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How sharp is this budget lens? (a technical post)

I have read posts that suggest that, when stopped down, a budget lens can almost equal a more expensive one. I have decided to put this to the test with my old, budget, film era, 75-300mm Canon lens.

For this test, the camera was mounted on a sturdy tripod, the mirror was locked up, and a 2 second self timer was used – all to minimise camera shake. In addition, the focusing point was ‘micro adjusted’ to its optimum (see this previous post). Here are some of the results:

75mm f4 cornerThis shows the corner at 75mm f4. Bear in mind that this is a full format lens, so with the cropped format sensor of my camera, this  section is about half-way out to the edge (all lenses tend to be sharper at the centre compared to the edge). This is unacceptably soft. This next one is at F16:

75mm f16 cornerA lot sharper, but not pin-sharp. This would probably withstand printing up to A4 size. Zooming in to 135mm and looking at the centre of the frame, at F8:

135mm f8 centreModerately sharp, again this would probably print OK up to A4 size. But when stopped down to f32, although the depth of field has increased, sharpness has taken a hit:

123mm f32 centreOK for a postcard size perhaps… Going now to 300mm, this shows the corner at f5.6:

300mm f5.6 cornerQuite blurred with purple fringing – not nice. Stopping down to F16 improves things a little:

300mm f16 cornerBut this is still rather soft with purple fringing. This might print OK at A4 but it is pushing it a bit.

So my conclusion? Much depends on the final use. With care, stopped down to about f11-16 and in the middle of the zoom range, with a bit of sharpening I might be able to get an acceptable A4 print. Otherwise I better start saving for an L series lens (all donations gratefully accepted!)