ISO sensitivity in photography is certainly the most misunderstood part of the photography knowledge for a novice and even from a lot of professional photographers! It seem even quite incredible that the latter can misunderstand what is exactly ISO.
You might have read almost everywhere that digital ISO is about the capability of increasing or/and decreasing the sensitivity of the sensor to catch the light, this is wrong, totally wrong!
A camera sensor do not have any kind of potentiometer, to increase or decrease it's sensitivity nor the sensor will add or withdraw electrons to or from itself!! ISO is all about software, firmware or whatever you call a written language for computers.
To understand exactly what is ISO, i need to bring you in the "geek" side of the camera to let you see what happen with the sensor.
A sensor is made with a fixed ISO sensitivity (also called "native ISO"), roughly the job of the sensor is to catch light (photons/electrons) and give the amount of electrons it has catch to the processor of the camera, so the camera will count how much electrons the sensor caught and this count will be transformed in number, in other hand it's what we call "converting from analogical data (the electrons) to digital data (the number of electrons, written as 0 and 1 in computer language).
And it's there, during the conversion, that ISO shows up, because it's all about scaling and re-scaling the signal from the sensor and the abilities of the software-firmware to read and re-scale properly the amount of light the sensor have received.
Having said that, here is a video from Dylan Bennett explaining in a very simple way what is ISO, and as i always said, if someone is better than you doing something, let her or him doing it, so better i let him explain what happen in the camera and i will see you after the video(just below it) to speak how to use ISO in real life photography, with the ins and outs and the side effects of increasing ISO.
So now that you know exactly what happen at the sensor level with the ISO, let us speak about the real life application in photography.
The beauty of high ISO is where you are in a situation you cannot use flash for any reasons and there is not enough light to have a "fast" shutter speed (to not get a blurred picture).
First: when do you not need high ISO?
You do not need high ISO when your shutter speed is more than fast enough to freeze the motion (if you don't know things about shutter speed look in the sub-menu on the right side (soon to appear).
Take a look at the picture on the side, i froze the pelicans during their flight. No motion blur at all.
It was a very sunny day, no clouds, harsh light, meaning i had a lot more light that i even needed, at ISO 100 (the minimum) and i got a shutter speed of 1/4000 of a second!
When you are under a bright light outdoor put your ISO at the minimum, it is were you will got the best quality of the photograph in term of no noise.
But outdoor with bad weather, bad light condition or indoor, it's were you will need to increase your ISO, and some time it's even better to increase a bit more than needed and to be sure to not have a blurred subject due to the lack of the speed of the shutter.
High ISO have a counter effect on the quality of the picture itself. Like you saw on the video, more the ISO get high, more the parasite called "noise" or "grains" show up.
On the side is a typical example where i have no choice to bump up in extremely high ISO (EOS 5D Mark II).
I was on a boat, we needed to get back to the base camp before we can't see anymore where we are going in this maze between the mangroves.
I liked a lot the reflection of the sky on the channel, but the light, as you can see, was not enough, and in same time i should counter the speed and the shakes of the boat.
So no choice, the lens wide open at f/2.8 (What is aperture?) ISO 6400, with a 15mm fisheye, and a shutter speed of 1/125s only, it's not bad, but just enough to not have too much blur, thanks to the high ISO capability.
As you can see the ISO 6400 on the above picture make it ugly, i needed to post process this picture to "melt" the noises and the grainy things, because of that, the sharpness and the "matter" took a hit, high ISO is nice but you should use it at the last hope.
In fact as you can see this picture have a lot of dark and shadows and it's where mostly the noise and grains appear, in the dark.
Depending of your camera and it's capacity, there is a level where you can go without having those huge grains and noises. Here on the right side is a good example of my camera where it handle quite nicely ISO 4000 (EOS 5D Mark II) which is my usual top limit to not cross with this camera.
We was diving in a cave system, and as you can see, it's very dark, i am at 1/200s, even if my buoyancy is not bad, i needed to compensate for my movement and her movement too (yes she is a cave diver). I know that with this camera, up to ISO 4000 it is not a big deal at all, so i increase the ISO to 4000 letting me the option to have a good shutter speed and not have a blurred picture.
In event or concert photography, you do not have the right to use flash or strobes. Often the lights condition are awe-full for a photographer. Here also you need to increase ISO!
Usually with a fast lens as fast as f/2.8, i do not need to increase more than ISO 3200 to have a shutter speed fast enough to freeze the motion.
And for the camera i own, ISO 3200 it's a piece of cake without those grainy noises.
Extremely high ISO is very destructive in term of quality, but it gave us the possibilities to take a picture (a non blurred picture) where before the digital era, we cannot.
On the picture on the side, i was also playing with the extreme high ISO possibilities of my camera (EOS 5D Mark III). The setting are ISO 25600 for a shutter speed of 1/640s (kids are fast, very fast !), but as you can see the color cast is not so nice, and the skin tone is not great at all (no worries, i am not so crazy, i did it for the only purpose of this special photography high ISO page).
So at the end, what we have learned ? High ISO is nice to have and use, specially if you need to freeze action and movements but it will be dependent of the quality of the picture.
If you have enough light for the shutter speed you need, with lower ISO, do it, lower your ISO, the quality of the picture (color, sharpness, noises, etc...) will be better, by far better!
But if you do not have enough light, it's by far better to have a noisy, grainy picture than a blurred picture, we can always post process the photograph to eliminate the noise, but when it's blurred, it's done, and nowadays there is no software which is able to get back a blurred picture... at all, but flashes exist... from a very long time now, yes i know... it's another topic. ☺
You might also be interested with the last entry in the blog:
Nowadays our smartphones have more computer power than a 10 years old desktop computer. Technology is evolving fast and with it, the language acquiring new words like smartphone or selfie.
We are doing everything with our phone, from a GPS assistant to photograph or record a movie in full 1080p HD, even post processing those images, and i don't even speak about playing 3D games, read some news on internet and so on...
At the end, when we compare how we use our phone the most, in percentage, I can say safely that we barely use it to call someone, and this is why i call mine a photophone.
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