As most people who have a digital camera will know , ISO is an important part of the settings in your camera. The large majority of camera users will shoot in Jpeg if for no other reason than expediency. They want to print photos, share on social media or use images in presentations, without the time consuming post processing steps. Therefore they need to get their exposure & shutter speeds right and this is the domain of ISO.
If you are shooting RAW only then ISO has less impact on the RAW file as the RAW file is composed of the incident light that it records when the shutter is pushed, and has no added sensitivity ( amplification ) applied to it, therefore all RAW images need to be post processed in whatever manner the photographer feels fit. For the sake of clarity we will consider ISO in relation to Jpeg output from the camera. Having set the camera as desired you are now ready to venture out and take some photos.
The factors that influence you choice of ISO will be how much light and shadow you are working with and the subject. We will leave out other variables for now and assume that you have set the camera according to your preference ( film sim, metering, etc ) and concentrate on the ISO.
There are many camera users who prefer the set and forget method when it comes to ISO, as most camera are reasonably good at picking appropriate ISO settings for any given amount of light. For general photography, snaps at the beach on holiday etc, this can be a good option. I and many others would select Auto ISO 3200 on the XA2 and this would work well for most if not all X series cameras.
This method frees you up to concentrate primarily on what your subject is, and whether you are shooting P Mode or perhaps Aperture Priority, where you are concentrating more on the exposure. This mode doesn’t always work so well if shooting in Shutter priority where high shutter speeds are required.
What does this mean? Simply put you decide on the ISO to be used and alter other settings to suit as necessary.
We can liken this style of setting to those from film cameras. In 35 mm roll film you have a choice of speeds (sensitivity to light ) that is not changeable. Therefore your base ISO ( sensitivity ) would be 64, 80, 100, 400, 800, 1600, and the rare 3200 ASA film speed. And thats it. For general photography the most popular films were ASA 200 & 400, which gave you a nice midpoint between fine film grain ( noise ) and retaining good shutter speeds and detail. The higher you went in the film grades the more granular the images could look .
Although sensitivity in digital cameras is derived from the amplification of the signal on the sensor, the resultant process and compromise between detail and noise (grain) is basically the same in concept for the Jpeg shooter as it was for the film user, although the underlying processes are quite different.
However we know that the more you crank up the ISO the more data is revealed in a much shorter exposure time. Up until recently when aps-c cameras like the Sony A series and the Fuji XA became readily available the smaller sensored cameras ( point & shoot as well as superzooms ) used tiny sensors that exhibited unpleasantly noisy images above ISO 400, and sometimes even below this level. As time progressed this situation improved considerably and today a lot of these tiny sensors acquit themselves well in the noise area, producing good images printable at least to A4 size.
Noise however is much less of am issue in today’s aps-c cameras and the Fujifilm XA2 has an extremely low noise sensor up to at least ISO6400. What this means is that you have a lot more flexibility in your settings.
I prefer the Selected ISO method as this offers more control. For example on a bright sunny day I will typically set the ISO to 500. This allows for good shutter speed without the risk of badly blown highlights, and the Ev can be adjusted to suit differing conditions.
On a variable light day I may bump the ISO to 800 just to ensure I maintain a good shutter speed. This holds as well when I shoot in Aperture priority mode and gives the additional advantage of using both aperture and Ev to suit the conditions.
Heavily overcast days will see me opt for ISO 1600 or higher again depending upon subject and conditions.
The advantage of choosing a set ISO is the control you get especially if shutter speed is going to be an issue as it will be on a dull day.
The two images above both taken on heavily overcast days show little or noise discernible noise. However the image of the bike was taken while braced to reduced any motion blur and to help with Image Stabilisation.
The image of the Roses was chosen deliberately as the creamy apricot colour and leaf detail is delicate. This image was shot at ISO 1600 and the rose stem was moving quite rapidly in a breeze making this harder to get a clean shot. This is where having the set ISO helps as the Auto ISO was trying to lower the ISO to 400 as it did in the previous image. At that shutter speed (1/85s) all I would have had is a blurred image, as the camera was assuming that the light level was sufficient for the lower ISO.
This type of scenario is something you will encounter often if using Auto ISO and allowing the camera to make the call.
There are of course other influencing variables as well, such as the lens selected, how much Ev adjustment your camera has and what metering mode you may have been using as well as what AF setting you were using at the time.
The final image below was shot on a heavily overcast and wet morning at ISO 1250 and 1/1100s. The settings and shutter speed were high which yielded a very nice noise free image. Shot at a lower ISO this image would not have been as light in appearance but would have yielded a much darker and lacklustre background. Image captured with the XF 27 mm f2.8 pancake lens. This is where the value of a brighter lens comes into the equation, although he XC 16-50 working at 16 mm in macro would yield a similar result.
Of course these are my own personal preferences and approach, yours may well differ in some respects. I would however encourage you to try the Set ISO method and see how it works for you.