On Sat, Aug 29, 2009 at 1:13 PM, J.C. O'Connell<hifis...@gate.net> wrote: > I don't follow your logic, the higher the overall sensor resolution > and sensor pixel densities get with time, the more the overall system > resolution depends > on the lens' absolute resolution AND FORMAT SIZE. If you use two lenses > with > same across the board resolution ( say 60 lp/mm), the larger FF sensor > will approach > 50% higher linear resolution (2.25X total resolution) with infinitely > good sensors. Conversely, with infinitely good sensors, a FF lens with > only 66% percent of the APS lens would give same overall system > resolution.
This is only the case if the lens is the limiting factor across the board for resolution, the field of view is the same and the pixel density is the same. One of those factors is generally not the case in any given situation. The biggest problem is that lens resolution usually isn't the limiting factor except at the edges and degradation of edge performance is unfortunately obvious in scenes with high-frequency detail near the edges of the frame (as is quite often the case in Landscapes, the most common application for high-MP digital). Note I'm not quibbling with your math, it's correct and would apply in an ideal system. > Now of course the sensors are not infinite resolution, but the same > logic > applies, all else being equal, the larger format will give higher system > resolution > for a given lens resolution or for same system resolution a lower > resolution > lens on the larger FF format can give same system resolution. This > sounds > like talking in circles I guess. All things being equal, this is certainly the case. > > Anyway, sure if you have FF lenss that in the corners are less than 66% > of the APS lens minimum, then nothing is to be gained. And sure, I guess > on some mediorce or really wide angle lenses not designed for digital > that may be occuring, but there are also many great FF lenses than have > corner to corner performance at some apertures that exceeds 66% of the > APS lens counterpart's resolution, and with those lenses at those > apertures > the net result is higher overall system resolution with FF vs APS. > > J.C. O'Connell (mailto:hifis...@gate.net) The problem is that even most 'good' wide-angles from the film era are being shown to be less than satisfactory on high-MP FF. What was sufficient performance to be considered an excellent performer on film is simply not enough for 20+MP digital. There are rather few wide-angle designs, even in the 28mm range which have edge performance suitable for high-MP FF. Once you hit around 20mm, you're starting to run into options being countable on a single hand. Really, the only ~20mm primes that are up to high-MP digital are the Contax Distagon 21mm f2.8 T* and the second version of the Leica Elmarit-R 19mm f2.8. Even superb performers like the Olympus 21's are running out of resolution at ~21MP. You can, in some cases, recover the performance in post (Canon's DPP software does an excellent job of this with Canon's L wide zooms, as does DXOptics with any lens/camera combination they have a profile for) Most of those lenses which are 'good' on film do have enough centre resolution for APS-C. But the rolloff in performance into Zone C is the real issue. The problem is making lenses which do not show visual degradation of edge performance vs centre performance on the two formats because there is now enough resolving power at the edge of the sensor for this to matter. The increased resolution across the smaller imaging circle of APS-C is easier to achieve than maintaining consistent resolution across the larger image circle of FF. This is exaggerated by the fact that FF lenses must also be able to perform to APS-C requirements due to the market bias towards APS-C (all the FF makers also have APS-C lines which sell far more cameras and are a major market for both APS-C and FF lenses). A lot of this comes down to the fact that a number of the limiting factors to resolution in 35mm film photography have been removed with the advent of digital and that's making lenses which delivered superb performance on film into mediocre performers on digital. This is because film flatness and registration error for 35mm film are no longer an issue. A sensor is nearly perfectly flat and even with body-IS the sensor registration really cannot vary anywhere near as much as film thickness and flatness allow. In other words, the rolloff on the MTF graph in zone C for 35mm lenses is becoming visible because the factors that used to hide it are no longer an issue. But the primary limiting factors are not the math and physics behind resolution, but a combination of cost and market factors. You can make lenses up to APS-C performance cheaper than you can for lenses up to high-MP FF. And because of that, the available inexpensive FF lenses are disappearing because they're visibly outperformed by the high-MP FF sensors in most applications where you'd want high MP and the fact that there really isn't a market for inexpensive FF lenses and won't be until there's inexpensive FF bodies, which is unlikely to happen anytime soon. So a given level of IQ costs more from FF(often significantly so), but the absolute maximum IQ can be higher than is achievable from APS-C. Low-MP FF is an odd exception here because the primary applications for it generally aren't as affected by edge performance or absolute resolution, they tend to care more about absolute speed (AF, FPS) and high ISO performance. But it doesn't present IQ improvements over APS-C outside of high ISO performance. -- M. Adam Maas http://www.mawz.ca Explorations of the City Around Us. -- PDML Pentax-Discuss Mail List PDML@pdml.net http://pdml.net/mailman/listinfo/pdml_pdml.net to UNSUBSCRIBE from the PDML, please visit the link directly above and follow the directions.
