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What does the ISO affect?

Duckhunter50

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So I grew up using film cameras and understand the concept of different film speeds. I typically used ISO 100 for brighter, slow or stationary subjects and ISO 400 for faster or lower light situations. But I don't understand how the ISO affects photos in a digital camera.
 
So I grew up using film cameras and understand the concept of different film speeds. I typically used ISO 100 for brighter, slow or stationary subjects and ISO 400 for faster or lower light situations. But I don't understand how the ISO affects photos in a digital camera.
In digital photography, ISO measures the sensitivity of the image sensor, but works the same as a film camera. The higher the iso the more ability to film in low light and the more grain you get.
 
I agree with Repaid1
Same thing with film or digital, lower ISO better quality higher ISO less quality but it allows you to get a shot in low light with the available aperture.
 
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Like the others have said, ISO is the digital equivalent to Film Speed.
With Film, ISO 64 or 100 were ideal for color. ISO 400 was considered high speed and could be "Pushed" to 800.
In extreme conditions like sporting events, ISO 800 could be Pushed to 1600. "Pushed" is a dark room technique used to double the film speed however, it caused a grainy affect. Of course the higher ISO allowed faster shutter speeds and/or a higher F/Stop. Black & White film always showed less grain.

With Digital, many factors determine high ISO image quality. The main factor is the size of the digital sensor. In DSLR cameras, you see the term "Full Frame". Full frame sensors have the same dimensions as 35mm film or 24mm x 36mm. Smaller sensors are considered "Crop". Full frame sensors give a wider view through the lens. In Nikon DSLR, FX is full frame, DX is Cropped and will give a zoom affect with the same lens. Full Frame will always have the better quality image and less Grain or Noise at higher ISO. With a top of the line Nikon DSLR like a D6, I'm not afraid to use ISO 6400.

Grain is a film term, Noise is more of a digital term. Noise is seen as a lack of color or details in a pixel. Noise is directly related to the size of the pixel / resolution of the sensor.
Resolution is the number of pixels. A sensor that is 4,000 x 3,000 pixels is a 12 MP sensor.
4,000 x 3,000 = 12,000,000 = 12 megapixels .

The number of pixels in the image is directly related to the size you can print. Most digital printers and print services consider 360 pixels per inch as photo quality. Divide the resolution by 360 and it will give the size of the available print. I minimum is 150 pixels per inch.
4,000 pixels / 360 pixels per inch = 11 inches

Pixels are different than dots per inch. Pixels are use to measure resolution in sensors, TVs, and monitors. Dots per inch are the number of ink dots a printer can produce.

Noise can also be caused when you edit the image. Sharpening is the most common cause of noise in editing.
With software like Photoshop, research a technique call "High Pass Filter Sharpening". This technique is the best I've found for sharpening without introducing noise. You can create a Photoshop Action that records your steps and reduce the time it takes. Play the Action and it will repeat the steps.

In DJI Drones, the sensor is measured in Video standards. A 1" sensor will have more resolution and less noise than a 2/3 sensor.
 
Great explanation Electromen
The fastest film I ever used 30 + years ago was ISO1600 or was it ASA back then , wanted the birth of my boys but the hospital wouldn't allow flash in the delivery room because of the oxygen, not the best quality pictures but at least I got them.
You could probably do better with the camera in a phone these days
 
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@Electromen provided a very complete description of film vs digital. And while there are many technical differentiations, on a practical level, for the most part, you can still think conceptually as you would with film. In many ways it's pretty much the same. Higher ISO's provide more sensitivity, whether film or electronic sensor, and introduce artifacts (grain/noise) into your images. There is one major difference...

Assuming that with a film camera you would shoot with negative film (as opposed to chromes... aka slide film/positives), with negative film, modest overexposure was rarely a problem and with many films often encouraged to preserve shadow detail. With digital you have to be very careful not to overexpose so as not to blow out highlight detail. If you've shot chromes then you know, like digital, your exposure has to be more precise. Fortunately with digital we have instant access to our images and video and have the luxury of the histogram to tell us when our exposure is proper.
 
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