mborn Loc: Massachusetts

Right on

jerryc41 Loc: Catskill Mts of NY

"The teleconverter reduces the maximum aperture of the lens by one stop (1.4x converter), 1.5 stops (1.7x converter) or 2 stops (2x converter). An f/4 lens becomes an f/5.6 lens with a 1.4x mounted. An f/5.6 lens becomes f/8."

Good article -
https://www.lensrentals.com/blog/2009/01/teleconverters-101/

ole sarg Loc: south florida

Why does it matter? You are looking through the lens!

John_F Loc: Minneapolis, MN

The focal length of a lens is function of the array of spherical surfaces and their radius of curvatures and the spacing of the centers of radii. There are strict mathematical formulae derived in physics of optics books. Those radii of curvature and spacing of the centers will result a number of lens cardinal points: two principal planes and two focal planes. Distances from subject are measured from the front principal plane and distance to image is measured frim the rear principal plane, if you choose to use the lens formula - 1/s + 1/s' = 1/f. If you choose to meaure from the focal planes, then the lens formula is xx' = f^2. The connector between the two is: x + s = f. If the lens surfaces are not perfectly spherical and the centers of curvature are not perfectly co-linear, then you have a cheapie lens and the formulae inexact.

The aperture is just a set of adjustable leaves - set at a very particular location in the lens. It can have no effect whatsoever on focal length. The f-stop is just a measure of aperture diameter relative to focal length: f-stop = f/D.

Rongnongno Loc: FL

You just forgot to mention the 'side effect' which is adjusting the depth of field...

wingclui44 Loc: CT USA

It does in some of them. Like my Kenko Teleplus Pro 300 DGX 1.4xAF tel-converter.

whitewolfowner

To answer your question. The common teleconverters out there are 1.4, 1.7 and 2.0. The reason is simple because the 1.4 takes away 1 f stop (f2.8 is now f4.0...f5.6 is now f8.0), a 1.7 teleconverter adds 1.5 stops to the lens and and a 2.0 adds two stops to the lens. So no matter what f stop you go on the lens, those factors apply. Many cameras will tell you the accurate f stop in the viewfinder. For the new mm range of the lens multiply the lens mm by the number of the teleconverter. So a 1.4 teleconverter will make a 50mm lens a 70mm lens and a 2.0 will make it a 100mm lens. If you have a zoom, multiply the zoom range at the low and high ends to find the new values; an 80-200mm f2.8 with a 1.4 teleconverter will become a 112 - 280mm f4.0 lens. A 2.0 teleconverter will turn that 80-200 f2.8 into a 160-400m f5.6 lens. Keep in mind that a good 1.4 teleconverter will have very little loss and used with a pro lens will produce professional results. As you increase the teleconveter power, the quality goes down. The results from teleconverters very greatly with lenses used too form lens to lens so do your research before plunging or you could end in your own pew all alone (from your GAS).

jackpi Loc: Southwest Ohio

I agree.

It is also important to be aware of the downsides of teleconverters. At f8, your camera may or may not autofocus and, if it does, it may do so only with the central autofocus point (the Nikon D5 and D500 will have a few more central AF points available). It will also be difficult to autofocus with a long focal length lens handheld, because the combination of narrow depth of field combined with slight movement of the lens will cause the lens to continuously hunt for focus. Also note that any teleconverter will cause a 15-25% loss of sharpness on a zoom lens..

If you use a 1.7 or 2.0 TC on anything but a non-zoom lens with a focal length of f4 or less, you will need to manual focus with a tripod.

amfoto1 Loc: San Jose, Calif. USA

Teleconverters limit the light passing through them. While you gain focal length, you "lose" effective aperture.

First we need to correct your math. No, that 1.5X doesn't just "add 150mm to the 200-500mm Nikkor". Actually, a 1.5X on a 200-500mm zoom lens makes for an effective 300-750mm combo. (200 x 1.5 = 300.... 500 x 1.5 = 750)

But it also changes the effective aperture of the lens, making it smaller...

A more common 1.4X or less common 1.5X teleconverter "costs" one stop of light (an f5.6 lens becomes an f8 lens).

A 2X teleconverter "costs" two stops worth of light (i.e., an f5.6 lens becomes an f11).

1.7X teleconverters are less common, but cost 1.5 stops of light (so an f5.6 lens would become about f9).

This is important particularly because it will effect your camera's ability to autofocus. Most cameras require f8 or better to be able to auto focus in most situations. Go beyond that very much and your AF system will start to struggle and hunt. At some point and under certain lighting conditions it might fail completely.

If using a full frame (FX) camera, one way to get "more reach" without any "light loss penalty" is to get a crop sensor (DX) camera!

If already using a DX camera, well 500mm is an EXTREMELY long focal length on it and maybe you just need to get closer to your subjects!

John_F Loc: Minneapolis, MN

I did not mention DOF as that is an artifact that arises only because the human eye has a limit of resolution which anounts to one arcminute of angle. When the rear lens principal plane is at a specific distance from the sensor plane, there will be one and only one distance from the front principal plane to the in-focus subject. The rest if the so-called DOF is 'tolerably' in focus.

MtnMan Loc: ID

The aperture is the hole in the lens the light goes through. It does not change by putting the lens on a different camera with a different size sensor.

But the field of view changes if you put the lens on a camera with a smaller sensor. Thus the use of the term "crop factor" and using it to calculate an "equivalent mm" for the lens with a smaller sensor.

The smaller sensor also yields a larger relative depth of field. You can approximate that by multiplying the f-stop times the crop factor. So a DX cameta at f8 will have a depth of field equivalent to an FX camera at f12 with the same lens and zoom setting.

Rongnongno Loc: FL

So irrelevant and of no importance????

Come on now, you are better than that.

PHRubin Loc: Nashville TN USA

Really? You don't know what he means? I thought we all agreed to assume the word "effective" or "equivalent in angle of view"!

Jackdoor Loc: Huddersfield, Yorkshire.

Agreed that DOF is in effect the range that is tolerably in focus, but that is a feature of the sensory apparatus, and its 'circle of confusion'. Since we're talking cameras and lenses, that means the camera's sensor. Absolutely nothing to do with the human eye, unless discussing direct vision devices such as telescopes, binoculars and microscopes, although then things are confused by the eyes' own focusing abilities.