Composition and depth of field (DoF) are the most important parameters to play with once you have some control over shutter speed, aperture and ISO. While there is no hard and fast rule (other than guidelines like rule of thirds) governing what is the best composition technique, DoF is something that is well defined (mathematically) and is independent of artistic bias. While most amateur photographers do know the most basic principle that aperture controls DoF; There knowledge of DoF ends there. Nonetheless DoF is a quite complex function of several parameters other than aperture. In this article I will give ten facts about perceived DoF and how it can improve your photography without going into intricate technical details. Some of the facts can be used by users of point and shoot cameras even if they don't have any direct control of camera parameters!! Before that some basics for those who do not know anything about photography. If you cannot understand any technical term just ignore it.

Focal Length of a lens

Photography basics - Lens Focal Length

Focal length of a lens is a measure of how strong a lens bends light. For simple lenses this is the distance after which parallel lights converge. Normally focal lengths are expressed in 35 millimeter equivalents. So the focal length expressed on your lens is exactly same as stated on the cover only if you use a full frame 35mm sensor camera. In case you are using a smaller sensor your lens' focal length is more than what is stated on the cover. To be exact the focal length is larger by multiple of crop factor (check your manual to find out what is the crop factor of your camera). So,


for example the 18-55mm canon kit lens on a crop body 1000D camera with 1.6 crop factor is equivalent to (18*1.6=)28.8-(55*1.6=)88mm lens. Apart from these details the most important thing one must know is that a small focal length lens (like 18mm) captures a WIDE scene while a large focal length lens (like 100mm) captures a zoom telephoto.

Comparison of focal lengths

Photography basics - Aperture

Aperture is the physical opening in the lens that allows light to come in and fall on the sensor. It is very similar to the iris which controls light falling into the human eye. A larger aperture allows more light to come in while a small aperture allows less light. Many low end point and shoot and even high end mobile phone cameras have a fixed aperture. What makes things confusing is how aperture is expressed. It is always expressed in terms of the lens' focal length instead of an absolute diameter. so \frac{f}{2} means aperture is half the size of focal length. Therefore for a 100mm lens it means the aperture is 50mm. To give a a sense of standard apertures \frac{f}{1.4} (read f one point four) is a very large aperture and \frac{f}{22} (read f twenty two) is a very small one. Important thing to keep in mind is large f number means small aperture and small f number means large aperture (simple). Why aperture is expressed in such a weird term will be clear once you go through fact number 6.

small f number: large aperture; large f number small aperture

Photography basics - Depth of Field (DoF)

A lens can focus at only a single plane and everything that is not in that plane will be blurred. Depth of field (DoF) is the distance between the nearest and farthest objects in a scene that appear acceptably sharp in an image. The distance of the object from the camera which is in perfect focus is called subject distance. Refer to figure below. As you can see DoF is not symmetric about the focal plane. Under normal circumstances more objects are in focus behind the focal plane than in front.

Object in focal plane is in perfect focus.

A shallow depth of field can highlight the subject and blur the background which is normally expected in portraits while a large depth of field will ensure everything is in focus which is a requirement in landscapes.

Shallow or low DoF and High or large DoF

So now that you have some basic understanding of the terms I am going to use, lets jump to the facts and to exploit them to your needs:

1. For a  fixed focal length, larger aperture will result is shallower depth of field.

This is something that most amateur photographers know. A large aperture (small f number) will result in shallow depth of field and a small aperture (large f number) will result large depth of field. Very large apertures like (f/1.4) can achieve very shallow depth of field, so shallow that your nose could be in focus but your eyes could be completely out of focus! Moreover large aperture allows significant amount of light to enter the lens and hence allows for faster shutter speeds and lower ISO. Large aperture lenses are exorbitantly costly, for instance 80mm f/1.4 lens costs around 3000$.  It becomes essential that you know other ways to control DoF. The picture below compares the effect of aperture for a fixed focal length of 18mm.

Both taken with a 18mm lens at f/3.5 and f/11 aperture

2. For same aperture, a larger focal length will result in shallow depth of field.

While a 50mm f/1.4 may be costly, you can get same (apparent) DoF using a 200mm f/9! Of course there is a catch. The images captures with different focal length lenses are completely different and in true sense there is no increase in DoF but if you try to capture the same subject by adjusting subject distance the result will have much shallower depth of field for higher focal length mainly because of the zooming effect. Users of point and shoot who normally don't have any access to aperture can exploit this by zooming all the way out and moving away from the subject a bit to achieve a shallow depth of field! Figure below shows a comparison - one taken with 18mm lens with f/3.5 and another at 55mm with f/5.6 - The 55 mm lens appears to have shallower depth of field even though the aperture is smaller than that of 18mm!

Apparent increase in DoF due to zooming effect of 55mm lens

3. Depth of field is inversely proportional to sensor size. A smaller sensor will have large depth of field while a large sensor will have shallower depth of field

That is the reason getting a shallow depth of field is very difficult with a point and shoot or a mobile phone camera. These cameras have a very small sensor compared to a SLR. So a full frame DSLR is better than a crop frame DSLR is better than a point and shoot is better than a mobile camera when it comes to shallow DoF.

A high end mobile camera Vs crop frame DSLR (ignore contrast difference)

4. Cropping causes decrease in depth of field. This is important in cameras with ability to take picture and videos in crop mode.

Just the act of cropping decreases DoF, this is where large mega-pixels can be helpful. You can take a high resolution photo and crop it to increase the apparent DoF. This is also helpful in cameras that can take videos and photos in crop mode.

Effect of crop - 3X crop causes apparent increase of DoF

5. Setting aperture to its lowest setting WON'T result in highest depth of field. Diffraction is a competing phenomenon that causes decrease in depth of field at very low apertures!

This is a bit contradicting to fact 1. You may think highest DoF (say for a landscape) can be achieved at lowest aperture (say f/22) but wrong; at very low apertures due to diffraction DoF decreses. Therefore the aperture for highest DoF is somewhere close to highest f number. So instead of f/22; f/11 may result in higher DoF!

6. Same aperture (in terms of f-Number) will result in same illumination of the sensor independent of the focal length

This is the reason why aperture is expressed in terms of f-numbers. No matter what the focal length is, same f-number = same illumination of the sensor. For same aperture in absolute terms a large focal length will have much lower illumination. So if you are using a telephoto lens keeping at the wider end will give more light and allow for slower shutter and lower ISO.

7. A telephoto lens (large focal length) if reversed will act as a super macro lens with very shallow depth of field.

This is a trick you can use to convert your existing telephoto lens to a super macro lens with extremely shallow depth of field. All you need to do is remove the lens and hold it backwards to the SLR body. Off-course auto-focus, aperture control and image stabilization will seize to work as there is no electrical connection to the body. If your camera has a live view mode it can help take focus the image. See in the image below which is taken with a 55mm lens in reverse, the DoF is extremely narrow probably 2-3mm only.

DoF is less than a few millimeters!

8. Depth of field decreases rapidly as subject distance from the lens is decreased. A close subject will have a very shallow depth of field while a far away object will have large depth of field

So if you want to get a shallow depth of field move closer to the subject; Using a large focal length at the highest possible aperture and moving as close as possible to the subject can result in a very shallow depth of field.

9. Shape of the aperture affects the quality and shape of blur. An artificial, creatively shaped aperture placed over a lens can create interesting effects on blur (bokeh photography).

shape of aperture

This fact can be used to get some very interesting and artistic effects from your camera. In fact you don't even need a DSLR. This can be achieved using any camera with a paper cup attached over its lens with a particular shape cut over its face. Blurring patterns will then have that characteristic shape. One thing to keep in mind is that this paper aperture will reduce light thereby asking for fast shutter and increased ISO.

Bokeh effect. The shape used as aperture is shown.

10. Depth of field can be used to create illusion of miniaturization. This type of photography is known as miniature faking.

This is a wonderful optical illusion. We as humans are hard wire to see macro shots with a shallow DoF while a landscape with large DoF. So if you can achieve a very shallow DoF with a landscape you can fool the brain to think that its a macro shot! Although achieving very shallow DoF for landscapes is impossible with a normal camera. To achieve that you need what is know as a tilt camera. But still even if you don't have a tilt camera you can always achieve that using software like Photoshop or GIMP. This technique is known as miniature faking. The shot below is a miniature faking of the city of Hyderabad. Click on the image to see the original one.

Miniature Faking - click on the image to open the original one

I hope these facts can help you understand photography a tad bit better. If you like it please share it with others and comment.

Want to read more? Take a look at this excellent article by PhotographyTalk.

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