This essay is intended to give beginners to photography a brief understanding of the out of focus parts of a photograph. The out of focus elements of a photograph are sometimes described in terms of good or bad “bokeh”. Bokeh (also called “boke”) is a Japanese descriptive term for how the out of focus elements of an image look. This essay attempts to explain what affects and how to obtain certain characteristics in the out of focus regions of a photograph. Please keep in mind that although it is generally held that the smoothest out of focus transitions are optimal, bokeh is subjective and you should determine how you want your finished photo to look.

The Aperture

As the reader probably knows, the aperture of a lens controls the depth of field and the amount of light reaching the sensor. Depth of field increases when the lens is stopped down, (larger f number means more depth of field, less light reaching the sensor).

Generally speaking, the larger the number of aperture blades in the aperture of a lens, the smoother the out of focus parts of a photograph look. More aperture blades make a rounder aperture which tends to make a smoother and more natural looking bokeh. However, a large number of aperture blades do not guaranty good bokeh. Optical formula also affects how the out of focus parts of a photograph are rendered. Some lenses are inherently better in the bokeh department than others because of the lens formula. Generally, the wide aperture (f2.8 or wider) short to mid range telephoto and macro classes of lenses put out by the major manufactures often produce the most pleasing bokeh, and the slower variable aperture wide range zoom lenses produce the worst. For example, the Tamron 180mm f3.5 macro lens with its seven bladed aperture has much nicer looking bokeh compared to slower variable aperture telephoto zooms that have eight and nine bladed apertures.

Shooting at the largest available aperture setting on a lens produces the smoothest out of focus regions because of the minimal depth of field and of the optimum roundness of the aperture.

Magnification and Depth of Field

Depth of field (not to be confused with depth of focus) refers to the distance in front and behind the point of focus that's in acceptable focus. What's acceptable varies for print size: the depth focus that might be suitable for a 4”x6” sized print would most likely not be good enough for a 12”x18”. One important fact to keep in mind is that the depth of field behind your point of focus is double the depth of field in front of your point of focus.

Magnification and the aperture setting used determine the depth of field of a photograph. Lens focal length does not affect depth of field. Assuming the subject remains the same size in the viewfinder and the camera sensor is the same size, the depth of field will remain the same regardless of the lens focal length used.

Lens focal length does not affect depth of field. Magnification does. But lens focal length does drastically affect how the background of a photo looks because of the difference in perspective. A photo taken with a wide angle lenses will have a background composed of a much wider field of view and thus be more complex than a photo taken with a telephoto lens when the subject is the same size in the viewfinder. The smaller field of view is what gives photos taken with a long telephoto lens the “compressed” flat look and is why a telephoto photo would have a simpler and less distracting background.

It's often impossible to get detail-less smooth out of focus backgrounds with compact cameras simply because the working magnification is so low as compared to cameras with larger sensors. This is confusing, so I made some examples.

I took two photos, one on a digital SLR camera with an APS-C sized sensor with a 60mm lens, and the other with a compact digital camera zoomed to 60mm. The photos were taken at the same focal length, only the 60mm lens on the DSLR can cover the whole APS-C sensor whereas the 60mm on the compact is only large enough to cover the tiny sensor. The different aspect ratio in the first set of photos is due to the fact that the compact digital uses a 4:3 ratio format and the DSLR uses a 3:2 ratio format. Both photos are uncropped scaled down versions of the originals.

Both photos were taken at f5.6. You can see that DSLR has a much wider angle of view. This is because the larger DSLR sensor uses "more" of the 60mm lens than the compact digital. This is the same case as DSLR's with APS-C sized sensors compared to 35mm film SLR's and "full frame" DSLR's. The larger formats use more of the lens.

Here I cropped the DSLR photo to show the same angle of view as the compact digital camera. The Compact digital photo was not cropped. You can see that the depth of field is the same for both images.

The 60mm lens on the small compact digital camera sensor has the angle of view of a 200mm lens on the DSLR. It's working at a much lower magnification.

Because of their small sensors, compact digital cameras and DSLR's are great for close-ups. The 35mm macro lenses in the 50-200mm focal length range usually go to 1:1, or life size. This means that the lens can focus down to an area the size of the sensor. Because of this, it's often much easer to do close-ups with a DSLR or a compact digital camera. A macro lens at the minimum focus distance on a DSLR will get 1.5X “closer” compared to the same lens on 35mm film camera because of the smaller sensor, although on both cameras you'd be working at life size. A common misconception is that the long macro lenses such as the 180 and 200mm's have less depth of field at life size than shorter macros. As stated above, at the same magnifications and f stop, the different focal length lens will have the depth of field, but the longer focal length lens will have a simpler looking background. One thing to keep in mind though, is that the longer macro lenses are much more prone to sharpness killing camera shake, so it's much easer to get sharp handheld macros with the shorter macro lenses.

The Background and Light sources

The background obviously plays an important part in the composition of a complete image. You should always consider the non subject areas carefully. Composing your background so that there is some rhythm or pattern in the out of focus elements can make a huge difference. Small adjustments often produce big out of focus results.

Colors tend to blend together when thrown out of focus. Objects such as tree bark or rocks with lots of color texture when out of focus will get rendered as one mix. Look for colors that will blend in an interesting way; red-brown mud with the sky, golden sand with the blues of water. Try avoiding backgrounds that are mainly sky or light gray.

When shooting at higher magnifications, such as when photographing small animals, birds or macros, it's often easy to drastically change the background appearance and color by slightly moving the camera position. Experiment with a background color that contrasts the subject. Try to make interesting color gradients in the out of focus elements.

A technique that often produces interesting results is to choose backgrounds with spots of opposite or contrasting colors, such as groups of flowers. This produces backgrounds that often compliment wildlife and nature subjects very nicely, especially where the depth of field is low enough to render them as spots of color. Such techniques are helpful when the main subject lacks enough interest to provide a complete image.

Points of bright light will always show up quite clearly whether their in focus or not. In my opinion, most of the time for macro and wildlife photography it's very difficult to make a nicely balanced a photo with bright points of light in the out of focus regions complement the main subject. I nearly always find it to distracting from the main subject for my tastes. Backlighting out of focus objects such as leaves and flowers can bring out saturated and vibrant colors.

Sometimes, when shooting macros with a flash at small apertures, obnoxious specular highlights can be a problem in the out of focus parts of a photo if there are reflective points, such as water drops. These are always apparent, but especially so when a less than optimal roundness aperture is stopped down producing extremely unnatural looking geometrical highlights in the shape of the aperture. Ways to eliminate or reduce this problem might be to not use the flash, use a diffuser on the flash, or use a polarizer on the lens and polarizer gels on the flash. Changing the angle of light from the flash can also make a difference. These highlights can also be seen in non-macro photos, but they are much less noticeable due to the much lower magnification.

Bokeh controlling lenses and out of focus blurring with Photoshop

Nikon DC (defocus control) lenses can control how the out of focus elements are rendered by adjusting the spherical aberration of a lens. DC lenses (and other manufacturer's equivalents) are specialty items and are relatively uncommon and will not be discussed further here.

 It's actually quite easy to smooth backgrounds with Photoshop's Lens Blur and Gaussian Blur. The Lens Blur filter is the more useful of the two for this purpose and has a whole dialog box where you can enter information about the aperture and highlights, add sensor noise and use depth masks to control the depth of field. The simplest way to add out of focus blur is to apply Lens Blur to a duplicate background layer and then use a mask to keep the subject sharp, although sometimes it's necessary to use complex selections. The tricks in making this look convincing is to not overdo the blurring, keep the focus plane in line with the camera sensor, and add camera noise if needed to keep the image uniform.

 That's all for this essay!