Focal Length and or Crop Calculations with Adapted Lenses

robperks

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Dec 15, 2009
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I tried looking this up, and either it is so obvious I was missing it, or I was searching with the wrong key words.

In particular I use a GF1. When I use adapted lenses is there a rule of thumb to determine the 35mm equiv. focal length? I know the standard lenses you multiply the focal lengthn by 2. What about when I use a standard 4/3 or 35mm lenses with an adapter?

Thanks for your help in my pursuit of lens knowledge.
 

Amin Sabet

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Same exact thing. Whether you use a native :43: lens or any other 4/3 or adapted lens, the focal length written on the lens is the actual lens focal length, and if you want to get an idea of which focal length would give you the same angle of view in 35mm frame terms, multiply by two.

One good thing about having a site/forum/commuinity of :43: users is that we don't need to think in 35mm frame equivalents at all! In fact, we should be translating other formats to :43: equivalents :wink:.

I look forward to the day when someone posts here something like "I was using my Canon 5D III and 85L (42.5mm equivalent AOV) and..." :smile:.
 

BBW

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Eventually I am sure I'll learn to speak in Micro, but until then here's a question: if one is using an older regular lens of say 50mm on one of these new 4/3 cameras with an adapter what does that do? I thought I understood it but now am getting a bit confused.
 

Amin Sabet

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It's very simple. Say you're using the following two cameras:

  • OM-1 (35mm frame) with OM 100mm f/2.8 lens mounted
  • E-P1 (4/3 frame) with OM 50/1.4 lens mounted via adapter
Now use those two cameras to take a picture of a given subject at a given distance will have the following effects:

  • Framing (angle of view) will be the same
  • Depth of field will be the same if the E-P1 lens f-number is set to half the f-number of the OM-1 lens. Ie, the 100mm lens at f/4 on the OM body will have the same DOF as the 50mm lens at f/2 on the E-P body
  • Speed (as it relates to exposure) is based on the nominal f-number, and no calculation is needed. Ie, for a given ISO speed, setting the OM lens to f/2.8 and the E-P1 lens to f/2.8 will result in the same exposure for any given shutter speed
Note that all of the above are true whether you use an OM lens via adapter, regular 4/3 lens via adapter, native :43: lens, or medium format lens via adapter. Any 50mm f/1.4 lens used in any fashion on a :43: body will give you the same framing and DOF as a 100mm f/2.8 lens on a 35mm frame (135 format) body, and the speed of the lens is based only on the actual f-number without any calculation factor.
 
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