A mixed bag of lens knowledge – lens quiz answers

A range of Lenses - get the lens quiz answers here

A range of Lenses • On Photokonnexion we give away knowledge – Today we give the lens quiz answers.

Answers to the Lens Quiz…

The lenses quiz a few days ago supplied links to interesting resources on Photokonnexion. Here is all that knowledge in one post. Check your answers. Enrich your knowledge! We are generous with facts!

Lets get started!

Lens quiz question 1. Why do you use a lens hood?
A lens hood helps to prevent light from entering the lens at a sharp side-angle which is likely to cause lens flare. An occurrence of flare can create ugly artefacts on the image.

Lens quiz question 2. Name the phenomenon that causes light to bend when it hits a lens surface?
When light passes through a surface into a new medium it is refracted. The phenomenon that causes light to bend on entering a new medium is refraction.

Lens quiz question 3. How does the optical path differ from the optical axis?
The optical axis is an imaginary axis of rotation for the symmetry of the lens. A perfectly round lens can be rotated about the axis and not change its image in any way.
The optical path is the path light follows through the camera.
The optical path coincides with the optical axis through the lens. On entering the DSLR camera body, if the mirror is down, the light is directed up to the pentaprism and reflected through the viewfinder, into the eye. It is in this case the path of the light differs to the optical axis.
If the mirror is up light coming through the lens will strike the surface of the sensor. This path also coincides with the optical axis.
The optical path stops at the sensor. The axis of rotation theoretically passes through the lens and the sensor and out of the back of the camera.

Lens quiz question 4. What is another name for an individual glass lens inside a photographic lens?
Each single glass optical lens in a photographic lens is an “element”. In any one photographic lens there may be any number of lenses, some forming groups.

Lens quiz question 5. Is the focal length of an optical lens the same as the focal length of a photographic lens?
Focal length measures the power of an individual optical lens. It measures how much the lens bends light. The point where bent light intersects with the optical axis of the lens is the focal point (in the case of a convex lens). The distance from the focal point to the centre of the lens is the focal length of a lens (measured in millimetres).
A photographic lens uses a number of elements to create an image. The focal length is measured from the image. The centre of the photographic lens is deemed to be the centre of the array of elements that make up the lens. So, the focal length is measured from the sensor to the centre of the elements that make up the photographic lens.

Lens quiz question 6. How many lenses in a lens group?
A photographic lens can have as many elements and element groups as appropriate to achieving the designed purpose of the lens. The design dictates the number of lenses and lens groups required.

•  Theoretical Lens diagram • Theoretical lens layout showing elements and groups.

Click image to view large
Generalised photographic lens layout showing showing the principle features.

Lens quiz question 7. Describe what “chromatic aberration” looks like in a picture.
Most often a chromatic aberration is a slight rimming of colour(s) around brightly coloured objects. Sometimes it is barely visible.

Lens quiz question 8. What is projected onto the sensor plane from the lens?
The image. The sensor plane is a flat plane which is the surface of the sensor. The image from the photographic lens is projected there. The light energy hitting the sensor is converted to electrical signals and stored as an image file.

Lens quiz question 9. What is the diaphragm in a photographic lens?
Somewhere within the body of a photographic lens is an internal “wall” which crosses the path of the light. It has a hole in the centre. This allows light to pass through to the camera from the lens. This wall is called the diaphragm. It also supports the iris that creates the variable sized gap called the aperture.

Lens quiz question 10. Can the focal length be greater than the measured length of the lens body?
Very long focal lengths make very long lenses. Some would be unwieldy to use. Many consumer DSLRs have 200mm focal length lenses but the whole lens often measures less than 200mm. This is because the combination of certain elements creates an “effective” focal length. We refer to elements mimicking a true focal length as “telephoto” elements and they create a class of “telephoto lenses”. Telephoto elements do lower optical quality. Modern engineering makes the images acceptable.

Lens quiz question 11. What would you normally use a macro lens to do?
To take photographs of very small things. The macro lens projects an image onto the sensor at a ratio close to 1 to 1. This is normally printed at a much larger size therefore giving the impression that there has been a magnification of the object. In fact macro lenses rarely actually magnify. The correct name for a lens that enlarges beyond 1 to 1 is a photomicrographic lens.

Lens quiz question 12. Does a wide aperture have a low ‘f’ number or a high ‘f’ number?
A wide aperture is indicated by a low ‘f’ number. As the ‘f’ number goes up the aperture gets smaller.

Lens quiz question 13. Is the shutter in the lens or the camera?
It depends on the camera. DSLRs normally have the shutter right next to the sensor, in the camera body. The cost of putting shutters in all interchangeable lenses would increase the price, weight and size of lenses.
However, shutters are slow physical mechanisms. So in fixed-lens cameras (bridge cameras and point-and-shoot cameras) the shutter is in the lens. It is smaller and therefore faster, and is cheap since you only buy it once.
Medium and large format cameras have large digital ‘backs’. Shutters would be very slow for such large sizes. A slow shutter makes many types of exposures impossible – e.g. action shots. So the shutter is in the lens where the light beam is thinnest.

Lens quiz question 14. Which is the fastest lens an f1.2, f5.6 or f4.0?
A fast lens is one that lets lots of light in allowing short shutter opening. The fastest lenses have the widest apertures. A low ‘f’ number indicates a wide aperture. So F1.2 would let the most light in. Thus, the f1.2 would be the fastest lens.

Lens quiz question 15. What controls the depth of field?
A lens can only resolve light to a sharp image at one point. Sharpness drops off either side of that point. The depth of field is the zone of acceptable sharpness (to the eye) either side of the sharpness point. However, as a lens uses a wider and wider aperture its control of sharpness deteriorates making the zone of acceptable sharpness shallower.
You get a deeper depth of field with a small aperture. But small apertures restrict the incoming light requiring slower shutter speeds to get the same exposure.

It’s what you learned in the lens quiz that counts

I know you got some of these right if you have been reading Photokonnexion for a while. If you got them all right then well done. Even more well done to those who read around my links and learned new things. As in most things in life it is not what you know that takes you forward, it is what you are prepared to learn that counts.

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Damon Guy - Netkonnexion

Damon Guy (Netkonnexion)

Damon is a writer-photographer 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 photogs.
See also: Editors ‘Bio’.
By Damon Guy see his profile on Google+.

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