Question for Great Bustard

Started 9 months ago | Discussions thread
HumanTarget Senior Member • Posts: 1,632
Re: Question for Great Bustard

kiwi2 wrote:

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.

No disagreement there.

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

Yes, but "correctly exposed" is, as Richard pointed out in another post, a can of worms. And what's "correctly exposed" following your method will result in different results on different  formats  (say, f/1.8 on the iPhone vs. f/1.8 on full frame, to continue our comparison). If you're less concerned with what your exposure settings are and instead more interested in the results you'll get (say, in terms of DOF or noise), it helps to know what's really going on.

Understanding that the amount of light is more important than exposure in terms of noise is kind of useful, too. Like, if you need a certain shutter speed and DOF in a given limited amount of light, a larger sensor's not going to really gain you anything.

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