Hot answers tagged

82

In order to get an image, both the subject and the "camera" must be much larger than the wavelength of the light that you use for imaging. The wavelength of visible light is between approximately 400 and 800 nm, i.e. smaller than a µm. Radio frequencies go up to several GHz, which corresponds to wavelengths of many centimeters. For example, the 2.4 GHz ...


22

There are two hard limits on how fast a lens can be: The first is a thermodynamic limit. If you could make a lens arbitrarily fast, then you could point it to the sun and use it to heat your sensor (not a good idea). If you then get your sensor hotter than the surface of the Sun, you are violating the second law of thermodynamics. This sets a hard limit at f/...


21

You're probably comparing a linear polariser with a circular polariser. The linear polariser is a basic filter that only passes light waves polarised in a particular direction. That works either way round, and you can combine two of them to produce a variable density filter - by rotating the second polariser, it passes most of the light when the polarisation ...


20

I disagree with the answer with many upvotes. Physical lengths can be "swindled" in a number of ways and theoretically it would be possible to build a portable camera that snaps images of a very tiny portion of the electromagnetic spectrum. Plus, you are not considering that there are not only high-band signals, but also ultra-high-band signals that could be ...


18

Your observations of the lens leads you to both a correct, and incorrect, conclusion. Correct: the aperture (i.e., mechanical iris) of the lens is substantially smaller than the 10 cm it supposedly should be. Only the front element is anywhere near 10 cm diameter. Where the iris mechanism is in the lens barrel, the diameter is substantially smaller than 10 ...


17

why does shutter speed modify picture sharpness/detail? Why do pictures get darker with faster shutter speeds, and brighter with slower shutter speeds? These things happen because the light sensor in the camera doesn't measure the intensity of light instantaneously, but rather measures all the light received during the entire exposure. You could say that ...


13

If you simply want visual examples with commonly available lenses and resolutions the webpage: "Guide to Identifying or Recognizing a Face: Resolution, Focal length, and Megapixels" has a number of examples. Axis Communications has what they call a Pixel Density Model: Examples of maximum distances for identification (500 px/m or 80 pixels/face). The Axis ...


12

Physics plays a role in answering your question and that information is out there. The basics from that linked discussion are that the index of refraction of the lens material will affect the maximum aperture you can achieve, so for pure glass that has an index of refraction of 1.5, the maximum aperture would be f/0.5 or thereabouts. Better substances, such ...


11

An aperture could be closed which is effectively an infinitely large f-stop number since no light gets through. The fastest possible (smallest f number) is a bit harder. The speed of a lens is limited by the ratio of the entrance pupil to the focal length of the lens. The longer the focal length, the bigger the entrance pupil must be. In theory you could ...


11

I read online that diffusion filters do not add data as the entirety of the effect can be recreated in post with more flexibility. Yes and no. But in practice mostly no. Yes, in principle, the effect of a diffusion filter is equivalent to applying some kind of blur to the image in post-processing — most typically gaussian blur, although other kinds of ...


10

Your link discusses how a CCD (charge coupled device) image sensor works. Note, CCDs have applications besides images sensors, but the vast majority of CCDs are used as image sensors, and that is the only primary application I will be discussing. CCDs In typical CCDs used for color image sensing each CCD cell has a color filter over it. The most commonly ...


9

No. This is like saying you can determine the height of a house by counting steps to get up. The size of steps matters and so does the number. Dynamic-range should be determined by the well-depth (size of capacitors at a photosite) and noise floor (how much noise in the system when these is no signal). Knowing these two, one can compute the dynamic-range of ...


8

What you're seeing is smoke what I am assuming is coming from the hot metal as the worker hit it. This smoke hit a light beam (probably coming from a window somewhere in the room) and was lit up, just as you shot the picture. Not an artefact, just reality and in my opinion a nice addition to the photo.


7

Sort of. Not a "camera", but a computational imaging technique. We explore the feasibility of achieving computational imaging using Wi-Fi signals. To achieve this, we leverage multi-path propagation that results in wireless signals bouncing off of objects before arriving at the receiver. These reflections effectively light up the objects, which we ...


7

No, you won't obtain the photon count directly. Also, a camera sensor has noise, not just from photon counting but also from electrical circuits. Also, a DSLR has a color filter on top of the pixels, even if you take only grayscale images. It will probabilistically filter away some photons. If the photon is of the correct color, chances of it passing the ...


7

Diffusion filters cause a portion of the light to refract/bend by creating some kind of obstruction. The amount (percentage) of the light that is scattered is indicated by the filter's strength rating. And because it is a relative amount of scattered light, stronger light sources will show more halation/bloom. There are different ways of causing the effect ...


7

I've actually done this myself. The first time I was not successful. The second time I was successful. Considerations There are a few things that go into this. Planning -- finding a transit near you Equipment -- what you'll need and/or considerations for what would work Exposure -- considerations of the ISS that will influence your exposure Planning You'...


6

It depends: On the sensor design, the ISO setting selected, the length of exposure, the intensity of the light entering the camera, etc. Thermal noise can be anywhere from almost none of the noise in a digital photograph to almost all of the noise in a digital photograph. If the photo is taken relatively quickly with moderate light intensity at high ISO ...


6

No, it's a photography question alright. But I assume that by "clarity" you mean "sharpness", otherwise the question makes no sense. If your object is 30 m away, light from it will reach the sensor in 100 ns (billionths of a second). That's several orders of magnitude faster than the shutter's speed, we can actually ignore the 100 ns, and say light arrives ...


6

The obvious answer is that the light from a rainbow, or at least the portion that is fitting in your viewfinder at the time you are rotating the polarizer, is all polarized in the same direction.¹ That is, the light reflecting off of millions of water droplets falling in the atmosphere is polarized in the same way by the more or less uniform shape of those ...


5

Okay, so, the first thing to understand is that the textbook¹ is trying to get you to understand a theory put forward by Christiaan Huygens in the late 1600s. It turns out he was (generally) right about the wave nature of light, but the actual specifics are iffy. Don't get too bent out of shape trying to make everything make sense, because... well, it doesn'...


4

For phase detection AF to work properly, the camera needs more than just a clear glass replacement for the removed filter, it needs a replacement of a different thickness. IR rays don't focus on the same point as visible light, which is why lenses have (used to have?) a red dot to tell you how to adjust focus from the visible light distance to the correct ...


4

Why is the area in focus in front of the focus distance narrower than behind it? It isn't. Not always. Just usually for landscape shooters using wider lenses aiming somewhere not that close. :) The 1/3-2/3 proportion rule is basically a rule of thumb that does not apply in all cases. The larger your aperture, longer your lens, or closer your shooting ...


4

You can imagine light as an electromagnetic wave, but for this question I'll use its second "state" as a (humongous) set of particles - photons. Why do pictures get darker with faster shutter speeds, and brighter with lower shutter speeds? In a given period of time some amount of photons pass through the lens and excite parts of the semiconductor chip (...


4

The digital camera sports automation that adjusts exposure and focus. These mechanisms are likely dependent on semi-silvered mirrors. These work like mirrored sunglasses; they pass some light and reflect the remainder. When you mount a polarizing filter, it can be an impairment diminishing the effectiveness of this wonderful automation. Because mounting a ...


4

Flashes and studio strobes use arc light: a glass tube with electrodes at both ends containing a gas that can be ionized at high voltage to produce light (e.g. Xenon). Arc lights can fire incredibly quickly (the Paul C Buff Einstein studio strobe can fire in 1/13500 of a second at low power). The tube is also balanced to emit white light at daylight color ...


4

The actual focal length if the lens depends on the focus, so you cannot even trust was is reported in the EXIF data or even the markings on the lens. If the settings are repeatable (prime lens...) your best bet is to calibrate the image by shooting a known object (typically a rule/tape measure), and measuring its size in pixels in the resulting shot, and ...


4

Get an idea on the spectral sensitivity and quantum efficiency of your camera. The data are provided by the camera manufacturer and/or chip manufacturer. It's easier for monochrome cameras as there is no colour filter (bayer pattern) involved. They are also more light sensitive. CCDs are more sensitive than CMOS and CCD is also more linear. (Your Canon ...


3

A 'circular' polarizer is just a linear polarizer with a quarter-wave plate behind it that repolarizes the light in a circular kind of way. But the polarized light removed by passing through the linear polarizer is gone, and thus is not reintroduced by the "re-polarization" of the remaining light. A quarter-wave plate is not another linear polarizer. It is ...


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