can someone explain the new dslr lenses designed for aps-c sensors?

[unsped]

http://www.sigmaphoto.com/lenses/lenses_all.asp

an example, the DC lenses are designed for aps-c sized sensors... so does that mean that a 18-200 is a 18-200 on a d50/d70/rebel? and not 1.5/1.6 * 18-200 like a regular lense?

i was pretty sure it would offer 18-200 equiv, but noticed this paragraph

http://www.dpreview.com/news/0502/05021402sigma18-200dc.asp

"This new eleven times zoom lens although a little slow at telephoto (F6.3) would offer an impressive 27 - 300 mm equiv. on a 1.5x FOV crop camera (such as the Nikon D70)."

im just guessing dpreview had incorrect information at the time?

 

[DBL]

No, you still multiply by your crop factor to get the effective field of view or focal length. An 18-200 acts like a 27-300 on a 1.5x crop body. The difference is that these lenses cannot be placed on a FF DSLR. The image circle produced by the lens is not large enough. Theoretically, this enables the lenses to be designed lighter and smaller.

 

[unsped]

if the reason the crop factor exists is because the image circle is 1.5x times the aps-c sensor. how does a lense creating a image the exact same size of the aps-c sensor provide the same 1.5x magnification.

how do you have 'crop factor' when the image is no longer cropped.

 

[JMWarren]

More like the sensor is 1.5x smaller than a 35mm frame....You use sensor size and focal length to determine FOV (ok its more complicated than that but i'm not getting into it).

 

[OdiN]

Originally posted by: DBL
No, you still multiply by your crop factor to get the effective field of view or focal length. An 18-200 acts like a 27-300 on a 1.5x crop body. The difference is that these lenses cannot be placed on a FF DSLR. The image circle produced by the lens is not large enough. Theoretically, this enables the lenses to be designed lighter and smaller.

But then you have the Diffractive Optics lenses...which are tiny for their focal lenghts.

 

[unsped]

so the idea is that the lense only has to produce the cropped portion of the image... so they dont waste plastic/lenses producing the portion of the image thats not shown?

but that its still cropped because all focal ranges are listed in 35mm equivalents?

 

[JMWarren]

Yeah, the lenses just produce an image circle that is correctly sized for a APS-C sensor. Focal Length is not related to sensor size. Check out the points and shoots with 5-15mmish lenses and small sensors that work out to a 35-105mmish. Or go the other way and try medium or large format cams. 80mmis is considered normal on a Blad.

Mike

 

[jiwq]

does APS-C reduce manufacturing costs, thus passing the savings on to the consumer?

 

[JMWarren]

In general that seems to be the case.

 

[OdiN]

Yes APS-C sensors are smaller and thus cheaper in most situations. Depends though.

The other reason that it's less expensive on a dSLR is because the smaller sensor means that the prism doesn't need to be as large.

 

[OdiN]

Originally posted by: DBL
No, you still multiply by your crop factor to get the effective field of view or focal length. An 18-200 acts like a 27-300 on a 1.5x crop body. The difference is that these lenses cannot be placed on a FF DSLR. The image circle produced by the lens is not large enough. Theoretically, this enables the lenses to be designed lighter and smaller.

Oh the Sigma lenses can be put on a full frame camera but you will get vignetting.