Teleconverters, all over again.
Jun 26, 2015 12:13:35 #
One of the most common lens questions I get asked is "Why can't I use a TC with this lens?"
There is LOTS of mis-information on the internet about TC's, their compatibility, and their usefulness. I have had a discussion on my website about just this issue for a couple of years now and get a lot of positive comments about it, so I decided to post it here again. I say again as I posted it here when I originally wrote it but after two years the number of questions on this forum about TC's shows its clearly a subject that needs to be refreshed regularly. Here it is, and it can also be read on the Equipment page of my website listed below this post:
An IMPORTANT discussion about Teleconverters (Tele-extenders)
I have been inundated with questions recently about people wanting to use TCs (Teleconverters) on kit, or slow, lenses. I feel this is a large enough issue to be addressed directly.
TCs are designed to magnify the image projected by the lens, they do so by extending the lens itself a set distance further out from the sensor depending on the magnification power of the TC (1.4X, 1.7X or 2X magnification, each one is longer than the one before it). The nature of these TCs causes them to cost the user a certain amount of light in the process. A 1.4X will cost you one full stop of light (100mm F2.8 lens becomes a 140mm F4 lens). A 2X will cost you two full stops of light (100mm F2.8 lens becomes a 200mm F5.6 lens). These numbers do NOT represent a "crop factor" but are a true magnification of the lens in use.
Most modern DSLR cameras will not Autofocus a lens with a maximum aperture of smaller than F6.3. SOME Pro and Semi-Pro models will Autofocus up to Maximum aperture of F8 under very particular circumstances. This leads to loss of autofocus function when a slower lens is used with a TC. A typical 18-55mm F3.5-5.6 kit lens when used with a 2X TC becomes a 36-110mm F7-11 lens, and will not autofocus at all.
Now for the DANGEROUS part. Almost all "kit" lenses, as well as most longer VARIABLE APERTURE zooms, have a rear optical element that actually extends back beyond the lens mounting plane and into the body of the camera when zoomed back to its widest focal length. This causes an "interference fit" situation. In short, that means that the rear element of the lens will actually crash into the optical element of the TC and damage either the lens, the TC, or in a worse case scenario, BOTH.
As a general rule DO NOT use any TC an any lens with a variable aperture (F3.5-5.6 or similar) as these lenses almost all give you an interference fit condition. Fixed aperture lenses almost always accept a TC, and if they are "fast" enough, autofocus will continue to function. An F4 lens will AF with a 1.4X TC, but seldom with a 1.7X or 2X model. An F2.8 (or faster) lens will almost always retain AF with any TC on any camera. Fast lenses are the only ones designed by the manufacturers to work fully with TCs and each and every lens manufacturer will have a full listing available that designates TC compatibility with their lenses.
I pretty much only recommend using a manufactures TC on their lenses as the use of third party TCs can be a gamble. Some third party TCs may mount on "interference" type lenses and not have interference issues, but you will still usually lose AF as well as the ability to effectively focus in lower light situations because of the loss of stops the TC causes. The old split prism viewfinders found on film cameras makes this function easier, but the flat screen viewfinders typically used in DSLR cameras makes manual focusing more difficult, and nearly impossible in low light.
Good luck to anyone using TCs, they can be a useful tool on the right lenses, but can cost you a small fortune if used on the wrong lenses, and offer very little potential benefit.
There is LOTS of mis-information on the internet about TC's, their compatibility, and their usefulness. I have had a discussion on my website about just this issue for a couple of years now and get a lot of positive comments about it, so I decided to post it here again. I say again as I posted it here when I originally wrote it but after two years the number of questions on this forum about TC's shows its clearly a subject that needs to be refreshed regularly. Here it is, and it can also be read on the Equipment page of my website listed below this post:
An IMPORTANT discussion about Teleconverters (Tele-extenders)
I have been inundated with questions recently about people wanting to use TCs (Teleconverters) on kit, or slow, lenses. I feel this is a large enough issue to be addressed directly.
TCs are designed to magnify the image projected by the lens, they do so by extending the lens itself a set distance further out from the sensor depending on the magnification power of the TC (1.4X, 1.7X or 2X magnification, each one is longer than the one before it). The nature of these TCs causes them to cost the user a certain amount of light in the process. A 1.4X will cost you one full stop of light (100mm F2.8 lens becomes a 140mm F4 lens). A 2X will cost you two full stops of light (100mm F2.8 lens becomes a 200mm F5.6 lens). These numbers do NOT represent a "crop factor" but are a true magnification of the lens in use.
Most modern DSLR cameras will not Autofocus a lens with a maximum aperture of smaller than F6.3. SOME Pro and Semi-Pro models will Autofocus up to Maximum aperture of F8 under very particular circumstances. This leads to loss of autofocus function when a slower lens is used with a TC. A typical 18-55mm F3.5-5.6 kit lens when used with a 2X TC becomes a 36-110mm F7-11 lens, and will not autofocus at all.
Now for the DANGEROUS part. Almost all "kit" lenses, as well as most longer VARIABLE APERTURE zooms, have a rear optical element that actually extends back beyond the lens mounting plane and into the body of the camera when zoomed back to its widest focal length. This causes an "interference fit" situation. In short, that means that the rear element of the lens will actually crash into the optical element of the TC and damage either the lens, the TC, or in a worse case scenario, BOTH.
As a general rule DO NOT use any TC an any lens with a variable aperture (F3.5-5.6 or similar) as these lenses almost all give you an interference fit condition. Fixed aperture lenses almost always accept a TC, and if they are "fast" enough, autofocus will continue to function. An F4 lens will AF with a 1.4X TC, but seldom with a 1.7X or 2X model. An F2.8 (or faster) lens will almost always retain AF with any TC on any camera. Fast lenses are the only ones designed by the manufacturers to work fully with TCs and each and every lens manufacturer will have a full listing available that designates TC compatibility with their lenses.
I pretty much only recommend using a manufactures TC on their lenses as the use of third party TCs can be a gamble. Some third party TCs may mount on "interference" type lenses and not have interference issues, but you will still usually lose AF as well as the ability to effectively focus in lower light situations because of the loss of stops the TC causes. The old split prism viewfinders found on film cameras makes this function easier, but the flat screen viewfinders typically used in DSLR cameras makes manual focusing more difficult, and nearly impossible in low light.
Good luck to anyone using TCs, they can be a useful tool on the right lenses, but can cost you a small fortune if used on the wrong lenses, and offer very little potential benefit.
Jun 26, 2015 12:24:47 #
Jun 26, 2015 14:24:53 #
MT Shooter wrote:
One of the most common lens questions I get asked ... (show quote)
Here is a link to NikonUSA's compatibility chart for lenses, teleconverters, and cameras. This is for Nikon only.
http://www.nikonusa.com/en_INC/IMG/Assets/Common-Assets/Images/Teleconverter-Compatibility/EN_Comp_chart.html
Jun 26, 2015 16:47:02 #
Jun 27, 2015 13:00:21 #
Nice write up MT ! - Here - http://www.dyxum.com/lenses/teleconverters.asp - is some useful data on TC's. Most of it pertains to ALL make cameras not just Minolta/Sony.
Jun 28, 2015 18:12:09 #
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