Why zoom lens specifications for digital cams are different

[edkel]

I look at ads for digital cameras. They would say something like "30-100mm; 40-120 film cameras equivalent"

Why are lens specifications for digital cameras different from, lens for film cameras. Can you mount an old zoom len from a film camera, onto a new digital camera?

[framewerkz]

Simply put, it's because the imaging sensor in most digital SLRs is smaller than a regular 35mm frame, which results in a cropping of the image - resulting in an equivalent focal length multiplying effect.

Most Canon DSLRs have a crop factor of 1.6 - which means that a nameplate focal length on a 35mm lens will result in a multiplying factor of 1.6 times. It's good at the long end, not so good at the short end.

And yes, most 'old' film lenses work perfectly well on DSLRs. Some of the older Sigmas need re-chipping, though.

[edkel]

I understand the first part about the image sensor in a digital slr is smaller than the 35mm film frame of a film camera.

I still don't understand the focal length explaination. Is the distance between the len and the image sensor in a digital slr, different than the distance between the len and the 35 mm film frame in a film camera?

[framewerkz]

Try this.

[ukexpat]

I understand the first part about the image sensor in a digital slr is smaller than the 35mm film frame of a film camera.

I still don't understand the focal length explaination. Is the distance between the len and the image sensor in a digital slr, different than the distance between the len and the 35 mm film frame in a film camera?

See the link posted above. If you think of it as a field of view crop factor rather than a focal length multiplier, it should be easier to understand, ie the effect of the smaller sensor is to crop the field of view of the lens giving you a field of view that, if you were using a 35mm camera, would look like it came from a lens with 1.6x the focal length.

[castledude]

I always explain it this way ...

Take a 35mm negative and blow it up to 8x10.

Now cut the negative in to four pieces and blow up one of the pieces to 8x10.

The net effect is that a smaller part of the film is going onto a larger area (of paper) so you end up with a "magnification".

With a DSLR the lens to Focal Plane is the same distance as the lens to film in a normal SLR so the system with most DSLRs the focal plane is smaller than the film so it has a "magnifying effect". To keep it straight in our poor little brains the camera makers give you the conversion with respect to 35mm film.

In the case of the P/S cameras they will set the focus point on the lens closer (to make the camera smaller) but give you the conversion so you can make comparisons.

[ukexpat]

I always explain it this way ...

Take a 35mm negative and blow it up to 8x10.

Now cut the negative in to four pieces and blow up one of the pieces to 8x10.

The net effect is that a smaller part of the film is going onto a larger area (of paper) so you end up with a "magnification".

With a DSLR the lens to Focal Plane is the same distance as the lens to film in a normal SLR so the system with most DSLRs the focal plane is smaller than the film so it has a "magnifying effect". To keep it straight in our poor little brains the camera makers give you the conversion with respect to 35mm film.

In the case of the P/S cameras they will set the focus point on the lens closer (to make the camera smaller) but give you the conversion so you can make comparisons.

Yup that does it too. As we sometimes say, I think we are all in violent agreement on this one!!