How to choose the objective you need

Written by 杨 道波 Thursday, 28 October 2010 15:34

When asked what is the best car, I bet you can not answer this question. If you want to give a sensible answer, you need to ask further: What budget? Spacious or compact? For the city or the road? Sports or family? Designed for professional use? ...

Many people unfamiliar with the picture facing a similar question: What lens to choose for my camera?

Again, it is not possible to answer without knowing more about the use we want to make this goal: Photo-fun? Set memories of football matches of the youngest? Photographing my stamp collection? Record memories of my hiking in the mountains? What budget?

Convinced that the answer lies on the internet, it embarks on the research and found! Yes, but ... we finally found this.

But in all this, we can not find the answers to questions that arise.

It is faced with a jumble of numbers cabalistic units and esoteric symbols.

In short, the information collected may be relevant but it remains to decipher and translate this obscure language in terms of needs.

That is why I propose, unpretentious, help prospective buyers to navigate a bit better in the descriptions. And at the same time, (re) see some of the terminology used in the language of Shakespeare (requires internet!). When I quote William, I'll do it in italics (the "anglique" does not exist).

The pros forgive me, I will not go very sophisticated concepts, my goal (sic) being only a little thinning down the field. I am also unable to venture into discussions pointed, being myself an amateur who has struggled before I end up in the jungle of figures, fractions and units of measurement.


So you've bought a camera with detachable lens, a "reflex."

If you want to buy a lens (lens, plural lenses) further, the first thing to ensure is that you can attach your device. A target for Nikon will not be attached to a Canon camera and vice versa. The goal that you will buy must have a mount (mount) your expected housing. This does not mean that the objectives of a brand are the only ones that can be attached to a device from the same brand. There are manufacturers who produce objectives for multiple brands of devices. So check first that the mount is compatible with your device.

Example: In the instructions of a manufacturer, we find

This means that the manufacturer offers the same goal with different frames by device (SIGMA, CANON, NIKON (D), SONY, MINOLTA PENTAX, or).

Also note in passing the acronym AF (Auto Focus) or autofocus. If the lens is designed to allow autofocus, it must be able to "communicate" with the housing by means of electrical contacts, as illustrated by example below. Therefore make sure that this is the case.


You will always, among the characteristics of an objective, one or two numbers expressed in millimeters, such as 50 mm, 28-50 mm, 55-200 mm ...

These numbers have nothing to do with the physical dimensions (length, diameter) of the target. They describe what is called the focal length of the lens (focal length or focus length).

The focal length determines how the lens enlarges or reduces the image when taking a photo.

If only one number is specified, the length is fixed (fixed focus length) and the magnification (magnifying factor) is fixed too.

If two numbers are specified, the variable focus lens is (length variable focus or zoom). The smaller of two numbers is the smallest focal length, the greater the greater length.

The objective can then choose any focal length between the smallest and largest.

To determine how the objective magnifies or reduces the resulting image, we must first know the magnification - or-magnification factor of your camera. You'll find it in the following list:

Coefficient = 2.0 for Olympus and Panasonic
1.7 Sigma
1.6 for Canon (except 1D, 5D and 1D)
1.5 for Pentax, Samsung, Sony, Minolta, Nikon and Fuji
1.3 for Canon 1D

Then check this table



You're the proud owner of a Nikon. You saw on a website a 24-105 mm lens priced into your budget.

The list shows you the coefficient of expansion for Nikon is 1.5.

In the table, so you see the column "Factor: 1.5" and you see that 24 mm is reduced to 0.72 (about 3 / 4 the size of the normal image) and 105 mm at a magnification 3.36 (slightly more than 3 times the normal size image).

To better visualize this, go "play" a bit on the simulator focal TAMRON at
Focal length comparison tool, Tamron USA. By moving the small blue cursor at the bottom of the picture, you'll see the effect of the zoom function of the length.
See also simulator focal SIGMA (French): 20image.swf
For pictures taken with a particular brand and target for focal data, see full-size sample photos from lenses.

In the table, all sizes are proportional to the zoom range. If you want to know how such an expansion of 450 mm focal length, multiply the result by 3 to 150 mm. For a focal length of 12 mm, divide by 2 the result for 24 mm.

To find out what (s) length (s) correspond (s) your needs, try to determine what kind of pictures you'll take most often. In general, a fixed lens with a magnification close to 1 or zoom with a magnification / reduction of 0.7 to 3 covers most needs. If you want to shoot long range (animals, sports), you will need a longer focal length. If you make more pictures of large groups (architecture, great monuments, panoramas), opt for shorter focal lengths. For portraits, a magnification factor of about 1.5 to 3 you will not get too close to the subject.

Of course, one might say that a 12-300 mm zoom lens covers all the needs. But you know that every coin has another side. First, the price of such a goal will be high (about 600 to 800 U.S. dollars - prices in January 2008). Then, a goal may not have the same optical qualities for all focal lengths. You'll no doubt a loss of quality or brightness in the extreme focal lengths.

You should also know that the focal length has an influence on the background of your photo. With long focal lengths, the subject and the background will tend to coalesce into a single plane. With short focal lengths, the subject seem less "stuck" in the background.

F / F 2.8 or / 3.5?

The figures given by F / X (sometimes f / x or 1: x) express the amount of light passing through the lens. This is the diaphragm (diaphragm) or opening (aperture). Diaphragms that you find most often in the notices or advertisements are 1.4, 2.8, 3.5, 5.6, 6.3, 8, 11, 16, 22 and 32. To simplify, we can say that the diaphragm is the size of a hole through which light passes. Over the hole is small, less light will pass.

Unlike defining mm focal length, the numbers are inversely proportional to the amount of the amount of light passing through the lens. To continue the comparison with the hole over the hole diameter is large (and it lets light) plus the number that characterizes it is small.

A small picture is worth a thousand words, here is an illustration of what is a diaphragm (in the illustration, the diaphragm is represented by the white circle on a blue background).

You will say that I tire you with English authors, but you may remember that in "Alice in Wonderland" a character celebrating her non-birthday. Well, the number that characterizes the diaphragm corresponds to its "non-luminosity." The higher the number increases, the objective is "not bright". Well, I stop is my comparison.

To summarize:

- A large number indicates a smaller amount of light that can pass through the target and corresponds to a small diaphragm

- A small number indicates a greater amount of light that can pass through the lens and is a large diaphragm

You'll understand the goal in most cases to have a goal as light as possible, it should opt for an F / x where x is the value of the smallest possible (more like F / 2.8 that F / 4.0 for example).

Once again, the coin has two sides and the cost of a lens with a diaphragm can be opened up to F / 1.4 is greater than that of an open goal at F / 4.

To illustrate this and see the implications on the kind of photos you want to do.

You will find in catalogs or ads mentions combining focal and diaphragms, such as:

- 55-200 mm F / 2.8: this means that all the diaphragm focal largest is 2.8

- 18-55 mm F / 3.5 - 5.6: this means that larger apertures are respectively 3.5 for the focal length of 18 mm and 5.6 for the focal length of 55 mm

In general, a long focal length means a smaller aperture.

Reproduction ratio or maximum magnification ratio
(Maximum magnification ratio)

No, it's not about demographics or diet! There are two numbers indicating how far the objective lends itself to macro or close to the picture. If you do not get these kinds of photos, will expect the following paragraphs.

When you make a picture, the image of the object photographed is formed on the sensor of your digital camera. If you want to make a macro, it is necessary that the image of your subject (insects, water drop, stamp, ...) on the sensor is as large as possible and contain as much detail.

The reproduction ratio gives an indication about this. It is expressed as 1: 1 for example. You will find most often in records of the manufacturers, more rarely in ads. It expresses the ratio between the size of the object and the size of its image on the sensor.

It can be likened to a fraction. Thus, 1: 2 means that the size of the image on the sensor is equal to max 1 / 2 times the size of the object. 2: 1 means that the size of the image on the sensor is equal to a maximum 2 / 1 or 2 times the size of the object.

In the illustrations below, if your sensor is represented by the rectangle is what would the reproduction ratios.

It is clear that your picture is more detailed with a 1: 1 with a 1: 2. And more with a ratio of 2: 1 with a 1: 1.

It is generally considered that the Macro is located between the ratios 1: 1 and 10: 1. From 10: 1 it says "microphotography. Below 1: 1, the notion of "proxiphotographie.

Forums packed with information about it. Use the search function to the home page and enter the keyword "macro" or "proximity" to learn more.

Note also that manufacturers have the tendency to quickly qualify a target of "macro" and writes that word on the lens body. This information is not always reliable. If we stick strictly to the definition, to be called "macro" goal is to present a reproduction ratio of 1: 1 at least.


The minimum distance of focus (minimum Focusing length) is expressed in cm and indicates how far one can approach the subject without the image is blurred. It is a parameter to take into account if we want to engage in close-up photography.

It is possible to approach at a distance less than indicated, but it will have to use props and implement other techniques that are beyond the scope of this article.

Still mm

Another characteristic of a lens is the diameter of the screw thread at its front end. This setting is useful if you want to buy a sun visor or put filters on the lens. The diameter is expressed in mm and is represented by "O" or by the words "diameter filter (filter diameter).


The weight (weigth) of a target can also influence your choice. If you enjoy hiking or trekking, it is perhaps better to buy a lens 'zoom' which covers a focal range of data rather than two fixed lenses. You will and a goal to wear.

"Good Vibrations"

If you plan to use your frequent target in difficult conditions (picture taken from afar with a long lens, macrophotos, photos taken at night or in low light, ...) a reduction of vibration can be useful.

There are various names to describe such a reducer: VR for Vibration Reduction, VC Vibration Compensation, OS Optical Stabilizer, ...

It is very difficult to summarize the various possibilities in this field, since for some manufacturers this feature is included in the camera body, while in others it is foreseen in the objectives.

At first, it can happen to such a function. There are alternatives to stabilize an image, like using a tripod, mini tripod, or simply support your camera on a stable element (tree, fence, table, wall, column, ...).

For an interpretation of the various acronyms used by manufacturers, take a look at
Glossary of Acronyms - Talking Photo.


Good shots!


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Last Updated on Thursday, 28 October 2010 16:04