Reading about tele extenders for lens and how they change the focal length/field of view I came up with the idea if there is something/some way to shorten the focal length of the lens. And the question is: Is there something like this?

Unfortunately search engines do not provide me information about this :)

Edit: I do not care about the light. My target is to find way to replace for example quite expensive lens like Canon 11-24/F4 with Canon 16-35/F4. Some degree of degradation of IQ is expected and accepted

  • \$\begingroup\$ Michael, I am sure you make difference between light and focal length. I search for a way to decrease focal length! \$\endgroup\$ Commented Dec 8, 2017 at 10:58
  • \$\begingroup\$ I agree, this is a different question. \$\endgroup\$ Commented Dec 8, 2017 at 12:38
  • \$\begingroup\$ The accepted answer to the duplicate makes very clear that the size of the image circle and the brightness of the light field are directly related. You can't have one without the other. \$\endgroup\$
    – Michael C
    Commented Dec 11, 2017 at 8:02
  • \$\begingroup\$ @MichaelClark, I browse the mentioned answer, but I did not find good explanation as answer below. And for me this knowledge (below) is just different (in sense of point of view) from the answer you mention \$\endgroup\$ Commented Dec 11, 2017 at 8:48

1 Answer 1


It's called a focal reducer or speed booster, metabones makes many, however they come with caveats.

A teleconverter makes the image circle of a lens wider, so it can work on the same sensor/film size as the lens you attach it to was originally intended for. You can make the image circle smaller but it won't work anymore on the same kind of sensor/film the lens you use it with was originally designed for. So you can only use it with smaller sensor, e.g. full frame lenses on crop bodies.

However, it works, it makes the lens (when attached to that smaller crop sensor) effectively wider and faster, but it will only make it as wide as it used to be in the first place on the full frame sensor. However, it will be faster than it used to be on the full frame sensor.

Btw, a focal reducer is nothing but a teleconverter mounted in reverse. Just like a good teleconverter is expensive, you can expect a focal reducer to have the same (expensive) price.

Edit after your edit:

In the general case, you can't do what you want. What you put behind the lens can make the image circle larger or smaller (or achieve some other kind of effect), but it does nothing in terms of changing the lightrays captured by your lens, which is what you want to do. You'd need something in front of the lens. Such elements exist, for example the WCL-X100 Wide Conversion Lens for the Fuji X100 series of cameras, or the various offerings for smartphones, but these elements are lens-specific, not generic elements. I suspect that producing such an element for the Canon 16-35/F4 would be extremely difficult and expensive.

  • \$\begingroup\$ Thank you, Aram, my idea was a bit different and will not work (based on your explanations) \$\endgroup\$ Commented Dec 8, 2017 at 9:57
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    \$\begingroup\$ Unfortunately yes, see my edit. \$\endgroup\$ Commented Dec 8, 2017 at 12:38
  • \$\begingroup\$ The design of a focal reducer is very different to the design of a teleconverter. \$\endgroup\$ Commented Dec 8, 2017 at 18:34
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    \$\begingroup\$ Another drawback of the focal reducers/lens turbos/speed boosters is that they have different mounts on both ends. For example I didn't find a Sony-A to Sony-A lens turbo to mount some A-mount lens on my APS-C Sony Alpha camera. Or some Sony FE E-mount lens on an APS-C E-mount camera. All models I found also include some adapter for different mounts. \$\endgroup\$
    – Gerhardh
    Commented Dec 11, 2017 at 9:37

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