# Questions tagged [physics]

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### Why does viewing the sun through a telescope burn your eyes?

First a disclaimer: I KNOW that viewing the sun through a telescope will burn my eyes, so no need to convince me of that. I'm not going to try it. This question is about the optics and physics of why ...
• 155
133 views

### Why doesn't a shorter shutter time cause less capture time/ readout time?

If I make a video @ 120 FPS and a shutter speed of 1/12000 from my pwm controlled lamp, which flickers at around 1300 Hz, I get those 11 rolling shutter bars: (11 Cycles in 1/120s equals around ...
• 155
257 views

### How can I measure the flicker frequency of a light source with my phone?

I have a Hue light and want to measure the flicker frequency (due to the PWM) without buying an expensive photometer. Is it possible, to calculate it with such an Image, by counting the bars from the ...
• 155
701 views

### What determines the image circle diameter?

Anywhere in the web it's stated that the image circle diameter depends on the lens design. I'm curious to understand it a bit better. Let's start with basics. Let's consider a single thin lens ...
• 514
160 views

### Calculate the ideal geometry of a Bahtinov mask for 200 mm MFT

I want to use a Bahtinov mask to focus stars with a Panasonic Leica DG Elmarit 200mm f2.8 MFT. So far I found out that the geometry of the perfect Bahtinov mask ...
• 143
123 views

### Why does the rainbow change into a green light when zooming using iphone

The rainbow is created with Kikkerland rainbow maker. When zooming with an iPhone the image abruptly changes at 2x zoom to a green light. What is the physics behind this?
276 views

### Is SNR dependent on unit area ('per pixel')?

I understand that shot noise will be dependent only on final photon count, and thus is invariant under area changes (shot noise = sqrt(total_photons)). Is the same true for final SNR, if considering ...
314 views

### How (the heck) did NASA photographer Joel Kowsky take this amazing photograph of the International Space Station transiting the Sun?

The NASA Image Feature Space Station Transits the Sun shows seven shadows of the ISS transiting the face of the Sun. In order to discuss the technical aspects of the process I've cropped bits of the ...
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### What's the actual physics of diffusion filters?

Recently, I purchased a Tiffen Black Pro Mist which adds halation to highlights via diffusion. I read online that diffusion filters do not add data as the entirety of the effect can be recreated in ...
• 193
481 views

### Why does light-colored skin look different in black and white versus digitally desaturated versions of color originals

One friend, who was a photographer, commented that a desaturated photograph of light-skinned people looks like they have a suntan compared to a true black and white picture of the same skin, and ...
1 vote
227 views

### Why does temperature in Lightroom go from blue to yellow instead of orange to bright blue?

I was wondering why Adobe chose to have color temperature compensation be the B-channel of the LAB spectrum, as it does not look at all like the kelvin spectrum. Does the B-channel compliment Kelvin ...
• 67
540 views

### Can I hang a heavy lens vertically from the body?

I recently purchased a Cotton Carrier system which uses a clip to hang the camera vertically from the chest area. I am using this with a D500 and a 200-500 lens, which has a tripod foot. When I am ...
• 121
1 vote
278 views

### How Schlieren Photography can be done using mobile camera?

Can we do Schlieren Photography using good mobile phone camera (13-20 MP)? If yes how or can we do this photography only using professional digital camera? Schlieren Photography
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### How can we calculate effective megapixel loss due to diffraction?

How can we calculate an image's effective megapixel count at each aperture setting, specifically as loss relates to diffraction? Assume that we already know the sensor size, its true megapixel count, ...
• 739
1 vote
1k views

### Physical luminance and the CIE XYZ color space

In reading up on the XYZ color space you regularly come across something to the effect that "Y is taken as the luminance" or "relative luminance" whether it be for the XYZ color space or in xyY. I'm ...
1 vote
1k views

### photon counting with a DSLR

I'm quite new to photography, but was wondering what settings I should have in order to have a good approximation of the relative number of photons that creates each pixel? i.e. best settings such ...
60 views

### How do i find the distance required to resolve a point on a page from a mobile camera?

Given all details such as focal length , aperture etc , how can i find the size of a point on a paper that is at a given distance from a mobile camera ? Or vica versa , the distance required to ...
16k views

### What is the farthest a camera can see?

At what distance can a face no longer be identified using a camera? At what distance can a figure of a person no longer be captured?
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### Does a sensor count the number of photons that hits it?

I'm interested in the grayscale image case. In a sensor there is an array of cavities which collect photons. (Source: Cambridge in Colour Digital Camera Sensors) Does each cavity count the number of ...
1 vote
119 views

### What is the flame-like pattern detected in this image near the hammer of a metal worker?

First, I'm new to StackExchange, so I apologize if I'm posting in the wrong area... What is the strange visual element emanating from the hammer in this picture? It looks like an upside-down white ...
453 views

### Is there theoretical limit on the minimal focal length for a given sensor size?

There seems to be a limit, around 0.33, on the fastest possible aperture. Is there a theoretical limit on how wide a non-fisheye lens can be? That is, what is the minimal possible focal lens for a ...
• 339
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### How do apertures which seem too large to physically fit work? [duplicate]

A 400mm f4.0 lens is supposed to have a maximum aperture diameter of 10cm. Looking at such a lens they don't seem big enough to fit that kind of aperture including all the mechanics for control inside ...
• 243
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### Camera image in pixel to real image in meters size relation [closed]

first post so please be nice. I am doing an image processing project and my problem is now that I would like to know how big my digital image is in the real world. I have: digital image: 336x256 ...
163 views

### What technology do modern flashes use to emit light?

Today, practically all lamps that produce continuous light are using LEDs. Indoor lighting, street lighting, flashlights, etc. all use LEDs. However, the flash in a camera has to create a huge amount ...
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303 views

### Light falloff - until lens surface, focal center, or camera sensor?

Preface: if there is a better-suited forum for this question, please direct me. I figured it'd be better to post into Photography and not Physics as some of you might have already examined this ...
• 165
545 views

### Polarization and Rainbows

Why when I use a polarized lens if I turn it one way the rainbow disappears and the other way it brings it out better to the point you can see the second double rainbow when the naked eye can't? Is ...
• 1
641 views

### Calibrating monitor with lab spectrometer

I'm trying to calibrate my monitors. But I'm wondering if I can avoid buying another piece of equipment by using a lab spectrometer (in the past I've always just calibrated by eye). I'm a PhD student ...
743 views

### Cross Polarization: Are linear and circular polarizers compatible?

Can cross-polarization be achieved with two linearly polarized (flash) lamps; one either side of a circularly polarized lens? Lens attachments tend to be circularly polarized, while the PVA sheets ...
• 615
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### How to get Canon 1200D quantum efficency plot?

I've searched quantum efficency of Canon 1200D and I found some images like here: http://www.deep-sky.co.uk/imaging/dslr/dslr.htm . How these people get quantum efficency plots for mentioned Canon 40D?...
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### Why do polarising filters only work in one direction?

So I bought a polarising filter yesterday that has a dial you can turn to increase/decrease the polarisation effect and mount directly onto the lens. I also have an old polarising filter that was one ...
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### Polarizers: Linear vs Circular. How does it affect a photo? [duplicate]

Trying to decide which type of polarization filter I should get. Some information I've read suggests that there is no difference and that a circular polarizer is just an ordinary linear polarizer, ...
• 615
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### Why is the area in focus in front of the focus distance narrower than behind it?

I guess the title and the picture explain my question well.
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### Since the speed of light is so high, why does shutter speed even matter?

When a camera's shutter is opened, if light reaches the sensor instantaneously (speed of light = 300.000 km/s), why does shutter speed modify picture sharpness/detail? Why do pictures get darker with ...
• 259
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### How much of digital sensor noise is thermal?

Is all digital sensor noise thermal? And what is the relationship between temperature and noise? For example, as we cooled a CMOS towards absolute zero, would we discover that high-ISO images would ...
• 3,603
1 vote
591 views

### How do wavefronts/wavelets propogate in three dimensions, and why is this important for photography?

The photography book I have talks about waveforms, but it doesn't do a good job of explaining them. So, here's my understanding from the book and google searches (it could be unbelievably wrong.) In ...
• 739
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### Are there cameras that can photograph Wi-Fi/WLAN or mobile phone radiation?

Considering that there are cameras for infrared, X-ray and ultraviolet, I wonder if there are also cameras that can picture the WLAN or mobile phone parts of the electromagnetic spectrum. Considering ...
82 views

### Low-light photography: How much light reflects off a surface

I'm looking at light levels for photography and I'm trying to figure out how much light hits my sensor. There are plenty of tables online which give light levels, but that's only directly relevant if ...
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### Converting cameras for infrared use: When does the filter have to be replaced, and when is simply removing it ok?

When converting a digital camera for infrared use, the general approach is to remove any filters that limit its sensitivity to the visible range of light wavelengths. Some conversions replace the ...
• 494
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### In what way does the lens mount limit the maximum possible aperture of a lens?

In many answers to questions about different aspects of really large aperture lenses it's pointed out that the lens mount sets a hard limit on the maximum possible aperture of the lenses to that ...
• 8,288
484 views

### What are the main limiting factors for dynamic range in modern cameras?

Another question got me thinking about what is limiting dynamic range in digital cameras. From my understanding the precision of the analogue-to-digital converter, i.e. how many discrete levels it can ...
• 3,196
1 vote
2k views

### How do I know how much light will be needed for a given film speed, shutter speed, and aperture?

For a given film type, shutter speed and aperture, how do I calculate the light needed to get a good exposure? For example, if I use a f/8 aperture, an exposure time of 100 ms and a ISO 800-film (...
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### What are the theoretical minimum and maximum apertures?

Is there such a thing as a maximum aperture that a lens can be open to? What about a minimum aperture that it can be closed to? Do these concepts even make any sense? Is there a lens with the ...
590 views

### Does an aperture value exist, where a lens creates a brighter image than the real view?

So (without limitations of the viewfinder for example) looking through the viewfinder creates a brighter image than normal view would be. The lens would then work like a funnel(collects light from ...
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### Why is the focus distance for a mirror image further than the surface of the mirror itself?

When I take photographs of a subject reflected by a surface (water, mirror, glass) I notice that the focussing distance is not the distance from my camera to the surface, but mostly it is close to ...
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