Lens Ratio

Hello all
Why when reviewing a lens,do they state a 300mm, say on a k-r is equal to 450mm on a full frame camera.
Does this mean if i go out with my made up friend who has a pentax full frame camera? and i have my k-r with a 300mm lens and we take the same shot at the same distance, does he need a 450mm to take the same size image. As the sensor on a full frame is about 2.3 times larger than on the k-r i would of thought he would only need a 130mm to capture an image of the same size, or am i missing something.
Cheers
Dave

Dave, I think you are more or less right. As far as I know (and I could well be wrong), I think the logic is: The focal length of the lens is not related to the size of the sensor. However, 35mm format is generally used as the baseline against which everything else is measured hence things tend to be referred back to that format although I don't know if it is all that helpful.

If you imagine the 300mm lens mounted on the full frame camera, it will produce a particular image, when that same lens is mounted on a crop-sensor DSLR, the image is a cropped version of the same image - i.e. the edges will have been cropped off relative to the full frame image - but everything else will be the same; the image of the central subject of the image on the crop sensor will be the same actual size as on the full frame sensor - but the full frame sensor will have more background around it . (i.e. the issue is image crop rather than magnification).

Therefore the full frame shooter can produce an identical image, at the same size, to the cropped sensor user by cropping the image in post-processing. If he wanted the same picture in the sense that the central subject took up the same proportion of the overall frame (bearing in mind that the full frame is bigger), then he would need the extra magnification of a longer lens, and given the crop ratio of 1.5 then 450mm would be needed.

I think that's how it works anyway - someone will correct me if I'm wrong

Cheers

Imagine you have a standard 6x4 print. Now take a knife and cut out the middle 2/3rds of the print (at the same ratio). The original print is what the "full frame" camera would have taken, and the middle 2/3rds is what you would get on APS-C. Everything else remains the same.

Hope that helps.

Mat W and Kris
Thanks for the explanation, i think i get it.
Its been such a long time since i used a 300mm on 35 mm film camera the images look smaller than i remember but mybe a lot of things are like that as you get older.
Cheers
Dave