Exposure Triangle

How To Use AE-L/AF-L Button Correctly?

This post is a part of our Q&A section. If you want to submit your question, please use the form in the Contact page.


Darren (London, UK) asked : What is the function of AE-L/AF-L button which is on the right side of the viewfinder on my Nikon D60. I read the camera manual but couldn’t grasp the concept of it. Can you write about its feature or may be how to use it and when to use it?

I wrote a blog on how to set AE-L/AF-L button on Nikon D90 and how does it work a while ago. Nikon’s all DSLRs including D60 share same theory about how it works but it depends on the situation when do you want to use it. Basically AE-L/AF-L stands for Auto Exposure Lock/Auto Focus Lock but you can use AE-L/AF-L button for many other different functions. You can even use this button to lock Flash value which we discussed in a earlier blog. Camera menu option and the button position on the camera may vary depending on the camera model, but you will find it under the CSM menu and probably you have to go under the Control option. If you want to see the details on how to set AE-L/AF-L button in your camera, you may want to check the blog post on How to set AE-L/AF-L button on Nikon D90. In this article, I am going to demonstrate the effect of Auto Exposure Lock feature with the help of couple photographs I took. Once you grasp the concept of it, you can apply the technique in similar situations.

Before applying Auto Exposure Lock (AE-L)

Before applying Auto Exposure Lock (AE-L)

My camera was in Aperture Priority mode and the metering system was in Matrix metering mode. That means camera metered the exposure based on the overall scene inside the frame. The white snow background gave the sense of a bright situation to the camera metering system and it calculated the Shutter speed accordingly. But when I took the picture, the main subject came out underexposed because the camera metering system couldn’t figured it out that the main subject was not as bright as majority of the frame.

After applying Auto Exposure Lock (AE-L)

After applying Auto Exposure Lock (AE-L)

To overcome this issue, I thought Auto Exposure Lock (AE-L) feature would be the best choice in this situation. Before activating the feature, I zoomed in little bit so that the brighter area in the frame will get reduced and the camera metering system won’t be fooled. After that I focused on the main subject and press the AE-L/AF-L button to lock the exposure at the current values. At this point, the camera metering system calculated the exposure values based on the relatively darker area because I eliminated the most of the snow by zooming into the subject. I then zoomed out (recomposed the shot) and took a picture with the locked exposure. This gave me a nicely lit subject and well exposed background. If you are not careful enough with the framing, you might end up getting some overexposed areas in the frame as well.

[Updated on 4/30/2019] Someone might argue that why don't we use spot metering instead of matrix metering and lock the focus on the subject’s face and meter the exposure. By doing so, camera is going to meter the light based on the spot where the focus is locked on and not the entire frame, which will then properly exposed the subject. Yes, it might work in some cases but not a good choice in every situation, and that is the exact reason I chose this picture to demonstrate the effect. If I use spot metering in this particular scenario, it will properly expose the subject but will also overexpose the snow area way too much than I would like it be.

Understanding An Aperture And The Depth Of Field

In my previous blog post, we discussed about the shutter speed and it's effect on the exposure. Now, lets discuss about an Aperture, one of the three pillars in photography along with the shutter speed and an ISO, and it's relation with the shutter speed in exposure setting. To set an aperture value, your camera should be either in Aperture Priority mode (A) or in Manual Mode (M). If you are using an Aperture priority mode, you change an aperture value and the camera adjusts the shutter speed automatically to balance the exposure. And if you are using Manual mode, you can set the shutter speed and an aperture value independently. I already discussed about ISO and shutter speed in my previous blogs and today I am going to explain about an Aperture and how it affects our photographs.

What is an Aperture?

When the shutter is opened to allow lights into the censor, light must pass through a window called an Aperture. Sometime it's compared with the pupil in a human eye. Wider the aperture, more lights come into the censor and vice versa. An Aperture is also called a F-stop and it's value is called F-stop value. F-stop value is indicated by f and followed by the number value.

F-STOP Values

F-STOP Values

For example f2.8 (also written as f/2.8) means wider aperture and allows more lights. Similarly f22 means narrow aperture and allows less lights into the censor. Difference from one aperture value to immediate next value is called one full stop value. This term is frequently used when we deal with the exposure setting. For example, if your lens F-stop value is in f2.8, f4, f5.6, f8, f11, f16 and f22 series, moving an aperture from f2.8 to f4 is called one full stop and decreases the brightness of your picture by one full stop (it will allow only half amount of lights than in f2.8). So once again, changing an aperture value by one full stop up (from 2.8 to 4) will allow half the light and one stop down (from 4 to 2.8) will double the amount of light hitting to the censor of the camera. To calculate the aperture values in a full stop value (1 EV), you can use the formula below.

Full Stop.JPG

Nowadays, in modern digital cameras, aperture value, shutter speed and ISO values can be set in a fractional value rather than one full stop value. Almost in all modern digital SLRs, these exposure values can be adjusted and decreased or increased in the faction of one-third, two-thirds or one full stop value. The most common fractional value is one-third stop (1⁄3 EV) but some cameras use half-stop value as well. If you look at the diagram above, we jumped the aperture value from f/2.8 to f/4 which is one full stop change. But, if you want the aperture one-third stop smaller than f/2.8, you would set the aperture value to f/3.2 and if you want two-thirds smaller, it would be f/3.5, and finally one full stop would be f/4. The next few f-stops in this sequence would be f/4.5, f/5, f/5.6, f/6.3, f/7.1, f/8, etc. which can be calculated by using the formula below.

One-Third Stop.JPG

And if you want to calculate the aperture values in half-stops, you can use the formula below.

Half Stop.JPG

What is Depth of Field?

Depth of field is an effect of using different aperture settings. When you use a small aperture value (that means wide opening of the lens and allows more lights), you will get the sharper image of a small area or the foreground area and the blurry image of the background area. Less aperture value means less area of the subject will be in focus and results into shallow depth of field. But when you use big aperture value (that means narrow opening of the lens and allows less lights), more wider area is in focus and everything that you see through the view finder will be in equal focusing mode resulting into deep depth of field. Generally, we use shallow depth of field while shooting macro objects like flowers, insects etc and deep depth of field is required while shooting landscape to be everything from foreground to background in focus.

Shallow depth of field

Shallow depth of field

Shallow Depth of Field

Focal Length - 32mm     Shutter speed - 1/60    Aperture - f5     ISO - 200

Deep depth of field

Deep depth of field

Deep Depth of Field

Focal Length - 85 mm    Shutter Speed - 1/2    Aperture - f/16     ISO - 200

Take and Give relation

Exposure can be set by changing an aperture value, shutter speed or different combination of shutter and aperture value. When you take one F-stop from an aperture, you can give one F-stop to the shutter speed to match the same exposure. Moving from one shutter speed to another one is also one full F-stop jump like in an aperture value.

Aperture Full Stops:

1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32, 45, 64

Shutter Full Stops:

1/1000s, 1/500s, 1/250s, 1/125s, 1/60s, 1/30s, 1/15s, 1/8s, 1/4s, 1/2s, 1s

Fundamentals Of The Shutter Speed

In last few weeks, I have got several requests from the readers to write about Shutter Speed, Aperture and ISO; three pillars of photography also known as exposure triangle. If you are reading about photography or want to learn the basics of photography, you would see these terms almost everywhere. These are the foundation and very fundamental settings for your camera. When you shoot in Auto mode, camera calculates these parameters for you but if you want to have more control of the lighting or exposure, you have to use either semi-manual or Manual mode and select these values manually. To change the shutter speed, your camera should be either in Shutter Priority mode (S) or in Manual Mode (M). If you are into Shutter priority mode, you change the shutter speed and the camera adjusts an Aperture value automatically to balance the exposure. And if you are using Manual mode, you can set the shutter speed, an aperture value and ISO independently. In this blog, we will discuss more about the shutter speed.

What is the Shutter Speed?

Shutter speed indicates how fast the shutter of your camera opens and closes while taking picture. When you press the shutter release button, it opens the shutter of the camera, allows lights from the subject to the censor of the camera and closes it. The time difference of the opening and the closing of the shutter depends upon the shutter speed. Higher the shutter speed faster the shutter opens and closes. Similarly, lower the shutter speed, shutter will open for a long time. Shutter speed is directly related to the exposure settings of your camera. Higher shutter speed means it will allow less lights into the censor but captures fast moving subject (freezes the motion of the moving subject) by closing the shutter quickly. Lower shutter speed is used when you want to allow more lights into the censor and capture the activity. While using the slower shutter speed, you have to be careful of the camera movement. If you are shooting with the slower shutter speed without using tripod, you might get blurry picture because of the camera shaking.

Tip: Relationship between the Focal Length and the Shutter Speed

The general rule of thumb is: If your lens is not a VR (Vibration Reduction) lens and you are not using a tripod, your shutter speed should be at least 1/(focal length) of the lens to avoid blurriness on the picture due to camera shake. For example, if you are shooting handheld with 105mm non-VR lens, you should use 1/105th seconds of the shutter speed to get sharper image. If you are using a VR lens, you can decrease the shutter speed up-to 2 to 4 stops depending on the lens because you would gain that much light by turning the VR feature on.

Now, let’s look at how the shutter speed is indicated on camera and what does it mean to you as a photographer.

Shutter speed is indicated by the whole number or the fraction or special notation, BULB. The whole number is generally followed by a double quote to indicate that it is in a second format. The fraction value is represented as a number without a quote. For example, if you see 2" in your viewfinder, it means the shutter will open for 2 seconds. But if you see 60 in your viewfinder, that means your shutter will open for 1/60th of a second. Generally in a viewfinder, you won't see it in a 1/60 format but if you see a whole number without a quote, you have to understand that it's a fraction of a second. When you see a BULB, it means that your shutter will be opened as long as you press the shutter release button and until you release it.

Faster shutter speed to freeze motion

Faster shutter speed to freeze motion

I took this picture above with the shutter speed of 1/1000th of a second and you can see how the birds’ movements are frozen.

Slower shutter speed giving silky water effect

Slower shutter speed giving silky water effect

This picture was taken at the shutter speed of 10 seconds using a tripod. I wanted to get the silky water effect with a long exposure setting and I was shooting in a very low light condition (at night) as well.

You want to use a faster shutter speed to freeze fast moving subjects like running players in a sports events or playing kids or racing cars etc. But you should use a slower shutter speed to capture a motion of your subject or if you are shooting in a low light condition or setting up for night time photography or to get silky effects of water fall using ND filter etc.

1" - Slower shutter speed and allows more light come into the censor.

1/4000 - Faster shutter speed and allows less light come into the censor.

I will discuss about shooting fireworks and it's relation with the shutter speed for the exposure settings in my next blog post.

Understanding ISO In Digital Photography

ISO is one of the three pillars of the photography along with the shutter speed and the aperture. When we are talking about the camera with digital censor, ISO refers to the amount of amplification of the signal. It affects the exposure of the image by affecting the shutter speed and the aperture combination used in camera. ISO value indicates the camera's image censor's sensitivity to the light and gives us an idea of how sensitive the image sensor is to the light. High ISO value means image sensor is more sensitive to the light and similarly, low ISO value means sensor is less sensitive to the light. You can take pictures in a low-light condition when the ISO value is set to high in your camera. But at the same time, setting up high ISO value also introduces the digital noise in the picture because when we increase ISO value, artificial electric gain is added to the signal to provide more lights into the image. It's not coming from the natural light source but the light is being produced artificially and amplified by the camera using the signal booster.

When the ISO is set to Auto, camera’s metering system will automatically detect the light condition and increase or decrease the ISO value as needed. If you are using your camera into semi-manual mode or Manual mode (M), you can also manually set the ISO value to adjust the proper exposure under different lighting condition. Higher ISO value allows you to set the faster shutter speed so that you can freeze the motion when your aperture couldn't be more wider to allow more lights into the sensor. But you also have to consider the noise factor when increasing the ISO after certain value. Some cameras specially consumer level DSLRs are highly susceptible to the noise with higher ISO value whereas full frame cameras can take much higher ISO value without producing noticeable noise in the picture. When you have enough lights available, you should use lower ISO value and adjust the proper exposure using the combination of faster shutter and wide open aperture (smaller f value). Increasing or decreasing ISO value by double (from 200 to 400, 400 to 800 etc or reverse) is same as increasing or decreasing the exposure by one f-stop.


Focal Length - 50mm     Aperture - f/1.8     Exposure - 1/1.3 sec     ISO - 800

All of these pictures are taken in an Aperture Priority (A) mode with f/1.8 aperture and only changing the ISO value. The first picture was taken with ISO 800 and the camera adjusted the shutter speed to 0.769 seconds automatically to balance the exposure. This picture is almost noise free and much more cleaner than the second shot where the ISO was bumped to 6400 and then the camera accordingly adjusted the shutter speed to 0.0769 seconds to adjust the exposure. By increasing ISO value, I was able to shoot with much faster shutter speed but it produced the noise in the picture as well.


Focal Length - 50mm     Aperture - f/1.8     Exposure - 1/13 sec     ISO - 6400

I cropped the corner area of the second image to demonstrate the digital noise in the picture. If you look at the picture below more closely, you can clearly see the grainy noise all over the picture introduced by high ISO.


Focal Length - 50mm     Aperture - f/1.8     Exposure - 1/13 sec     ISO - 6400

Using higher ISO value allows your camera sensor to capture images at very low light condition but also produces noises (grains) in the picture. You won't probably notice it in a small size picture but if you enlarge the image, you can easily notice the effects of high ISO vs low ISO. It is always good practice to shoot with lower ISO value possible and increase it if you cannot get more lights by adjusting the Shutter speed and the Aperture value. But with more and more innovation everyday, modern DSLR cameras produce acceptable pictures quality even at higher ISO.

If you are looking for Nikon D90 specific ISO settings, please click here.