Originally posted by starbase218 True, but: a given focal length will produce different perspectives on fullframe and APS-C.
no. please, perspective depends _only_ on distance to subject, not on lens, not on format or alignment of planets. lens specific distorsions are not part of perspective (they are part of optical aberrations
)
Quote: So you adjust focal length to e.g. 200mm on fullframe and 135mm on APS-C.
about right, but this is because of field of view.
Quote: But now the DOF is shallower on FF. So you stop down to F5.6 on FF while you keep F4 on APS-C.
not entirely sure, but an online dof calculator could clarify that (goes and checks)
nikon aps-c, 135mm f/4, 4m subject distance:
Depth of field
Near limit 3.93 m
Far limit 4.07 m
Total 0.14 m
In front of subject 0.07 m (49%)
Behind subject 0.07 m (51%)
Hyperfocal distance 227.9 m
Circle of confusion 0.02 mm
nikon d800 &co, 200mm, f/4, 4m
Subject distance 4 m
Depth of field
Near limit 3.94 m
Far limit 4.07 m
Total 0.13 m
In front of subject 0.06 m (49%)
Behind subject 0.07 m (51%)
Hyperfocal distance 235.9 m
Circle of confusion 0.03 mm
so pretty much spot on (courtesy of dofmaster.com)
Quote: To be able to use the same shutter speed, you increase ISO to 200 on FF while keeping it at 100 for APS-C. The FF sensor is larger anyway, so it can cope better with noise.
i wouldn't bet on that. this has been the case so far, but keep in mind much larger sensors are notoriously bad at higher iso's (medium format).
Quote: However, the difference between F4 and F5.6 is approximately 1.4 (square root of 2).
no. the ratio of light gathering between each "well known" apertures is 1/2, that's precisely why they are well known, and if you want to verify, you need to square them:
5.6^2/4^2
1.96000000000000000000
Quote: But APS-C sensors are more than 1.4 times smaller than FF sensors. In case of Pentax, it's a 1.5x crop.
we're mixing units here. 1.5x crop is a linear measurement, light gathering capabilities are a factor of surface (so linear squared, one might say, except squaring the crop factor is inaccurate unless you're talking about a rectangular frame). in short, assuming the same aspect ratio, the "35mm" sensor is twice the surface of aps-c, and this is why it's usually "one f-stop away" in nearly everything (like dof at same field of view). this is also why f-numbers need to be squared to obtain the exposure ratio (they are a linear ratio between focal length and aperture size)
Quote: So F5.6 on FF will still give a slightly shallower DOF than F4 on APS-C, given equivalent focal lengths of 200 and 135mm respectively. And noise performance on the FF sensor at ISO 200 will be slightly better than the APS-C sensor at ISO 100, assuming the sensor technology is comparable.
dof: no, as you see from above, it's pretty much the same (close enough it makes no difference)
comparing noise performance of ff at iso200 to aps-c at iso100 on modern sensors is a purely academic exercises imho.
Quote: However, this point is moot, since we were trying to equalize the DOF. So although the FF performance is slightly better, its DOF will still be a little shallower as well.
A few cases where fullframe is better than APS-C:
- The detail level at the same ISO levels
- Ability to create a shallower DOF at wide viewing angles with the same F-number
- Ability to shoot in darker environments by using higher ISO's and the same F-number if you don't mind (or are actually aiming for) the shallower DOF
all these "all else being equal" comparisons have two problems: all else is never equal, and comparative discussions might not be meaningful for practical purposes from a point on (for instance, comparing the output of a sensor like the one in the k-5 with one in a "comparable" 35mm camera, using good quality (read: excellent) glass, one might be surprised how close they will be). Technology is at a point where one no longer _needs_ bigger sensors for image quality purposes (exceptions are rare, and tend to drift towards phaseone and the likes anyhow), you might _want_ it for a bigger, brighter viewfinder, you might dream of it because you have some special pixie dust lens which only really works "properly" on a 35mm frame, or just because you can afford it. but need it? nope. sensor technology has reached "maturity" a few years ago (about the time k-5 was released), in other words. that's not to say improvements are no longer possible, but rather that you probbaly don't need them
now i should run...
ps: sorry for the rant, nothing personal, i have a compulsive reaction to misuse of the term "perspective" in particular