|
|
Thread Tools | Display Modes |
#21
|
|||
|
|||
One thing that still bothers me about these digital cameras is how they
assign film speed to them. The way I understand it a ccd does not have variable sensitivity. How can you state that the film speed is say ISO400 and you can also select ISO1600? How is this accomplished, by binning, or some other way? With film, fast films are coarse grained and slow films are fine grained. What is the base film speed for the 300Dm Canon D20 etc? I see they use ISO100 for flash. Is this the base speed for a digital? |
#22
|
|||
|
|||
On Fri, 28 Jan 2005 18:19:11 GMT, Ed Majden wrote:
One thing that still bothers me about these digital cameras is how they assign film speed to them. The way I understand it a ccd does not have variable sensitivity. How can you state that the film speed is say ISO400 and you can also select ISO1600? How is this accomplished, by binning, or some other way? With film, fast films are coarse grained and slow films are fine grained. What is the base film speed for the 300Dm Canon D20 etc? I see they use ISO100 for flash. Is this the base speed for a digital? The ISO equivalents are assigned by empirical comparison to film; that is, a digital camera set to ISO 100 will deliver an image similar to a film camera using ISO 100 film, given equivalent exposure settings. In the case of a digital camera, the adjustable ISO setting is accomplished by changing the gain of the output amplifier (that is, the amplifier that boosts the signal as it is read from the sensor). There is no such thing as a "base speed". with a CCD or CMOS sensor. When you increase the gain, you are also amplifying noise. In the case of the 300D, the optimal ISO setting appears to be 200. At that setting you get the best S/N ratio. So for astronomical imaging, it is better to work with a low ISO setting and a longer exposure if your tracking can support it. _________________________________________________ Chris L Peterson Cloudbait Observatory http://www.cloudbait.com |
#24
|
|||
|
|||
On Fri, 28 Jan 2005 19:12:04 GMT, Ed Majden wrote:
Hello Chris: What you say is correct for deep sky imaging. I want to try one for meteor spectroscopy where the duration of a meteor track is rather short. Normally I use TriX Pan and this will record a spectrum of a meteor that is ~ -2.0 magnitude or brighter. I wonder if the digital will respond the same as there is no time for image build up! The narrow spectral range is also troublesome but it should be okay centered on the visual region on the spectrum. I just bought a Canon D20 which has better noise characteristics than the Rebel 300D and D10. One could remove the IR blocking filter but I don't want to void warranty just yet! The blue end response is also limited. The ISO values should be equivalent for short exposures. Where the ISO equivalents fall apart is for long exposures, where the digital camera will continue to deliver the same sensitivity, but the film's sensitivity will drop because of reciprocity failure. The sensor in the camera has at least ten times the quantum efficiency of film. You do have some sensitivity loss because of the color sensor in the Canon, however. The little filters used over the pixels aren't the greatest in the world. It's a shame you can't get the Canon cameras with B&W detectors- that would make them very attractive for astronomical imaging. Still, I think you can expect that your 20D will prove at least as sensitive as the Tri-X for your meteor spectroscopy. I've found that my 300D does a better job catching meteors than Ektachrome, both at ISO 200. _________________________________________________ Chris L Peterson Cloudbait Observatory http://www.cloudbait.com |
Thread Tools | |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Thread Starter | Forum | Replies | Last Post |
pics with Canon Digital Rebel - CORRECT link | Etienne Ely | Amateur Astronomy | 2 | October 28th 04 06:55 AM |
pics with Canon Digital Rebel | Etienne Ely | Amateur Astronomy | 2 | October 23rd 04 05:47 PM |
Canon D300 and Digital Rebel questions | Ryan Walters | Amateur Astronomy | 16 | August 27th 04 04:58 PM |
Digital Rebel Canon - how to take dark frame? | Chotechai | Amateur Astronomy | 16 | February 16th 04 03:20 PM |
Canon EOS Rebel Digital | Tdcarls | Amateur Astronomy | 9 | September 22nd 03 07:58 PM |