16-45

I use a da16-45 on my K200D.
Is this a true 16mm width??
regards
chris

With the 1.5 crop factor of your camera it's equivalent to 24mm on full frame if that's what you are asking?

I thought this lens was designed specifically for pentax digital, how would I achieve a true 16mm??
Many thanks
Chris

It is designed for Pentax digital. However, the focal length for digital cameras is still predicated on 35mm terms. If you put your lens on a full frame (35mm) camera (assuming it worked, which it wouldn't), it would be 16mm.

To get a true 16mm you will have to get a 10mm, or 10.66666 mm to be precise.....

To be more precise:

The focal length of the lens used on a full-frame sensor or an APS-sized sensor is the same (i.e. achieves the same magnification) but since the smaller sensor only captures a smaller central portion of the image projected by the lens, the field of view obtained in the latter case gives the impression that the lens is acting as one with focal length times the crop factor.

You live and learn.
aliengrove many thanks.
Chris

Are you saying that with this lens, 16mm cannot be achieved with any pentax digital camera.if so why call it a 16mm lens.
Regards
chris

Because it is a 16mm lens at its widest end. On digital that equates to a near-ultrawide.

The effect of a particular focal length depends upon what format you project its image onto. The focal length of a given lens remains the same.

Pentax are labelling their lenses correctly.

Thanks all.
Why dont they just state it as 24mm then-sorry to labour the point
Regards
chris

All digital camera manufacturers use full-frame as the reference for focal length. On Olympus, for example, the crop factor is 2x, effectively doubling the apparent focal length. The crop factor becomes an advantage at telephoto, when the crop factor gives a field of view of a 300mm lens on an APS-sised sensor equivalent to 450mm full-frame, ie smaller lenses for the same magnification compared to full-frame.
As John says, the focal length is still 300mm, it's just that the sensor is smaller than 35mm so giving a field of view equivalent to 1.5x the focal length.

Thanks.Trying to get my head round it,probably be easier to nail a blancmange to the ceiling
Regards
Chris

A lot of lenses (including a lot of older Pentax lenses) will work on both full-frame and APS sensors. The focal length is the same, it's just the field of view that changes. There has to be a standard reference, and that is based on 35mm when it comes to crop factor.

Here's a definition of focal length (or more correctly, equivalent focal length)

In a hypothetical ‘thin’ lens (i.e. a lens that can be approximated on a ray diagram to a single plane at which refraction occurs), the focal length is the distance from the lens to the point at which a collimated beam of light entering the lens is brought to a point. In Newtonian optics this point defines the focal plane, perpendicular to the optic axis. In practice the focal length is measured from the rear nodal point, or node of emergence.As the rear nodal point is the position of the equivalent thin lens, i.e. the Newtonian lens that would produce the same effect, this distance is properly called the equivalent focal length.

cheers aliengrove,dont really understand the technical stuff but your explanation made me smile a lot
regards
Chris

Just get yourself a Sigma 10-20 and enjoy it....

That'll be a 16mm then
Thanks for your help
Regards
chris