DOF and Cropping take 2

Started Feb 11, 2014 | Discussions thread
Great Bustard Forum Pro • Posts: 42,548
On understanding.

James O'Neill wrote:

Ian Stuart Forsyth wrote:

awaldram wrote:

  • The DOF is the same for all systems for a given perspective, framing, aperture diameter, and display display size.

This one seems wrong.

But it isn't.

And then you realise he isn't talking about aperture in the same terms as everyone else. He talks about Aperture DIAMETER not an f/ number, but the width of the hole in mm. Stop and read what I just said again. when you or I might talk about a 50mm lens at f/2 he talks about a 50mm lens with a 25mm aperture.

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.

Hands up who says "two lenses have the same aperture when their f/ numbers are the same"
and who says "two lenses have the same aperture when the diameters in mm are the same"

Hands up for those that understand why distinguishing between the virtual aperture (entrance pupil) and relative aperture (f-ratio) is important.  No one?  Bueller?

  • The same total amount of light falling on the sensor will result in the same noise for equally efficient sensors, regardless of pixel size or the ISO setting. Typically, sensors of the same generation are rather close in efficiency, but there are most certainly exceptions.

At best this is unproven.

What is unproven?  That the same total amount of light falling on the sensor results in the same noise for equally efficient sensors?  Well, not only has it been demonstrated countless times, what else, besides the amount of light falling on the sensor and the efficiency of the sensor would be the source of noise?

I think you can say if sensors have the same efficiency, and the same pixel count, and same well depth, and same read noise then then one with 4 times the pixel area will have a base ISO 4 times higher. But when you have different well depths and pixel counts I think this his assertion breaks.

Equally efficient sensors have:

  • The same QE (Quantum Efficiency -- the proportion of light falling on the sensor that is recorded).
  • The same read noise for the same area of the photo.

The FWC (full well capacity) doesn't figure into efficiency, but the sensor with the greater FWC per area on the photo will be able to absorb more total light by using a longer shutter speed.

  • Larger formats do not necessarily have a more shallow DOF than smaller formats. Larger formats have the option of a more shallow DOF than smaller formats for a given perspective and framing when using a lens that has a larger aperture diameter, as the lenses for larger formats usually, but not always, have larger aperture diameters for a given AOV. However, people using Auto, P, or Tv modes on the camera will likely find that the larger format camera will choose a wider aperture in many situations, resulting in a more shallow DOF. In addition, many choose to use a wider aperture (resulting in a more shallow DOF) to get more light on the sensor and thus less noise.

Again by talking about diameter (mm) not f/ number tends to cause confusion.

What causes confusion is people not understanding the difference between the relative aperture (f-ratio) and virtual aperture (entrance pupil), and the role each play in the visual properties of the photo.

I think what he is saying is that if you double the focal length, say from 50mm f/2 to 100mm the aperture is not a fixed at 25mm so you get something wider than f/4 though maybe not as wide as f/2 And when the cameras are in auto it will tend to select the same f/ number regardless of format, which means a wider diameter on the larger formats.

What I am saying is that the same aperture diameter results in:

  • the same DOF for a given perspective, framing, and display size
  • the same total amount of light being projected on the sensor for a given scene luminance and shutter speed
  • the same noise for equally efficient sensors

regardless of format.  Note how often the word "same" comes up -- thus the term "Equivalence".  Note how all these things are the same for the same aperture diameter, not relative aperture.  Thus the importance of the virtual aperture (entrance pupil).

There is a very basic argument that more light falling on sensor means less noise.

Recall what determines the amount of light falling on the sensor for a given scene:

  • perspective
  • framing
  • aperture diameter
  • shutter speed

Recall what determines the noise in a photo:

  • the total amount of light falling on the sensor
  • the efficiency of the sensor

It's really quite easy to understand, but there are two hurdles to overcome:

  • Understand the difference between relative aperture (f-ratio) and virtual aperture (entrance pupil)
  • Understand that there are two primary sources of noise in a photo:  the photon noise (noise from the light itself) and the read noise (additional noise from the sensor and supporting hardware).

Once those are understood, then understanding Equivalence is quite easy.

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