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Photo Forum / Digital Photography / Digital Photo / August 2004Digital night photography
Hi, I am very much interested in night photography. By that, I mean long, long exposures. Exposures like minutes and longer. I have always taken such pictures by manual film cameras and although it should be possible to find out the right exposure, I have always tried to guess. Consequently the results are very much by chance. From time to time I think about going digital and today I begin to wonder how this type of photo works in digital photography. I just wonder as most decent digi cameras feature an LCD monitors the photographer should be able to observe the process of shaping image. That way the photographer can see exactly when the exposure time in enough and stop the shutter at that time? Does is really work like that? Can someone who have actual experience tell me a little about such experiment? Regards, Mojtaba > Hi, > [quoted text clipped - 14 lines] > > Mojtaba Well, you won't be able to see the image as it comes along, but then again, you will get to view the final result when it is done on the LCD. No waiting for the film to develop and such. I've found that digital has made me a better judge at exposure times in different situations. Having the instant feedback of the LCD screen helps a lot, even though it might not be a "real time" view like you were hoping for.  Signaturehttp://www.pbase.com/bcbaird/ > Hi, > [quoted text clipped - 10 lines] > that time? Does is really work like that? Can someone who have actual > experience tell me a little about such experiment? I haven't found the LCD display to be very helpful when shooting at night. It gives you an instantaneous image of what it sees right now, not a building up image as light accumulates on the sensor. It has to work that way because if it allowed light to accumulate to build the image everything would be a blur as you move the camera. One of the difficulties of taking night photos with film is "reciprocity failure". Film responds to changes in light in a relatively linear way at the exposures that the film is most optimized for - i.e., subsecond exposures. At the extreme ends of high and low light, film response is no longer linear. So if, for example we can take a particular scene with a 10 second exposure, then cut the light in half, it might take a lot more than 20 seconds to make an equivalent exposure. I'd be very curious to know if there are analogous effects with digital sensors. That might be an important question for you. Another issue is "noise". Digital noise can look like grain on a digital image. It seems to be my experience that noise levels can increase at low light levels. But I don't know the theory behind it. This too is something you might need to know about if you want to do a lot of night work. Unfortunately, I don't know the answers to these questions, but maybe I've helped add some more specific questions for you. Perhaps one of our experts can give us the answers. Alan >> Hi, >> >> I am very much interested in night photography. By that, I mean long, >> long exposures. Exposures like minutes and longer. I have always taken <snip> > One of the difficulties of taking night photos with film > is "reciprocity failure". Film responds to changes in light [quoted text clipped - 5 lines] > it might take a lot more than 20 seconds to make an > equivalent exposure. CCDs don't do this. 1/2 the light * 2 times the exposure = same picture. However the dark noise increases in direct proportion to the time too. For good CCDs this isn't a problem. For the best CCDs (not yet in cameras AIUI) the readout noise is under 1 photoelectron, so they can with not too much penalty simply add short frames, instead of taking one long exposure. This has a number of benefits. >>>Hi, >>> [quoted text clipped - 23 lines] > frames, instead of taking one long exposure. > This has a number of benefits. Amateur astronomers are doing just this (adding multiple shorter exposures) with DSLRs. The cameras of choice seem to be the Canon 10D, 300D and Nikon D70. With the canon, people are doing 1 to 6 minute exposures at iso 800. Here are some results with a 10D, adding 40 some 1-minute exposures: http://clarkvision.com/galleries/gallery.astrophoto-1 This works better than film! Roger >>>>Hi, >>>> [quoted text clipped - 30 lines] > Here are some results with a 10D, adding 40 some 1-minute > exposures: Not quite what I meant. To expand. A photon hits the sensitive part of a CCD, and if lucky (50-90% or so, depending on design) generates one electron that is then stored in the CCD until it's read out. AIUI, the current generation of DSLRs have readout noise well over 1 electron per pixel. For example, if the average noise is 5 photoelectrons per pixel, and you add 10 frames, then you have a random noise that will average 50 counts, but there will be significant numbers of pixels with 20 or 30 counts different than the average. You'r not going to be able to reliably pick out a star that's put a total of 20 photoelectrons into one pixel over the many exposures. If the dark current is low enough, then one long exposure will let you pick it up. However, if you have an average readout noise that is much lower, you can stack frames without boosting the noise in the same way. Even if taking several frames boosts noise, it can be worth it. If you are taking a long exposure, and a plane flies over with running lights on, or a cloud passes over, or ..., then your exposure is ruined. If you are taking several shorter exposures, you can simply throw out bad ones, and add the rest together. Also, you can correct to an extent if your pointing is not perfect, by moving each frame slightly before addition. There are two main sources of noise other than photon noise to worry about in obtaining long exposures: readout noise and dark current or dark noise. Readout noise is the uncertainty in the amount of charge in each pixel as it is being read out. This is typically a fixed amount, and clearly it is best to have a signal (total number of electrons in a pixel) that is far greater than the readout noise of the CCD. Thus, if one takes many short exposures, there is the danger that on each of them many pixels have noise that is dominated by readout noise rather than photon noise. Therefor it it is best to expose long enough so that the readout noise is of no consequence. I don't know what the readout noise is on consumer digital cameras, but I uspect it must be tens of electrons per pixel. A high grade astronomical CCD can have a readout noise of less than 2 electrons per pixel. The dark current will ultimately be the killer in long exposures. At room temperature a CCD is constantly generating large signals all on its on, and as the exposure goes on, this signal can completely fill up every pixel on the CCD. How long this will take depends on the particlar CCD, but exposures of several minutes must be the maximum for most cameras. The escape from this is to cool the CCD, and astronomers cool CCDs to near liquid nitrogen temperatures. Then dark currents as low as one electron per pixel per hour can be obtained. In these long exposures, however, another source of "noise"is highly prominent: signals generated by cosmic rays. Astronomers generally take more than one exposure and have automated cosmic ray recognition and removal software. So, for long exposure with a consumer digital camera, you have to trade off longer exposures with lower effects of readout noise, but higher dark current, vs. many shorter exposures with higher readout noise and lower dark current. It's simplest to use trial and error to figure out what is best unless you know the readout noise and dark current values your camera has. Then, given the brightness of a scene, you could calculate the optimum combination of number of exposures and expposure lengths to get the deisred result. CCDs do not suffer from reciprocity failure, and silicon is much more sensitive to light than film. Recent DSLR cameras have taken increasing advantage of the high native sensitivity of CCDs, though the "ISO" rating of the best astronomical CCDs is greater than 35,000. > There are two main sources of noise other than photon noise to worry > about in obtaining long exposures: readout noise and dark current or [quoted text clipped - 36 lines] > advantage of the high native sensitivity of CCDs, though the "ISO" > rating of the best astronomical CCDs is greater than 35,000. I have quantified the signal to noise per exposure at: http://clarkvision.com/astro/canon-10d-signal-to-noise With multiple exposures the readout noise gets averaged as square root number of frames, wheres the signal builds as the number of frames. You can produce clean images of intensities less than the readout noise by adding multiple frames. It is being done every day by amateur astronomers. The differencees between cooled CCDs and DSLRs has become very small with this technique. Roger >> There are two main sources of noise other than photon noise to worry >> about in obtaining long exposures: readout noise and dark current or >> dark noise. Readout noise is the uncertainty in the amount of charge in <snip> > I have quantified the signal to noise per exposure at: > [quoted text clipped - 7 lines] > amateur astronomers. The differencees between cooled CCDs > and DSLRs has become very small with this technique. Fascinating page. Have you put a number on the absolute values of the noise? For example point it at a star, defocussed, with a known brightness, and measure the total signal, along with the lens details to get a ratio of pixel value to incoming photons. I'd do it, if someone'd donate a 10d :) >>With multiple exposures the readout noise gets averaged >>as square root number of frames, wheres the signal builds >>as the number of frames. You can produce clean images of >>intensities less than the readout noise by adding >>multiple frames. Exactly as you have stated is not complete, unless you you do a noise frame subtarction. In each frame the signal has noise as well, so in each frame, the noise is composed of both the readout noise and the photon noise. Imagine that on average you detected 10 photons per pixel in an exposureso that the photon noise is the square root of 10 in a pixel and the readout noise is 20. On average you will have a signal of 30 electrons. Now add together 100 frames. The total will be about 3000 electrons with an uncertainty of 55 electrons. A accurate measurement indeed! But roughly 2000 electrons in that signal were contributed by readout noise, while only 1000 were contributed by photons. That's a pretty low contrast image. But wait! You can measure or estimate the contribution of the readout noise to an accuracy of 45 electrons or much better if it's constant and you you take the time to measure it very carefully. That means you can SUBTRACT the readout noise from the final summed image and really win. The point is, by summing many frames, you measure both the signal and the readout noise to high accuracy. The photon noise and the readout noise both go down as the square root of the number of frames. Now many CCDs that amateurs use have such constant readout noise per pixel that they just need to do a simple subtraction of some average value, and they are done. Exactly the same thing is done with sky background subtraction, but in this case you have to measure the sky background very carefully while observing. I have been able to observe and obtain good measurements of objects that were only a few percent of the brightness of the sky itself by this method. >>> With multiple exposures the readout noise gets averaged >>> as square root number of frames, wheres the signal builds [quoted text clipped - 26 lines] > able to observe and obtain good measurements of objects that were only a > few percent of the brightness of the sky itself by this method. Yes, dark frames are also done and subtracted. There are "hot" pixels, those with higher dark current, so the dark subtraction step is important, as you suggest. If you look at my astrophoto page, you will see the dark subtraction mentioned. Roger Hi Mojtaba... It works quite well. You can't see the image as it "develops" but you can certainly see the final result on the LCD screen. Some cameras allow you to boost the brightness of the LCD screen, which helps in determining if everything is ok. Here are some photos I took of Sydney Opera House / Harbour Bridge: http://www.shuttertalk.com/forums/viewtopic.php?id=96 I think with digital, it actually helps you learn faster - when I was taking those photos, I took a set, changed the exposure time and took another set, and could see the difference straight away. Regards, Julian |
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