Understanding ISO, Shutter Speed and Aperture – A Beginner’s Guide

Without a good understanding of ISO, Shutter Speed, and Aperture, it is hard to take great pictures. These are the Three Kings of Photography also known as “Exposure Triangle”. Most DSLRs now have Auto modes which automatically select the shutter speed, aperture, and ISO to expose you. However, an Auto mode limits what you can do with your camera. The camera must determine the exposure by looking at the light passing through the lens. Photographers can fully control the situation by understanding how ISO, shutter speed, and aperture interact. It is important to know how to adjust settings on the camera to achieve great photos.

As a refresher, let’s briefly review the Exposure Triangle.

  1. Shutter Speed is the time that a camera shutter is opened to allow light into the sensor. Shutter speeds are usually measured in fractions of seconds, whereas they can be under one second. Slow shutter speeds let more light enter the camera sensor, making them useful for night and low-light photography. Fast shutter speeds freeze motion. Example shutter speeds are: 1/30 (1/15th of an second), 1/30 (1/60), 1/125.
  2. Aperture is a hole in a lens through which light passes into the camera’s body. The bigger the hole, the more light is allowed to reach the sensor. The aperture also affects the depth of field. This is the area of a scene that appears sharp. The depth of field will be larger if the aperture size is too small. However, a large aperture will result in a smaller depth of field. Aperture is often expressed in “f” numbers. This is also known as “focal rate”, because the f-number (or the ratio of the lens aperture’s diameter to the length) is used to describe photography. Some examples of f-numbers include f/1.4 and f/2.0.
  3. ISO – This is a way to increase the brightness of your photos when you don’t have a longer shutter speed and a wider aperture. The ISO number is usually expressed in numbers. A lower number means a darker image while a higher number means a brighter one. But, increasing your ISO can come at a price. The visibility of noise and graininess in your images will increase as the ISO increases. Examples of ISO are 100, 200 and 400. 800 and 1600.

If you want to learn more about exposure, please take a look here.

1) How do ISO, Shutter Speed and Aperture Work Together to Create Exposures?

We need to be able to comprehend what happens inside the camera when we take a photograph.


When you point your camera at your subject and hit the shutter button to take a picture, light from the subject enters your camera lens. If the subject is well lit, the light travels through the lens in great quantities. However, if the subject is in dim lighting, less light travels into your lens. The light that enters the lens passes through several optical elements made from glass. It then goes through the lens aperture (a hole in the lens that can be adjusted from small to large). The light passes through the lens aperture and then hits the shutter curtain. This is a similar window to a window that can be opened or closed as needed. In a matter of milliseconds the shutter opens, allowing the light to hit the sensor for a specific amount of time. The shutter speed is also known as “Shutter Speed”. It can be very short (upto 1/8000th) or it can last for up to 30 seconds. Your “ISO” will brighten the image if needed (this again makes grain and other issues with image quality more obvious). The shutter closes, and the light stops reaching the sensor.

Nikon D5000 Viewfinder

Shutter Speed and Aperture are all important to properly expose the image so it isn’t too bright or dark. What happens if the aperture/hole of the lens is too small when there is a lot of light entering the lens? A lot of light is blocked. The camera sensor will need to take longer to capture the light. What must happen to allow the sensor to capture the correct amount of light? The shutter must stay open for longer periods of time. With a small aperture lens, the shutter would require more time to open. The shutter speed must be set at a slower speed to allow the sensor to capture enough light to create a properly exposed image.

What would happen if your lens aperture/hole were very large? A lot more light will hit the sensor so we need to use a shorter shutter speed in order for the image properly exposed. The shutter speed should not be too slow. If it is too slow, the sensor will get more light than it requires and the image will start to “burn” or overexpose. This is similar to a magnifying glass burning paper on a sunny afternoon. The image that is too exposed will appear brighter or more white. Contrarily, if your shutter speed is too fast, the sensor will not be able to capture enough light, and the image will appear “underexposed” (or too dark).

Let’s do a real-life example. Set your camera’s camera mode to “Aperture Priority”. Your lens aperture should be set to the minimum number your camera can handle, such as f/1.4 for fast lenses or f/3.5 for slower lenses. Make sure you have “Auto ISO” turned off and your ISO set to 200. Point your camera at an object other than a light source, such as a wall picture. Half-press the shutter button and the camera will determine the best exposure settings. Keep your camera pointing at the same subject, but don’t move it! You should see multiple numbers if you look in the camera viewfinder or on the back LCD. The first number will display your aperture. This should be the same as the aperture you have set. Next, it will display your shutter speed. This should be something like “125” (means 1/12th of a second), and “200”, which represents your sensor ISO.

These numbers should be written on a piece paper. Then take a photo. Properly expose the image when it appears on the rear LCD. Although it might appear blurry, properly exposed means that the image should not be too bright or dark. Let’s assume that the settings you have written down are 3.5, 125 and 200 ISO. Change your camera mode to Manual Mode. Now, manually set your aperture to the number you have written down. This should be the lowest aperture your camera lens can allow (in this example, it is 3.5). Next, set your shutter speed at the number you have written down (in this example it is 125). Keep your ISO constant at 200. Check that the lighting conditions are the same in your room. Take another photo by pointing at the same subject. The results will look similar to the one you took before, except that you are setting the shutter speed manually instead of leaving it to your camera. Let’s now block light from entering the lens by increasing our aperture. We will see what happens. You can increase your aperture to “8.0”, but keep all other settings the same. Take another photo by pointing at the same subject. What happened? Now, your image is too dark and underexposed. What is the reason for this? You blocked some of the light from hitting the sensor, but did not alter the shutter speed. The camera sensor didn’t have enough time to collect the light, so the image is underexposed. This would have been avoided if you had decreased your shutter speed to a lower number. What is the relationship?

Change your aperture to the previous value (the smallest number), but decrease your shutter speed to a smaller number. My shutter speed will be 4 (quarter of second) in my example. Take another image. Take another picture. What happened? Let your lens take in all light available without blocking it. Then you can decrease the shutter speed to let your sensor receive more light. This is an extremely basic explanation of the relationship between aperture and shutter speed.

What does ISO do and when will it come in to play? We have so far kept the ISO constant at 200 and did not change it. Remember, ISO means sensor brightness. Higher numbers indicate more brightness. Lower numbers are associated with lower brightness. You could make the photo two times as bright if you increased your ISO value from 200 to 400. The above example shows that an aperture of f/3.5 and a shutter speed of 1/125th would be sufficient to expose the image properly. However, if the ISO were increased to 400 it would take half as long. You could also set your shutter speed at 1/250th of an second, and still have your image properly exposed. You can try it: Set your aperture to the number you have written down, and then use a shutter speed twice as fast. Then, change your ISO setting to 400. The final image should be the same as the one you took earlier. You would need to use a shutter speed twice as fast if you wanted to increase ISO to 800. This could be from 1/250 to 1/550.

You can increase ISO from 200 to 800 to get faster shutter speeds. In this case, it will go from 1/125th to 1/500th. This is enough speed to freeze motion. However, ISO increases come at a price: the higher the ISO the more noise and grain will be added to the image.

This is basically how the Three Kings work together in order to create an exposure. To see the effects of shutter speed, ISO and aperture changing, I recommend that you practice with your camera more.

2) Which camera mode should I use?

As I mentioned in my article “Understanding Digital Camera Modes”, I recommend that beginners use “Aperture Priority” mode. However, any mode can be used as long as they are familiar with what they are doing. This mode allows you to set the aperture of your lens, and the camera will automatically determine the shutter speed. You can adjust the aperture to control depth of field. However, this is dependent on other factors like focal length and camera distance. It is fine to use “Auto” and “Program” modes. This is especially true when you consider that modern DSLRs allow for a lot of control, including the ability to adjust the shutter speed or aperture. However, most people end up using Auto/Program modes without knowing what’s happening inside the camera. I strongly recommend that you learn how to use all modes.

3) To what ISO should I set my camera?

You should enable the “Auto ISO” feature on your Nikon camera. This allows the camera to automatically determine the correct ISO in different lighting conditions. Auto ISO is easy to use and safe for all lighting conditions. Your “Minimum ISO/ISO Sensitivity”, set to 100 for Canon cameras and 200 for Nikon. Next, adjust your “Maximum ISO/Maximum Sensorsitivity” setting to 800 or 1600 depending on the level of noise you are comfortable with. If you have a shorter lens than 100mm, set the “Minimum Shutter speed” to 1/100th. If you have a longer lens, you can adjust it to a higher value. The camera will monitor your shutter speed and adjust the ISO accordingly if it falls below the “Minimum Speed”. Your shutter speed should be set to the longest focal length of your lens. If you have a Nikon 70-30mm f/4.5-55.6 zoom lens, your minimum shutter speed should be 1/300th of one second. Why? Why? Because the more the focal length of your lens increases, the chance of blurry images is increased. This rule is not always true. There are many factors that can affect whether or not you introduce camera shake. Camera shake can be caused by shaking hands or improperly holding the camera. However, a lens with Vibration Reduction (also known to Image Stabilization) may help reduce camera shake. You can play around with the “Minimum shutter speed” option, change numbers, and see what works best for you.

Auto ISO on Nikon

You can start with the lowest ISO setting and then increase it until you get the desired shutter speed. You can increase the ISO until you reach an acceptable shutter speed.

4) Compensation for Exposure

Modern DSLRs have the amazing feature of being able to adjust the exposure using the “exposure compensate” feature. Exposure compensation is available for all modes, except manual mode. You can adjust the exposure compensation by dialing up or down (+/-) the Auto/Program mode. This will allow you control the exposure and override camera-guessed settings. Exposure compensation can be used to adjust exposure, even if your image or parts are under- or overexposed.

5) Should I Increase ISO or Use Flash?

It all depends on the subject you are trying to capture. Sometimes, it’s not possible to use the built-in flash camera in low-light situations. If your subject is far away from you, your flash might not reach them. If this happens, you have two options: either move closer to your subject or use a higher ISO. For architectural or landscape photography, it is important to turn off the flash completely. It will not be capable of brightening up the whole scene. In low light situations, you have two choices: either increase your ISO to allow for hand-held photography or use a tripod.

6) What is a “Full Stop”?

A term “full stop” is a term used in photography. In photography, each increment between ISO numbers is called a “full stop”. One full stop is found between ISO 100, ISO 200, and two between ISO 100, ISO 400. What are the stops between ISO 100 & ISO 1600? There are four stops in total. What is stopping? It might be mentioned in photography literature, or even by photographers. Sometimes it can be confusing to know what it really means. The term “full-stop” doesn’t just refer to ISOs. It also applies to shutter speed and aperture. You can easily remember the full stops between shutter speeds by simply starting at one and then dividing it by two: 1, 1/2. 1/4. 1/15. 1/30. 1/60. 1/125. 1/250. 1/500. 1/1000. To make it easier for photographers, the numbers are rounded (beginning at 1/15 which should be 1/16). Because the numbers are calculated differently, it is more difficult to remember stops in apertures. For more information on stops, please refer to our Exposure Stops article.

7) Case Studies and Specific Examples

Let’s look at what you can do with your camera to expose an image properly in various lighting conditions.

  1. What should you do in low light situations? Use the Aperture-Priority Mode, and set your aperture at the lowest number. If you are using a fast lens like the Nikon 50mm F/1.4, be careful as setting the aperture to the lowest possible number (f/1.4), will result in a very shallow depth of field. If you have an auto ISO, set it to “On”. Make sure the minimum shutter speed and maximum ISO are defined as per section 3. If you still get small shutter speeds despite increasing your ISO, which can indicate that you are working in very dim conditions, you have two options: a tripod or flash. Flash is required for moving subjects.
  2. How do I freeze action? You will need lots of light. It is very easy to freeze action in broad daylight, but it can be difficult in low-light conditions. If you have lots of light, ensure that your aperture setting is the lowest. (Again, be cautious about depth of field). Next, turn your “Auto ISO” on (if it’s available) and increase your shutter speed to a very high number, such as 1/500th or even 1/1000th of an second. My shutter speeds for bird photography are 1/1000th or faster.
Caspian Tern - 1/2000th of a second
  1. How do I create motion blur? Disable Auto ISO. Set your ISO to the lowest setting. If you cannot create motion blur because the shutter speed is too fast, increase the aperture until it drops below 1/100 to 1/50 seconds.
  2. What should I do if I can’t get the exposure right? You have either overexposed or underexposed the image. Your camera meter should be set to either “Evaluative” or “Matrix”, depending on whether you are using Canon or Nikon. If your exposure is correct but it’s not set, this means you are taking a picture with a lot of contrast between different objects. You can still take a photo if your camera meter is set to “Spot”. Then, point your focus point at an area that isn’t too bright or dark. This will give you the “sweet middle”.
  3. How do I separate my subject from the background? Use the smallest aperture of your lens to get closer to your subject. Different lenses render backgrounds differently. You might not like the bokeh of your lens. Consider getting a portrait lens, such as the Nikon 50mm F/1.4 or the Nikon85mm F/1.4. These lenses are considered to be the best when it comes to bokeh.
  4. What can I do to reduce noise/grain in my photos? Turn off Auto ISO and set your ISO at the base ISO of your camera (ISO 100 for Canon and ISO 200 for Nikon).