Yet another crop vs FF factor.

Ok, correct me if I'm wrong. Been thinking about this for some time, and I just wanted to confirm or proven wrong.
When comparing full frame cameras against crop cameras, I never found the following factor mentioned or described, I don't know if everyone is assuming it already, or is something that isn't often remembered.
The thing is, when you have a lens like 17-55mm f2.8 (just an example) on a crop camera, and you apply the necessary crop factor conversion (x1.6), to make it an effective 27-88mm, you should also taken in consideration that tis conversion is also affecting your aperture. the f2.8 is right to 17-55mm but not to 27-88mm.
Obvious, i think, or am I all wrong?
BTW, the math:
17-55mm gives you an aperture diameter of 6-20mm for f2.8
Converting it to crop (x1.6)
27-88mm with an 6-20mm diameter gives you f4.5

Right, wrong?

There is no "effective focal length".
F ratio or "aperture" as it is often called, is ALWAYS the focal length divided by the entrance pupil diameter.

Wrong, and welcome to the forum.

1. Focal length DOES NOT CHANGE, the field of view changes, and the field of view is the same as focal length x crop factor

2. Aperture stays the same, no change. DOF changes, but aperture does not.

ill try to answer, i think it give you a bokeh like an aperture of 4.5 but the quantity of light that pass troug your lenses remain unchanged so its luminous like a 2.8.

Ok, all valid, but explain me this. What happens when you use a teleconverter, when you use a 2x teleconverter your aperture is reduced by 2 stop because your focal length is duplicated, this is the thing that a crop cameras do, right?

subpixel wrote in post #11385911
Ok, all valid, but explain me this. What happens when you use a teleconverter, when you use a 2x teleconverter your aperture is reduced by 2 stop because your focal length is duplicated, this is the thing that a crop cameras do, right?

No, crop frame cameras do not change focal length, period. The sensor is just smaller so the FoV is smaller.

subpixel wrote in post #11385911
Ok, all valid, but explain me this. What happens when you use a teleconverter, when you use a 2x teleconverter your aperture is reduced by 2 stop because your focal length is duplicated, this is the thing that a crop cameras do, right?

No. A teleconverter increase the focal length of your lens but using a camera with a smaller sensor doesn't.

themadman wrote in post #11385932
No, crop frame cameras do not change focal length, period. The sensor is just smaller so the FoV is smaller.

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

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

think about, the sensor is still the same size when you use the converter. nothing is cropped.

This is painful. It's been explained.

HyperYagami wrote in post #11385988
think about, the sensor is still the same size when you use the converter. nothing is cropped.

Ok, all good. But why is it said that the teleconverters reduce the amount of light?

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

17-55mm focal length is still 17-55mm focal length no matter what camera it's on, crop or full frame. The field of view changes, not the focal length.

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

That's why they call it a CROP factor. The sensor effectively crops the image that the lens is seeing... So it becomes a "longer" lens, essentially...

It's a crop, not a change.

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

Focal length DOES NOT CHANGE with crop factor!!!

It is only the FIELD OF VIEW.

A 17-55mm lens on a APS-C camera has the same FIELD OF VIEW as a 27-88mm lens on a FF camera.

The reason a teleconverter changes aperture is because it DOES change the focal length so the opening to focal length ratio changes, hence reduced light and aperture.

themadman wrote in post #11386046
Focal length DOES NOT CHANGE with crop factor!!!

It is only the FIELD OF VIEW.

A 17-55mm lens on a APS-C camera has the same FIELD OF VIEW as a 27-88mm lens on a FF camera.

The reason a teleconverter changes aperture is because it DOES change the focal length so the opening to focal length ratio changes, hence reduced light and aperture.

Thanks for that las explanation, I didn't realized that using a teleconverter and crop factor where completely different things, sorry