2: A smaller sensor reduces the focal length allowing smaller
bigger zooms, that are also faster. Common is th 36-432mm super
zoom class. Try lugging full frame lenses about for that focal
range!
I have to disagree with this: The zoom lenses on small-sensor superzoom cameras are NOT faster than SLR telezooms, not in any meaningful sense.
They may have smaller F-numbers, which means that they provide brighter illumination at the sensor; but when the illumination is multiplied by the sensor area to obtain the rate of light accumulation, the advantage goes to the larger sensor. For instance, the Panasonic FZ20 (f/2.8) has 2-3 stops LESS light-gathering ability than the Canon 28-300mm f/3.5-5.6L IS on the 1.3x crop 1D Mark II.
To turn this increased light-gathering ability into higher shutter speeds, you must turn up the ISO on the digital SLR. But that's okay; for instance, the FZ20 at ISO 80 forms its image using approximately the same amount of light as the 1D Mark II at ISO 1600. As such, it should not come as a surprise that ISO 1600 is quite usable on the 1D Mark II; maybe a touch noisier than the FZ20 at ISO 80, but it's not a massive difference. Certainly not 4.3 stops' worth.
On the other hand, if you stay at the lower ISO settings on the SLR, you get more dynamic range / less noise than is possible on the superzoom cameras. You are still benefiting from the greater light-gathering ability of the SLR lens, just in a different way. With enough light or slow shutter speeds, the superzoom could do the same, say, at ISO 12, but unfortunately smaller pixels have lower full-well capacities, so with current sensor technology that is not feasible.
Because the difference is in the light-gathering capability of the lens, it will never be neutralized by technological changes. If the next-generation superzoom can produce cleaner images with less light, then the next-generation DSLR will likely do the same. The ISO disparity will remain. And don't expect order-of-magnitude improvements: because of photon shot noise, there is a physical limit to how much image information can be extracted from a given amount of light.
Partly for the reasons described above, it is my opinion that the use of F-numbers to measure lens speed and ISO to measure sensitivity is misleading when comparing cameras with different sensor sizes. These parameters should instead be converted to 35mm equivalents as described at
http://forums.dpreview.com/forums/read.asp?forum=1000&message=14967397 ;.
As you mentioned, the superzooms also lack the ability to produce shallow depth of field comparable to a digital SLR. On the other hand, the SLR can produce deeper depth of field by stopping down. This does mean throwing away its light-gathering advantage; that advantage comes from its larger apertures, which also bring shallow DOF. But the superzooms have no optical capability that the SLR + telezoom lacks.
What the superzooms
do have is much lower size, weight, and cost than digital SLR setups with comparable telephoto reach. Many also have image stabilization systems that partially offset their poor light-gathering ability, permitting their telephoto reach to be used with static subjects in moderately low light. They have their niche in the market, and for some people they are a good choice. But they also have their limitations, and it is important to remain aware of them.
--
Alan Martin
