Understanding "crop factor" with 16mm lenses on micro 4/3 sensor?

A

Andreus

Well-known member
I am using the GH1 with lenses from old 16mm movie cameras; primarily in C mount, some Arri mount.

I am trying to determine the FOV of my lenses in 35mm equivalent terms, and I'm stumped because of the way different lenses crop/don't crop on the m4/3 sensor.

Lenses _< 25mm tend to vignette on the sensor. So these don't have a crop factor, right? (or do they have a negative crop factor?) I am seeing more than what the lens was designed to show. So is a 25mm lens then really giving me the FOV of 20mm?

Lenses > 25mm don't vignette. So 50mm, 75mm, 150mm lenses have full coverage. It is said that the GH1 has a 2x crop factor, but I believe that is for 35mm format glass? These lenses were all designed for regular 16mm or super 16mm format. Yet they are not vignetting like the 25 and below. Still, with m4/3 larger than 16mm format -- does it mean my 50mm lens is giving me the FOV of a 50mm, or more? eg, 45mm? 40mm?

How can one know??

Thanks!
 
Any lens marked as 25 mm focal length will have the 'normal' field of view on GH1. Any lens shorter will be a wide and any lens longer will be tele. The focal length on a lens is a constant number, it's the size of the sensor that determines what FOV it will yield.
 
A 50mm lens is 50mm whether it's made for 16mm or 35mm. Put it on a GH1 and the same crop factor applies (i.e it'll be that of a 100mm lens) The difference is going to be in optimization for your particular format. For example, for smaller sensors you don't need to worry about things like vignetting or edge to edge sharpness so much, so using a lens that is designed for smaller sensor on a large sensor camera can be problematic.
 
It wouldn't be a negative crop factor if you want to deal with numbers. It would just be a number smaller than 1 but greater than 0. For the lenses that are vignetting, that just means that the image that lenses can produce is smaller than the sensor size you are shooting on.

Honestly unless it's critical to your work, don't worry about conversion rates with crop factor. When changing formates it's easier to just think within that formate. When shooting on 16mm film I don't try and convert the focal length into 35mm terms. There's no reason to in my mind.
 
Your confusion starts here:

Andreus said:
It is said that the GH1 has a 2x crop factor, but I believe that is for 35mm format glass?
Click to expand...

It is not a 35 mm format glass, it is a the size of the frame. The diagonal of the 24x36mm frame is 43.266 mm, ~ 2x the diagonal of the 4/3 format.

A 50 is a 50 is a 50.
If you take an 50mm lens from let's say Leica, Nikon, Hasselblad and put it on GH1 you get the same image and perspective as you get from a 50mm Panavision, Zeiss, or Cooke designed for 35mm-16mm motion picture format, or Panasonic stock zoom set at 50mm.

The sensor of the GH1 compared to a FF 35mm stills camera has a ~2x crop factor, it has ~ 1.4X crop factor when compared to a 35mm motion film camera's film gate.
 
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