Originally posted by mikesbike But putting a 300mm f/2.8 lens on your K-1

Actually, that lens would give a different image than a 200/2.8 on a K-5. The difference is DOF, depth-of-field. A 300/2.8 has a shallower DOF than a 200/2.8.

Let's compare the numbers, assisted by the online DOF calculator* on

http://www.dofmaster.com/dofjs.html
300/2.8 on K-1, 10m subject distance gives a DOF from 9.91m to 10.1m, resulting in roughly 18cm.

200/2.8 on K-5, 10m subject distance gives a DOF from 9.83m to 10.2m, resulting in about 35cm

Thus the DOF is nearly twice as deep.

Again, with a larger subject distance:

300/2.8 on K-1, 100m subject distance gives a DOF from 91.4m to 110.4m, in total 19m

200/2.8 on K-5, 100m subject distance gives a DOF from 85m to 121.4m, in total 36.4m

Here the DOF is also twice as deep.

Let's fiddle with the aperture, step down on the K-1 and look at the resulting DOF:

(for sake of brevity I'll only list the total values)

300/2.8 - 19m

300/3.2 - 21.3m

300/3.6 - 24m

300/4.0 - 27.1m

300/4.5 - 30.5m

300/5.0 - 34.5m

300/5.6 - 39m

To obtain an image similar to one taken with a 200/2.8 on the K-5, you'll need a 300mm lens stopped down (or just not being faster than) something between F5.0 and F5.6.

This is between 1⅔ and 2 f-stops slower than the lens on the K-5.

You could spare both money and weight ...

were it not for the higher sensor sensitivity you'd need shooting with a lens 2 f-stops slower.

*) Just for clarification purposes, I've chosen "35mm film" and "aps film" for the comparison