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Lens review

Tamron SP 24-70 mm f/2.8 Di VC USD

23 June 2012
Szymon Starczewski

7. Coma, astigmatism and bokeh

The coma is practically imperceptible on the edge of the APS-C/DX sensor. In the corner of that detector the image of the diode remains circular and its deformations are very slight. A bit different situation can be see in the corner of full frame. The coma is noticeably bothersome there at the widest angle of view; fortunately for bigger focal lengths it becomes smaller. Even at the shortest focal length it would be difficult to call it exceptionally high.

Tamron SP 24-70 mm f/2.8 Di VC USD - Coma, astigmatism and bokeh

The astigmatism is corrected well. The average difference between vertical and horizontal MTF50 values amounted to 4.3%. It is worth noticing, though, that at 24 mm the results were below the average and at 45 mm – above it. At that latter focal length the achievements of the lens are the weakest and it must be said that the astigmatism contributes to it although it is hardly the most dominant factor.

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Let’s check what information defocused images the tested lens can provide.

Tamron SP 24-70 mm f/2.8 Di VC USD - Coma, astigmatism and bokeh

Very sharp and distinct concentric rings in all defocused images are a real surprise. On stopping down their intensity seems to increase – that change, along with the change of their structure is especially pronounced while passing from f/4.0 to f/5.6. It is exactly that place where the spherical aberration made itself felt the most keenly, all of it confirming our observations from chapter 5. Fortunately there are a lot of that concentric rings with a changeable light intensity and they are densely packed so the blurs in real life photos won’t necessarily look bad. With such a strong background blur and not exactly point-like sources of light those defocus light rings might actually seem to be quite smooth.