Re: Focal length limits
Depends on the application, of course. Amateur astronomers regularly use 500mm-2000mm lenses (telescopes) at typically f/4-f/11. No AF, rarely IS, and never hand held - a rigid mount, sometimes motorised, is required.
There is an upper useful focal length limit, but it depends on the pixel pitch of the detector. The limit is set by the resolution deterioration from the atmosphere and depends greatly on local conditions. For astronomical applications at good sites (e.g. nothing extreme like Antarctica or remote locations in the Andes) you are usually limited to a resolution of ~ 1 arcsecond (1/3600 degree), which means that there is little point in having smaller pixels than 0.5 arcsec/pixel. For terrestrial daytime applications this limit is generally much higher, as the air close to the ground is denser and more affected by turbulence than if you look straight up.
For a 7D with 232 pix/mm, a resolution limit of 0.5 arcsec/pix corresponds to a focal length of 1780 mm, while for a 5D2 you have 156 pix/mm and thus can increase the focal length to 2640 mm before getting resolution-limited by the atmosphere. The opening aperture also needs to be larger than ~10 cm to not limit the image resolution by diffraction.
The record-braking 120 MP APS-C sensor Canon showed off a while ago has 455 pix/mm, which would result in a maximal useful focal length of 900 mm. It would put very severe constraints on tolerances and the quality of the optics, however. In principle one could go even further with even higher MP detectors, but tolerances and dynamic range per pixel would likely be limiting factors. With "perfect" detectors the resolution would be completely determined by the optics. Still, I don't think it would be practically possible to get 1 arcsec resolution on anything shorter than 200mm, because already that would require an f/2 lens (to not be diffraction limited).
Looking at distant objects on the ground limits useful focal lengths to much shorter than the above estimates.
Depends on the application, of course. Amateur astronomers regularly use 500mm-2000mm lenses (telescopes) at typically f/4-f/11. No AF, rarely IS, and never hand held - a rigid mount, sometimes motorised, is required.
There is an upper useful focal length limit, but it depends on the pixel pitch of the detector. The limit is set by the resolution deterioration from the atmosphere and depends greatly on local conditions. For astronomical applications at good sites (e.g. nothing extreme like Antarctica or remote locations in the Andes) you are usually limited to a resolution of ~ 1 arcsecond (1/3600 degree), which means that there is little point in having smaller pixels than 0.5 arcsec/pixel. For terrestrial daytime applications this limit is generally much higher, as the air close to the ground is denser and more affected by turbulence than if you look straight up.
For a 7D with 232 pix/mm, a resolution limit of 0.5 arcsec/pix corresponds to a focal length of 1780 mm, while for a 5D2 you have 156 pix/mm and thus can increase the focal length to 2640 mm before getting resolution-limited by the atmosphere. The opening aperture also needs to be larger than ~10 cm to not limit the image resolution by diffraction.
The record-braking 120 MP APS-C sensor Canon showed off a while ago has 455 pix/mm, which would result in a maximal useful focal length of 900 mm. It would put very severe constraints on tolerances and the quality of the optics, however. In principle one could go even further with even higher MP detectors, but tolerances and dynamic range per pixel would likely be limiting factors. With "perfect" detectors the resolution would be completely determined by the optics. Still, I don't think it would be practically possible to get 1 arcsec resolution on anything shorter than 200mm, because already that would require an f/2 lens (to not be diffraction limited).
Looking at distant objects on the ground limits useful focal lengths to much shorter than the above estimates.
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