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EF To EF??

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John Russell
EF To EF??
on Feb 18, 2014 at 2:30:21 am

So, I know this is a bit odd, but I could really use some help with this.
I own a 5Dmk3 and most of my lenses are L-Series. BUT I also own the Rokinon 8mm fisheye. Now I know it is meant for APS-C sensors. So with my full frame mk3 it vignettes around the edges of the image.
Today I was on a shoot with a friend of mine that was shooting on a sony nxcam 100. He had a metabone adapter for ef mount lenses. When he put my 8mm on the mount, the image filled the frame! No vignette whatsoever. I would like to replicate the effect on my mk3 but I can't find anyone that makes an ef to ef adapter.
Any help with this would be GREATLY appreciated!
Thanks much
-Russell


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Chris Evans
Re: EF To EF??
on Feb 18, 2014 at 6:09:31 pm

The Metabones Speed Booster doesn't actually give the camera a Full Frame field of view. A telephoto extender should get rid of the vignetting, but it zooms your image, making the angle of view of the 8mm pointless.

TL,DR: you can't.


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John Russell
Re: EF To EF??
on Feb 18, 2014 at 9:06:53 pm

I understand that, but I'm confused as to why there was no vignette if the Speed Booster does not give a full frame field of view.


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Chris Evans
Re: EF To EF??
on Feb 18, 2014 at 10:14:39 pm

A full frame sensor (35mm) is actually 36mm by 24mm. The diagonal length is 43.3mm.

If you remove a lens from a camera and place a piece of paper at the focal plane of the lens (where the sensor would be), you will see an image of what the lens is seeing. The image will be a circle because lenses are round and the iris inside the lens is a circle.

For a lens that is designed for full frame cameras, the diameter (distance straight across) the circle produced by the lens at the focal plane will be greater than 43.3mm. That means that the light coming through the lens will cover the entire 35mm sensor when the lens is placed on the camera. For the sake of explanation, the edge of the circle is 1mm away from the corner of the sensor.

The Rokinon 8mm lens would produce a circle at the focal plane that is larger than 30.1mm (the diagonal size of an APS-C sensor. Again, for the sake of explanation, the edge of the circle is 1mm away from the corners of the sensor. If you put that on a full frame camera, the circle only covers most of the lens and the corners of the sensor don't receive any light (the edge of the circle is inside the corners of the sensor, instead of outside), thus causing the vignetting. (you probably know this, but it's important that you know all of this up to this point to understand the next part)

The FS100 (I assume that is the one you were talking about) has a super 35 sensor, which is about the same size as an APS-C sensor, so you will get full sensor coverage by lenses designed for APS-C sized sensors (assuming you have an adaptor that allows you to put them on the camera). It is important to note that the reason you can do this is because EF lenses have a longer flange distance than E-mount lenses. The flange distance is the distance between the back of the lens and the focusing plane. An adaptor adds the difference between the two flange distances so that EF lenses will focus on the same plane as the E-mounts do.

The Metabones Speed Booster is two things. First, it is an EF to E-mount adaptor. This means you can put EF lenses on cameras with an E-mount. Secondly, and more importantly, it is effectively a reverse telephoto extender. This means that it takes the circle that EF lenses produce on the focal plane and makes it smaller. Basically, it takes the light and compresses it so that it all fits on a smaller sensor. However, for a full frame lens, it does not compress the circle to a point where the edge of the circle is 1mm away from the corners of the sensor (which would give it the same coverage as a full frame sensor), the edge of the circle would be more like 2 or more mm away from the corners of the sensor.

The Rokinon 8mm produces a circle on the focal plane that is large enough that when it is compressed by the metabones speed booster, it still is larger than the sensor in the FS100.

That is why there is no vignetting on that camera.

Note: I don't actually know how far away the edge of the circle produced by the lens is from the edge of the corners of a sensor, that was just for illustrative purposes to help you understand better.

Note #2: The edges of a the circle produce by a lens are not hard circles, they are soft. That is why a vignette isn't a hard line. You could actually be seeing some vignetting on the corners of the image of the FS100, but you may not be noticing it because it's just the edge of the vignette and doesn't darken the corners very much.

Now you know.


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