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Old July 17th, 2018, 01:32 AM   #1
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The mathematics of lenses on super 35 sensors

Hi
Looking for confirmation that I've successfully gotten my head around effective focal lengths when using full frame lenses on my FS5.
I'm looking at buying the Canon 24-105mm f4.0, along with a speedbooster adapter. Am I right in thinking that because of the 1.4x crop factor of super 35 sensors, the lens is effectively a 34-147mm? But than with the 0.7x speedbooster, the lens reverts back to something like 24-103mm? Plus, the f4.0 will become f2.8 with the speedbooster.
Is this all correct or have I terribly misunderstood?
Many thanks
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Old July 19th, 2018, 02:07 AM   #2
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Re: The mathematics of lenses on super 35 sensors

If you're used to the so called "full frame" field of view, then yes more or less the smaller sensor will be cancelled out by using the focal reducer. *But* you will get a one stop bright exposure.

If you're used to S35 FoV, then the focal reducer will make it *WIDE* for that focal length than you're used to.
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Old July 22nd, 2018, 11:41 AM   #3
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Re: The mathematics of lenses on super 35 sensors

Nearly there. A FF sensor is considered to be 43.3mm in diagonal measurement. The sensor used in the FS700, FS5, FS7 and F5 has a diagonal of 27.1mm. To find the exact crop factor it is a simple exercise of dividing 27.1 into 43.3. The resultant crop factor is 1.5977, let's say rounded up it is 1.6.

In that case your 24-105mm is effectively a 38.4-168mm lens without a Speed Booster.

Add a Metabones Speed Booster Ultra and you introduce a .71 x reduction factor. Quote:

"Like the revolutionary original Metabones Speed Booster announced in January 2013, the Speed Booster ULTRA has a magnification of 0.71x."

Metabones®

The math then would be as follows: 24 x 1.6 = 38.4 x 0.71 = 27.26mm. So in reality your 24-105 would become a 27.26-119.28mm lens. Let's call that a 27-120mm lens for arguments sake. So not quite as wide as it would be on a FF camera.

My experience with the Canon f4 24-105mm Mk 1 on the FS7 was that the lens actually ramps to close on f5.6 at 105mm and is not the sharpest tool in the kit. I believe the Mk II is better but I can't confirm this as I haven't tried one. I ended up going with the Sigma ART 24-105mm. It is heavier but it holds f4 through to 105mm and it is noticeably sharper especially on the edges on S35 sensors wide open. Which is how I use it on interviews quite often. It also has good contrast and pretty acceptable colors and to date I have not seen any CA to speak of. The thing that took a little while to get used to was the fact that on the Sigma the focus ring is the rear ring not the front ring as one would normally find with this type of lens. Value for money a great lens.

This is worth a read:

https://www.lensrentals.com/blog/201...t-series-lens/

Chris Young
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Old July 26th, 2018, 09:24 AM   #4
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Re: The mathematics of lenses on super 35 sensors

Thanks for the replies.
One question about the Sigma lens. Would getting a Canon EF mount version and using the speedbooster then make it wider than getting the Sony E mount version and also get the extra stop of light?
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Old July 27th, 2018, 09:57 PM   #5
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Re: The mathematics of lenses on super 35 sensors

In a word yes. Both lenses are native f4/0 full frame lenses but in the case of the Sony as beautiful a lens as it is it's a FF E-Mount and therefore cannot use anything like a Speed Booster. With the Sigma in either the Canon or Nikon mount yes you will pick up an extra stop as effectively the lens becomes an f2/8 lens and you recover the bulk of the FF width with a Speed Booster.

In my case I went for the Nikon option as it is extremely rare that I ever use auto focus or iris and all my life I worked with non stabilised lenses so that wasn't an issue for me.. On the Nikon version of the Speed booster Ultra you have an "iris" ring that couples up with the mechanical iris actuator of any Nikon lens. Full manual focus and iris were my main requirements and the Canon option doesn't offer manual iris. All iris control is either done from the roller wheel on the side of the camera or the handle if you have it programmed that way.

Either way the Sigma lenses are a very good performance to price option. I've used lenses that cost way more than the Sigma and you would be hard pressed to say they were substantially better. All said and done though my main jobbing lenses though it's only f4.0 which I find no problem for general work is Sony's 18-110mm servo zoom lens that was designed to work with the FS7. Again an amazing lens considering its performance for the price you pay for one.

I don't think you would regret going with either Sigma though as it is a great value/performance option.

Chris Young
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