The Street Photographer's Guide to Auto ISO
Introduction
When you are shooting Street photography, you want every opportunity to capture your subjects in that special moment. When they walk into the correct portion of the frame that gives the composition balance; or when they are doing something humorous or out of the ordinary. As Henri Cartier-Bresson said, you want to capture that 'decisive moment'.
This means that, unless you are truly comfortable with changing the camera settings on the fly, you really don't want to be faffing around with altering settings when your subjects are right in front of you. This is how the street photographer can easily miss that 'decisive moment', and potentially walk away without a photo that would have been extra special.
However, many photographers want to maintain a certain degree of control too. They don't want the camera to have full control of the shot about to be taken. Some photographers want to be able to change Shutter Speed depending upon how fast the subject is moving. Also, they might want to change the Aperture, depending upon how much depth they want across the image.
So how can the street photographer maintain a level of control over their settings but at the same time not spend too much time changing settings, so that they miss a vital shot?
This is where Auto ISO comes in.
So what is it and how exactly does it work?
Let's get into it.
What is ISO?
The International Standards Organisation (ISO) is simple to understand - it is the sensitivity to light. This can be applied to film or a digital sensor.
A low ISO means there is less sensitivity and if you choose a high ISO there is more sensitivity.
If you have a digital camera, you can choose the ISO you need to make up a proper exposure, along with your Aperture and Shutter Speed. Cameras have a native ISO range, depending upon the model. The Sony A7III has a native ISO range of 100 - 51200, which is the range that is considered optimal for the sensor hardware. However, the usable ISO range of the A7III is 50 - 204800 for still images.
If you are using a film camera then you buy a specific roll of film which has a constant ISO, such as ISO 100 film.
When talking about stops of light as it pertains to ISO, a stop of light is equal to double or half the number at any given ISO value. For instance, ISO 200 is one stop darker than ISO 400. ISO 800 is one stop brighter than ISO 400.
One Part Of the Exposure Triangle
A narrow aperture, i.e, f /11 - f /22 will give you a greater depth of field. That's great for Landscape Photography.
A wide aperture (f /5.6 - f /1.2) is going to give you a shallow depth of field. At wide apertures, the background will more likely be blurred out. Wide Apertures are typically used for portraits and macro photography.
A fast Shutter Speed is great for stopping action - 1/250 sec and faster will be a great starting point for daytime street photography.
For long exposures and making people disappear in your scene, you would use a slow Shutter Speed. (However, if you are hand-holding the camera, slow Shutter Speeds will cause image shake as it is difficult to hold the camera steady whilst the shutter is open for a longer period of time, i.e. 1/30 and slower.)
The hole in the opening of your lens' iris lets more light in at wide apertures. And when you are shooting at slow shutter speeds the shutter is open for a determined amount of time, letting light hit the sensor for a longer amount of time. Because of these factors you typically won't need to use a high ISO, as there is so much light hitting the sensor.
However, the main caveat is you are shooting at nighttime or in dark conditions. (Even with wide Apertures and Slow Shutter Speeds, when it is dark there is less ambient light able to hit the sensor, therefore you need to raise your ISO to allow more light onto the sensor.)
On the flip side, when you are shooting a scene that requires more depth of field, you will be using a narrow aperture. (The iris opening will be narrow, letting less light onto the sensor). And/or if you are using a fast Shutter Speed to freeze fast-moving action, the shorter the time light will be hitting the sensor. (The camera's shutter will be open for an extremely short time depending on the fast Shutter Speed you pick. Fast Shutter Speeds are measured in fractions of a second, i.e. 1/4000 sec).
Having less light hit your sensor will make the image darker. This is when you will need to raise your ISO, thereby increasing your camera sensor's sensitivity to light, in order to get a proper exposure.
What is Auto ISO?
Semi-manual Shutter Priority allows you to change the Shutter Speed whilst the camera automatically changes the aperture for you.
Aperture Priority does the opposite. You set the Aperture and the camera alters the Shutter Speed to get a correct exposure.
Auto ISO, on the other hand, takes away the choice of the ISO so you don't have to worry about that particular element of exposure.
It actually goes further than this in a couple of ways. You can set a maximum ISO. A limit that you don't want the camera to go beyond, such as ISO 6400 (i.e. if you don't want your images to have too much grain). You can set a minimum ISO as well, which is very handy if you are shooting Street Photography at night. (More on this soon).
The next function in this equation is the ability to set a Minimum Shutter Speed.
Let's delve into this further and have a look at how it works in Aperture Priority.
Auto ISO in Aperture Priority
Using Auto ISO you can select ISO 100 as your base ISO and 6400 as your maximum ISO. You can then set a minimum Shutter Speed of 1/250 sec. You are choosing the aperture, so you pick f /8. If you choose wider apertures and let more light in (f /5.6, f /2.8) the camera will pick a faster Shutter Speed (darker) in order to keep a correct exposure.
On the other hand, if you are in fairly dark conditions and if you choose to go for more depth, with a narrower aperture, once you choose an aperture value that might require a lower Shutter Speed (e.g. f /16) than the minimum to get an accurate exposure, the camera will increase your ISO from the base of 100, as it won't go below the minimum Shutter Speed that you selected (1/250 sec).
The one issue you might find is that if the camera's Auto ISO settings are maxed out and the maximum of ISO 6400 has been reached. Plus you have selected the widest f-stop of f /2.8, that your lens allows, and you are in a position where the camera needs more light to get a decent exposure, it will then actually drop below your minimum Shutter Speed (i.e. from 1/250 sec to 1/125 sec) in order to do so. It will not, however, increase the ISO over the maximum of 6400.
Because the camera is willing to go below the minimum Shutter Speed, in these cases, it is highly unlikely that you will underexpose the image. However, you can overexpose the image - if I am at the widest aperture of f /1.8, that my lens can go, and I have the lowest ISO (100), and the fastest Shutter Speed is just not enough (1/4000sec), then the image will be overexposed. Then my only option would be to choose a narrower f-stop.
Auto ISO in Shutter Priority
In Shutter Priority the camera selects and adjusts the f-stop because you are selecting the Shutter Speed. If your base ISO is 100 and you have an aperture of f /8, as it gets darker the camera will select a wider f-stop, until it reaches the widest aperture on your lens. Once the widest aperture has been reached the camera will increase the ISO. When it is completely dark and the ISO is maxed out, the camera has no exposure mode to choose from, so unfortunately it will underexpose.
Auto ISO in Manual Mode
Imagine that you are on the streets and you come across an interesting painted face and this painted face person is juggling red balls. You're using Auto ISO in Manual Mode. So you know that you want to freeze those balls so you set your Shutter Speed at 1/1000 sec. At the same time, you want the busy background to be blurred so you change your Shutter Speed to f /4. Even though the sun suddenly goes behind a cloud and it starts to rain, and there are lots of shadows, you don't need to worry about changing the ISO as Auto ISO has this covered by increasing the ISO above the base level that you initially set in order to get a correct exposure. Then suddenly juggler stops juggling, so you change the Shutter Speed to 1/250 sec. But then the sun comes back out and once again, you can keep the same Shutter Speed and Aperture and let Auto ISO decrease the ISO back to the base level. But hang on, the sun is too bright now and you have blinkies on your LCD screen. You have the solution - dial back on the Exposure Compensation dial to underexpose, in order not to overexpose the sky. This is the beauty of Auto ISO with Manual Mode.
However, you need to be careful, as you can underexpose and overexpose in this mode. Typically, this is when the ISO falls outside of the Auto ISO range (the camera won't increase or decrease the ISO past the levels that you set). E.G. If you have selected too small of an Aperture and a too fast Shutter Speed, that would require an ISO greater than your max level, the image will be underexposed. This can also happen in the opposite direction, the camera can overexpose if you are using too slow of a Shutter Speed and too wide of an aperture, and the base level Auto ISO is just not low enough, so you need to be aware of your exposure and adjust it accordingly.
Auto ISO in Program Mode
Program mode is one of the simplistic options for beginners or for those who are not fussed about camera settings, choosing solely to focus on composition, subject and light. Alongside Auto ISO, Program Mode will choose the f-stop and the Shutter Speed and stay above the minimum Shutter Speed. Once the exposure is too dark, the widest lens aperture is selected and then your base ISO starts to increase. But this time, just like Aperture Priority, when it gets too dark to maintain proper exposure, and your maximum ISO threshold has been reached the camera will sacrifice your minimum Shutter Speed, dropping below it to get the right exposure.
Exposure Compensation
As described, with some cameras, you can use Exposure Compensation whilst the camera is in Manual Mode with Auto ISO. The camera will adjust the ISO when using Exposure Compensation in this mode, but only within the Auto ISO range, that you have set. So it is limited in usability.
You can also use Exposure Compensation when the camera is using Auto ISO in Aperture and Shutter Priority. But be aware of which exposure parameter the camera the exposure compensation dial is changing.
How to use Auto ISO with Street Photography
Auto ISO is great for night street photography. Imagine you are shooting in a brightly lit area of your city at night and your camera is set to Aperture Priority Auto ISO. Your ISO range is set for 800 - 6400 and you have a minimum Shutter Speed of 1/160 sec. You are outside a shop with beautiful window lights and people milling about inside. You are shooting at f /2.8 and your camera has picked 1/500 sec and ISO 800. You take a shot and you then walk down the road. There are less lights on in this area, it is very dark, but you quickly see a couple kissing and want to capture the special moment. You change the aperture to f /8, therefore the camera has dropped to the minimum Shutter Speed of 1/160 sec and it automatically increases the ISO to 6400. The couple is just standing and kissing, so these settings will be fine. You can also reduce the Exposure Compensation if there are any blown-out highlights from streetlamps or any remaining blown-out highlights in the sky.
You are now using Auto ISO with Manual Mode. It is daytime and you are walking down the high street looking for some great street photos to take. You see a glamorously dressed woman drop her handbag, which startles her Poodle and the dog jumps up in her arms. Because you are using Manual Mode, and it's clear and sunny you have selected an aperture of f /4, 1/250 sec. The Auto ISO is at 100. You immediately take the shot, but then you hear a hubbub to your right. There is a dark alleyway and there is a well-dressed man, looking cool, whilst talking on his mobile and smoking. No need to worry about raising the ISO as you quickly enter the alley. Auto ISO has you covered. You keep the same settings and Auto ISO increases the ISO to 1600. You didn't even give to think about taking that shot. You just took it.
Setting up Auto ISO
Each camera manufacturer has its own menu settings and layout so if your camera has Auto ISO, check the manual.
Some cameras have a minimum Shutter Speed setting that you can choose (Standard) which is based on the reciprocal rule which means that if you are using a 50mm lens then the camera determines that the slowest Shutter Speed that you should be holding the camera at is 1/50 sec.
Other settings that you can pick instead of choosing the ISO range yourself, include Slower and Faster.
On a Sony camera, 'Slower' is 'The ISO sensitivity will start to change at shutter speeds slower than [Standard], so you can shoot images with less noise'.
'Faster' is 'The ISO sensitivity will start to change at shutter speeds faster than [Standard], so you can prevent camera shake and subject blurring.' These settings may differ with different manufacturers and models.
The first thing to do is to set up Auto ISO. I can do this on my A7RIII with ease. Menu 1, page 9. Select the ISO setting, then scroll up to Auto. For daytime street photography, I choose the base ISO value ISO 100 & for the maximum I choose ISO 6400. I click the button on the control wheel to set it.
Then go back to the menu.
Underneath I select ISO Auto Min SS.
Then there are a number of choices but I select 'ISO SS 250' for the minimum of 1/250 sec. I click set.
And that's it I'm all set up for daytime street shooting. If I wanted to shoot solely for night street photography. I would pick a minimum ISO of 800 and I would pick a slower minimum Shutter Speed to give me more light. I know that I can get sharp shots holding my camera at 1/125 sec.
When not to use Auto ISO
If your camera doesn't allow Exposure Compensation with any Auto ISO function you might struggle in areas where the camera gives you a proper exposure, yet you find that you still need to underexpose the image to stop a sky from being blown out. This also applies to any other tricky lighting conditions.
If, like myself, you enjoy shooting Urban landscape street photography on a tripod, particularly at night, you do not need to worry about having long Shutter Speeds as you have a solid base for your camera, then Auto ISO is not an issue. You can quite easily keep the camera at the lowest ISO your camera allows and wait patiently for the shot.
Conclusion
I hope that you try Auto ISO and that you find it helps speed up your Street Photography. As you can see it takes some of the fiddling with your camera settings away, depending on which Mode you are shooting in, and allows you to focus on that decisive moment.
Go forth and create.