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Lotusm50 wrote:
It still is a f1.7 lens and it's not clear it makes a difference in the use of the lens. If lighting conditions call for an exposure of f1.7 at 1/250 sec on full frame, it will also be an exposure of f1.7 at 1/250 on 4/3rds, for a given ISO.
The thing is, all ISOs are not created equal, i.e. the given ISO will give better quality (including lower noise) on the full frame camera. That means the smaller sensor camera would have to use a lower ISO to compensate (if the goal were to make a similar-quality image), which would have to be made up in another area.
Of course, the noise advantage of the full frame camera will only come if you're willing to use wider apertures than with the smaller-sensored camera, because otherwise the latter could always take a roughly equivalent image.
Lotusm50 wrote:
And while more light will hit a full frame sensor (well, becuase it is bigger) the exposure does not change.
The light intensity per unit of area will not change, and the exposure settings will not change. However, the total amount of light forming the exposure does of course change (and a widely repeated loose photographic definition of exposure is "total amount of light hitting the medium", which is actually more accurate in terms of results than one based on the flux).
Lotusm50 wrote:
In some sense, the full frame sensor requires more light (total number of photons collected across the entire sensor).
Not really. Take the same number of photons and distribute them across any area and you should (theoretically) be able to achieve similar image quality. The larger sensor doesn't require more light-- that's why bigger sensors are better in lower light, and at least no worse. You'd be just fine using a two-stop higher ISO and stopping down two stops.
Lotusm50 wrote:
For a given pixel count and all other things equal, the lower total amount of light collected by the 4/3rds sensor will just result in a smaller signal to noise ratio (which for a given ISO, may or may not noticeably bind on IQ).
Right, and here is one of the most important things: it only matters if it matters. I don't foresee myself generally shooting m43 handheld indoors with an f/2.8 lens (even though it's f/2.8 stabilized, which should be fast enough, right?), but outdoors a lot of these differences won't matter. Indoors or out, wider aperture lenses like f/1.7 should work fine in a wide range of conditions.
Lotusm50 wrote:
So that I might better understand your argument, could you give me an example, other than DOF, of how your perspective on the "slowness" of this lens would manifest itself in use, relative to a 34/1.7 lens on full frame? Other than for DOF, how is using these 2 lenses different in use?
First, I didn't say the lens is slow. It's not as fast, considering the system, as a fast prime on full frame, but it should be adequate for a wide range of conditions. It is of course pretty fast stacking it against other lenses made for crop cameras in general, not just for 4/3. On the Pen it will be roughly equivalent to a stabilized 25mm f/2 on APS-C.
A good example of what you asked for is that one would be able to get higher-quality results using an f/1.7 lens on full frame under the same dim conditions, but it would come at the expense (or benefit depending on how you see it under the circumstances) of shallower DOF. So one could either achieve higher-quality results at any group of settings, and/or shoot under harsher conditions, using the FF camera-- as long as you are willing to accept narrower DOF.
Equalization of settings only happens, incidentally, because the camera makers apply differing amounts of gain to their sensors so that they will be roughly consistent in use. That doesn't mean that any old camera will function just the same when set the same way. Try taking just about any pocket cam and shooting it at ISO 800, then shoot with the same settings on an SLR. You will notice a difference, even though the exposure is "the same".
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