As this is half over I'll start with a controversial phrase to attract the attention of readers:
" Upload ISO does not increase noise in the pictures "
Noise:
In a photograph taken at normal shutter speeds, ie less than one second 99% of noise in the image has two origins:
- photon flux Noise Noise reading
sensor noise photon flux
This is the main source of noise because it is the self-noise of an image is independent of the camera and use and is absolutely inevitable, even with perfect technology.
Given a scene point its luminosity is given by the number of photons originating from the same, suppose you have a point that has a brightness of 1000 photons per time interval, if the photon flux was always constant camera sensor provided 1000 photons would arrive within a fixed time. However, the photon flux is variable and the variation is given by the square root of luminosity. This means that the photons to the sensor 1000 will be arriving 1000 + / - 31.62 photons. This difference between the photons should reach the sensor and really get is noise.
From here we can draw several interesting conclusions
The first is that as you increase the brightness of the image (signal) noise increases by 100 photons have a variation of 10 as noise photons variation in 1000 is 31.62.
However, in a photo which affects the image quality is not the absolute amount of noise but the rate signal to noise ratio, it is understandable if we consider the following:
Suppose a pixel having a brightness value 1000, if we vary 1 point this light the result is totally invisible to the naked eye. If the pixel brightness instead had varying 1 point 1 would be equivalent to doubling its brightness or to turn off the pixel completely and this is much notary. From here that the more obscure the larger picture will be the noise due to an unclear image signal is low and the rate signal to noise ratio will be much higher. Many say "a picture is more noise in the shadows ", this is false, in the headlights there is a lot more noise in the shadows but as the signal rate is high signal to noise is less and less noticeable. This serves to impress in talks amateur photographers coffee.
As the noise is the square root of the signal as the signal rate increases signal to noise ratio decreases, in 1000 there are 31 noise photons for a 3.1% signal / noise in 2000 photon noise is 44.7 2.2 % signal to noise ratio.
Conclusion
simple: You have to take a picture with as much light as possible. Viva light.
The "cult" to expose to the right is based on this, there are many articles that explain erroneously that expose on the right is good at talking of levels in the RAW and the like, the truth is that the idea is simply to maximize the rate signal to noise and better capture more light. The image quality "optimal" in a camera at ISO100 is achieved with the histogram as far right as achievable. More on this a little later.
Another thing we also see is how to do "stacking" of images helps to eliminate the noise if the brightness of a point is 100 + / - 10 due to the noise taking enough pictures we can make the average brightness point is as close to 100 as we want all of this is eliminated completely, the image noise.
The noise reduction camera can not eliminate this type of noise it is impossible to know whether a point sensor that receives a certain number of photons should actually receive more or less. An advanced camera could eliminate noise taking 100 pictures instead of one and averaging values \u200b\u200bbut by the time the cameras do not do this and must be done by software.
noise sensor reading
This source of noise is due to the imperfection of technology. If the sensor 100 photons will reach the sensor does not always register 100 photons but the value varies a bit registers. The overall oscillation is constant and not dependent on the brightness of the scene. You can eliminate much of this noise by taking a picture with the lens cap position; under the assumption that no photon should be recorded by the sensor
everything is recorded sensor noise and can be removed from a photo taken with identical exposure parameters. This is what makes the noise reduction of the camera, just take a picture with the diaphragm closed for the same exposure time and everything that appears in this picture if I was also in the original photo deletes the same as it is known is noise.
Read noise sensor has 2 components one is reading the number of photons that makes sensor is imperfect and the other is the analog / digital sensor that makes the number of photons at a certain voltage.
RF + RL = RL = RC
read noise RF = read noise photon noise
RC =
conversion depends on the RF signal is amplified by increasing the ISO. But RC is constant.
Then:
RL = G * RF + RC
where G is the amplification factor due to the ISO.
Note that as RL increases the signal increases as RC is constant but the increase is not proportional. Sean
100 photons with an error + / - 10 and G = 1 (ISO 100). Sea RC = 5 (arbitrary)
RL = 1 * 10 + 5 = 15
As the rate signal 100 signal to noise ratio is 15/100 = 15%
If we double G (ISO 200)
RL = 2 * 10 + 5 = 25 (Signal = 200) as a sign
now the rate is 200 SNR is 25/200 = 12.5% \u200b\u200b
Sean now 1000 photons with an error + / - 31 and G = 1. Sea RC = 5 for the same sensor
RL = 1 * 31 + 5 = 46
rate / N = 46/1000 = 4.6%
If we double G RL = 2 * 31 + 5 = 67
rate signal to noise ratio = 67/2000 = 3.35%
Conclusion: As we raised the ISO read noise increases but the proportion of noise with respect to the signal decreases (!).
then return to our phrase controversy
" Raise the ISO does not increase noise in the pictures "
The main reason for which to raise the ISO the pictures we leave is more noisy simply that we get less exposure time and reduce the signal.
1 / 100 ISO 100 has less noise than 1 / 200 ISO 200. This is nothing new.
While reading the sensor noise increases to raise the ISO we have learned that what really matters is the rate signal to noise when turning up the ISO helps to raise the signal is then up the ISO continued imposition of the time we reduce the noise exposure photo.
No, I'm not crazy. Here is an example.
Be the histogram:
The histogram corresponds to a normal picture as we see the dynamic range of the scene is smaller than the sensor and is "space" in the histogram to disclose in a different. The farmers of exposing to the right will immediately recognize that increasing the exposure of the photo could be reduced noise. Pompously we know that all noise actually increased the signal but also increase the rate decreasing signal / noise and that noise is a function of the square root of the signal.
Now suppose for the photo in question can not be changed exposure, was the maximum opening of the lens and if you change the shutter speed something terrible would happen, for example, a blurred picture. Does it make sense then increase the ISO to right the histogram? The answer is yes and this is the great "idea" presented in this article.
If we can increase the ISO move to this type of histogram:
What we do is increase the signal by the ISO and as we have demonstrated timely read noise increases but does not increase in proportion to how the signal increases due to the constant component RC.
This has immediate applications for those who take moving objects in poor lighting conditions, concerts, sports, etc.
While the optimum to expose to the right is out at ISO100 at slower speed we can in these conditions there is some shutter speed limit, if the histogram permits must then increase the ISO to lower noise photos. Also applies to the moon and astronomical photography in general, if the histogram permits, ie without burning your headlights must be removed at the highest possible ISO if the shutter speed can not change because, for example, stars would show trace or the moon would move us in the image. Conclusion
practice
Suppose we have a picture at f2.8 and 1 / 100 ISO100 and assume that the histogram supports 1 stop change the headlights without burning.
The optimum is then draw the same picture on 1 / 50 ISO100 increasing the signal noise without generating extra reading out in ISO minimum.
If you change the shutter speed is not possible then it is better to get in 1 / 100 ISO200 and then adjust the RAW 1 / 100 ISO 100, noise increases reading but is offset and exceeded by the increase of the signal. The rate signal to noise ratio is lower.
repeat:
f2.8 ISO100 1 / 100 is "noisier
that
f2.8 ISO200 1 / 100
Assuming that in both cases there is no high beams SETTINGS burned and both are correct for the scene.
References:
"Noise Dynamic Range and Bit Depth" by Emil Martinec ( Noise, Dynamic Range and Bit Depth in Digital SLRs )
"Expose Right" Cambridge in Colour ( Expose Right )
"Rawanalize" A RAW software to analyze raw images. ( http://www.cryptobola.com/PhotoBola/Rawnalyze.htm )
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