Every lens has the following attributes and lens performance depends on the below parameters
- Focal length
- Minimum focus distance
- Maximum Aperture
- Sweet spot (lens is sharpest at a particular aperture)
Lenses in general produces excellent images when
- When the subject is at minimal focus distance of the lens (or)
- When the subject is filled most of the frame, require less or no crop (or)
- When the subject is lit with soft or diffused light
The image quality suffers when the subject is in shade or when the light is low or when the lens is focusing at larger distances where cropping is required etc. In general, lens focusing speed also will be reduced in low light and also with larger apertures like f/8, f/11 etc.
Lenses produce best Bokeh when the distance of the subject to the lens is less and the subject to background distance is more.
Some sample shots explaining the above.
displayimage.php?id=150133 (shot at minimal focus distance with no clutters in the Background which yielded smooth Bokeh)
displayimage.php?id=144947 (lens produced sharp details as the subject is within the lens reach)
displayimage.php?id=150357 (when the subject is small and at a distance, image quality suffers due to cropping).