Increased image circle with teleconverter?

<p>Hi,<br>

A very silly thought occurred to me: If you put a teleconverter on the back of a 35mm lens. does its image circle increase? Will this combination then be useful on a medium format back with or without further extension (would extension be required in any case to focus on a near object?). What about just a 35mm lens with extension... or would the barrel of the extension tube cause vignetting (that could be got around by using a bigger dia bellows or cardboard tube, perhaps!). Does this work with APS-C /DX format lenses on FX/full-frame 35mm cameras for instance?<br>

The reverse combination, putting a medium format lens on a smaller camera, is of course doable (extension would be required in any case?)<br>

Just an idle weekend thought....</p>

<p> </p>

<p>Unless I'm misunderstanding something, the image circle is inversely related to the focal length of a lens, not directly related.</p>

<p>I seem to remember diagrams of teleconverters showing that they basically spread out the light rays, as they are 'negative' elements... so the central part is captured by the sensor or film, but the outer parts are wasted... if you put your film a little farther away, you should be able to capture a bigger image size... does this make sense? </p>

It does make sense, Dilip.<br>Except the bit about putting the film farther away: the film has to stay put, else focus changes.<br>Whether you indeed get a bigger usable image circle depends on how much physical vignetting the converter itself produces. Something to spend an idle weekend on testing... ;-)

<p>Ah, I see the point. I was focusing (so to speak) on focal length, not teleconverters.</p>

<p>You need to take a wide view on these matters :)</p>

<p>But seriously, I think it will work on near objects (macro), with some image degradation on the periphery (obviously). A cheap man's medium format lens...</p>

<blockquote>"If you put a teleconverter on the back of a 35mm lens. does its image circle increase?"</blockquote>

<p>Short answer - No!<br>

Long answer: The useable image circle may well decrease, because the combined optical system will almost certainly have worse lateral colour aberration and curvature of field, as well as overall poorer definition and vignetting.</p>

<p>Contrary to popular opinion a teleconverter doesn't just magnify the image circle. What a teleconverter does is to turn a normal lens into a telephoto one - i.e. an optical system where the rear node of a lens is forward of its physical distance from the focal plane. Or to turn an already telephoto lens into one with an even greater telephoto ratio, and having a large telephoto ratio isn't compatible with having a wide coverage circle.</p>

<p>Short answer - Yes!<br>

<br />I just tried it. I put my 18-55 DX lens on my F3, and found that it didn't cover the full image in the viewfinder (which has 100% coverage) below about 22mm focal length. But with a TC-200 2x teleconverter, the full image was covered at all focal lengths.</p>

<p>This shouldn't be surprising. With a teleconverter, the focal length increases, so the field of view decreases. If on an FX camera, the field of view has decreased to what the lens covers on DX, why wouldn't it fill the image? Where would the light coming from those directions be going? (Well, it could end up hitting the side of a barrel somewhere, but that would be a special property of some particular teleconverter, not of teleconverters in general.)</p>

 

<p>Hi Neal, that was a good idea... I was thinking more in terms of FX lens on medium format, but DX to FX makes sense! Thanks for the trial!</p>

<p>I think it sounds right that it does work, but the TC has to have a big enough lens.<br>

<br />I have a TC that claims to be for digital (APS-C) cameras, so maybe it won't. A full frame TC should.</p>