What Is Depth Of Field (DoF)?

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.

Manjit Singh (Delhi, India) asked : Can you please explain me about Depth of Field and it’s relation to the camera settings?

This is what Wikipedia says about DoF - In photography, depth of field (DoF) is the distance between the nearest and the farthest objects in a scene that appear acceptably sharp in an image. Although a lens can precisely focus at only one distance at a time, the decrease in sharpness is gradual on each side of the focused distance, so that within the DoF, the unsharpness is imperceptible under normal viewing conditions.

Sometimes you want to focus everything inside the frame (for example you are doing landscape photography), and a large DoF is appropriate. And in other cases where you are doing macro photography or portrait photography and want to blur the background, a small DoF may be more effective, emphasizing the subject while de-emphasizing the foreground and the background. Usually, a large DoF is often called deep focus or deep depth of field, and a small DoF is often called shallow focus or shallow depth of field. Now let’s discuss briefly about these terms.

Shallow depth of field: When you selectively focus one part of the image and let rest of the image go out of focus, you will get shallow depth of field. This is pretty famous when you are shooting macro or portrait and want to blur the background or produce nice bokeh in the background.

Shallow depth of field

Focal length : 28mm     Aperture : F/4.5     Shutter speed : 1/40 sec     ISO : 200

Deep depth of field: When you want to keep everything inside the frame from the foreground to background in focus using correct combination of camera settings, it is called deep depth of field or larger depth of field.

Deep depth of field

Focal length : 32mm     Aperture : F/8     Shutter speed : 8 sec     ISO : 200

Now let’s take a look at the camera settings that affect depth of field. The DoF is determined by an Aperture, lens Focal length and the physical distance from the subject.

1. DOF and its relation to an Aperture

To get shallow depth of field, you need large opening of the Aperture. And please keep in mind that large opening of Aperture means small f-stop value. Smaller the f-stop value, larger the lens aperture opening will be (allows more light and faster shutter speed) and larger the f-stop value, smaller the lens aperture will be (allows less light and slower shutter speed).

In another word, we can say: for a given subject magnification, increasing the f-number (decreasing the aperture diameter) increases the DoF; decreasing the f-number decreases DoF.

If we keep the focal length and the distance from the subject fixed: larger the opening of Aperture (small f-stop value), shallower the depth of field you will get and smaller the opening of Aperture (large f-stop value), deeper the depth of field you will get.

2. DoF and its relation to the Focal length

Focal length is something that depends on type of the lens you are using. If you are using zoom lens, you can zoom in and zoom out to increase and decrease lens focal length.

If we keep an Aperture and the Distance from the subject fixed: larger focal length will give you shallower depth of field and smaller focal length will give you deeper depth of field.

3. DoF and its relation to the Distance

Physical distance from the subject to the camera also affects depth of field.

If we keep an Aperture and the focal length fixed: closer you are to the subject, shallower your depth of field will be and farther you are from subject, deeper your depth of field will be.

Conclusion: Larger aperture opening (small f-stop value), closer to the subject and larger focal length will give you the shallowest depth of field possible.

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)

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)

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 Nikon's Flash Metering Mode

Metering is the technique, which is used to determine the optimal exposure for the subject and the overall scene based on the information camera gets from available light sources. Nikon uses the flash metering system to determine the flash output when the flash is set to auto (also called TTL or i-TTL) mode, and the camera metering system to determine the overall exposure of the image if the camera is set to A, S or P exposure mode. In manual (M) mode, you would take the reading from the camera metering system as a reference whether it is negative (underexposed) or positive (overexposed) and set the exposure values (Aperture, Shutter speed and ISO) manually to adjust the exposure. Nikon D90 and most of the other Nikon DSLRs support three types of Metering Modes, and they are Matrix Metering, Center-Weighted Metering and Spot Metering, which we discussed in previous blog.

In TTL mode, the built-in flash, as well as the external flash unit attached to the camera, adjusts the flash power to properly expose the scene by using the information from one of these three available metering modes. When the flash is set to TTL mode, choice of the camera metering mode determines whether the flash metering system, which shares the same metering sensor with the camera metering system, should consider the ambient light and balance the overall exposure or ignore the ambient light all together and only count the light from the main subject to determine the flash output power.

What is TTL mode?

Nikon’s latest DSLRs and the flash units use the TTL technique to determine the amount of lights reflecting from the scene and adjust the flash power accordingly. TTL stands for Through The Lens and it refers to the process of gathering information through the lens and passing that information to the metering system in order for the camera to deploy the appropriate flash power. Older camera systems either used manual calculations to determine the flash exposure or used a sensor in the flash itself rather than the metering sensor in the camera. And, i-TTL (intelligent-TTL) refers to the latest TTL flash system from Nikon. In latest Nikon cameras, TTL refers to i-TTL by default and I am using TTL and i-TTL interchangeably in this blog.

There are two types of i-TTL methods used in Nikon’s lighting system and the DSLRs, which are iTTL Balanced Fill-flash and Standard iTTL Fill-flash. Now, let’s talk about these two different TTL methods and discuss how they work differently.

1. iTTL Balanced Fill-flash (i-TTL/BL or TTL/BL)

Nikon's i-TTL (intelligent through-the-lens) Balanced Fill-Flash automatically balances the output of the Nikon Speedlight to match with the scene's ambient light. This flash mode is selected by the camera automatically when the flash is in TTL mode and the camera metering is in either Matrix Metering mode or Center-Weighted Metering mode. If the camera is set to A, S or P exposure mode and the flash is in i-TTL/BL mode, Nikon Speedlight fires the series of pre-flashes to determine the exposure value for the subject by taking the ambient light into account and send that information back to the flash metering system in the camera. The flash metering system then combines that information with the metering information from the camera metering system and the focal length information from the D or G lens to analyze and calculate the final flash output in order to balance the overall exposure. All of this complex processing happens in a fraction of a second, before each exposure, to provide unprecedented levels of flash precision and performance. This is a very powerful and easy to use technology from Nikon and probably one of the best in the industry.

iTTL Balanced Fill-in flash

In short, using this technology, Nikon DSLR measures the available light and then adjusts the flash output to produce a natural balance between the main subject and the background. It will reduce the harsh shadow as well as the highlights in the subject caused by the over powered flash.

2. Standard i-TTL Fill-flash

This mode is activated when the flash is in TTL mode and the camera metering is in Spot Metering Mode or if you are using the external flash and the flash is set to standard mode. In this mode, the flash output is adjusted only for the main subject in the frame, and the brightness of the background is not factored in while calculating the flash power. If the standard i-TTL Fill-flash is chosen by the camera by dialing the camera metering into the spot metering mode in A, S or P exposure mode, the background light is ignored and the camera will fire the flash to give you the correct exposure for the main subject only. If the overall exposure for the scene and the main subject is properly balanced without using the flash (shooing outside in a bright daylight for example), using the standard i-TTL Fill-flash may overexpose the subject by adding the extra light to already properly exposed main subject. You should use this mode only when the flash is main source of the light for the subject (shooting in a dark room for example) and you want to emphasize the main subject at the expense of the proper exposure for the background.

Standard i-TTL Fill-flash

In this picture above, the exposure on the main subject was metered by the flash metering system by firing the pre-flashes on the main subject and then analyzed the light reflected from the main subject along with other information that camera provided, including the metering mode and the focal length. The flash metering system then adjusted the flash power and fired the flash just to illuminate the main subject. If you notice the background, which is relatively darker, was metered by the camera metering system and exposed by the ambient light only. The flash power was calculated by the flash metering system to expose the main subject only which was weakened by the time it reached to the background, and hence flash didn’t contribute for the background exposure at all.