Please tell me I am correct & the pro Is wrong???

Started Jan 29, 2013 | Discussions thread
plevyadophy Veteran Member • Posts: 4,258
THE definition Re: Cha-ching!

Great Bustard wrote:

tko wrote:

You are right, it's FOV. I would also say it's equivalent focal length. As in 100,000 pounds of TNT is equivalent to an atomic bomb.

Not the same, but equivalent.

That's why it's called the equivalence theory, a word many don't seem to fully grasp. It's not the "same thing theory" for a reason.

And just to complete the discussion, an F-stop is always an F-stops, ISO is always ISO, and focal length is always focal length. But there are equivalences. Sometimes people forget or never learned and leave off the equivalence part.

Exactly right:

A 50mm f/2 lens is a 50mm f/2 lens regardless of the sensor that sits behind it. However, the effect of 50mm f/2, in terms of the visual properties of the recorded photo, depend very much on the sensor that sits behind the lens:

50mm f/2 on 4/3 (mFT) is equivalent to 62.5mm f/2.5 on 1.6x (Canon APS-C), 67mm f/2.7 on 1.5x (APS-C for everyone else), and 100mm f/4 on FF (FX), where "equivalent to" means:

  • The photos all have the same AOV (diagonal angle of view) and aperture (entrance pupil) diameter: 50mm / 2 = 62.5mm / 2.5 = 67mm / 2.7 = 100mm / 4 = 25mm.

  • The photos all have the same DOF (as well as diffraction softening) when they have same perspective (subject-camera distance), AOV, aperture diameter, and display size.

  • The photos all have the same motion blur and the same total amount of light falls on the sensor when the aperture diameter and shutter speed are the same (e.g. ISO 100 on mFT, ISO 160 on APS-C, and ISO 400 on FF).

  • The photos all have the same same noise when the same total amount of light falls on the sensor if the sensors are equally efficient (less noise if the sensor is more efficient, more noise if the sensor is less efficient).

  • Other elements of IQ, such as resolution, bokeh, flare resistance, etc., as well as elements of operation, such as AF speed/accuracy, size, weight, etc., are not covered in this use of the term "equivalent".


I think that's the best written definition I have ever seen (succinct, clear and accurate).

I will bookmark your post as a reference should anyone ever ask me.



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