Signal to Noise Ratios in Photography
When taking a photo, you need to get a good exposure. And to get a good exposure, you need to get as much light into your camera as you can, without over exposing the shot and blowing out your highlights.
There are some exceptions to this but in general the more light you let in to your camera (as long as you’re not over exposing the photo), the better the signal you will have for the camera to build that image, however, there is a little more to it than that and the signal to noise ratio is a really good thing to understand to really have an idea of what the camera is doing and how to get the best results from it.
First of all it’s good to know what noise is.
Noise is a slight variation in the colour and luminance between pixels in your photo. The greater the variation, the more it becomes visible as a texture.
When you get a lot of variations in the colour, you’ll tend to see greens and reds in your solid colours. And when you just get a variance in luminance, it gives the look of a texture in those solid colours.
There are three different ways noise is generated.
You first of all have your background noise, this is the noise that will be there in every single photo you take, to see this, take a photo with the lens cap on, bring it in to your editing program and then bring up the exposure by sliding your whites and blacks to the right…this will vary from camera to camera but you can see on my camera, in the video above, how much it produces…and this is pretty standard for digital cameras nowadays.
Next we have High ISO noise, and this can be seen the same way, but by cranking up the iso on your camera, taking a photo with the lens cap on and then looking at it in your editor. This will be a combination of the background noise and the high iso noise, but the high iso noise will be the difference between the two.
Third of all we have long exposure noise. This tends to get worse with hotter climates, so I’m guessing is a byproduct of heat buildup in the sensor. Obviously not all pixels will be the same across your sensor and some react a little worse than others to heat. The ones that don’t do so well will sometimes give you red, green or blue pixel and the longer you leave your shutter open for, the bigger they seem to get…this isn’t so important unless you are doing exposures of a minute or longer usually.
I did a video on long exposure noise a while ago, so if you haven’t seen that yet and want to learn more, click here
Noise tends to be more visible in what would be a single coloured area of your photo…so if you’re taking a photo of a blue sky or painted walls any noise will be more visible in those areas, whereas if you photograph a dense forest, or a textured surface where there is a lot of detail and texture, the noise will be less obvious.
Also in the shadows of an image, or where the background might be blurred out and give you a more even colour, where there is less detail, the noise will be more obvious.
Next you need to know what signal is…this is basically the light being let in to your camera. This is what you want and there more you get the better…as long as your image isn’t over exposed.
Let’s say you take a photograph with settings of f8, 1/200 second at iso 100 and get a good exposure, you might return to the scene later on when it is darker and to get the same exposure with f8, 1/200 you’d have to bump up your iso to 6400, so there is less light about and you need to open up the camera by 6 stops to get the same exposure.
This would mean that the signal would be 64 times weaker than the first exposure where there was more available light.
And this brings us on to signal to noise ratios.
First of all we need to understand ratios. With ratios, a 1:1 ratio means that there is the same amount of both…and in photography, if you had a signal to noise ratio of 1:1, it would be a ridiculously noisy image…whereas a 5:1 ratio would be a much better image as there would be 5 times the amount of signal (good stuff) compared to 1 part noise (bad stuff).
So when I say a high signal to noise ratio, I mean an image with a good exposure and low noise and when I say a low signal to noise ratio, I mean an image with a lot of noise.
So to recap, the signal is the data you are collecting to make up your photo and the noise is the interference created when collecting this data.
Now there is plenty of geeky stuff you can go away and research on signal to noise ratios on line, and there’s a lot of maths behind it, but I am going to stay in the laymans terms here to make it as easy as possible to understand.
If there are any engineers out there, I apologise if I mess any of this up, but this is just for photography, for us average folk to understand our cameras a little better and ultimately to be able to get better photos from our cameras.
As I’ve already said, the closer the signal to noise ratio is to 1:1, the noisier your image will be…as a photographer, you want to aim to get as strong a signal as possible whilst keeping the noise down…and without over exposing your photos!!
Increasing your signal to noise ratio
The first thing you could do is open up your aperture to let more light in, which will increase the signal without increasing the noise, but we all know that lenses with wider apertures get expensive, really quickly.
If you decrease the shutter speed this will also help, but if you decrease it too much and you’re shooting handheld, you’ll be limited by how long you can make your exposure time before getting blur from the movement of your hands. If however you’re on a tripod and your subject is static, you can increase your exposure time a lot more…but not too much…otherwise you’ll start to get long exposure noise.
The next thing is to drop your ISO and this will reduce the amount of high iso noise compared to your signal, but if the shot then becomes so underexposed that you have to really bring back the exposure when editing, this will mean you’ll start off with a weak signal and as you increase the exposure in your editing program you will also bring up the background noise pushing your image closer to that 1:1 ratio. SO you can only really drop your iso when there is enough light about to do so.
So as you can see it is a little bit of a juggling act to get a good signal to noise ratio…and it comes down to a good execution of your principle settings, Aperture, shutter speed and iso.
The main thing to think about is how much signal do you have and how can you increase this signal without blowing out the highlights?
And not only the signal strength but how much detail and texture is there in that signal?
If it is a solid colour, lets say just after sunset in the blue hour….with a pretty boring cloudless sky…you really need to keep your iso levels down…because noise really shows itself in solid or uniform colours. And in this instance, it would be worth shooting from a tripod and increasing your exposure time (longer shutter speed), to increase the signal.
Noise also hides in the darker areas, so in getting a longer exposure, if you still have dark areas in your shot and you cannot increase the signal any more by just opening the aperture and increasing the exposure length because the highlights will blow out, it might be worth bracketing your shots and this will mean you’ll take separate shots for your highlights, mids and shadows…this is a slightly more complex technique but I have done a video on this, so click here if you haven’t seen his yet. This will basically give you more signal data in your shadows that you can blend into your final shot.
If your image has a lot of detail in it, you will be able to push the iso’s much higher before starting to see that noise. Also, if your subject has a lot of detail in it but it is surrounded by more uniform colours, try getting closer to the subject, filling the frame with that detail and losing any areas that are less interesting that might show the noise more and could distract the viewer away from the main subject. This will mean you’ll increase the detail throughout your composition hiding more of that noise.
So how much noise you can tolerate is a mixture of exposure levels, personal preference, composition and the detail and texture of the subject or scene that you are photographing.
It also depends on what camera you have, how old it is and how high you can push your ISO before the noise becomes too much, with the sony A7iii which is what I am using today, I can comfortably push this up to about 12,800 iso before worrying too much when I get a good exposure of a detailed subject or scene.
However with the older GH4, I can’t push the iso up as much as it is an older camera.
If there are some solid colours in the scene and the light is dropping, I will then change my settings or composition accordingly. As I raise my iso levels, I’ll look for more detailed or textured subjects or I might start shooting off a tripod.
Another thing to take into consideration is how you are shooting the subject. If you are shooting handheld, you will be restricted with your shutter speed so there will come a point where you will have to increase the iso to get a good exposure…But it is better to get a slightly noisier image than a blurry unusable image.
Now on-top of all of this is the ISO invariance of the sensor which means that the camera boosts the iso just like the program would in the simplest terms…there is a little bit more to it than this but I covered this in a previous video so it you want to learn more about that, click here.
Basically cameras with an iso invariant sensor, will cope much better being under exposed than ones with a non iso invariant sensor. If your camera is a sony or Nikon and I think fuji as well; as you boost an under exposed shot, it won’t bring up the background noise as much as say with a canon camera…again there is a lot on this on line if you search for iso invariant sensors, but I’m not going into detail about it today.
…and one other thing…if you take a shot with a high iso and it looks ok but you need to crop in to make the framing look a little better…this will magnify the noise in your shot and it will become more obvious, so zooming in optically when taking the shot is preferable over cropping in post. SO again this comes down to the basics in photography of getting the composition right in camera.
Noise in the shadows
Next we’ll have a look at noise hiding in shadows…the reason why this occurs is because the signal is low in these areas…and this means the background noise will be amplified when you slide the shadow slider to the right…right?
Like I said earlier, to overcome this, you can bracket your shots, and this just means normally taking three exposures, one for the highlights, one for the midtowns and one for the shadows. Then when blended together, the image will have much more data in the shadows as well as not blowing out the highlights…just be careful when using this technique as it really is easy to go overboard with the editing.
This video was spurred on by Joe Harris’s post on my Learn Photography Now Facebook page about getting good shots with high ISOs…I know some of you don’t like Facebook and that’s fine, but for those of you that haven’t signed up yet and are ok with Facebook, head over to the link by clicking here to join in with the discussions, and under topics, search for noise to find Joe’s post. It’s interesting and he has some great examples of what I’ve covered here. So thanks Joe for spending time on putting the post together.