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| Re: Sigma 18-35 f/1.8 hands-on and samples online |
The myth is that f/1.8 = f/1.8 = f/1.8 on different size sensors or films. This is the thing that trips up so many people.
Understanding the concept of equivalence can only help photographers. They will be able to understand why an 18-35mm f/1.8 zoom on a Canon 7D is approximately equivalent to a 29-56mm f/2.8 zoom on a 5D Mark III. Knowledge of the physics and math behind photography is no way harmful or detrimental to the art of photography. The two can and SHOULD compliment each other. Photographic Equivalence explained here.
It's no myth at all. The problem stems from people arguing two different facets of the same thing (really probably three facets), and being either too stupid or proud to realize it. They either don't comprehend the nuances of the arguments (to realize they're both right, conditionally), or they're too proud to admit it.
There is an operational argument (how the camera acts when determining shutter speeds, how many photons are collected per sensor-mm^2 per second, etc) and there is a results argument (how the image actually looks on screen or paper, but let's keep to screen). Lastly, there's a common-denominator physics argument facet that is almost completely f-ing pointless (in at least two ways), but people keep bringing it up.
The operational "camp" is correct - assuming 15 camera bodies from various brands will meter the same scene the same way, assuming the T-stops of various lenses are in fact "true" to the f-stop (which hardly happens, if ever), and assuming these 15 bodies aren't playing games with their indicated-vs-true ISO settings, then f/1.8 = f/1.8 = f/1.8 in the sense that the attached cameras will give exactly the same shutter speeds. Because, ultimately, everyone on planet Earth is concerned about shutter speeds when taking a photo. So, they're worried about the operational characteristic of COMBO A vs COMBO B and they find they are, in fact, equivalent in that sense.
The results camp begs to differ. They make the argument that the f/2.8 lens on FF is equivalent to the f/2 lens on the APS-C camera, so that f/1.8 != f/1.8 != f/1.8. They are conditionally correct, just like the operational people. The results camp seems to assume the FF camera is always at least 1 stop better than the APS-C camera, therefore the end results are equivalent -- the FF camera can just bump up ISO to gain an equal shutter speed and equal noise level as the "faster" APS-C combo. In their mind, the f/2 lens has to be f/2 on APS-C just to "catch up" to the FF camera.
This is also a flawed argument, but conditionally correct - Again, this assumes the FF camera is ALWAYS at least 1 stop better than the APS-C camera. This argument fails/will fail when considering old FF vs modern APS-C. It also fails when considering cameras react differently to increased ISO levels. I once compared my Panasonic GX1 vs NEX5n vs Canon 5D3 all at ISO400 (I think) and was completely unimpressed with the 5D3 - I literally had a hard time telling them apart (dark shot, not brightly lit stuff). I then performed the same comparison at ISO 3200. The 5D3 was more than 1 stop ahead of the other two cameras, but you didn't hear me shouting from the rooftops that my 40/2.8 pancake was actually a badass 40mm/1.4 in "light gathering" for only $150! (and yes, this took resampling into effect, but let's not go there)
Then you have the "common-denominator physics" arguments that are completely f-ing pointless, even if they ARE, technically true. YES Captain of the Obvious, we KNOW that a PHYSICALLY LARGER LENS (f/2.8 vs f/2.8), such as a FF lens vs an APS-C lens, will actually capture more photons/second. No-freaking-duh.
This argument is pointless, but people keep bringing it up as "light-gathering". First, while YES the FF lens has "scooped" more photons/second, it has to distribute them over a larger area in order to get the equivalent shutter speed. This is true as you go up and down the sensor size. Nobody really gives a damn about how many photos the unmounted, physical pieces of glass and plastic could collect in 1/100 of a second - they only care about where those photons actually go (the results camp) or how it affects the metering of the camera (the operational camp).