The Nikon D count rates are generally higher when compared to the Canon 40D, except for the wavelength regions mentioned above. The spectral responsivity and the quantum efficiency may now be solved according to Eqs. Based on the above findings, the 3 second exposure by the D and the 4 second exposure by the 40D camera are chosen to represent the raw count rates in the calculations. The net result is shown in Fig. The dotted lines are for the Canon 40D camera.
Panel B shows the corresponding calculated quantum efficiency. The calculated spectral responsivities and the quantum efficiency curves are compatible in shape and amplitude. Note that the quantum efficiency calculation is a more direct and robust method since it does not depend on the Singular Value Decomposition. It is now clear that both cameras have their peak sensitivity in the blue and minima in the red channels. The color balance between the channels is, in other words, the same for both cameras.
However, as expected from the level of the count rate profiles, the Nikon D is the most sensitive camera. The above is a surprising result. We double checked our experimental setup to make sure that we actually used the same settings on both cameras.
The difference is so small that it is hard to see it if we overlay the curves of the 40D source functions in Fig. One factor that could explain the discrepancy, especially in the red, could be the difference in the spectral transmission of the lenses.
Figure 7 shows the spectral transmission profiles using the FICS spectrograph as the detector and a flat Lambertian surface as the target reference. The surface was illuminated by a Tungsten lamp. The setup is identical to our narrow field of view intensity calibration procedure [ 3 ]. The shape of the transmission profiles is more or less equal for both lenses. The dotted black line is the difference in transmission between the Nikon and Canon lens, respectively. If we take into account the transmission of the lenses, the difference between the cameras in spectral responsivity and quantum efficiency is only changed by a few percent.
The above result indicates that the main difference between these two cameras is in their detectors. The main difference in spectral responsivity is found to be in the red channels, and could well be related to the transmission of the red elements in the CFM, the infra-red filter or in the semiconductors used.
This is an interesting topic, but it is beyond the scope of this paper. A digital camera image could contain more information than just relative scaled intensities or color coded pixel values that have little physical meaning in terms of brightness on a quantitative scale. The recent improvements in both sensitivity and dynamic range enable the DSLR camera to be used as an intensity tool as well.
But the lesson learned from this exercise is that there seems to be a lack of a common standard for camera sensitivity given by the manufacturers. As shown above for Nikon and Canon, the ISO International Organization for Standardization values, originally defined as the speed of photographic film, may not be the optimum parameter representing the sensitivity of a digital sensor. The spectral responsivity or the quantum efficiency is the parameter that should be used in the future to characterize the sensitivity of a camera.
We hope that the manufacturers can provide this information in future. It would increase the usage and potential of these fantastic devices. Sigernes, J. Holmes, M. Dyrland, D. Lorentzen, T. Heia, T. Aso, S. Chernouss, and C. Express 16 20 , — Baker and G.
Lorentzen, S. Chernous, T. Moen, and C. Berry, and J. Optica participates in Crossref's Cited-By Linking service. Citing articles from Optica Publishing Group journals and other participating publishers are listed here.
Alert me when this article is cited. Click here to see a list of articles that cite this paper. Learn more. Allow All Cookies. Optics Express Vol. Open Access. Express 17 , More Like This Sensitivity calibration of digital colour cameras for auroral imaging. Autonomous absolute calibration of an ICCD camera in single-photon detection regime.
Standardized spectral and radiometric calibration of consumer cameras. The topics in this list come from the Optics and Photonics Topics applied to this article. Short background 2. The experimental setup 3. So how can they see these distant objects more clearly?
This is one of the key functions of a telescope - to resolve celestial objects. The higher the resolution of a telescope, the more details we can see from the images obtained on it. Technically we are referring here to the spatial or angular resolution of a telescope.
The three images below simulate the effect of differing resolution for the galaxy NGC The left hand image has low resolution, the middle image better resolution and the right hand image high resolution so that detail can be clearly seen. The ability of a telescope to distinguish between, that is, resolve , close objects. For circular apertures, such as in telescopes, where the light rays from a source are parallel, as is the case for distant point sources of light such as stars, the light will be diffracted so as to form an Airy disc.
It is the size of the Airy disc that imposes a limit on resolution. Two objects are said to be resolved if their Airy discs are sufficiently separated to be seen as distinct.
Rayleigh proposed the criterion that two point objects are just resolved if their angular separation is such that the central maximum from one point source lies on the first minimum of the other as shown in the image below:. The image below shows.
A more practical version of this equation expresses the theoretical value of the resolution in units of arcseconds.
This is given by equation Note that this equation is not specified in the Board of Studies Physics syllabus or formulae sheet but understanding it will help you discuss the concept of resolution for telescopes. Firstly, resolution is inversely proportional to the size of the primary mirror.
A large telescope therefore theoretically can resolve more detail than a small telescope at a given wavelength. How does an 8m telescope compare with the human eye when it comes to resolving detail? If we assume that a fully-dilated pupil has a diameter of 7mm, ie 7 x 10 -3 m and we are observing in yellow light at a wavelength of nm 5. The longer the wavelength, the lower the theoretical resolution for a telescope of given size.
Hence an optical telescope such as Gemini that can also observe at near-infrared wavebands should theoretically obtain lower resolution observing an object in the IR than at shorter wavelength visible light. As we shall see below, however, other factors come into play that reduce the actual resolution obtained by telescopes.
If an optical device such as an eye or telescope achieves its theoretical resolution in operation it is said to be diffraction limited. In practice, this is not always achieved. The human eye, for instance, has imperfections on the cornea which normally degrade its resolution to about 1 arcminute, compared with the Modern optical telescope mirrors generally approach their theoretical limits for smoothness so should not suffer from this problem.
Large mirrors traditionally were made very thick to avoid the problem of flexing which would distort any image. Glass is very heavy, necessitating heavy mounts and drives to support the telescope, and also retains heat quite well.
This is a problem as it takes a long time to cool down at night. The warmth of the mirror can heat the air above it, causing turbulent convection cells that diminish the seeing of the telescope.
Turning on that feature allows the camera to push the ISO up when it decides the shutter speed is getting too low for a good picture.
This takes Auto ISO and lets you have some say about what happens. Using it, you set the limit for how high it can go ? The amount of control this feature allows means more photographers will start taking advantage of it. A solid understanding of ISO will help you make smart decisions about how to set your camera. And that, in turn, will lead to better pictures. By clicking Sign Up, you are opting to receive educational and promotional emails from Nikon Inc. You can update your preferences or unsubscribe any time.
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