Question for Great Bustard

Started 10 months ago | Discussions thread
OP kiwi2 Veteran Member • Posts: 3,517
Re: Question for Great Bustard

HumanTarget wrote:

kiwi2 wrote:

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.

All true, I don't disagree.

But using a longer focal length on the same camera is also how we 'zoom' in on a narrower field of view and increase chosen subject size in the frame. That longer focal length reduces light intensity at the sensor, so a larger aperture is needed to compensate. The f/ratio describes that relationship between focal length and aperture size.


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?

Photography is based around the f-numbers being the same. It has practical applications like being able to take a reading with handheld light metre, that may give you a reading of say 1/250 sec, ISO 100, f/5.6... and no matter what lens you put on, be it a wide angle or telephoto or of any focal length on any format size, it will come out correctly exposed if set to f/5.6.

It's kind of useful

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