[vodanh1982]

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?

 

You're saying

 

a 50mm becomes a 75mm (not true)

f/1.8 becomes f/2.8

ISO 200 becomes 300

 

Left numbers FF, right numbers APS-C

 

That doesn't make any sense

 

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:

Let's compare 50 f/1.8 vs 75 f/2.7

From http://en.wikipedia.org/wiki/F-number

 

D (crop)=f/N=(50/1.8) = 27.7

D (FF) = (75/2.7) = 27.7

 

So two lenses have the same "opening"

 

Because the FF is larger the light intensive is lower. At the same shutter speed and ISO the FF image is underexposed. To make 2 images at the same exposure, we need to increase ISO of FF. Now the FF has higher ISO than the crop.

The question: will the FF at higher ISO has more noise than the crop at lower ISO?

There are two factors that affect the noise

-The sensor technology.

-The total amount of light get to the sensor.

Assume two sensors are made with the same technology. Because of the same "opening", two sensors receive the same amount of light with the same shutter speed. In conclusion, two sensors has the same noise even at different ISO.

In reality, the result maybe a little bit off because the crop and the FF are not manufactured using the same technique, micro-lenses, spacing, light fall off sensor (non-rounded sensors), etc

 

Other factor that is not equivalent is the resolution. Lens is limited by their own resolution in term of LP/mm. Assume the same sharpness and even sharpness distribution for the whole area of the lens, the FF will have more resolution because it is bigger. The total resolution = lens resolution x picture height.