Lenses, a beginner tale....

In the good old days of film, SLR’s tended to come in two flavours. Body Only, and with a "Standard" Lens, usually around 50mm focal length. Any other lenses, zoom or prime, came separate.

Zoom and what? Prime, what’s that then?

Lenses can be basically placed into 2 groups, Prime lenses, and Zoom Lenses. A prime lens is a lens which cannot change its focal length, e.g. the 50mm f1.8 "standard" lens that came with the camera in those good ole days above, is a 50mm and will always be a 50, and nothing else.

Zoom lenses can change their focal length, e.g., the now much more common 18mm-55mm Zoom lenses that come as part of an SLR Kit these days. A Zoom lens like that can be an 18mm, 20mm, 24mm, 28mm, 35mm, 50mm, just by turning the grip.

As you can see, the zooms can cover the focal lengths set by the prime lenses, and more. In the last few years developments in lens design and glass quality means that most of the zoom lenses can at least match the quality of the prime lenses of not very long ago. The photographer on the left would have to carry 8 lenses to cover all those focal lengths, the one on the right would need to carry only 4. The quality and convenience means that even the most die hard prime lens users are now using Zooms.

MOST, but not all of the lenses that come in kits are not really the best you can buy. Some suffer from distortions, (vertical straight edges become bowed for example, due to barrel distortion, where a square would be photographed, but the final result is barrel shaped), horizontal lines suffering the same effect( pin-cushion distortion) Chromatic aberrations(colour fringing along edges).

The more expensive lenses, surprise, surprise, tend not to suffer from these problems, or at least suffer much less from them. Prime lenses hardly suffer from these problems at all by virtue of the fact that they are much simpler design, and tend to have fewer pieces of glass for the light to pass through..

There are also differenced between similar zooms and primes. The most obvious are, 50mm f1.8, 50mm f1.4, 85mm f1.8, 85mm f1.4. In the world of zooms there are, 24-70 f3.5-5.6, 24-70mm f2.8, 70-200mm f4, 70-200mmf2.8. These are called "slower and faster" lenses. The smaller the f number, (Aperture), the more light the lens lets in, which means you can shoot "faster".

Aperture? What's an Aperture?

No matter what type of lens you have, they all have basically the same engineering. You have a case, with grips and mounts. You have a collection of pieces of glass,(elements), you have a diaphragm. WAIT, I have a WHAT???

A diaphragm is a wonder of engineering. Several flat, weirdly shaped pieces of metal are placed around the outside of where the light travels along the inside of your lens. (where this placed is calculated to such a degree it’s quite frightening). The ring to which these pieces of metal are attached is rotated, and the blades move to form as close to a circle as they can, blocking the light. This ring gets smaller and smaller until it forms as close to a circle as it can, and still let light through, and can’t get much smaller.

The diaphragm is made so that for a certain amount of adjustment, it reduces the amount of light travelling through the lens by half. These are given values, and each one is called a stop, sometimes and f stop, these are apertures.

f stops (full and half, oh, and there are thirds and, uch, you get the idea…)

f1 (supposedly lets ALL the light seen through it pass though to the film/sensor)

f 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32, 45, 64, 90……. Reading from the left, each one is half as bright as the one before.

SO, if you have an 70-200 f4, and an 85mm f1.4, the latter lets in 8 times more light than the zoom.

So, when you look at the lens that came with your camera and it says 18-55 f4-5.6, and you fancy the latest 24-70mm f2.8, the new lens will let in 2-4 times MORE light.

When you stop both lenses down to 5.6 or 8 or 11, they should both be letting the same amount of light through.

Apertures (now we know that they're all the same) and Shutter Speeds ( also incrementing by halves, 1/1000th sec is half as long as 1/500th sec, is half as long as 1/250th. This is called the Reciprocity Law, if you half the light through the lens, and double the shutter speed, then the exposure should be the same. Simple, innt?

What can I see though the lens...

Simple rule of thumb, the smaller the number the more (wider) the view through the lens.
E.g.
A 20mm lens is considered a "Wide Angle" Lens.
A 300mm is a “Telephoto” Lens.

The wide angle lenses quite simply can “see” a wider angle, around 95 degrees. The telephoto can be said to act like a telescope and can only see 8 degrees, but the image is magnified by about 6 – 7 times. Another obvious difference is that the 20mm will be quite small, and fit in the palm of your hand. The 300mm will be at least about 8 inches long, and, depending on the design, quite heavy.

Digital cameras and focal length.

Hmmmmm. This is where it gets tricky.
If I put a 50mm f1.8 lens on my Canon Eos33, FILM camera, it’s a 50mm and that’s all there is to it.
If I put the same lens on my Eos 30D, it becomes an 80mm f1.8.
If I put it on the Eos 5D, it’s a 50mm f1.8 again.

How the?????

Lenses are made and focal lengths are calculated for a piece of film measuring 24x36mm. Thanks to Leica, who invented the 35mm format, in an attempt to put cheep film into people’s hands via the movie industry. The reconned there was so much movie film lying around, if they could adapt a camera to use it, it would save a fortune and nobody would need to use the 5”x4” half plate film of that era anymore.

SO...

Jump forward 80 or so years.

There are about 4 cameras on the market that have a 24x36mm sensor. THOSE cameras act like 35mm cameras, and all the focal lengths are as they say on the box.

NOW, of course we have smaller sensors too. Make no mistake, these smaller sensors don’t cost that much less than the full frame ones, once the production lines are up to speed, those cameras are there to create a market. SO, the camera manufacturers have made 2 markets out of 1, and indeed between them all there are 4 sizes of sensor.

MOST are about half size, so the chips are 24x18mm ish. Which means you have to multiply the focal length, by x1.3, x1.5, x1.6, and x2 in the case of 4:3 cameras?

Eh, WHY?

Simply because the image being projected by the lens onto the focal plane, but instead of something the size of a piece of film, there’s a sensor half its’ size in the middle, so, in effect, it sees only the middle half of the image and is interpolated up to create an image as if it was taken on a full sized sensor, similar in comparison to the 20mm v 300mm comparison earlier.

Compact cameras have lenses like 7mm f19 and other odd sizes. The same rules apply here. To work out the actual focal length you have to find out the size of the sensor, not always that easy. However, most these days will have a specification, and a line something like Lens= 38-125mm (35mm equivalent) written somewhere in there.

Same rules for medium format sensors. These are measuring 36x48mm and such sizes, but the focal lengths also have to be calculated as there are not many 6cmx6cm or 6cm x 4.5cm sensors out there.

Because of this magnification factor, smaller sensor cameras had trouble when they came to very wide angle photography. E.g. A 24mm lens, previously seen as a mid range wide angle, suddenly became a 38mm lens on a 1.6 crop camera, not very wide.

Hence the number of new “Ultra Wide” lenses which have appeared over the last few years. Lenses down to a focal length of 4mm are not that uncommon any more. HOWEVER, some of these lenses are for digital cameras only. That means they do not project an image that fits on 35mm film in the camera, the image produced is only slightly larger than the most common cropped sensor sizes.