What Is Depth Of Field (DoF)?

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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.

Dealing With The Digital Noise

Visual image noise is the random grainy effect on the photograph which is disturbing most of the time rather than pleasing. If these grainy dots, also known as a digital noise, are noticeable or objectionable in the photograph, it can ruin the beauty of the photograph by distracting the viewers mind from the subject. Digital noise is mainly caused by either high ISO settings or a long exposure shooting. Let's discuss these two main factors which contributes to the production of the digital noise.

1. High ISO Setting

High ISO noise commonly appears when you raise your camera’s sensitivity setting above ISO 400. Different camera’s sensitivity level with light differs from one another and sometimes it determines how good your camera is with the lights. With the Nikon D90, noise may become visible at ISO 800, and is often fairly noticeable at ISO 1600. And at ISO 3200, noise is usually quite bothersome. Nikon advises that using ISO 6400 in special situations like very low light condition may help you but you should consider labeling it H1.0. You can expect noise in any pictures taken with that ISO level which is obvious. High ISO noise appears as a result of the amplification needed to increase the sensitivity of the sensor. While using higher ISO settings do pull the details out of the dark areas, they also amplify non-signal information randomly, creating the noise. You’ll find a High ISO NR option in the Shooting menu, where you can specify High, Norm, or Low noise reduction, or turn the feature off entirely. Enabling the noise reduction tends to soften the grainy look, you may want to disable the feature if you‘re willing to accept a little noise to get more details in your picture.

2. Long Exposure Shooting

A similar noisy phenomenon occurs during the long time exposures, which allow more photons to reach the sensor, increasing your ability to capture a picture under low-light conditions. However, the longer exposures also increase the likelihood that some pixels will register random phantom photons, often because the longer an imager is, the warmer it gets, and that heat can be mistaken for photons. There’s also a special kind of noise that CMOS sensors (like the one used in the D90) are potentially susceptible to. With a CCD sensor, the entire signal is conveyed off the chip and funneled through a single amplifier and analog-to-digital conversion circuit. Any noise introduced there is, at least, consistent. CMOS imagers, on the other hand, contain millions of individual amplifiers and A/D converters, all working in unison. Because all these circuits don’t necessarily call process in precisely the same way all the time, they can introduce something called fixed-pattern noise in the image data.

Fortunately, Nikon has done an exceptional job minimizing those noises from all causes in the D90 and other Nikon DSLRs. Even so, you might still want to apply the optional Long Exposure Noise Reduction that can be activated using “Long exp. NR” in the Shooting menu, where the feature can be turned On or Off. When the Long exp NR feature is turned on, D90 takes a second blank exposure and compares the random pixels in that image with the photograph you just took (first image). Pixels that coincide in the two pictures represent the noise and can safely be suppressed. This noise reduction system, also called dark frame subtraction, effectively doubles the amount of time required to take a picture, and is used only for exposures longer than one second. Noise reduction can reduce the amount of details in your picture, as some image information may be removed along with the noise. So, you might want to use this feature with moderation.

If you shoot in a RAW mode, you can also apply the noise reduction to a lesser extent by using post processing software like Photoshop, Lightroom, CaptureNX2 or Aperture later on the computer.

Source: David Busch's Nikon D90 Guide to Digital SLR Photography

Understanding Flash's Guide Number (GN)

Guide Number, usually abbreviated GN, determines power rating of flash unit that describes how powerful flash unit is and how far it can shoot. In another word, GN specifies the power of an electronic flash in a way that it can be used to determine the right f-stop to use at a particular shooting distance and ISO setting. GN is mainly used to calculate how your camera’s aperture should be set to get proper flash light. When there were no automatic flash units available for photographers, they used to do a manual calculation using the GN and the distance of the subject from the flash unit. GN is usually given in reference of ISO value 100. For example, Nikon D90's built-in flash has a Guide Number of 12 meters or 39 feet at ISO 100 in auto mode (i-TTL). And in Manual mode, it has a Guide Number of 13 meters or 43 feet. GN is slightly less in i-TTL mode than in Manual mode because it spends energy to fire pre-flashes before the main flash goes off (to meter correct amount of the light for the subject). Basically, to determine the right exposure at any given ISO settings, you would divide the guide number by the distance to get appropriate f-stop. We can write the mathematical formula as,

Aperture = GN / Distance

Using D90’s built-in flash as an example, at ISO 100 with its GN of 43 feet in Manual mode, if you wanted to shoot a subject at a distance of 10 feet, you would use f/4.3 (43 divided by 10), or close to this value in practice. Similarly if subject is at 5 feet, an f-stop value f/8 would be used to get proper exposure. But in practical, we may need to increase the ISO few stops to adjust with f-stop and the distance of the subject.

When we are talking about the Guide Number, we didn’t mention about the Shutter speed and we only talked about the Aperture. It is very important to remember that Shutter speed controls the brightness of the ambient light and Aperture controls the amount of light from the flash. We use Guide Number calculation to help control the flash exposure. Today, GN is mostly used for comparing the power of various flash units, rather than actually calculating the exposure value to use. You don’t need to be a math genius to see that an electronic flash with a GN in feet of, say 98 at ISO 100 (like the SB-600) would be a lot more powerful than the Nikon D90’s built-in flash. Using that GN as a reference, Nikon SB-600 allows you to shoot up to 22 feet using f/4.5 at ISO 100 easily.

Please refer to your flash user manual to get the exact GN value.