What is Teleconverter

What Is A Teleconverter? #
This is an optical device which includes optics and ways of transmission of electrical signals. The teleconverter is mounted between the camera and lens in order to extend the focal distance of the ensemble. Longer focal distance leads to a higher magnification ratio of the ensemble (the magnification ratio being the ratio between the size of the image on focal plane of lens and the size of the real image). "1,4x", "2x" in the name of the teleconverter is actually the multiplication factor of the focal distance. Thus, mounting a 1,4x teleconverter on a lens with focal distance f, result in an optical composed lens with focal distance fc = 1,4 x f. A lens with f = 200mm becomes a composed lens with fc = 280mm, and the ensamble will show a higher magnification ratio compared to the original lens alone.

Canon and Nikon offer teleconverters like 1,4x, 1,7x and 2x; other manufacturers offer also 3x power teleconverters.

Teleconverters are used only on some fix focals and zooms recommended by the manufacturer of the lens or of the teleconverter. For example Canon recommends EF 1,4x II only with fix focal lenses with f ≥ 135mm or with following zoom lenses: EF 70-200 f/2,8L, 70-200 f/2,8L IS, 70-200 f/4,0L, 70-200 f/4,0L IS USM, and 100-400 f/4,5-5,6L IS.

The ensemble lens / teleconverter will have a lower maximum aperture compared to the lens alone, by a factor given by the multiplication factor of the teleconverter. For example a lens with maximum aperture f/4 combined with a 1,4x teleconverter will result in f/5,6 maximum aperture system. The same lens with a 2x teleconverter will result in a system with f/8 maximum aperture. The more power the teleconverter has, lower quantity of light goes to the sensor / film of the camera.




Advantages Of Using Teleconverters
A brand name teleconverter is not cheap, but it costs much less than a lens. An ensemble lens - teleconverter will result in more focal lengths available and the ensemble will cost less and will be much slimer and lighter than 2 eventual tele lenses whith similar focal distances as the ensemble.

Using an ensemble lens - teleconverter you will get lower DOF. Yes, you read correct! Even if the maximum aperture of the ensemble is lower than the maximum aperture of the lens alone! Why this? Because the magnification ratio of the ensemble is higher and this has a biger influence on DOF than loss of maximum aperture. Of course, you get a biger image on the sensor / film.


Example:

● camera Canon 40D, sensor APS-C, crop factor 1,6

● lens EF 300 f/4L IS USM (f = 300mm)

● teleconverter EF 1,4x II (factor 1,4)

● ensemble lens - teleconverter will feature a maximum aperture f/5,6 and fc = 300 x 1,4 = 420mm

● s = distance to subject, say s = 15m

● lens alone at maximum aperture f/4 will give a calculated DOF of 37,2 cm

● ensemble lens - teleconverter at maximum apertuer f/5,6 will give a calculated DOF of 20,1 cm
Another advantage is that the minimum focusing distance of the ensemble is the same as the minimum focusing distance of the lens alone. Thus, at minimum focusing distance the ensemble can be used on macro work. The magnification ratio is not splendid, usualy below 0,5x. But it can help sometimes. Notice that for macrophotography magnification factor higher than 0,5x is required.




Disadvantages Of Using Teleconverters
A major disadvantage is that the ensemble lens - teleconverter will have a smaller maximum aperture compared to the lens alone. A 1,4x teleconverter on a f/2,8 maximum aperture lens will result in a system with maximum aperture of f/4. A 2x teleconverter on a f/2,8 maximum aperture lens will result in a system with maximum aperture of f/5,6. There are consequemces. You might need longer shuttrer speed or higer ISO to take the shot. A lens with optical image stabilisation might allow you to use longer shutter speeds and modern DSLRs have good image quality even with ISO400-800. So not always a seriou problem. Unless you need to shoot a fast moving subject in low light, when image stabilisation is useless (subject moves fast) and high ISO might induce image noise (in low light).

AF sensors in (D)SLRs need a certain amount of light to function quick and precise. The AF snsors do their job when the shutter release button of the camera is half depressed and the mirror is still down. AF sensor reach as much light as the lense, with complete open aperture, allows to get to them. Most cameras, especially the "low-end" ones, need at least the light amount equivalent to what a f/5,6 maximum aperture lens allows. If you will mount a 1,4x teleconvertor on a lens with f/5,6 maximum aperture then the ensaemble will have f/8 maximum aperture and the AF sensors will not have enogh light to function correctly. You will have only the option of manual focusing, but remember, the viewer image will be darker than usual, making manual focusing more difficult.

Finally, a teleconverter adds more "glass" between subject and sensor / film. This degrades image parameters - contrast and resolution and sometimes adds chromatic aberations. A more powerful teleconverter will degrade image parameters more than a less powerful one. Brand name teleconverters alter image parameters by 5-10%. "No-name" teleconverters may induce more alteration of image quality parameters.




How To Choose A Teleconverter?
It is important to consult the technical documentation of the teleconverter on the internet page of the manufacturer. Manufacturer will list the lenses which are compatible with each teleconverter.

Do not expect miracles from a system where a teleconverter is mounted on a lens with maximum aperure f/5,6 or higher. In best case you will end up to a system which focuses only in manual mode, and you will need to adjust focus in a dark viewer.

EF 1,4x II can be hired by the participants to fpix tours- see our section Rental Service For Photographic Equipment. You may also rent Canon lenses EF 300 f/4L IS USM and EF 70-200 f/4L IS USM, both lenses being compatible with EF1,4x II.

(info from www.fpix.eu)

wow, very informative...

Thanks for the information max... It somehow sounds similiar to my el-cheapo extension tube... just that mine dun hv any glasses in between... Hehehe....

heheh nvm , i post a thread for you about "what is extension tube" hehehhee