If there are any experts out there who can provide an authoritative answer to the meaning of magnification as far as teleconverters are concerned, it would be greatly appreciated. There has been a controversy raging on another site for some time now concerning how magnification is determined and whether teleconverter manufacturers are exaggerating the performance of their products.

Basically, there are those who believe that 2X means that the area of an image is doubled in size (length and width are increased to about 1.4 X 1.4, the square root of 2), while others believe that 2X means that linear dimensions are doubled (area is 4 times as large). This discussion came about because many of the teleconverters being sold appear to increase the size by less than what would be expected if linear measurement is considered. That is, 2x converters seem to increase linear dimensions of images only about 50% rather than the expected 100%.

All the research that I have done indicates that linear size should be the proper measurement. One article stated that there are actually two measurements, linear magnification and angular magnification. Linear magnification applies to an optical system which produces a real image from a real object. Angular magnification applies to a system through which one views a virtual image. In either case, I believe that magnification would refer to the apparent increase in height and width of the subject rather than the area of the image.

United States Optical Technologies, Inc. says that Power, or magnification, means that an image is magnified X number of times the normal size. That is, 15X means that an object 1500 yards away would appear to be 100 yards away. If this is a proper definition, a simple drawing shows that the linear height and width of any portion of an of an object at 1500 yards brought to 100 yards would appear to be 15 times as great, and with an area 225 times as great.

Are some teleconverter manufacturers stretching the truth, or are they technically correct in that using focal length measurements allow them to state magnifications that are different than what the general public understands them to be (but which they dont explain in their literature)? An authoritative answer to this question of magnification would be appreciated by all.

Rodger

-- Rodger Carter (rodger.carter@wpafb.af.mil), June 10, 1999

Answers

I'm not sure what 'authority' would be suitable here, as opposed to common practice, or the practice adopted by individual manufacturers.

Linear or angular measurement should normally give the same number. For teleconverters, this is the same as the multiplier for the focal length. A 2X teleconverter will multiply the focal length by 2, and will double the liear or angular dimensions of the image.

Squaring this number will give a multiplier for the area, of course. I've never seen this number used, but there is no reason why it shouldn't be. The advertiser should be honest, of course (here in the UK there is a legal requirement for honesty), and shouldn't quote an area mignification but state (or imply) that this is a linear magnification.

An advertiser might use deliberately vague terms like "2X larger". In such cases, the buyer should beware.

-- Alan Gibson (Alan.Gibson@technologist.com), June 10, 1999.

Teleconverters are not in powers of magnification, but in the multiplication of the focal length.

So a 2x teleconverter makes a lens work like it is twice the focal length. A 50mm ends up as an effective 100mm, etc.

They also change the effective maximum aperature, but do not change the closest distance the lens will focus.

-- Terry Carraway (TCarraway@compuserve.com), June 16, 1999.

Sorry about this late response. But I thought your original message was deleted last week when I couldn't find it. Anyway, At the risk of rendering this thread useless, and killing one of the greatest optical debates of our time, let me pose a simple solution to complex minds... 1)Get a camera and a tripod. 2) Focus on a wall large enough to fill your viewfinder. 3) Using tape, or a pencil, mark each corner of the wall that corresponds to the corners you see in your viewfinder, so that you can connect the dots, forming a square. 4) Now, without moving the camera, attach the 2x teleconverter, and repeat the exercise.(You'll have a square within a square.) 5)With a little math(subract "area" from "area"), you will have your answer. Take this exercise back to whatever site this controversy has been "raging" on, and I can assure you that soon after, it will be a non issue. *Courtesy of your friends at photo.net*. Darren H.

-- Darren Harris (Searcher7@mail.con2.com), June 17, 1999.

It's linear magnification - which is the same as angular magnification. It is NOT area magnification.

I've tested a few TCs. The Canon EF 1.4x and 2x TCs give 1.4x and 2x linear magnification. The Tamron 1.4x gives 1.4x.

However, some manufacturers "stretch" a bit. The Kenko 1.5x is really 1.4x and the Tamron 2x is closer to 1.8x.

-- Bob Atkins (bobatkins@hotmail.com), June 17, 1999.