How to get the ISO right every time: a complete guide to master exposure in your photos

ISO sensitivity is one of the things that changed forever with the advent of digital photography. Before we had to change film or adjust the development to get more or less light. Today it is only necessary to raise and lower this parameter to achieve a correctly exposed image. So we are going to learn to control the ISO of our camera.

In the past times, photographers had to carry two cameras with two reels of different sensitivities, in my case I was going with a slide of 100 ISO in one and a black and white of 400 ISO in another. This gave me the ability to shoot in all kinds of situations without fear of missing a shot.

But all that mess changed with digital photography, and in a short time it became possible to shoot at sensitivities that we did not even dream of on film with such quality. So we have to learn to take full advantage of a universe of possibilities compared to what we had before.

What is sensitivity?

Sensitivity is how sensitive the sensor is to light in causing a given load. To measure it we have the ISO standard (International Organization for Standardization), which measures, in a geometric progression, with a 1:2 ratio (100-200-400-800-…), the amount of light needed to cause a given density on the sensor surface.

The problem is that in the world of digital photography we find several ISOS standards, and each manufacturer can choose the one that is most favorable, but based on the standard that we have all known since the chemical era. In other words, the ISO is a reference that depends on the standard output sensitivity (SOS) and the Recommended Exposure Index (REI), something that many people do not know:

• sos: This is a rule that applies exclusively to jpeg files with sRGB color space and used by Olympus and Pentax. It has a value equivalent to 0.71 ISO sensitivity
• REI: norm that depends on the criteria of the manufacturer (Canon, Nikon and Sony) and that is based on its empirical experience in what the manufacturer believes gives the best exposure result.

This makes the subject of sensitivity enormously complicated. But we are going to continue thinking only in ISO, and we will start from the basis that we will always talk about an approximation and that the results can change from one brand to another.

How to work with sensitivity in today’s cameras

digital sensitivity It depends on many factors, from the size of the sensor and the pixels, the placement of the circuits, the digital analog converter, the processor. It even depends on the program with which we develop our photographs. Working with Adobe Camera RAW is not the same as with Capture One or Snapseed.

We should not talk about grain, as with the film, but about the signal to noise ratio. We must differentiate between signal, data that builds the image; and noise, which always accompanies the signal and affects it more or less in the transmission of information.

It is what causes an ISO to have more noise than another. In order not to complicate ourselves, we should always work with the nominal ISOthe lowest ISO that the camera offers us, without the help of the software (when you see the ISO value between two lines on your camera, it is a bad sign).

Every time we raise the ISO one step, that is, from 100 to 200, from 200 to 400… we make it twice as sensitive by a simple trick: gain. Raising the ISO is only amplifying the received signal, just like if we raise the volume of our stereo.

If the equipment is good, the result of this amplification will be perfect up to a limit that we must decide. And how do we know? There are two ways to do itone simpler and one more complicated.

Based on visual experience

We placed the camera on a tripod in front of a corner wall with a light and a dark area, for example. We need to see lights and shadows:

• We adjust the exposure with the nominal ISO in Aperture Priority (Av) at f11 with the shutter time you indicate.
• We are uploading the ISO without modifying anything else.
• Once the photographs are made, we open them in Adobe Photoshop or in any program that allows us to open all the files at the same time.
• We expand to 100% and visually we decide what is the limit that we want to use in our camera, comparing both the lights and the shadows.

Scientific method

This option is more complex, but it is not based on our visual criteria, but on the response of one of Adobe Photoshop’s histogram tools. The process is long and we tell it here. But let’s see a summary.

We have to take a picture of a gray card with a uniform lighting. And the most important thing is to center the exposure, so that the histogram is in the center. Then we go up the ISO to get the same exposure in all the shots.

We open all the files in Adobe Photoshop and in the Histogram window we mark the Expanded view mode. The idea is to see the data Standard deviation. This value must be in all the photographs with a value that increases slowly and progressively.

In that file in which I upload in a notorious way will be the ISO limit that we can actually use with our camera. And I assure you that it does not fail.

Some tips for using the ISO

In the end we have to use the ISO that allows us to take a photograph with the diaphragm that we need without excessively lowering the shutter time to avoid jerky photography. Of course, we will be clear about the highest ISO that we can use without being bothered by ‘noise’.

Personally, I always use auto ISO, setting the minimum and maximum I want, one of the most interesting features of today’s cameras. On my Sony A7III, I always shoot between ISO 100-6400; and on my Sony RX100 IV I move between 125-1600 ISO.

The ISO can help us a lot if we have a small flash. It is only necessary to increase the sensitivity to make the flash seem much more powerful and shine brighter. That is, the light will reach further with the same power. And so we also avoid wasting the battery of our lighting system.

In this way, after doing all the tests that we have indicated, I forget this parameter in my digital cameras. It is very comfortable when working with changing lighting.

Of course, if I use a tripod, I always work with the nominal sensitivity, the lowest of my camera, to make the most of the quality of the sensor. Although there are times, like with night photography, that it is necessary to increase the value to be able to register the dimmest lights.

Today it is something that is no longer a problem, unless we work with a mobile phone with high ISOS or with an old camera. And something very important to finish, noise is seen more on screens than on paper. If you print a photograph, the very texture of the paper will lower the noise without doing anything at all.

And if you ever have to upload the ISO, do it without fear. It is worse not to take a photograph or lose it because it is moved. In the past it was not so worrying to have noise, what mattered most was that the image was really powerful.