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Facts about focus… how much do you know?

Orb Out Of Focus

• Orb – Out Of Focus •

Focus is more complex than at first sight.

Focus, and the associated functions on a camera, take a little time to sort out in your mind. Here are some fundamentals and some obscure facts for you to think about.

Focus facts…
  1. When you point your camera at a single colour, blank space, solid blue sky or night sky the auto-focus will ‘hunt’ for some contrast. The lens will be unable to focus but keep trying. If it detects a difference of contrast in the frame it’ll be able to balance it out and use it to create a focus. If your lens is ‘hunting’, find something with a bit of colour or light contrast. Then it will stop hunting and focus. More…
  2. The light from your lens focuses on the “image plane”, the flat surface that is also the surface of the Digital Image Sensor. The circle of light it creates is bigger than the sensor. The light that falls outside the sensor area is normally inferior to the light which falls on the sensor surface. This is because the edges of the lens creates an inferior quality of light compared to the middle part of the lens. More…
  3. There is such a thing as a “fixed focus lenses”… they have no focus capability. They are put in the camera at time of manufacture and set to give a deep focus from a few meters through to infinity. Can you imagine why such a lens would be used in a camera? More…
  4. Bokeh appears in out-of-focus areas. Some people think that the prominent little circular highlights are the bokeh. This is incorrect. In fact bokeh is a statement relating to the quality of the out-of-focus blur. The blur itself is called… blur! More…
  5. Macro lenses are highly effective at focusing at short distances from the lens and to enlarging the image. Actually they will also focus at other distances too. Macro lenses produce a great portrait. More about macro lenses…
  6. When a lens lets light through the light follows something called the optical path. The focus is achieved on the optical path. However, in a DSLR when the mirror is down, the optical path ends up at the eye. The optical path is not the same all the time. More…
  7. Where you focus affects your exposure. Most people assume focus and exposure are two separate things. And, in fact, they are. But, your camera senses the light conditions where you are focusing and sets that focus at the same time as taking readings for the light meter. Change your point of focus and you may change the light reading. Even a tiny movement may change your exposure readings if your focus changes from a lighter to darker surface. Useful to know if you are in a darker environment with bright light sources around. Especially useful if you are trying to learn manual control.
  8. The ‘sensor plane’ is the surface on which the image is projected by the lens. It is the surface of the sensor itself. In former times it was the same surface that the film was laid upon. Any idea what other names might be applied to the same surface in a camera? More…
  9. Sometimes it is important to measure your focus distance precisely. For example if you want to repeat shots or vary the distance form camera to focus-point by a measured amount in a series of shots. Such measurements are often used in forensic photography, macro work and for working out scene sizes and distances. There is only one place where you can take a true measurement from on a DSLR. It is clearly marked. Know what it’s called and where it is? More…
  10. Consider turning off your auto-focus when using a tripod. The motor that works the auto-focus creates vibration that may affect your shot. It is less likely to have an effect on long exposures, but if you get into the habit of doing it you may help to sharpen your shots. Reducing any vibration is good when you are trying to sharpen your shots.
  11. The Plane of focus is a very important concept – it is about where you are focussing. It gives you an idea of what your focus is doing at any angle of the camera. You should really have a good idea about it if you want to make sure you are getting a proper focus on the subject of your shot. More…
Lots more to learn…

Focus is big subject and can become extremely technical. There is a lot more you can learn. These points are just some fun ideas to help you learn about different aspects of focus in general. If you have any comments, other ideas or want me to write about another aspect of focus – leave a comment at the bottom of the page.

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By Damon Guy (author and Photokonnexion editor)

Damon Guy - Netkonnexion

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’.
See also: Profile on Google+.

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Measure Up For Focus – New Definition

If you don't know what this funny mark is on the side of a DSLR then read on...

If you don’t know what this funny mark is on the side of a DSLR then read on…

Definition: Focal Plane Mark

That funny symbol on the side of most DSLR cameras is called a Focal Plane Mark. We have put two new definitions into the Photography Glossary today which explain this mark and how to use it.

Definition: Focal Plane Mark

Definition: Plane of focus; Focal Plane; Film Plane; Image Plane;
 

By Damon Guy (author and Photokonnexion editor)

Damon Guy - Netkonnexion

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’.
See also: Profile on Google+.

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