THE ORDER in which you set the aperture, shutter speed and ISO in Manual mode will depend on what the priority is for the picture. If you need control over the depth of field, such as in a close-up, portrait or landscape, set the aperture first. You can then adjust the shutter speed to make the image brighter or darker – or increase or decrease the ISO to do the same. If you’re shooting in low light, then you might need to increase the ISO regardless, in order that you can use a shutter speed fast enough to prevent camera shake.
If you’re shooting action, set the shutter speed first. You may need to open up the aperture or increase the ISO in order to let enough light into the camera to achieve a fast shutter speed. If you want to use a slow shutter speed for creative blur, choose a smaller aperture or lower ISO.