With “little” understanding of mine, I thought of sharing this about telephoto/telephoto zoom lenses…
Every lens has the following attributes
1. Focal length
2. Minimum focus distance
3. Aperture (Smallest Aperture less light required to capture the picture)
4. Sweet spot (Sharpest @some aperture settings)
Most of the times, lenses performance depends on the above parameters.
The following is some of my explanation of above wrt to 70-300mm f/4.5 to f/5.6 VR.
This lens produces excellent images (Full Frame) when
1. Around minimal focus distance-20 feet when subject is small and 20-40 feet when the subject is bigger.
2. Ultra sharp images when the light is very good/light is on the subject.
3. When light is good and aperture is around f/8
In the above scenario, the images are so sharp that no sharpening in PP is required.
The image quality suffers when the subject is in shade/low light, focus is at infinity etc. In general, lens focusing speed will be reduced in low light and also with larger apertures like F/8, f/11 etc.
Also the bokeh directly depends on the distance of the subject in these cases. When the subject is withing the reach of the lens as said above with less clutter in the BG, most of the lenses produces the best they can (very clean, also one should position themselves accordingly for this).
That is the reason, when shooting subjects at far distances, cropping yields less IQ. So in these scenarios better to use good prime telephoto lenses with good converters to get better pictures as their reach is more.
The above Oriental Magpie Robin image was taken with Nikon D800E camera with Nikon AF-S 600mm VR lens and TC 1.4 II tele-convertor in DX mode.
Some sample shots for the explanation.
Scenarios change for other lenses like Wide angle lenses (a future topic)