Canon EOS 20Da
From Wikipedia, the free encyclopedia
| Canon EOS 20Da | |
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| Type | Single-lens reflex |
| Sensor | 22.5mm x 15.0mm CMOS |
| Maximum resolution | 3,504 × 2,336 (8 million) |
| Lens type | Interchangeable (EF-S, EF) |
| Viewfinder | Optical |
| Storage | CompactFlash(CF) (Type I or Type II) |
The Canon EOS 20Da is an 8.2-megapixel semi-professional DSLR camera designed specifically for astrophotography. It was initially announced on February 14, 2005 for release only in Japan. On June 1, Canon announced that the 20Da would be made available world-wide. It is a variant of the Canon EOS 20D. It accepts EF and EF-S lenses.
One of the changes made to the 20Da over the 20D is the replacement[1] of the "hot mirror" IR filter which covers the CMOS sensor on the 20D. In conventional photography, the IR filter is used to make the spectral response of the CMOS sensor more like that of the human eye. In this way, the pictures gathered by the sensor more closely resemble the world as we see it. In astrophotography, many objects of interest emit strongly in the so-called H-α line, which is heavily attenuated by the IR filter on the 20D. The IR filter of the 20Da passes 2.5 times as much light at around this 656 nm wavelength as does the filter of the 20D, allowing more fine detail to be revealed in long exposures of emission nebulae. A separate IR filter must be attached to the lens in order to use the 20Da to capture images similar to those seen by the human eye.
One of the main barriers to producing quality astrophotographs is the difficulty of focusing accurately. The 20Da features 'live-focus' to overcome this problem. In a normal SLR, the light from the lens is diverted to the viewfinder except during the short period when an image is being captured. This means that the digital sensor cannot be used to provide a live image. Digital compacts do not have a mirror — light is always falling onto the sensor. The point of 'live-focus' is to allow the photographer to see the results of changing the focus of the lens and/or telescope in realtime on the LCD screen. However, autofocus cannot be used during live-focus, and the photographer has to focus manually.
It was initially unclear exactly how 'live-focus' is implemented. Early Canon literature suggested that a partially-transmissive mirror was used, whereas other sources implied it was implemented through mirror lock-up (so no image will be visible through the viewfinder). Canon's official website now says that the live view is accomplished via mirror lock-up. (The mixup may have come from the fact that the IR filter is more transmissive than before and a word or two was mis-translated from the original Japanese.)
See Canon's official website for full details of the differences between the 20D and 20Da.
[edit] See also
[edit] External links
- DPReview previews the 20Da
- DPReview covers announcement of world-wide release of the 20Da
- 20Da official website (Japanese)
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