Relative crop factor Alexa Mini LF and others

[PSil001]

Hello,

How do you calculate the equivalent focal length for a lens on a Super 35 sensor when using other ARRI cameras, such as the Alexa Mini LF, Alexa 65, and ARRI 16SR? What are the crop factors for each of these cameras?

For example, if you use an 18mm lens on a Super 35 sensor, what would the equivalent focal length be on each of these cameras?

[Simon Duschl]

Hi,

the crop factor is defined as:

Crop Factor = Diagonal of Smaller Sensor / Diagonal of Full-Frame 35mm Reference Sensor

where

Diagonal = √(Sensor Width²+Sensor Height²)
c = √(a² + b²)

Here's a good explanation:
https://shuttermuse.com/calculate-cameras-crop-factor/

In purely mathematical terms, the following Crop Factors result:

• 35mm Film (Reference) = 1,00

• ALEXA Mini LF = 0,967668935

• ALEXA 65 = 0,722735981

• 16SR (S16 mm) = 2,973959605

​Please note these values are related to the full sensor readout e.g. Open Gate or S16mm Film and not to smaller cropped sensor modes.




​

[KirkWilber]

Hi,

the crop factor is defined as:

Crop Factor = Diagonal of Smaller Sensor / Diagonal of Full-Frame 35mm Reference Sensor

where

Diagonal = √(Sensor Width²+Sensor Height²)
c = √(a² + b²)

Here's a good explanation:
https://shuttermuse.com/calculate-cameras-crop-factor/

In purely mathematical terms, the following Crop Factors result:

• 35mm Film (Reference) = 1,00

• ALEXA Mini LF = 0,967668935

• ALEXA 65 = 0,722735981

• 16SR (S16 mm) = 2,973959605

​Please note these values are related to the full sensor readout e.g. Open Gate or S16mm Film and not to smaller cropped sensor modes.
​

Thanks for the detailed explanation and the crop factor calculations! It's helpful to see how the equivalent focal length changes across different camera systems. The reference to the diagonal measurement formula is especially useful.

[PSil001]

thanks! I was reading the website, and I think your answer is a bit different. According to the website, instead of:
Crop Factor = Diagonal of Smaller Sensor / Diagonal of Full-Frame 35mm Reference Sensor

It would be:
Crop Factor = Diagonal of Full-Frame 35mm Reference Sensor / Diagonal of Any Other Sensor


For example, in the website's Canon APS-C Crop Factor calculation, they show this:
Crop factor = 43.27mm (Full-frame 35mm) / 26.68mm (APS-C) = 1.621814, rounded to 1.6.

It seems that the 35mm full-frame sensor is used as the reference at the beginning.

Also, when calculating the crop factor for Super 16 (S16), if I use your formula instead of the one on the website, I get:
Crop factor = 14.54mm (S16) / 43.27mm (Full-frame 35mm) = 0.33 for Super 16, instead of the 2.97 value you provided.

Would you agree with this? Or is there something I’m missing?