DOF and Cropping take 2

Started Feb 11, 2014 | Discussions thread
awaldram Forum Pro • Posts: 12,872
Re: Equivalence.

awaldram wrote:

Great Bustard wrote:

Here we are:

This nicely cover off everything in this thread

  • Equivalence is only relevant when comparing different formats. For example, if we are comparing the performance of a 50mm lens designed for FF to a 50mm lens designed for APS-C or mFT (4/3), both lenses being used on the same camera, Equivalence does not come into play.
  • Neither the focal length nor the f-ratio of a lens change as a function of format: 50mm = 50mm and f/2 = f/2 regardless of the format the lens is used on.
  • The effect of the focal length and f-ratio of a lens, however, do change as a function of format.
  • The DOF is the same for all systems for a given perspective, framing, aperture diameter, and display display size. For the same aperture diameter and shutter speed, the same total amount of light will fall on the sensor for all systems, resulting in a lower exposure for larger sensors systems (same total amount of light distributed over a larger area results in a lower exposure, since exposure is the density of the light falling on the sensors).
  • 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.
  • 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.
  • Equivalence says nothing about shallow DOF being superior to deep DOF, as this is entirely subjective.
  • The resolved detail is a function of the lens, the AA filter, the sensor, and the processing (RAW vs default jpg, for example). A sharper lens (greater lp/mm) on a smaller sensor will not necessarily resolve more than a less sharp lens on a larger sensor. Instead, we need to compare the resolutions in lw/ph, as DPR does with their MTF-50 tests (discussed in more detail here). Furthermore, the resolved detail is also a function of the number of pixels on the sensor (discussed in more detail here), and all systems suffer the same amount of diffraction softening equally at the same DOF, although the system that began with more detail will retain more detail (although the advantage asymptotically vanishes as the DOF deepens -- discussed in more detail here).
  • Equivalence makes no claims whatsoever about which system is superior to another system, especially given that there are so many aspects about systems that Equivalence does not address. For example, in terms of IQ, Equivalence says nothing about bokeh, moiré, distortion, color, etc., and in terms of operation, Equivalence says nothing about AF, build, features, etc. In fact, Equivalence can even work against larger sensor systems by denying them their "noise advantage" when they need to match both the DOF and shutter speed of smaller sensor systems.
  • However, Equivalence does make the argument that it makes no sense to artificially handicap one system or the other by requiring identical settings for a comparison, when identical settings result in different effects on different systems.

This point

The reason that smaller sensors are more noisy than larger sensors is not because they are less efficient, but because less light falls on them for a given exposure. If the larger sensor is more efficient than the smaller sensor, then the noise gap will widen, if the smaller sensor is more efficient, the noise gap will shrink.

I don't think tells the whole story , because we use ISO to define the sensitivity of the sensor then surely for a give ISO every sensor will be identical.?

So my view would be

The reason smaller sensors are more noisy than larger sensors is because less light fall on them (light density) they therefore have higher internal gain per ISO to compensate.

This gain increases the noise for any given exposure solution.

As per Nikon USA

How much light is needed is determined by the sensitivity of the medium used. That was as true for glass plates as it is for film and now digital sensors. Over the years that sensitivity has been expressed in various ways, most recently as ASA and now ISO.

So for any sensor size ISO 100 will deliver the same exposure (and why external light-meters work across formats)

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