The truth about extenders

[jjmcchesney]

Hi Folks,

So I have a Canon 7D and a 70-200mm EF lens. I got givena 1.4X EF extender.

Question. What's the overall focal length?

is it 1.6 (crop factor) X 1.4 (extender) x 70 (at low end) = ~157mm (or 448mm at high end)

or is it 98mm at low end 'cropped' to look like 157mm?

So, your not zoomed as much as you think.

I've googled this but all I can find is comments on extenders stopping down.

thanks,

J.J.

[kvbarkley · Accepted solution]

it is 1.4 * the focal length.

On an EF-S camera, it only looks like you have 1.6x the focal length because the sensor is smaller and you look at a smaller area of the image circle.

[jjmcchesney]

Thanks!

[ebiggs1]

"Question. What's the overall focal length?"

The FL of a Canon 70-200mm EF lens is 70-200mm. It never changes no matter what camera you use it on. It is the "equivalent" FL that appears to change only when you compare the Canon 7D and a 70-200mm EF lens to a 35mm size or so-called full frame camera. All cameras are in fact FF meaning you get exactly what you see in the view finder.

The part that does change is the angle-of-view, AOV. Since you are covering a smaller sensor the AOV narrows. You can find simple AOV charts on the web for you to look at. When you add a tel-con you must add it into the equation also. Again we are talking equivalent FL because, no matter, the FL of the lens does not change. All extenders lose light, the 1.4x by about 1/2, so 1 stop.

At 200mm on the lens and the 1.4x tel-con, you are looking at about 448mm equivalent FL.(200 X 1.6 X 1.4)  And, if you have a f2.8 zoom you now have a f4 zoom. If you zoom was a f4 zoom lens it becomes a f5.6 lens. These numbers are not equivalent but actual changes. It is best to just forget about this crop nonsense from a mathematical and magnification perspective and just find and use focal lengths that give you the framing (AOV) you want. People that never intend of owning a 35mm or FF DSLR camera never need to know any of this.