Connect and share knowledge within a single location that is structured and easy to search. In photography, ISO generally refers to a measure of " Film Speed ", which I use including reference to digital sensor sensitivity. Links on the Film Speed page. The standards for film and digital are technically different in ways I haven't investigated closely enough to report fully on , but generally they're similar enough that for practical purposes, they're essentially the same notwithstanding Reciprocity Failure , which many films are quite prone to, though digital generally is not.
So if you measure an exposure with your digital camera, you could use that exposure with a film of the same rating as the ISO setting you used in the digital camera, and expect to get a similar exposure unless the shutter speed is long or short enough for Reciprocity Failure to kick in for the film also assuming similar equipment otherwise -- no differences in filters, etc.
In both digital and film, a higher number indicates greater sensitivity. A number twice as high is twice as sensitive e. So, when shooting in relatively low light, and wanting a relatively-fast e. In digital cameras and similarly but differently in film , higher ISO ratings tend to create noise the related effect in film is increased graininess. So, while it's not always better that depends on what you're going for , lower ISO ratings are always or at least almost always lower in noise, which may be desirable.
In the case of low light photography where shutter speed is not a concern, long exposures mixed with lower ISO ratings will create a "better" image -- though it's conceivable that some may like the effects of the noise; certainly there's appeal at times to film grain.
As for how the scale is defined, it is based on measurements of an image produced based on a certain scenario of illumination.
The details are complex, so I'll leave the details as an exercise for the reader. A lay summary is that for digital it's a measure of how quickly the digital sensor becomes "saturated" with light. For film, the process is related but different. In summary: higher ISO is more sensitive but noisier but not necessarily worse , digital and film rated for the same ISO or ASA for the film will have similar sensitivity, and the scale is based on how quickly an image will become "saturated" given a certain amount of illumination.
Hopefully , my results will create a nice clear noise-free answer to the important point he brings up: that a high-ISO image with a low amount of light will be less noisy than a low-ISO image with the same amount of light getting to the sensor , which is later amplified in post-processing.
More to come, hopefully in I may still do it at some point. In the mean time, I'll also point to this article that talks about comparisons of native versus non-native ISO values, and amounts of noise in them, which, while the article doesn't exactly say so, I think is probably directly related to exactly this question. Then while keeping these settings constant, increase ISO as far as you can without clipping highlights as this will reduce the proportion of the signal that is read noise.
For a fixed amount of light coming into the camera, lowering the ISO will not result in a reduction of noise improved signal-to-noise ratio. The only way to reduce noise is to combine lowering the ISO with letting in more light by opening the aperture or leaving the shutter open longer. If the amount of light you can let in is limited you have hit the max aperture and can't use a longer shutter speed without introducing blur then using the highest ISO possible without clipping highlights will lead to the lowest noise in the image.
This is going to seem counter-intuitive and invite downvotes seven so far! High ISO values don't cause noise, lack of light causes noise. The reason people associate high ISOs with noise is because when you increase the ISO setting in shutter or aperture priority mode, the camera either closes the aperture or increases the shutter speed to compensate, both of which decrease the amount of light coming into the camera.
One important source of noise in images arises from the random nature of light and is called photon noise, or shot noise. Photons are emitted randomly from light sources. If you collect a lot of photons the randomness averages out and you get almost exactly the same number emitted in each direction. If you collect much fewer photons the number collected in neighboring pixels that should see the same color can differ giving brightness variations referred to as noise or grain.
This is how lack of light leads to noise. See Wikipedia: Shot noise. Another source of noise is read noise. Read noise happens when the analogue charges on the sensor are digitized read out. Read noise is approximately constant with respect to the number of photons captured. Increasing ISO amplifies the signal, and thus the photon noise, but the read noise stays the same. If you use a lower ISO with the same amount of light coming into your camera you will get an underexposed image and when you brighten it in post you will amplify both the photon noise and the read noise.
Your total noise will then be higher. These images were shot with identical settings bar ISO, and processed in exactly the same way. The bottom one is clearly noisier, despite being shot at ISO Therefore I don't see how you can argue that high ISO results in noisier images, given that when all else is equal lower ISOs give more noise!
Sorry, but your explanation doesn't change the fact that your basic statement is wrong. What you normally want to use is the lowest ISO that will give proper exposure with a usable combination of shutter speed and aperture.
So, less noise! Yes but you can't increase the ISO and at the same time change settings to let in less light and then conclude you have more noise because you're using a higher ISO. Especially when you can show that if you increase the ISO on it's own you decrease noise!
It's like buying a car with a smaller engine in order to reduce fuel consumption, driving everywhere with your foot flat to the floor and concluding that small engines increase fuel consumption!
Change one variable at a time. There seem to be a variety of opinions on this topic, and while they may seem mutually exclusive, I am not certain that is the case. The general consensus is that opting for the lowest ISO possible to achieve a proper exposure is the best approach. That statement is heavily laden with hidden meaning, however, as the lowest ISO possible may not necessarily be ISO You may be forced to use a higher shutter speed or a narrower aperture to achieve the necessary creative effects you desire, forcing you to use a higher ISO to maintain proper exposure.
You may also face issues with available light and reach the limits of your lens i. I think that is the point Matt Grum has been trying to make. Barring any specific creative needs i. This can be easily demonstrated, as the sample shots below show.
This set of examples is shot with several things in mind:. The clearest shot is, as you can see, the ISO shot. At maximum aperture, ISO required a 0. This is pretty long, but since there was no motion, a lengthy exposure is not an issue. The ISO shot is still properly exposed, and used a shutter speed exactly four stops faster than the ISO shot. Despite being properly exposed, there is obviously additional noise. The noise level in this shot is obviously FAR higher than the other two since it has been digitally enhanced.
I think a good way to learn what ISO settings are best for various scenes and lighting is to set your ISO to Auto, use manual mode, take a few shots, and review the results. The camera's automatic exposure metering will always try to create a "proper" exposure, and when you have a scene with a broad range of tones, it should choose the right setting most of the time.
You can also try manually setting a higher or lower ISO than the one the camera chooses automatically, and re-take a shot to see the results. Outside of maybe landscape and still life photography, you probably won't find a single "correct" or "best" ISO setting.
However, I do believe the general rule of thumb will always be:. Use the lowest ISO you can while maintaining a proper exposure for the type of shot you are making. For landscape, that will probably be lower ISO's, such as or For action photography, including sports, wildlife, birds, kids, etc. Regardless of what ISO you use for action shots, another good rule of thumb is:. Its always better to actually get a shot than miss one because you don't like the camera settings necessary to capture it.
Those ISO shots will have lower noise than the ISO shots that you underexpose then correct via post processing, as shown by the rather extreme example above. De-noising algorithms are also pretty advanced these days, and can greatly reduce the noise level of a high ISO shot to more acceptable levels.
That again makes using a higher ISO that will ensure a correct exposure a better option than a lower ISO that will likely underexpose and require post-process correction. Further investigations to cover Matt's assertions about ISO have lead to another sample image. The camera sensor itself always maintains the same degree of sensitivity; only the level of amplification changes. Depending on lighting conditions, the ISO setting will either be selected automatically by the camera or can be changed manually.
The drawback of increased light sensitivity is an increase in noise — the image loses contrast and appears blurry. Noise levels are strongly influenced by the quality of the camera used. High ISO settings are therefore only advisable with high-quality sensors , in order to avoid excessive image noise. In recent years, semiconductor manufacturing has become more advanced, and this has improved signal-to-noise ratios.
A large pixel area large sensor can capture light quanta more effectively. With this in mind, you exposed differently for the two mediums.
This gives the best signal-to-noise ratio and in turn shows a lower ISO number. Therefore, you may have to dig into how your own camera brand implements ISO so you know how to expose properly for your model of camera.
If the same tools were available for RAW files, this will give us a more accurate output, but how well is still debatable. For now we can only use what we are given to provide the correct exposure and not clip the highlight as a rule. In reality, how does this affect our day-to-day shooting? In many ways, you have to know how your individual camera model responds in the most common situations you shoot. I dug through a lot of the comments and feedback of articles on this matter and through all the discourse most are more interested in practical applications for ISO.
Overly simplifying, if the sensor gets too much light, it can oversaturate and blow out the highlights. Boost it to , and your image will become brighter still. Can you see why this might be useful? But boost the ISO, and your shots will brighten right up. Even in decent light, boosting the ISO can be beneficial. You might need an ultra-fast shutter speed to capture a moving race car, yet raising the shutter speed lowers the exposure and can create a too-dark image.
So instead of raising just the shutter speed, you increase the shutter speed causing your image to darken but also increase the ISO counteracting the darkness by brightening the image. The higher your ISO, the more noise or grain that will appear in your photos, which looks like speckles of color and light randomly strewn across your image.
Can you see the difference? Look at the middle few petals. So raising the ISO, while useful, is part of a tradeoff. Yes, you get a brighter image, but you also get increased noise. That said, camera sensor technology is always improving.
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