DOF and Cropping take 2

Started Feb 11, 2014 | Discussions thread
Great Bustard Forum Pro • Posts: 40,847
In fact...

awaldram wrote:

James O'Neill wrote:

By the same token when I compare two digital formats the sensor in the cameras are not identical in enough of their parameters to get the same DR, the Same noise, the same detail so it's massively unlikely that when if we can image equivalence we get the same results in the output.

Which takes us back to my first point that spawned this thread when Ian took exception to

"I don't recognize that as equivalent at all"

Because it isn't and can never be if you assume

Equivalence = Equal

...I spell it out rather clearly:

An important consequence of Equivalent photos is that since the aperture (entrance pupil) diameter and shutter speeds are the same, the same total amount of light will fall on the sensor for all systems, which means the photos will have the same noise if the sensors are equally efficient. In addition, if the mFT (4/3) lens is twice as sharp as the FF lens, the 1.6x lens is 1.6x as sharp as the FF lens, the 1.5x lens is 1.5x as sharp as the FF lens, and the sensors have the same pixel count and AA filter, then all will capture the same detail.

Of course, sensors are not equally efficient (although often, but not always, close for a given generation), nor are all lenses proportionally sharp, have the same color, bokeh, distortion, or flare characteristics, nor do all sensors have the same pixel count, CFA, and AA filter. However, that doesn't stop us from saying that a 50 / 1.4 on a Canon 5D (13 MP FF) is not equivalent to a 50 / 1.4 on a Nikon D600 (24 MP FF) or that the Canon 50 / 1.4 is equivalent to the Nikon 50 / 1.4G which is equivalent to the Sigma 50 / 1.4 (assuming, of course, that the lenses are used on the same format).

Indeed, according to Webster's, the primary definition of "equivalent" is:

  • 1: equal in force, amount, or value

So, "equivalent images" have equal perspective, equal framing, equal DOF, equal shutter speeds, and equal display dimensions, although other characteristics, such as noise, detail, etc., may be equal under specific conditions. The second and third definitions of "equivalent" also fits:

  • 2a: like in signification or import
  • 3: corresponding or virtually identical especially in effect or function

This is not to say that photos that are not Equivalent may not look more similar to photos that are Equivalent. For example, let's say we have an old FF DSLR and a modern mFT camera that has a much more efficient sensor. We may find that a photo at 25mm f/2.8 1/100 ISO 400 with the mFT camera may look more similar to a photo at 50mm f/2.8 1/100 ISO 400 on the FF DSLR than an Equivalent photo at 50mm f/5.6 1/100 ISO 1600, even though the f/2.8 photo on the FF DSLR has half the DOF as the mFT photo, since the f/5.6 photo is much more noisy due to the less efficient sensor. Likewise, we might find that differences in the colors between the photos matter more than the differences in DOF, noise, detail, etc. In other words, Equivalent photos are not always the most similar, but they are often quite close, as this quiz demonstrates (with one glaring exception). However, Equivalent photos do share important visual properties that are all independent of the technology used to create the photo, and it's safe to say that differences in technology may well make for differences that matter more than one or more of the parameters of Equivalence.

In addition, there is a small niggle in the parameter of "same framing" for systems with different aspect ratios (e.g. 4:3 vs 3:2). We can either crop one image to the the aspect ratio of the other (or crop both to a common aspect ratio) or compare at the same AOV and display with the same diagonal measurement. The details of this are discussed at the end of this section.

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