Confused again!

Hello all, Ok i have the sigma 17-70

http://www.sigmaphoto.com/lenses/lenses_all_details.asp?id=3315&navigator=6

and have now just bought the Sigma 28-300mm Hyperzoom DG f3.5, its for 35mm but when i had a play earlier today, i set the lenses on a tripod and the took images at 28,25,and 70 and they all look the same, i thought there should of been a cropped field of view/ magnification

28-300mm on 35mm film provides proper wide angle at the short end. You were mistaken if you were expecting the same on Pentax digital. That lens is not a good choice for digital unless you don't mind having hardly any wide angle.

ok so both images should be the same ?, i was under the impression that the 28-300 should have some of the image slightly cropped

i new i wasnt getting wide on the camera,hence why i have the 17-70 for my widest lens,the 300mm is just for me to have a go at captureing the moon!

As Pete says the focal length doesn't change, it's just the diameter of the image circle that differs due to the larger diameter optical path with 35mm format lenses.

Just confused my self again, i have been with a friend of mine who has a full fram 5dmk2, he has totally confused me though, even though he ment well. grrr

No, there's no cropping because the image circle is more than large enough to cover the digital sensor.

It's the other way round. A digital lens can cause cropping (vignetting) on a film camera.

Ok, good luck with that

You can forget about cropping issues unless using a "digital" lens on a film camera.

Reading between the lines I think organicimagery's use of the term 'cropping' is incorrect, but I think he's getting this from the frequently used term the 'crop factor'.

(For organicimagery's benefit): But this of course matters when you have a point of reference from a 35mm body. If you are only ever using digital, then there's no difference. It's when you place the same lens from a 35mm/full frame body to a cropped APS-C DSLR sensor that you notice the extra gain in apparant focal length.

I find it helpful to think of it like this...

Imagine you have a postcard. It doesn't matter what focal length lens it was taken on. Now take a scalpel and cut out a postcard-shaped hole in the middle, 2/3rds the area of the whole. The whole postcard represents what you would have got using a 35mm camera, the bit you have cut out represents what you get when putting the same lens on an APS-C digital camera.

The only wrinkle is that if the lens is designed for APS-C, the corners of your postcard are likely to be dark.

HTH. Kris.

moral of the story, dont listen to canon users lol(joke by the way)

Its all irelivant i know that, just wanted to clarify it in my head, phew.

This is what i read made life easier, http://www.kenrockwell.com/tech/crop-factor.htm

Ok so 28mm is wide on a full frame/35mm camera, however when on a dslr it applies a 1.5 field of view, to compensate for the smaller sensor,from old thread from hefty:

Focal length is focal length and a 300mm lens on film is still a 300mm lens on digital.

The difference is that your digital sensor "sees" a cropped part of the image projected by the lens (because it's smaller than a 35mm film area). Hence a 300mm lens fitted to a digital camera will "see" an area that's roughly the same as that "seen" by a 450mm lens on 35mm film.

A 300mm lens designed for digital will give exactly the same view as a 300mm lens designed for film on your GX10.

Does that make it clearer?