This is my final attempt.

Firstly, as I have stated before, I do not understand the optics or maths of depth of field calculations, so I can only rely on information from those that (presumably) do.

Secondly, the ONLY point I am trying to make is that the depth of field scale on an old lens made for 35mm full frame will indicate the wrong D.o.F. if it is used on a K-series (1.5 crop factor) dslr.

Thirdly, the evidence for that point only is from D.o.F. calculators, e.g. link 1; link 2; link 3; (a Google search will find lots more).

All I can suggest is that you use a D.o.F. calculator and see for yourself that the results (for same f, same A, and same distance) are different for full-frame and 1.5 crop. And if those results are correct, the D.o.F. scale is wrong when the lens is used on a Pentax K dslr.

So finally, if you two want to disagree with that point, you will have to argue not with me, but with the technicians who have produced the algorithms for the D.o.F. calculators, cos I can't explain their results.

Now that's it from me - life's too short.

Philip

The two Johns,
Your points might be relevant to practical photography but fail to support the original point that was made, quote - "Many of the older prime lenses actually show the depth of field available at each aperture". The usefulness of that statement is challenged by depth of field calculations, produced by science technicians who really understand this property of lenses, showing that for the SAME focal length lens, at the SAME aperture, focused at the SAME distance, the depth of field is DIFFERENT on a different format. Therefore, the depth of field scale printed on the barrel must only apply to the format for which the lens was designed.

Philip

Most of the optics/maths of this are beyond me, but a D.o.F calculator gives different results for different formats. E.g. Using the one at Cambridge in Colour for a 50mm lens at f/8, focused at 8 metres -

On 35mm (full-frame):
Nearest Acceptable Sharpness: 4.41m
Furthest Acceptable Sharpness: 43.03m
Total Depth of Field: 38.62m

On crop factor 1.5:
Nearest Acceptable Sharpness: 5.21m
Furthest Acceptable Sharpness: 17.24m
Total Depth of Field: 12.03m

So it would appear to me that reading the D.o.F scale of a lens designed for full-frame use would give the wrong D.o.F when it is used on a K-series body.

Philip

I agree with Carl. If you want to keep the cost down but extend the focal length range, you will probably have to accept the need to swap between the 18-55 and the 50-200.

Alternatively, you might like to try a prime as your walk-about lens - you can see the excellent quality of the relatively cheap Pentax da 35mm f/2.4 on other threads here, e.g. link.

Philip

An apt style name would be K-sd, akin to some modern music performers, but also similar to a Pentax - there is always endless speculation about the next model, it seems to take ages to arrive, and it might be King - some day.

Philip

The depth of field scales on these older lenses assume a full frame image (35x24mm on film). However, with a Pentax K-series crop frame sensor (24x16mm), the actual depth of field is only similar to that on the scale for wide apertures (e.g. f/2.8 ) and short subject distances, at which the D.o.F. is unreadable anyway, as it is almost nil.

Philip

The rules and calculations are not simple but there are several free apps for smartphones to work out depth of field for you instantly after entering the camera, focal length, aperture and subject distance. E.g. For Android phones open Google Play and search for "depth of field calculator"; I expect there will be similar for iPhones.

Philip

I agree with the others re. the SRS K-5 deal - £449 is a real bargain for such a good camera.

Philip

Not sure about the #1, Jon, but I like the other two. Perhaps #2 would be even better if the nearest pebbles were also in focus.

Philip

This is the front of Coleton Fishacre House, formerly the country home of the D'Oyly Carte family (of Gilbert & Sullivan operas):




Philip