DOF and Cropping take 2

Started Feb 11, 2014 | Discussions thread
Great Bustard Forum Pro • Posts: 42,575
Re: What it is vs. what it is thought to be

moving_comfort wrote:

Great Bustard wrote:

The point of photography is making photos. As such, one doesn't choose the particular system to get images which are equivalent to another system. A person chooses a particular system for the best balance of the factors that matter to the them, such as price, size, weight, IQ, DOF range, available lenses, and/or operation. By understanding which settings on which system create equivalent images, the difference in their capabilities is more easily understood.

One of the main sources of resistance to equivalence seems to stem from an aversion to it based on the false assumption that it exists to 'prove' one format is better than the other. Folks who have this aversion hear 'equivalence' and immediately go on the defensive, start refuting things that should be self-evident. Almost always, they're protesting what they think equivalence is, not what it actually is.

There are those, of course, and they are numerous. However, there are also those who can only think in terms of relative aperture (f-ratio) and ISO, and cannot understand, or simple refuse to understand, that, in terms of the visual properties of the recorded photo, it's about the virtual aperture (entrance pupil) and the total amount of light that falls on the sensor.

Interestingly, these two paradigms can be reconciled with the equivalence ratio (crop factor), but due to a complete and total misunderstanding of aperture, exposure, and total light, they absolutely refuse to apply this to the f-ratio, and only apply to the focal length.

So, once again, to make it painfully simple and clear:

Neither the focal length nor the f-ratio of a lens change as a function of sensor (for example, a 50mm f/1.4 lens is a 50mm f/1.4 lens, regardless of the sensor behind the lens). However, the effect of both the focal length and the f-ratio on the visual properties of the photo very much depend on the sensor, and scale in direct proportion to the size of the sensor:

25mm f/1.4 on mFT (4/3) is equivalent to 31mm f/1.8 on 1.6x (Canon APS-C), 33mm f/1.9 on 1.5x (APS-C for everyone else), and 50mm f/2.8 on FF (FX), where "equivalent to" means:

  • The photos all have the same diagonal angle of view (25mm x 2 = 31mm x 1.6 = 33mm x 1.5 = 50mm) and aperture diameter (25mm / 1.4 = 31mm / 1.8 = 33mm / 1.9 = 50mm / 2.8 = 18mm).
  • The photos all have the same perspective when taken from the same position.
  • The photos all have the same DOF (as well as diffraction softening) when they are taken from the same position with the same focal point and have the same display size.
  • The photos all have the same motion blur for the same shutter speed (regardless of pixel count).
  • The same total amount of light falls on the sensor for the same DOF and shutter speed.
  • The same total light falling on the larger sensor will result in a lower exposure than the smaller sensor (the same total light over a larger area results in a lower density of light on the sensor).
  • The larger sensor system will use a concomitantly higher ISO setting for a given brightness of the LCD playback and/or OOC (out-of-the-camera) jpg due to the lower exposure.
  • The same total light will result in the same noise for equally efficient sensors (regardless of pixel count and regardless of the ISO setting).
  • If the 25mm lens at f/1.4 has twice the resolution as the 50mm lens at f/2.8 on FF, 1.33x the resolution of the 33mm lens at f/1.8 on 1.5x, and 1.25x the resolution of the 31mm at f/1.8 on 1.6x, the sensors have the same pixel count and same AA filter strength, then all systems will also resolve the same detail.
  • Other elements of IQ, such as bokeh, color, distortion, 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 mean, it's all spelled out very clearly. And if a person doesn't understand one of the terms (for example, "aperture diameter"), there are links a plenty to explain in more detail.

Dangerous, indeed.

Thanks for stopping in, Joseph, as always one of the best resources one could ask for. And to think, you never charge speaking fees

It's only because of such obscene flattery that I forgo my usual charge. 

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