Understanding Depth Of Field Preview

Depth of Field (DoF) is the distance between the nearest and farthest objects in a scene that appear acceptably sharp in the image. It is widely used term in all types of photography and specially popular in Portrait and Landscape photography. We use small DoF (small aperture value), also called shallow depth of filed, in portrait photography to emphasize the subject and throw an unwanted background away from viewer's attention by blurring it; whereas, large DoF (larger aperture value), also called deep depth of filed, is widely popular in Landscape photography where everything from the foreground to the background should be in focus.

What is Depth of Field Preview and When do we need it?

Let's say your lens's widest aperture is f/2.8 and you want to shoot with f/11. In this particular case, when you close down your aperture value to f/11 from f/2.8, your camera will not close down aperture blades, also called diaphragm, immediately after you change an aperture value. This is because all camera bodies focus at the widest aperture of the lens and then stop down to the chosen aperture (f/11 in this case) when you press the shutter release button all the way down to take the picture (after focusing your subject obviously).

Note: I noticed that if you are using Nikon D90 and the D lens which has an aperture ring on it's barrel, you have to set the ring to f/22, lock it and set the actuator (at the end of the lens) which allows opening the aperture from the camera body.

DOF Preview Button on Nikon D90 and D80

Today's most advanced DSLRs come with a dedicated button (DoF preview button) to preview the Depth of Field and allows you to close down your aperture to the selected value by stopping down the lens's diaphragm. So, when you set an aperture to f/11, pressing DoF preview button closes down the aperture to f/11 and darkens your viewfinder by allowing the less light through an aperture. It is a really helpful tool when you are doing Landscape photography, and most importantly if you are using Hyperfocal distance formula. In often cases, you will get deceived by the widest open aperture and will be difficult to judge what is in focus and what is not through the viewfinder.

DOF Preview Button on Canon EOS 40D

Canon EOS 40D has a Depth of Field Preview button right below the lens release button. Every camera manufacturer has a different body design and their choice of placement for this button also differs by camera models (even on the camera from the same manufacturer).

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.

Lincoln Memorial Reflecting Pool

I took this picture of Reflecting Pool, which is located in between the Washington Monument and the Lincoln Memorial, last year. If I remember correctly, it was a cherry blossoms festivals 2010 and hundreds of thousands of tourists from around the world traveled to DC to see the beautiful cherry trees blooming to its peak around the Tidal Basin. At first, I was thinking to drive down there but a friend of mine suggested me to take a metro instead. So, I drove down to the nearest metro station, parked there and took a metro going towards the Smithsonian metro station. I am so glad that I took his advice and took a metro. It took me almost 30 minutes driving around a metro station parking lot to find a parking spot. Inside the metro station, I was overwhelmed by the crowd and then immediately got worried about the camera gears I was carrying with me. I had a heavy camera bag along with a tripod bag and it was getting very difficult for me to manage the space inside the metro. Somehow, I managed to get off at Smithsonian metro station after one and half hour, and then to my surprise, I saw the biggest crowd in DC ever. I was not able to walk on my own, the crowd was moving me and I was going wherever it would take me. When I reached the National mall, I got a little bit of a relief and got chance to breathe some fresh air but the crowd was still growing bigger and bigger.

You can also see crowds in the photograph below, which I took from the stairs of the Lincoln Memorial facing towards the Washington Monument. It was a beautiful scenery around the mall with all the green trees and the colorful dresses. The entire surrounding was joyful with happy faces and the whole environment was very encouraging to capture those beautiful moments. I took quite a few shots which I will continue to share with you in my future posts.

Lincoln Memorial Reflecting Pool

Focal Length : 60mm     Exposure : f/9     Shutter : 1/320sec     ISO : 200

This is one of the best photographs of reflecting pool I have taken so far. After I came back home and reviewed some of the pictures I had taken, I thought it was worth all the troubles I went through that day. If you are planning to take photos around the National mall area, I suggest you to plan your travel accordingly because DC gets very busy on special events days and I learned it the hard way.

Here is the cherry blossoms shot I manage to take that day.

Cherry Blossom

Focal Length : 90mm     Exposure : f/13    Shutter : 1/200sec     ISO : 200