f/4 picture on 2 different lenses

[debs1153]

Hi all

I notice that the same picture taken on 24-70mm zoom, constant f/4 lens (canon) at an aperture setting of f/4 shows sharper surrounding areas as compared to another 24-70mm zoom, constant f/2.8 lens (tamron) at aperture setting of f/4 - which shows softer surrounding areas. The area in focus in both the lenses are equally sharp.

Wanted to check if this is expected, i.e. due to bokeh, or is it an anomaly in the f/2.8 zoom lens which shows softer surrounding areas than the f/4 zoom lens.

Thanks

 

[Octarine]

First of all: Bokeh is a result, not the cause of anything. It describes the quality of the out of focus area. Highly subjective.
Secondly: if really all other factors are equal (no focus problems), then what you observed is a difference in border sharpness between the two lenses.
Sharpness is never equal across the image circle, having the highest figures in the center and falling off towards the borders. All lens reviews will show the sharpness as some form of diagram where the differences across the frame and between lenses can be clearly seen. Take note that there are different methods with different figures and scaling.
Let me guess which lenses you have used and link two reviews here:
Canon 24-70mm f/4 USM L IS: http://www.opticallimits.com/canon_eos_ff/798-canon2470f4?start=1
Tamron AF 24-70mm f/2.8 SP Di USD VC : http://www.opticallimits.com/canon_eos_ff/741-tamron2470f28eosff?start=1
The section 'MTF (resolution)' shows the difference across the frame using different columns/colors for Center, Border and Extreme Border.
Here, the Tamron lens shows lower results for Extreme Border areas at the shorter focal length than the Canon lens.

 

[debs1153]

First of all: Bokeh is a result, not the cause of anything. It describes the quality of the out of focus area. Highly subjective.
Secondly: if really all other factors are equal (no focus problems), then what you observed is a difference in border sharpness between the two lenses.
Sharpness is never equal across the image circle, having the highest figures in the center and falling off towards the borders. All lens reviews will show the sharpness as some form of diagram where the differences across the frame and between lenses can be clearly seen. Take note that there are different methods with different figures and scaling.
Let me guess which lenses you have used and link two reviews here:
Canon 24-70mm f/4 USM L IS: http://www.opticallimits.com/canon_eos_ff/798-canon2470f4?start=1
Tamron AF 24-70mm f/2.8 SP Di USD VC : http://www.opticallimits.com/canon_eos_ff/741-tamron2470f28eosff?start=1
The section 'MTF (resolution)' shows the difference across the frame using different columns/colors for Center, Border and Extreme Border.
Here, the Tamron lens shows lower results for Extreme Border areas at the shorter focal length than the Canon lens.

That’s very helpful! Thanks a lot