Comparing Olympus 4/3lenses to FX "Full Frame" offerings

Started Jan 25, 2014 | Discussions thread
Great Bustard Forum Pro • Posts: 40,751
Apreture, HST, and context.

Dimitri_P wrote:

tko wrote among other wierd stuff :

Do you think a F2.8 cell phone lens, the size of a thimble, is equal to a F2.0 M43rds lens?

Equality is a funny thing. 100 pennies is equal to 4 quarters which is equal to a soiled crumpled up 1 dollar bill.

Exactly. So, for example, it wouldn't make sense to say "2 coins = 2 coins", would it? That is, it would only be true if each coin had the same value.

F-stop is a NORMALIZED number, normalized to the sensor size.

You are funny.

Ah, here we are:

Understanding the fundamental concepts of Equivalence requires making important distinctions between various terms which people often take to mean the same thing. It is very much akin to making the distinction between "mass" and "weight", two terms which most people take to mean the same thing, when, in fact, they measure two different (but related) quantities. While there are circumstances where making the distinction is unnecessary, there are other times when it is critical.

The first of these distinctions that needs to be made is between aperture and f-ratio. The term "aperture", by itself, is vague -- we need a qualifying adjective to be clear. There are three different terms using "aperture":

  1. The physical aperture (iris) is the smallest opening within a lens.
  2. The virtual aperture (entrance pupil) is the image of the physical aperture when looking through the FE (front element).
  3. The relative aperture (f-ratio) is the quotient of the focal length and the virtual aperture.

Thus, the "f" in an f-ratio stands for focal length. For example f/2 on a 50mm lens means the diameter of the virtual aperture (entrance pupil) is 50mm / 2 = 25mm. Likewise, a 50mm lens with a 25mm virtual aperture has an f-ratio of 50mm / 25mm = 2.

So, the f-ratio is the aperture normalized to focal length, which will result in the same light per area on the sensor for all formats and focal lengths, but not the same total amount of light on the sensor for different formats.

The Hubble telescope is around F24. How can that be, when the lens is so huge and designed to gather light?

Once you figure this out, you will understand.

If I had to guess, I'd say he already had it figured out. The focal length of the HST is 76.6m and the aperture diameter is 2.4. This results in a relative aperture (f-ratio) of 57.6 / 2.4 = 2.4.

I'm sure you'll agree that the huge light gathering ability of the HST is a function of both the incredibly wide aperture (2400mm) and the incredibly long exposure times it can use.

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