sigma 24-70 f2.8 on a nikon d80

[tk shots]

<p>if i stop down on this lens,it's almost impossible to get it sharp...i find the best aperture is f11...anyone know anything about this?</p>

[peter_k4]

<p>Most lenses are sharpest between f5.6 and f11. beyond f11 an optical property called diffraction begins to reduce the contrast and resolution of the lens.</p>

<p>There are some websites with some elaborate flash animations that demonstrate diffraction better than I can explain in words.</p>

[colin carron]

<p>If you look at the photozone review of this lens (result - highly recommended)</p>

<p><a href="http://www.photozone.de/canon-eos/318-sigma-af-24-70mm-f28-ex-dg-macro-canon-lab-test-report--review">http://www.photozone.de/canon-eos/318-sigma-af-24-70mm-f28-ex-dg-macro-canon-lab-test-report--review</a></p>

<p>you will see in the analysis that the sharpmess shown in the MTF charts shoes the peak perhormance can be expected at f5.6 for all focal lengths. Like all lenses it is less sharp at full aperture because optical aberrations are more difficult to control, reaches a peak at f5.6 then drops off for smaller apertures. With a small aperture diffraction increases. This is the property of light where it bends around the edges of the aperture blades so blurring the image. Clearly for a small aperture a bigger proportion of the light will have to go close to the aperture blades so the diffraction effect increases with smaller apertures.</p>

<p>The happy mean for most camea lenses is about f5.6. I found my Sigma 24-70 was a good lens and performed well. I have uploaded an example shot taken at f2.8 with this lens showing the sort of detail I would expect to get. The inset is a 500wide pixel enlargement of part of the full frame.</p><div></div>

[tk shots]

<p>thanks guys....tommy</p>