# A few words on Equivalence and comparing systems

Started Apr 2, 2013 | Discussions thread
Re: No you did not, or ...

forpetessake wrote:

69chevy wrote:

Great Bustard wrote:

69chevy wrote:

Great Bustard wrote:

69chevy wrote:

So...

My camera phone has a crop factor of 7.64, the lens is 3.85mm and the aperture is f2.4.

The FF equivalent is 30mm f2.4?

No -- the FF equivalent is 29mm f/18 (3.85mm x 7.64 = 29mm, f/2.4 x 7.64 = f/18).

No disrespect, but if you are strictly talking DOF, this makes a little sense. If you are talking about diffraction effects or exposure effects it is not true.

...read the OP.  It's all discussed there (and yes, diffraction and exposure are discussed).

I did.

You can compare diffraction between two cameras with the same entrance pupil and different sensor sizes, but not with different flange distances and sensor sizes. This is where your equivalency formula falls short.

You either didn't read or did not understand. If it's the latter, then there is no shame in asking questions, but making arrogant statements is a completely different matter.

Diffraction isn't just related to aperture. It is also a function of sensor size. It is not arrogance, it is true.

When diffraction occurs the light is bent into a disc shape. As sensor size decreases, the pixel size does as well.

This means that the closer together the pixels are, the more diffraction affects the image.

This is why the equivalence is not accurate. Here is a little drawing I borrowed to illustrate. Credit goes to http://www.digitalversus.com for dumbing it down.

Full Frame 12 MP                       APSC 18 MP

1/2.3"  14 MP

I won't even get into the effects of how changing the flange distance changes the convex shape required by the lens. (Which also affects diffraction).