[Brightcolours]

What kind of equivalence? These number acrobatics are just ridiculous - for what reason has the ISO be multiplied? Does the sensor gets less sensitive or more just because of the sensor size?

The sensor gets MORE sensitive. If you have more surface, the sensor will catch more light. If you have bigger pixels, the pixels will catch more light. Hence: the FF sensor is MORE sensitive by definition. 

You're saying

 

a 50mm becomes a 75mm (not true)

No one says that 50 mm "becomes" 75mm. It is like this (for 1.5x crop compared to 135 format):

To get a similar field of view, you have to use an EQUIVALENT focal length (one that gives you the same image). So if you use 50mm on 1.5x crop format, you must use 75mm on 135 ("FF") format.

f/1.8 becomes f/2.8

Again the same: to get a similar DOF, you have to use an EQUIVALENT f-value, which will give the same aperture size. So if you use f1.8 on 1.5x crop, use f2.8 on FF for a similar DOF.

ISO 200 becomes 300

Wrong. The same applies again as above. To catch the same amount of light to form the image, and/or to take the image in a similar exposure duration, you will have to use an EQUIVALENT ISO setting. So if you use ISO 200 on 1.5x crop, you have to use ISO 450 on 135 format, for a similar amount of light forming the image and a similar exposure time.

Left numbers FF, right numbers APS-C

 

That doesn't make any sense

It does make sense in every way you look at it. But you have to not make thought process errors, like that f-values are linked to film sensitivity.

If you go on with that you end up with 500mm, f/18 and 2000 ISO for a normal point and shoot - what kind of equivalence that should be, I just fail to understand.   :blink:

I think you are confusing yourself here... "normal point and shoot" (I am guessing you mean compact digital cameras with a small sensor, here....) use lenses with super small focal lengths. Of course, if you take a compact digital ultra zoom model which reaches a 500mm FF EQUIVALENT, then yes, 500mm for FF for that 500mm equivalent setting on the compact digital. 

 

Lets take a "typical" compact digital as example, a Canon PowerShot S100. It has a 7.44 x 5.58mm sensor size. This S100 has a 5.2-26mm f2-5.9 lens.

And lets compare it to a "typical" 135 format FF DSLR, a Nikon D3s. It has a 36 x 23.9mm sensor size. Which lens would be equivalent on the D3s to the lens of the S100?

 

36 / 7.44 = ~4.8x crop. I take the width and not the diagonal, to get the same FOV when using the camera in horizontal orientation.

 

To get the same FOV, the D3s will need a 5.2x4.8= 25mm to 26x4.8= 124mm lens.

So, basically a Nikon 24-120mm lens.

To get the same DOF (same size aperture), we will need to use 2x4.8= f9.6 to 5.9x4.8= f28.3.

 

Of course, we can not get a lens with such small max. aperture on FF, so the lens will always have more capabilities than that of the compact digital camera (which of course is the reason both type of cameras exist). 

 

So, an equivalent lens to the S100's 5.2-26mm f2-5.9 on 135 format will be a 24-120mm f10-f32 lens, basically. One will have to settle for the Nikon 24-120mm f4 VR or its older sibling and stop it down accordingly.

 

To match the amount of light forming the image, and keep the same exposure time (both have nothing to do with the equivalence of the lens, we are talking about the equivalence of the sensor now), we have to set an equivalent ISO setting.

Suppose we use ISO 200 on the digital compact. We then have to use 200 x 4.8 x 4.8 = ~ ISO 4600 in the D3s.

This then will result in a similar exposure times, when we set the lenses at equivalent settings.