Knowing what each exposure setting does is one thing; choosing all three quickly in front of a real subject is another. This lesson gives a repeatable method and applies it to several common situations.
A four-step method
- Name the priority. Ask what would ruin this picture: a blurred subject, the wrong depth of field, or noise. That decides which setting you fix first.
- Set the priority control. Choose the aperture for the depth of field you want, or the shutter speed for the motion you want.
- Set the second control to its safe limit. If aperture came first, check that the shutter speed is fast enough for a sharp result. If shutter speed came first, open the aperture as far as the picture allows.
- Let ISO fill the gap. Use the lowest ISO that makes the first two settings possible. Take a test frame, check the histogram and adjust.
ISO comes last because it is the only one of the three with no creative benefit. Aperture and shutter speed shape the picture; ISO just makes the result bright enough.
Worked scenarios
Outdoor portrait in open shade
Priority: blurred background. Set f/2.8. The subject is still, so 1/250 s is more than safe. In shade that needs about ISO 200. Result: f/2.8, 1/250 s, ISO 200.
Landscape at sunset
Priority: sharpness from front to back and maximum quality. Set f/11 and ISO 100. The shutter speed falls to about 1/4 s, which is too slow to hold by hand, so the camera goes on a tripod. Result: f/11, 1/4 s, ISO 100. Without a tripod the compromise is f/8, 1/60 s, ISO 800.
Indoor sport
Priority: frozen action. Set 1/500 s. Open the lens fully, say f/2.8. The meter still shows underexposure until ISO reaches 6400. Result: f/2.8, 1/500 s, ISO 6400. The noise is accepted because the alternative is a blurred player.
Street photography on a cloudy day
Priority: being ready. Set f/8 for forgiving focus and 1/250 s to freeze walkers, then use Auto ISO. It will sit near ISO 400 and climb in darker side streets. Result: f/8, 1/250 s, ISO 400 to 1600.
Family dinner indoors at night
Priority: sharp faces without flash. People at a table move slowly, so 1/100 s is enough. Open to f/2 if the lens allows. ISO lands around 3200. With a kit zoom limited to f/4, ISO must rise two stops to 12800, or the shutter speed must drop to 1/50 s and ISO to 6400.
When the light runs out
Sometimes all three settings reach their limits and the photo is still dark. Compromise in this order:
- Open the aperture to its widest.
- Slow the shutter to the slowest speed that still keeps the subject sharp.
- Raise ISO to the highest value you find acceptable.
- If that is not enough, add light (a window, a lamp, flash), steady the camera on a tripod or a solid surface, or accept a slightly dark frame.
When there is too much light
The opposite problem appears in bright sun with a wide aperture. At f/1.8 and ISO 100, the correct shutter speed may be 1/8000 s, beyond what some cameras offer. The options are to stop down to f/2.8 or f/4, use an electronic shutter if the camera has one, or fit an ND filter.
Which mode makes balancing easiest
- Aperture Priority with Auto ISO and a minimum shutter speed: best for portraits, travel and general use.
- Manual with Auto ISO: you fix both aperture and shutter speed; ideal for sport and wildlife. Use exposure compensation to make the result brighter or darker.
- Full Manual: best when the light is constant, as in a studio, on a tripod or at night.
Common mistakes
- Setting ISO first out of habit and then struggling with the other two.
- Fixing brightness without checking the side effect, for example slowing the shutter to 1/20 s for a portrait.
- Using Manual in rapidly changing light and missing shots while adjusting.
- Reusing settings from an earlier scene without re-reading the meter.
- Stopping down more than necessary and driving ISO up for no visible gain.
Practice exercise
- Choose three subjects: a person by a window, a moving subject such as a pet or passing traffic, and a wide outdoor view.
- For each, write down the priority before touching the camera.
- Apply the four-step method in Manual mode and take the shot.
- Then photograph each subject with the wrong priority on purpose, for example the moving subject at f/16, and compare the results.
Next in this series: The Exposure Triangle Explained, How to Read a Camera Light Meter and Overexposure vs Underexposure Explained. You can also browse all photography lessons.
