# Canon APS-C<Speedbooster<EF Lens. Possible?



## Frage (Sep 24, 2014)

Hi people!

I am sorry if this a stupid question but I have to ask in order to learn.

Is possible such a combination?: A Canon crop camera with a (I think not existing)speedbooster and any EF lens in order to gather the "extra" light the lens is providing and the sensor, due to its smaller surface, is not able to use. Would it improve APS-C sensor ISO performance?

Thanks in advance and sorry if it is a completly dumb question.


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## KeithBreazeal (Sep 24, 2014)

Astro-photography has had these for many years.
http://handsonoptics.com/index.php?main_page=product_info&cPath=1_7_70&products_id=1036&zenid=52c8a711e5a1fcda746b09470e132bd8


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## sagittariansrock (Sep 24, 2014)

I believe it is technically possible with the mirror less EF-M due to the small flange distance from the mount to the mirror (so additional converging elements like a speed booster can be interposed). Most likely not with a EF-S SLR mount. 
I think the hangup with the EF-M is that Metabones probably doesn't want to commit resources into developing it until they know whether or not Canon will continue the line.

Edit: FYI, Metabones already makes Speedboosters for the NEX line of MILCs.


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## weixing (Sep 24, 2014)

Frage said:


> Hi people!
> 
> I am sorry if this a stupid question but I have to ask in order to learn.
> 
> ...


Hi,
Speedbooster is basically a focal reducer. IMHO, it's had nothing to do with sensor size as long as the original lens produce an image circle that can cover the sensor, but focal reducer will usually reduce the distance where the image is focus on, so it's difficult (if possible) to maintain flange focal distance of any lens and mount of the same type. That's why speedbooster only appear for SLR lens to mirrorless mount, so speedbooster for EF lens to EF mount might not be possible, but speedbooster for EF lens to EF-M mount is possible.

Have a nice day.


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## sagittariansrock (Sep 24, 2014)

weixing said:


> Frage said:
> 
> 
> > Hi people!
> ...




Actually, it's a question of a physical entity (set of lenses) being interposed between the rearmost lens element and the camera mount. It might not be impossible with the Canon telephoto lenses that can take a teleconverter (and consequently have free space behind the rearmost lens element). The Speedbooster is essentially the exact opposite of a teleconverter.

To answer a question the OP had asked, yes- it does increase the amount of light. The Metabones one increases the equivalent angle of view by .71x and the light by 1 stop IIRC. And I am probably generalizing- these numbers might be specific to FF>APS-C. Too late for me to think it out


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## Frage (Sep 24, 2014)

weixing said:


> Frage said:
> 
> 
> > Hi people!
> ...



So if Speedbooster (SB) is a focal reducer then






(Wikipedia)


> where f is the focal length, and D is the diameter of the entrance pupil (effective aperture). It is customary to write f-numbers preceded by f/, which forms a mathematical expression of the entrance pupil diameter in terms of f and N.[2] For example, if a lens's focal length is 10 mm and its entrance pupil diameter is 5 mm, the f-number is 2 and the aperture diameter is f/2



Does really work like that?


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## Ebrahim Saadawi (Sep 24, 2014)

Yes it cannot be physically done. Every lens is designed to project the image on a specific distance (distance from lens to sensor), this is called the flange ditance. Now when you put an adapter or a speedbooster, the lens will be pushed away from the sensor and will not be focusing on its correct plane. 

With mirrorless cameras, that flange distance is very shallow as there's no mirrorbox, so the flange distance of many lenses is larger therefore you can put an adapter on the back of the lens to push the lens away from the sensor making it focus on its correct plane. 

In short, to use a speedbooster, 
1-the lens must be designed with a longer flange distance than the camera it's mounted on. If it's designed exactly for the camera the SB won't work.
2-the lens must be designed to cover a larger sensor than the camera's sensor it's being used on, for example, a full frame lens on an APS-C body, because the idea of a SB is that it focuses the large image circle down to a smaller circle, giving an increase in light density and wider fov. If you use an APS-C lens with a SB on an APS-C body, the SB will shrink the image circle of the APS-C lens to a smaller m34s-ish image circle, therefore you will have a huge viggentte. 

The EOS M would work (with EF lenses, not EF-S or EF-M). With a SB when using with the Canon EF lenses the Eos m will effectively be a fullframe camera. It would make the camera look like the 5D/6D in terms if the image aesthetic (in both field of view and shallow depth of field) very interesting indeed. I hope they make it happen.


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## dgatwood (Sep 24, 2014)

Ebrahim Saadawi said:


> Yes it cannot be physically done. Every lens is designed to project the image on a specific distance (distance from lens to sensor), this is called the flange ditance. Now when you put an adapter or a speedbooster, the lens will be pushed away from the sensor and will not be focusing on its correct plane.



Well, you're partially right. That makes it harder, but not impossible. With a teleconverter, the lens gets farther away from the camera, but the focal plane shifts backwards by the same amount. With a wide converter, the focal length is reduced, so without additional correction, you'd have to somehow move the sensor closer to the lens, which is impossible (obviously). You can, however, compensate for that by adding extra optical elements that shift the focal plane further away.

The good news is that Kodak actually designed a telecompressor a couple of decades back that did exactly that. The bad news is that they patented it, and the patent (US5499069) didn't expire until four months ago, which is a big part of why such devices aren't on the market today. From the patent claims:



> 1. c) an optical adapter located between said lens barrel and said camera body for providing *a smaller size image than said objective lens system provides when mounted directly to the camera body*, said optical adapter including a lens attachment optical system, a front lens element of which is located in front of an image formed by said objective lens system exclusive of the lens attachment optical system, said lens attachment optical system having a plurality of lens elements which have radii of curvature and spacings *sufficient to create a back focal distance to clear the SLR camera body mirror.*



Emphasis mine.

More to the point, Kodak's patent covers *only* situations where you have to maintain a long back focus (flange focal distance). Wide converters that adapt lenses with a long flange distance to cameras with a shorter flange distance aren't affected by that patent, which is one reason that they're readily available. (That, and they're a lot easier to design.)

Now that the Kodak patent has expired, it seems fairly likely that we'll see these appear on the market in the not-too-distant future, assuming anybody is willing to spend the time to design the (significantly more complicated) optics.




Ebrahim Saadawi said:


> 2-the lens must be designed to cover a larger sensor than the camera's sensor it's being used on, for example, a full frame lens on an APS-C body, because the idea of a SB is that it focuses the large image circle down to a smaller circle, giving an increase in light density and wider fov. If you use an APS-C lens with a SB on an APS-C body, the SB will shrink the image circle of the APS-C lens to a smaller m34s-ish image circle, therefore you will have a huge viggentte.



This part is correct. The only way to avoid that is by putting a wide-angle adapter on the front of the lens, rather than the back, which is generally undesirable for any number of reasons.


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## Ebrahim Saadawi (Sep 25, 2014)

Yes of course you can shift focal plane farther by adding extra optical elements, like those Canon FD to EF adapters for example, but these optical elemnts significantly affect image quality therefore adapting lenses with a longer flange distance than the camera's is considered the only way. That's why Brian Caldwell (designer of the SB) told us it's not physically possible to make an EF to EF-S SB. 

The EOS M is the only Canon that we will be able to speedboost. It would effectively be a 250$ fullframe Canon that gives virtually identical images to the 5D mk III/1DX when used with the right lenses (EF+SB on the back). 

Another interesting speedbooster would be a medium format to Sony FE mount (A7 series). It would effectively give us a 1400$ medium format mirrorless camera. That would revolutionize the medium format industry.


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