# Question for Great Bustard

Started 9 months ago | Discussions thread
Re: Question for Great Bustard
4

kiwi2 wrote:

HumanTarget wrote:

kiwi2 wrote:

But asking the question doesn't change the lens's physical properties? It doesn't become a f/16 lens?

Just like a iPhone7 is still f/1.8 and doesn't become f/12.3 just because they put the 35mm equivalent focal length in the specifications?

Right, but it doesn't become a 28mm lens, either. It's a 4mm f/1.8. And the f in "f/1.8" is important.

4mm / 1.8 = 2.22mm
28mm / 1.8 = 15.56mm

So for the same angle of view, the 28mm's entrance pupil is 7 times that of the iPhone7.

The / is important too. 'f' focal length '/' divided by aperture diameter. Why? Because light fades over a longer distance. Remember me mentioning the inverse-square law in the other thread?

Light does not "fade." Light spreads out. So it's not that you're losing light, it's just being spread over 49x the area. All that means is that as sensor size increases you need a longer focal length to maintain the same angle of view. Or, to put it another way, if you want to maintain an angle of view using a longer focal length, you need a larger sensor.

The aperture may be 7 times larger, but the light had to travel 7 times further over the focal length. (28/4)

Again, it's also covering 49x the area with the same intensity, which means there must be 49x the light. So where is the light being lost by traveling 7 times as far? Why do you think a larger sensor will have less DOF than a smaller at the same aperture setting if it's not due to the lens providing more light?

The light intensity reaching the sensor/film is the same. That is why it gets the same f/speed.

Yes, but so what? If I'm more interested in noise or DOF than the settings recorded in the EXIF, why would I care if the f-number is the same?
If I need to move a heavy object and know that two people can each lift half their weight (w/2), wouldn't I chose the heavier person? Or would I say that, since both have w/2 strength that their weight is unimportant?

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