Yet another crop vs FF factor.

subpixel wrote in post #11385966
How can they not change, the example I gave of focal length is right, 17-55mm is 27-88mm in crop, and that is a fact.

It appears that you have misunderstood what you have read, because the above is NOT fact.

The truth is that the field (angle) of view provided by a 17-55mm lens on an APS-C format (often referred to here as a "1.6 crop" format) camera would be the same as a 27.2-88mm lens on a 35mm film format camera.

The only things that change when you put a given lens on different format cameras are the field (angle) of view and (because of the difference in the camera format) depth of field at identical aperture settings.

Read my article in your linked thread very carefully after you throw out what you "know" already. Lots of people have learned from it.

The focal length of a lens is a measurement of the distance from the rear nodal point of the lens to the point where the subject is in focus when the subject is at infinity. A lens with a 100mm focal length will be a 100 mm lens whether it is mounted on a crop body, a FF body, a medium format body, or a view camera, or just sitting on your desk. It is a measurement of the lens itself, and has no interaction with what, if anything, it is mounted on.

Perhaps it would be better if you thought of the so-called "effective" focal length for what it really is: a cropping of the image projected by the lens.

Thanks guys, my confusion is cleared
SkipD: very informative post on crop on the other link, thanks.

I'm not sure if this was covered.
The "aperture" changes when a TC is used because it is not really aperture at all. It is "F ratio", as I defined earlier. If you change the focal length (with a TC) and keep the entrance pupil the same (since it's the same lens), the F ratio (or "aperture") must necessarily change.

xarqi wrote in post #11386249
I'm not sure if this was covered.
The "aperture" changes when a TC is used because it is not really aperture at all. It is "F ratio", as I defined earlier. If you change the focal length (with a TC) and keep the entrance pupil the same (since it's the same lens), the F ratio (or "aperture") must necessarily change.

That's what I thought that happened with the crop cameras

subpixel wrote in post #11386343
That's what I thought that happened with the crop cameras

Hopefully you now understand that focal length of a lens does not change when it is moved to different format cameras. Thus the f-stop values also remain the same as the lens is moved from one format camera to another.

It's amazing how often we hear that camera sales people fed the buyers with the BS about a lens' focal length changing when used on different format cameras.

SkipD wrote in post #11386372
Hopefully you now understand that focal length of a lens does not change when it is moved to different format cameras. Thus the f-stop values also remain the same as the lens is moved from one format camera to another.

It's amazing how often we hear that camera sales people fed the buyers with the BS about a lens' focal length changing when used on different format cameras.

It's easy for misinformed people to fell for that one, just like I was.

xarqi wrote in post #11386249
I'm not sure if this was covered.
The "aperture" changes when a TC is used because it is not really aperture at all. It is "F ratio", as I defined earlier. If you change the focal length (with a TC) and keep the entrance pupil the same (since it's the same lens), the F ratio (or "aperture") must necessarily change.

Yep.

When the spec of a lens is given, the aperture is given using an "f/<number>" nomenclature. Ever wonder why that is?

It's because the "f" in that nomenclature stands for the focal length of the lens! The aperture of the lens is being given in terms of the focal length. If you have an f/2.8 lens, the actual diameter of the aperture is the lens' focal length divided by 2.8. If it's a "fixed aperture" zoom lens (which just means the ratio remains constant), then the diameter of the aperture changes as you change the focal length. This is one reason the cheapest zoom lenses tend to have "variable" apertures: the diameter of the aperture doesn't have to be as tightly controlled as a function of focal length.

This is also why when you change the aperture value (the focal length divided by the aperture diameter) to change the light by a stop, which is a factor of two, you don't change the f-stop value by a factor of two, you change it by a factor of the square root of two -- the amount of light being let in by the aperture is directly proportional to its area, not its diameter. It's directly proportional to the square of its diameter. If you were to double the diameter of the aperture, you'd be increasing the amount of light being let in by a factor of four, or two stops. Put another way, the diameter of the aperture is proportional to the square root of the amount of light being let in. Double the amount of light, and you have to increase (multiply) the aperture diameter by a factor of the square root of two. Halve the amount of light, and you have to decrease (divide) the aperture by a factor of the square root of two.

That relationship is why the f-stop numbers change by a square root of two per stop of light controlled.

subpixel wrote in post #11385966
How can they not change, the example I gave of focal length is right, 17-55mm is 27-88mm in crop, and that is a fact.
Crop factor thread:
https://photography-on-the.net …487&highlight=c​rop+factor

The 17-55mm lens on APS-C has the same AOV in its 15x22mm frame as provided by a 27-88mm lens in the 24mmx36mm FF frame. But it is a 17-55mm lens. period, end of sentence, end of paragraph

The crop factor is useful only to someone who has used FF, and resultantly knows the AOV which is achieved with different lenses, so that when they pick up the APS-C camera they know what FL to use to get the equivalent AOV. If you have no experience with FF, the crop factor of 1.6 is a totally meaningless and useless number.

Due to the smaller format size and the use of a shorter FL to achieve the same angle of view, at the same shooting aperture the DOF of the smaller format is deeper than with a larger format. One would need to shoot at about 1.5EV larger aperture size with APS-C to equal the DOF of the larger format FF camera.