Question about Speed Boosters

[mercer]

So, I recently purchased the G7 and it is the first camera where I can utilize a speed booster. I don't have the budget for a metabones, so I am looking at a Roxsen or RJ Turbo.

Obviously, I was curious what focal lengths could be achieved with a speed booster, so I hit Google, but with my research, I feel like I am even more confused. Basically, it seems there are a lot of misconceptions about what the formula is for determining focal length with a speedbooster.

I was under the impression it was...

• Lens Focal Length X Crop Factor X Speed Booster Focal Reduction = Speed Booster Focal Length?

Let's say I am using a 28mm lens on a MFT with a Roxsen speed booster...

• 28mm X 2 X 0.72 = 40mm

But then over at DPreview and a few other sites, forum members are stating that it's just 28mm X 0.72 = 20mm?

So, which is it... Do you still multiply the 2x crop factor for micro 4/3rds, or is that negated when you add a speed booster. 

Sorry, I am sure this has been discussed to death, but this is my first adventure with speed boosters and have never had a reason to investigate them. 

[dhessel]

You are correct if you want to know the new focal length in 35mm/Full Frame equivilant then you multiply the focal length x crop factor x focal reduction. However DPreview and the others are also correct in that a 28mm and booster will yeild the fov of a 20mm lens on a MFT camera. In turn that 20mm will have the fov of a 40mm on a Full Frame camera.

[Phil A]

I think this is a bit an issue of opinions as to what is scientifically correct and what is "practically accepted reality".

Focal lenght is fixed, it doesn't change depending on the camera. So 28mm is 28mm no matter if 35mm, APS-C or M43 sensor, but the crop of the image circle that is used obviously changes, so you will change the distance to compensate for this. Theory goes on and on but....

The practical answer you want:

28mm on M43 has the field of view like a 56mm on 35mm/FF. So the Speedbooster uses whatever modification, in your case 0.72, which will give you what a 41mm lens would frame on a FF sensor. 28mm x 2 x 0.72 = 40.32mm

 

EDIT: yeah, what dhessel said

[Guest Ebrahim Saadawi]

40mm Ff lens, abd a 20mm m43 lens. Equivalent that is.

[Cinegain]

Yup. The 0.71 or 0.72x, whatever really, just makes the 28mm lens itself act as if you'd fit on a 20mm lens on there. Optically.

The sensor crop of 2x still applies. So, equivalent to FF, it's still 40mm-like.

[mercer]

Thanks guys! I keep forgetting the FL/FOV differences. Makes perfect sense now.  

[bunk]

Add another 1.2 to the equation as you only use part of the sensor when filming in UHD (4592/3840)...

So it becomes  XXmm x 2 x 0.72 x 1.2 or  XXmm x 1.72

[mercer]

Add another 1.2 to the equation as you only use part of the sensor when filming in UHD (4592/3840)...

So it becomes  XXmm x 2 x 0.72 x 1.2 or  XXmm x 1.72

On the G7, the UHD crop is 2x. So, why would I have to add another 1.2?

[bunk]

The crop factor of an MFT sensor is 2. Since the G7 only uses part of the sensor you have to calculated that difference as well.

Nevertheless I stand corrected. I was under the impression the G7 didn’t do pixel binning in UHD. Turns out it uses  a 4120x2318 crop from the sensor to create UHD.
4592/4120=1,1145 so that’s an extra 1.1 to add to the equation. So it becomes 1,58 instead of 1,44 (or the 1.72 as I suggested).

http://www.cameralabs.com/reviews/Panasonic_Lumix_G7/       just after 3/4 of the page is a nice picture that shows the use of the sensor in UHD.

[M Carter]

The crop factor of an MFT sensor is 2. Since the G7 only uses part of the sensor you have to calculated that difference as well.

Nevertheless I stand corrected. I was under the impression the G7 didn’t do pixel binning in UHD. Turns out it uses  a 4120x2318 crop from the sensor to create UHD.
4592/4120=1,1145 so that’s an extra 1.1 to add to the equation. So it becomes 1,58 instead of 1,44 (or the 1.72 as I suggested).

http://www.cameralabs.com/reviews/Panasonic_Lumix_G7/       just after 3/4 of the page is a nice picture that shows the use of the sensor in UHD.

Beyond all the stabs at science - stick it on your camera and run some tests or shoot for a couple hours. Your brain should file it under the proper category and on a gig you'll reach for it as needed.