# Tag Archives: Focal length

## Understanding depth of field

• Pebbles •
Depth of field helps the background lose distinguishing features which makes the subject stand out.

##### It is an important artistic tool.

Depth of field (DoF) provides a way to isolate the subject from the background. Sharpness inside the DoF is of critical importance to our image. The blur outside the DOF helps divert the eye to the sharp subject. So how does DoF work?

##### Depth of field defined…

The depth of field (DoF) is an easy concept to remember. The DoF is the sharp part of a picture. It is defined by the out of focus parts of the picture on either side of it. A lens can only focus at one point. It is at that point that the image is sharpest. However, on either side of the sharp point is a zone where, to the naked eye, the sharpness is still good enough to be convincing.

The eye can distinguish sharp detail inside the DoF and we want to ensure our subject is in that zone. However, we also want to compose our picture so the unimportant parts of the picture are out of focus. To achieve these things we need to know how to control the DoF. In fact there are three ways to control it…

• Change the size of the Aperture.
• Change the focal length
• the distance of the subject from the lens

As the aperture changes size so does the zone of sharpness we call the DoF. As we open up the aperture wide the DoF gets shallow. As we make our aperture smaller the DoF gets wider and eventually we get sharpness all through to the horizon. Similarly changes to the distance from the subject affects DoF. If I walk away from a subject (and do not change the aperture) the depth of field gets wider (and visa versa). Focal length changes affect the DoF in a similar way.

##### A Simple Guide to Depth of Field

In this video Dylan Bennett provides the best explanation I have seen on why the DoF changes with the three factors I mentioned above. He uses a simple explanation and some great diagrams to show what is involved.

Dylan Bennett

##### Just to clear up a point…

The analogy that Dylan Bennett uses, “toothpaste squeezed in a tube”, works well for most people learning the idea of DoF. What really causes the DoF to elongate is related to something called the Circle of Confusion (CoC). For more detail you can see information in: “Definition: Circle of Confusion”.

The CoC projected onto the sensor is (notionally) a tiny point of light representing one point of light from the subject. When that part of the subject is in focus the CoC is very small and individually indistinguishable from those around it. Like this it’s a sharp representation of the subject point of light. However, at a point outside the DoF an individual point of light can no longer be represented by a sharp point on the sensor and begins to blend with points around it. It has lost its sharpness. This is because the lens focuses points outside the DoF slightly before the sensor or slightly after it.

Diagram showing various sizes of Circles of Confusion (CoC) on the sensor sized according to the focus (not to scale). Only CoCs projected from within the Depth of Field are sharp. Our eyes cannot perceive them well as they form sharp points. Ones projected from outside the depth of field are unsharp to our eyes.

Here is the reason why the DoF widens as the aperture gets smaller. When the aperture is small the angle of light that can pass through it from the subject is also small. As a result the CoC for each point of light is relatively small. This is because it can only originate from a small angle of light.

A very small aperture means that the circles of confusion are never big enough for our eye to see. This allows the lens to focus at infinity (say f11 and smaller). When the aperture is wide open the reverse is true. The circles of confusion can be much bigger. Only those rays that the lens will naturally focus will be sharp (a shallow depth of field).

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#### By Damon Guy (author and Photokonnexion editor)

Damon Guy (Netkonnexion)

Damon is a writer-photog and editor of this site. He has run some major websites, a computing department and a digital image library. He started out as a trained teacher and now runs training for digital photographers.
See also: Editors ‘Bio’.

## Ten simple ideas to improve your photography (and a fun quiz)

Ten Tips and 12 fun quiz questions.

##### Simple things help you…

We should all take a step back and think about the basics sometimes. It helps us remember essential techniques and keeps us on our toes. Here are the basics with some fun quiz questions too.

The simplest techniques in photography are often the most important ones. In this post we make sure we don’t forget them…

##### 10 essential things to know; 12 fun quiz questions
1. Not knowing your camera: This is really bad news. If you are hoping to improve your photography make sure you learn what every lump, bump, dial, screen, lens and twiddly bit does. Read your manual regularly. Practice with each function until you have got it right. Then practice it in the dark so you can do a night shoot.
Quiz Question 1: How many lenses are there on a camera? Answers at the end!
2. Poor stance: Most people when starting photography don’t realise that the way they stand and hold the camera creates all sorts of problems and poor performance. If you are a keen photographer a good stance can contribute to improved sharpness (hand-held shots), better focus, more steady hand and better shot timing. Learn to stand properly right at the start and you will save yourself lots of re-training time later.
Quiz Question 2: At what point in the breath cycle is it best to take your shot?
3. Not using a tripod: classic mistake. Tripods save you lots of time and give you pin sharp photographs. They give you an opportunity to set your camera up properly and ensures that your are ready for your shot.
Quiz Question 3: A monopod has one leg, a tripod has three legs. What is, and how might you use, a bipod?
4. Not giving the camera time to focus: When you press the shutter button halfway down it causes the auto-focus to cut in which focuses the camera. But if you punch straight through that to the shot the focus has not had time to do the full focus. This normally happens on the first focus attempt when the focus is right off. After that the lens in nearly focused and will adjust more quickly. So don’t make your first focus attempt too close to the shot or it will be blurred.
Quiz Question 4: Why do you have two rings on a modern auto-focus/zooming photographic lens? What do you call each of them?
5. Taking pictures against a bright light? Cameras don’t like very bright lights. Especially if there are also very dark spots nearby. Shooting indoors while looking at a window out to a bright sky will cause a strong white spot. This is very distracting and draws the eye away from the subject. Not good. There are Light and Lighting resource pages on Photokonnexion for you to learn more.
Quiz Question 5: How many stops of light can healthy human eyes see (20:20 vision)? How many can the camera (rough generalisation) cope with?
6. Relying on flash (especially pop-up flash): Pop up light has a very small concentrated source. It discolours faces, washes out colours, creates harsh, sharp-lined shadows and is badly placed (too close to the optical axis) creating nasty highlights on faces. Try to use natural light more. It is much more forgiving and does not produce such harsh shadows most of the time.
Quiz Question 6: What is often the result of using pop-up flash with respect to two parts of the face?
7. Dead centre subject: If you put the subject of your picture in the centre it will usually be boring. If you off-set your subject the eye will be looking to see why the symmetry is broken. That keeps the eye hunting around the screen. Learn about the “Rule of thirds” and other Composition principles. That will help you make the shot more compelling to the eye.
Quiz Question 7: What type of compositional perspective would you be working with if you want to promote a three dimensional feel to your picture composition?
8. Horizon control: Make sure your horizon is level, especially if it is a seascape. If you leave it on an angle the picture will be ruined because it will look like the sea is sliding off the page! Horizons also induce mid-picture viewer-stupor. Make a decision. Either shoot for the sky in which case place the horizon in the bottom third of the picture. Or, shoot for the ground in which case the horizon goes in the top third of the picture. An off-set horizon is more dynamic and keeps the viewers eye moving.
Quiz Question 8: If your main choice is to shoot for the sky, where would you take your exposure from? (Where would you point your viewfinder focus point?) a. The sky? b. The ground?
Quiz Question 9: Describe autofocus hunting and why it happens?
9. Simplify, simplify, simplify: The most effective way to show a subject to your viewer is to de-clutter the picture. Take out of your composition everything that is nothing to do with the subject. The more you make the viewers eye go to the subject the more effective your shot will be.
Did I mention that you should simplify your shot?
Quiz Question 10: What is it called when you paint out something from your picture in post processing to simplify a shot?
By the way, did I mention that you should work really hard to simplify your shots?
10. Go manual: Auto-modes on your camera are really best guesses about what the manufacturer thinks will be suitable for the average shots most snappers will take. Buy you are a keen photographer. To get the camera to do exactly what you want, and to make discerning choices about your images you should work on improving your manual control. Your understanding of photographic principles will improve, your skill at exposure will improve and you will find yourself making informed choices about how you want your picture to come out. You will turn from a snapper into a photographer.
Quiz Question 11: What does the ISO control do? a. Adjust the sensitivity of the digital image sensor or b. Change the aperture size?
Quiz Question 12: Does ‘shutter speed’ or ‘aperture’ control movement blur?
##### Answers to quiz questions
• Quiz Question Answer 1: I am talking about any camera that has a lens, not just DSLRs. the number of lenses is a matter of variation. If you are discussing photographic lenses then only that one will count (but read on). Some people think of each glass element in the photographic lens as an independent lens. Technically that is not true. They are optical lenses or glass elements, not photographic lenses. However, if the photographic lens (and elements if you included those) were all you counted you would be wrong. Here is a short list of Possible lenses on a camera of any sort…

There may be others.

• Quiz Question Answer 2: You should take a shot at the full inhale point or full exhale point before inhaling or exhaling in the next part of the cycle. You can choose which is best for you. All you do is delay the next part of the cycle while you take a shot. This is the point in the breath cycle when there is least movement of the shoulders/chest. Read more about it in Simple tips for a good stance
• Quiz Question Answer 3: A bipod is photographically uncommon. Understandably, it has two legs. Find out more here… Definition: Bipod
• Quiz Question Answer 4: The two rings on an auto-focussing photographic lens allow one ring to focus the image – the focus ring. The other ring is for zooming the lens. The latter changes the focal length and is called the focal length ring.
• Quiz Question Answer 5: Human eyes can see about 18 to 20 stops of light when healthy. However, by contrast the best commercially available cameras have to operate with a dynamic range of 8 to 12 stops of light. Research is pushing the boundaries but there is still a big gap to meet the dynamic range of the human eye (in 2013).
• Quiz Question Answer 6: Pop-up flash is very likely to cause red-eye.
• Quiz Question Answer 7: To make things look three dimensional in your image you should be working with three point perspective. Look for lines in your image that promote cube-like structures. For example buildings, walls and other objects with lines and shapes that have a solid feel in real life. This will trick the eye into believing that there is a solid object in the picture. Read: Simple ideas about perspective in photography and: Definition: Perspective
• Quiz Question Answer 8: If you shoot for the sky you will need to be taking your exposure from the sky as that is the brightest point. This will leave the ground darker in your exposure than you would see it with your eye. You can use one of a number of techniques to correct that later.
• Quiz Question 9: Auto-focus hunting is when the auto-focus in the lens cannot focus and will keep going up and down the focus range trying to get a focus. This is a common problem at night, in darker conditions, low contrast conditions and clear or totally grey skies. You can read more about it in: Auto-focus ‘Hunting’ Definition: Hunting, Auto-focus

• Quiz Question 10: when you paint out something from your picture in post processing to simplify a shot? You normally use a cloning tool. You can find out more in: Definition: Cloning; To Clone; Cloned; Clone Tool.
• Quiz Question 11: What does the ISO control do? It adjusts the sensitivity of the digital image sensor allowing you to work in bright light (low ISO setting) or low light (high ISO setting). There is an article on ISO here: ISO.

• Quiz Question 12: Shutter speed controls movement blur. Aperture controls blur (bokeh) created by the loss of sharpness outside the zone of acceptable sharpness. This is traditionally known as the depth of field. More reading on: Definition: Exposure and related to aperture: Definition: f number.

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#### By Damon Guy (author and Photokonnexion editor)

Damon Guy (Netkonnexion)

Damon is a writer-photog and editor of this site. He has run some major websites, a computing department and a digital image library. He started out as a trained teacher and now runs training for digital photographers.
See also: Editors ‘Bio’.

## Understanding the photographic lens in simple terms

The lens is a complex piece of equipment with some really easy ideas behind it.
Click image to view large
• The Lens •b# By Netkonnexion on Flickr

##### The lens – probably the second most important piece of gear.

The lens has become a most interesting piece of gear in its own right. The DSLR would be lost without it and the wide range of possible compositions we can access would be greatly reduced. So, in honour of the modern lens here are a few definitions and resources that I have put online in the Photographic Glossary (P.S. no brain cells will be tortured to death reading these explanations). Take a tour by clicking the links as you read through…

##### It pays to know what you have in the arsenal

So, there are basically three types of lens as far as the way the lens sees the world. These are…
The long focus lens is the lens that you will use to get out there almost to infinity and see things large!

Then again, it pays to have a view of the world where your eye is King (or Queen of course). The Normal Lens is operational in the range of perspectives and focal lengths of our own eyes.

Then, down at the lower end of the scale is the issue about wide angles. The humble wide angle lens is able to provide wide access to the background.

##### Have you ever wondered…

I was perplexed for years about this… just what is focal length? Every book I read seemed to make it so difficult. It’s simple really.

Oddly however, on the one hand our lenses are marked something like 50 : 250mm, which is the focal length; but the actual lens is no more than 100mm long. How does that work? Well, its turns out to be something simple called Telephoto lenses.

The whole focal length thing seems to be related to the way that a lens sees the world. So, how do a zoom lens and a prime lens differ?

##### Has that made it easier to understand about lenses?

I know it is difficult to come to terms with the optics in photography, its a bit dry and technical. I hope that by stripping away the jargon I have made it easier for you. Please leave a comment if you are still in need of help. Tell me what you want to know. I will try to clarify or extend the lens entries in our Photographic Glossary.

Oh! And, don’t be afraid to point out my mistakes and any explanatory shortcomings. It helps us all if I get it right! LOL.

#### By Damon Guy (author and Photokonnexion editor)

Damon Guy (Netkonnexion)

Damon is a writer-photog and editor of this site. He has run some major websites, a computing department and a digital image library. He started out as a trained teacher and now runs training for digital photographers.
See also: Editors ‘Bio’.