Why a crop factor with 4/3rds


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Pablo

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Sep 1, 2004
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Hi all,

I understand the crop factor with a film based lens on a digital body (1.5 1.6 etc)

But, when a lens is made as a digital lens. as for the 4/3rd system,
where there is a standard mount and a standard sensor size.

Why is there still a crop factor ?

Why is a lens marked as 40mm to 150mm not really 40 to 150, but 80mm to 300mm.

I thought that if a lens was designed to put the image on a pre set area, there would be no crop unless you changed that area.

Can someone put my mind to rest on this one ?


Thanks all in advance for sharing knowledge.

:)
 

Scaglietti

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Jan 14, 2005
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The "crop factor" is used just for reference to the "more familiar" 35mm format.

The __mm is the focal length of the lens. 40mm is still 40mm. It is just equivalent to 80mm in the 35mm format. That means a 40mm lens will project a picture on the 4/3 sensor which an 80mm lens will on a 35mm film.
 

ykkok

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Feb 24, 2004
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There's no crop factor.

40-150 aka 80-300 is only for reference only.

It's the equivalent "angle of view" in terms of 35-mm format. Meaning, what you would see in terms of 35-mm format.

So, 40mm on 4/3 will have the equivalent angle of view of 80mm in 35-mm format.
 

Cactus jACK

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Jul 12, 2004
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i think that you're mistaking the "crop factor" with the "magnification factor".

the Oly DSLRs (4/3 system) are full-frame (1.0x crop factor) but 2x magnification. that is because the image circle from the digial lenses fit nicely around the sensor (thus full-frame), but the sensor is only 1/2 the size film FF (thus 2x mag).
 

Pablo

Senior Member
Sep 1, 2004
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Hi Scaglietti and ykkok,

I am happy you both pointed the same thing to me.

In reference to 35mm is what I was hoping to hear, as I could see no other reason for such meaning of crop factor.

Thank you again for settling my mind on this !


Pablo :)
 

AReality

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Jun 9, 2003
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All frames are full frame. Unless u get 1/2 the frame black, then it's not full frame, but 0.5x frame.
 

AReality

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Regardless of how large the diameter of the lens, a 40mm lens is still a 40mm lens.
 

Hacker

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Dec 6, 2005
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Think medium format......same principle applies.
 

ykkok

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Feb 24, 2004
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AReality said:
Regardless of how large the diameter of the lens, a 40mm lens is still a 40mm lens.
Absolutely right! Olympus don't really say that 40 = 80mm, but "equivalent" in terms of 'field of view'.

That's why, on the lens marking, it's always the actual focal length.

Nobody is going to understand, say 56 degree - 20 degree angle. Do you think this is appropriate?

By the way, diameter and focal length are two different things, although they are related.
So, you are referring to 40mm diameter or focal length?

The "crop factor" term arises when a lens projects an image circle larger that the targetted film/sensor.

If 4/3 lens can be mounted on a 2/3" sensor, say Olympus 8080, then, it's a 2X crop factor - because 2/3 is half the size of 4/3.

As for Olympus 8080 which has a 2/3" sensor that's 4X smaller than 35mm format, do we refer it's lens (although fixed) carries a "4X crop factor" ? I'm afraid not and not realy relevant.

If we insist on this, then the 35-mm format has 2X or more as compared to Medium or Large Format, as mentioned by hacker.

My 2cents.
 

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