition to metric has made certain imperial-based phrases somewhat perplexing to the metric generation. I was with a friend and his two teenage boys (here in NZ) and one of us said "give him an inch and he'll take a mile". The boys wanted that to be explained, but the direct translation: given them 25.4mm and the'll take 1.6093km just doesn't have the same appeal!
Hi Tim:
Thank you for your kind thoughts and gracious comments. I thought your responses were business-like but not at all offensive. There has been a lot of stress for me lately and I know that it impacts one's ability to think well and express oneself effectively, so I am glad we all came out of what is a tricky discussion unscathed! 
Being able to escape to the forum for a while and immerse myself in things photographic is a welcome relief right now. I just wish I could go out and do some more photography - that is, for me, a really relaxing activity.
