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FORUMS Canon Cameras, Lenses & Accessories Canon EF and EF-S Lenses 
Thread started 19 Apr 2009 (Sunday) 09:15
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Reverse Teleconverter?

 
Stuart ­ Leslie
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Apr 19, 2009 09:15 |  #1

Curious what the experts here think regarding the feasibility of making something like this:

teleconverter for crop bodies (1.6 ex) that allows a 300 f/2.8 lens to be converted to an equivalant say 200mm f/2. My thinking is that by "using all of the glass" to fill the crop sensor, you would essentially be making the lens more "wide"- like if it were on a FF camera. The extra light gathered on the sensor should translate into a faster lens right? I know it would not be a "true" 200mm f/2 but would add a lot of versatility to this and other lens.

Probably missing something obvious but thought I would throw it for comments.


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BrianXTi
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Apr 19, 2009 09:21 |  #2

unfortunatey, I do not think this is possible.


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gasrocks
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Apr 19, 2009 09:31 |  #3

Agreed, probably not possible and with low market value besides.


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krb
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Apr 19, 2009 09:48 |  #4

Yeah, don't think it's possible as you describe.

What might be possible would be a reverse TC that corrects for the crop factor. Something that makes a 300mm f/2.8 lens behave like a 300mm f/2.8 rather than a 480mm f/2.8 lens. I doubt you'd find any takers for using it on the long end. If such a thing could be made to work on the short end, for example making my 17-40 act like a 17-40 instead of a 27-64, then maybe you'd be on to something.

I suspect that such a device would not only maximize any barrel distortion present in the lens but would also introduce a lot of distortion of its own.


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nureality
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Apr 19, 2009 10:24 |  #5

krb wrote in post #7760617 (external link)
Yeah, don't think it's possible as you describe.

What might be possible would be a reverse TC that corrects for the crop factor. Something that makes a 300mm f/2.8 lens behave like a 300mm f/2.8 rather than a 480mm f/2.8 lens. I doubt you'd find any takers for using it on the long end. If such a thing could be made to work on the short end, for example making my 17-40 act like a 17-40 instead of a 27-64, then maybe you'd be on to something.

I suspect that such a device would not only maximize any barrel distortion present in the lens but would also introduce a lot of distortion of its own.

Actually on a crop that 300mm f/2.8 is acting like a 480mm f/4-f/4.5

The crop does affect "effective DOF" because you're cropping away information of the shot, you are cropping away area of the OOF that calculate into how deep we feel the DOF is.


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XsiBrian
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Apr 19, 2009 14:05 as a reply to  @ nureality's post |  #6

They exist for astrophotography. It is called a telecompressor. (external link) I believe it would work in the way you describe. Turning the effective 480mm f2.8 (300mm f2.8 on FF) to a ~300mm f1.75 at the most. It would degrade quality because of the added glass but unlike the teleconverter it would not be as sensitive to defects in the attached lens. I think they don't make them because they did not build in support for them in the EOS system and canon probably thinks the market would be too small to consider.




  
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xarqi
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Apr 19, 2009 15:35 |  #7

nureality wrote in post #7760771 (external link)
Actually on a crop that 300mm f/2.8 is acting like a 480mm f/4-f/4.5

The crop does affect "effective DOF" because you're cropping away information of the shot, you are cropping away area of the OOF that calculate into how deep we feel the DOF is.

The first part is wrong: since neither the focal length, nor the aperture is changed by mounting the lens on an APS-C body, the f ratio isn't changed either.

The second part is just silly. Sensor size affects DoF through the degree of magnification required to reach a print/display of a standard size.
Yes, the APS-C sensor will crop more "information" from the edges of the image circle than would a FF sensor, but this is not necessarily the OOF area. Even if it were, and I accept that often it does tend to be, that has no effect on our perception of the degree of focus of the remainder, except that to obtain the same size, it must be magnified more, and so the blur is also magnified, and the DoF appears to be shallower.




  
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Kruzkal
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Apr 19, 2009 15:55 |  #8

I think a reversed tele-converter would need to be mounted at the front of the lens and therefore need to be filter size specific.




  
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KenjiS
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Apr 19, 2009 16:44 as a reply to  @ Kruzkal's post |  #9

I think i get his concept..

By taking the gathered light from the larger image circle and then using additional elements to focus that light and create a smaller image circle, you would end up with an effectively brighter lens because you're using the same ammount of light, but concentrating it on a smaller point..

....After all thats why big large aperture lenses like a 300 f/2.8 have a massive front element, they're acting like an optical "funnel" funneling the light down the tube and concentrating it to the comparatively small EF-mount opening....


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hollis_f
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Apr 20, 2009 06:31 |  #10

I can see how it would be quite possible on a camera with a crop sensor. In that situation the image circle from the lens is larger than the sensor size (Upper Diagram).

With a normal TC the image circle is enlarged so that a smaller portion of the image falls on the sensor (Middle Diagram).

With the reverse TC the image circle is shrunk even smaller, so that it just covers the sensor (Lower Diagram).

With the light being focussed down to a smaller area it would increase the brightness of the light falling on the sensor. It would also induce a reverse-crop effect, so that the lens would have the same field of view as if it were used on a FF sensor.

IMAGE: http://www.frankhollis.com/temp/TCs.jpg


Does that look/sound sensible or have I cocked up somewhere?

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Stuart ­ Leslie
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Apr 21, 2009 08:34 as a reply to  @ hollis_f's post |  #11

Thanks for the illustration Frank, that is exactly what I was thinking but more clearly explained with your drawing. Seems to be feasible doesn't it?


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Reverse Teleconverter?
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