Aperture, Shutter Speed and ISO The Mystery Decoded

Recently, it become apparent that some students have trouble understanding the relationship between Aperture, Shutter Speed and ISO and how they effect exposure for a given photograph.  First let’s define each:

Aperture or f Stop – is the size of the opening in the lens that allows light to hit the camera’s sensor. f stop is expressed as a fraction. An  f/3.5  lets in a larger amount of light than f/16. Aperture also effects the Depth of Field (DOF).  Depth of Field is area in front of the subject and behind the subject that is in focus. More about DOF in a later blog, except to say that larger f stops give smaller/narrower DOF’s.

Shutter Speed – is the length of  time the shutter is open allowing light coming through lens to the camera’s sensor.  If you are taking photos handheld, my recommendation is to never use a Shutter Speed less that 1/50 second, often displayed as 50 on your camera’s LCD and/or in the view finder.

ISO – is the sensitivity at which the sensor operates. The higher the ISO number used, the greater the sensitivity, the better the camera’s ability to shoot in low light conditions. However, higher values may give the photo a grainy appearance.  I recommend not using Auto ISO. From my observations of students photos, the quality of the photos suffer because Auto ISO often allows for too slow a Shutter Speed and results in a blurred photo when the camera is hand-held.

When shooting in Automatic Mode the camera controls all three, Aperture, Shutter Speed and ISO.  Automatic does not necessarily give a good photo.  I’m an advocate of taking control of the camera. Once you understand how your camera works, you will take better photos.

Below is a student photo taken in Automatic Mode. This photo is one of the first pictures taken with a new camera. The photo is dark and blurred. Note the Shutter Speed of 1/20 second.

1/20 Sec. @ f 3.5 ISO 3200


Aperture, Shutter Speed and ISO  are all tied together to get a photo with the correct exposure.   In Automatic Mode the camera makes all the choices.  When you shoot in any mode other than Automatic, such as Aperture or Shutter Speed Mode you have more  control over the resulting photo.

In manual mode, you have total control of all three which gives you the greatest amount of control, but also the opportunity for failure.  I recommend new students start with Aperture Mode and use Shutter Speed Mode when you want to stop the motion of the subject.

Where do you start?

When you use your camera in Aperture Mode, denoted as an A on your camera’s mode dial, you control the size of the lens opening and the camera sets the Shutter Speed. Remember you want the minimum Shutter Speed to be 1/50 or faster for hand-held photos.

In Shutter Speed Mode,  denoted as S or Tv, you are selecting the Shutter Speed and the camera controls the Aperture.

First set your ISO. What setting do you use?  Are you outside on a sunny or partially cloudy day? Try ISO 200.  If you are inside a building with good lighting ISO 200 or 400 could be a good place to start.  In early morning or evening around sunrise or sundown you may have to use something like ISO 800 or higher.

100 sec f/7.1 ISO 2500

In a concert hall you may have set the ISO much higher depending on the lighting.  The concert photo to the left  ISO 2500 was used.





In Aperture Mode, remember that the camera determines the speed for the ISO and Aperture you set. Let’s look at the Photos 1 & 2  below.  Do you see any differences in the photos?

Photo 1 – ISO 200, f/5.6, 1/2000 sec
Photo 2 – ISO 200, f/22, 1/125 sec.

Hopefully your answer is no.  In Aperture mode, the camera compensates for the different values in aperture f/5.6 and f/22  by a corresponding change in the time the shutter is open.  You see that for the smaller aperture f/22 the time was 1/125 much longer than the 1/2000 for f/5.6. Let’s see what happens when we change to ISO 800, but still use the same Apertures as before.

Photo 3 – ISO 800, f/5.6, 1/8000 sec
Photo 4 – ISO 800, f/22, 1/350

All four photos look about the same.  Why?  Because the camera compensates for changes in Aperture and ISO to always yield a good exposure.

What would happen if you chose to operate the camera in manual mode, which would allow changing Aperture, Shutter Speed and ISO independently?  How would Photo 4 change if the  Shutter Speed was 1/125?  Would it brighter or darker?

Scroll down to see.










ISO 800, f/22, 1/125 sec
Photo 5 – ISO 800, f/22, 1/350

The answer is “Brighter”. Did you get the answer correct? The slower speed 1/125 lets in more light than 1/350, so the photo would be over-exposed, i.e. Brighter.

Had an ISO higher than 800 been selected the resulting photo would have been darker than in Photos 1, 2, 3, and 4.  At a higher ISO, the sensor would have been more sensitive to light and would require a Shutter Speed faster than 1/8000, or an Aperture smaller than  f/22 both of  which are beyond the capability of the camera and lens used in the photos above.

I hope this helps you better understand the relationship between Aperture, Shutter Speed and ISO.

The link below from PetaPixel.com should help with your further understanding of Aperture, Shutter Speed and ISO.

Making Sense of Exposure

If you have any Questions please feel free to contact me





2 thoughts on “Aperture, Shutter Speed and ISO The Mystery Decoded

  1. I am no longer sure where you’re getting your information, however great topic. I needs to spend a while learning much more or figuring out more. Thanks for magnificent information I was looking for this information for my mission.


  2. Alex:
    Thank you for your comment. Understanding Aperture. Shutter Speed and ISO are fundamental to getting away from using your camera in Automatic Mode. I stress these concepts in my Introduction to Digital Photography class, along with the use of Aperture and Speed priority modes and choosing the correct ISO for the lighting conditions you experience.



Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

This site uses Akismet to reduce spam. Learn how your comment data is processed.