Depth of Field Definition and Perspective – Turn You into Pro Now

Depth of Field Definition and Perspective – Turn You into Pro Now

In this post, I will focus on something that can make you look like a pro easily: the depth of field. After you know the depth of field definition and perspective, your image will immediately upgrade to the next level. First, let me ask you a question. Have you seen this kind of picture before?
Depth of Field Definition
What similarity can you find in the images? Yes, a nice blurry background. People love the blurry background which called “bokeh“. This is a Japanese word of blur.

Depth of Field Definition

First of all, I want to introduce the depth of field definition. The depth of field is how depth of sharp image the lens can give. There are two elements which control the depth of field, namely, aperture size and distance between the object and the camera. If you don’t want to read about the technical detail, you can directly go to the conclusion.

Aperture Size

An aperture is a hole or opening through which light travels into the camera sensor or film. In the old days, aperture is always set manually. You can see the below images for different aperture size of the Nikorr 50mm f1.8D.

Dof Compare

When the aperture size is larger, shallower depth of field images are produced. However, aperture size cannot be used as measuring perimeter in photography because different light intensity reaches a camera under same aperture size but different focal length. Two other perimeters, f-number and focal length, are therefore use to do photometry. In other words, there are few people mentioning aperture size in photography. Photographers normally mean f-number but not the actual aperture size when they say “aperture“.


In optics, the f-number (also known as f-ratio, f-stop or relative aperture) of an optical system is the ratio of lens’s focal length to the diameter of the entrance pupil, i.e. actual aperture size. The smaller the f-number is, the shallower the depth of field. F-number is calculated from the following formula:

f number = focal length / diameter of aperture

Sorry to introduce mathematic here but it is really important in photography. From the formula, we can see that a smaller f-number gives a larger actual aperture size with a fixed focal length. For the same focal length, therefore, a smaller f-number means a larger aperture size and a larger aperture size means a shallower depth of field. Thus, smaller f-number gives shallower depth of field.

Focal Length

In the f-number formula, we can see that a larger aperture size is required to result in the same f-number for a longer focal length. In other words, you need a larger actual aperture size for a 300mm lens than a 50mm lens to get a f2.8. That is why super-tele lenses are not only long but also big. It is more difficult to manufacture such a large piece of glass and therefore super-tele lenses are expensive.

bokeh compare

For f2.8, a 300mm lens gives much shallower depth of field than a 50mm lens. It is because a 300mm f2.8 lens has a lager aperture size then a 50mm lens.

Distance between Object and Camera

distance and dofThis is another parameter which affects the depth of field. The closer the object is located to the camera, the shallower is the depth of field. In macro photography, photographers seldom use f8 or smaller f-number. It is because the depth of field is too shallower which maybe less then 1mm. From the above images, you can see that when you get closer to your subject, you can get a shallower depth of field.


This is all about depth of field definition. To wrap it up, there are three main points you must remember for depth of field.

  • The smaller is the f-number, the shallower is the depth of field
  • The longer is the focal length, the shallower is the depth of field
  • The closer is the object to the camera, the shallower is the depth of field


It is time to talk about perspective. This is a topic which many beginner photographers miss out.

Perspective means a warping or transformation of an object that differs from what it would look like and the size ratio between objects in the front and the back.

Wow, it sounds complicated. Don’t worry, I will make it simple enough for you to understand. Different focal length gives different perspective. I try to separate them into four groups, wide-angle, normal lens, telephoto lens and super-telephoto lens. The focal length which I am talking about is referred to full frame (35mm) format.

Wide-Angle Lens

wide angel exampleA wide-angle lens means a lens with less than 35mm. For example, Nikon AF-S Nikkor 14-24mm f2.8 ED, Canon EF 15mm f2.8 Fisheye, Tokina Fisheye 10-17mm f3.5-4.5 DX, etc. Wide-angle lenses are usually used in landscape, cityscape, architectural, etc. This type of lens allows more of the scene to be included in the photo. The relative size is exaggerated so photographers can make foreground object more standout while including an expansive background. Also note that the distortion is serious in using wide angle. Photographing a portrait is not recommended as the lens distorts the person who is being shot, especially at the two sides.

Normal Lens

Normal lens produces images which generally looks “natural” to naked eyes. Normal lens refers to 50mm focal length. Photographers usually consider 35mm to 70mm as normal range.

Telephoto and Super-Telephoto Lens

focal length and lengthThis is a specific lens type which the physical length is shorter than the focal length. In the picture, you can see that 300mm lens is just double of 90mm. Long focal length is great for wildlife, sports, and others. Due to the long focal length, the technology which makes physical length shorter is important. Otherwise a 500mm lens will be half meter in length! 70mm to 200mm range is great in taking portrait, especially 85mm. The perspective is wonderful to present a head shot or half body portrait. For longer than 200mm, it belongs into super-tele family. Lens is more expensive and bigger in size. These lenses are wildlife photographers’ lovers. They can shoot the target in distance while not disturbing it.

Perspective VS Focal Length

shooting angleA wide-angle lens produces image with a larger angle of visibility and vice versa. Another important point you need to know is that different focal length gives different background. Take a look at the image below on focal length vs. depth of field to see how the background changes while keeping the subject in the same size.

perspective comparison

Wow, I am very happy if you have gone through the entire article. It is more than a thousand words. I am surprised by myself because I have been writing only a few long tutorials with over a thousand words. Great job Kevin! :D So, if you think I am doing a great job too please help me to spread my work to your friends by clicking the share buttons below. Stay tuned for the next epic tutorial.