All this talk about "equivalence" is silly

Started Jun 10, 2011 | Discussions thread
Great Bustard Forum Pro • Posts: 42,267

Fabian628 wrote:

is this part true as well?

Case 2: if your primary concern is reach, then a 300 f/4 lens is decidedly similar to a 480 f/4 lens on a 1.6 crop camera, not 480 f/5.6.

If your primary concern is reach, then your primary concern is AOV, not aperture. So your "primary concern" would be on comparing 300mm to 480mm, and not care about whether it's f/4 or f/5.6.

But the fact of the matter is that the aperture diameter of 300mm f/4 and 480mm f/5.6 are nearly the same, which means the DOF will be nearly the same (for the same perspective and framing).

More than that, [nearly] the same aperture diameter also means [nearly] the same amount of light falls on the sensor for the same shutter speed. Thus, for equally efficient sensors (such as the 5D2 and 7D), 300mm f/4 on crop not only has the same AOV as 480mm f/5.6 on FF, but basically the same noise as well.

Of course, another consideration is AF. But operation is outside the scope of Equivalence:

h ttp:

Among the most critical of the operational differences to consider is the camera's AF system...

which is concerned entirely with IQ:

h ttp:

It is my hope that this essay is useful and informative in explaining the differences between formats (sensor sizes), and what role both the sensor and glass play in terms of image quality (the section on IQ is a "must read" to keep the relevance of these differences in perspective, if you'll pardon the pun). This is a very technical essay that explains technical aspects of photography, most notably noise, exposure, and DOF. The target audience is those who want to understand the physics of photography and how this applies to the engineering of modern digital cameras, and, more specifically, to how this relates to the different formats. This essay is not targeted to people who want to know how to use their cameras to create "good" photographs. As long as this essay is, that essay would be quite a bit longer.

However, since I brought AF up, both a 300 / 4 and 480 / 5.6 would use the f/5.6 AF sensors, so there's no reason to believe that the 300 / 4 on crop would have superior AF to a 480 / 5.6 on FF. In fact, some argue that even the extra precision of the f/2.8 AF sensors is not what it's advertised to be:

On the other hand, a 400 / 5.6 on crop is equivalent to a 640 / 9 on FF, and there is definitely an AF advantage for the 400 / 5.6 on crop. Of course, there is not 640 / 9 for FF -- the closest is a 600 / 4 which ain't small, light, or cheap.

This brings up:

h ttp:

  • 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.

And that brings up:

So, I'd say that what's "weak" is people misrepresenting what "the author" said, and did not say. Whether that misrepresentation (and/or confusion) comes from laziness (criticizing a document they've not read), poor reading comprehension, ignorance, or contrary agendas, well, I'm sure that depends on the individual. Shame, really.

In other words, the Essay addresses all these topics in detail , but because people don't read, don't pay attention, or actively ignore, "misunderstandings" are inevitable.

Anyway, I'm off to sleep.

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