LensTip.com

Lens review

Tamron SP 85 mm f/1.8 Di VC USD

12 April 2016
Arkadiusz Olech

5. Chromatic and spherical aberration

Chromatic aberration

The Tamron 1.8/85 VC doesn’t seem to have any longitudinal chromatic aberration problems. Even at the maximum relative aperture the colouring of out-of-focus images is slight – no reservations here.

Tamron SP 85 mm f/1.8 Di VC USD - Chromatic and spherical aberration


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The lateral chromatic aberration correction might be described in similar terms and the graph below is the proof of it.

Tamron SP 85 mm f/1.8 Di VC USD - Chromatic and spherical aberration


As you see that aberration depends weakly on the aperture value and most of results we got here range from 0.04 to 0.06%. It means the aberration is simply low and very difficult to spot in real life photos. The Tamron optics specialists did a very good job here.

Canon 5D III, f/1.8 Canon 5D III, f/2.8
Tamron SP 85 mm f/1.8 Di VC USD - Chromatic and spherical aberration Tamron SP 85 mm f/1.8 Di VC USD - Chromatic and spherical aberration



Spherical aberration

The spherical aberration correction is, for a change, a weak point of the Tamron. A slight focus shift is visible already in the first photo of this chapter and that effect only increases with the stopping down as shown in other photos below.

Tamron SP 85 mm f/1.8 Di VC USD - Chromatic and spherical aberration


When you pass from f/2.5 to f/3.5 the depth of field moves distinctly toward the longer distance. That effect is really pronounced when it comes to firm numbers - in order to show to what extend we made a simple experiment. We set the focus by f/1.8 very precisely and then, without changing it, we took a whole series of photos by higher aperture values. If there were no spherical aberration problems at all the graph should look almost exactly like the resolution graphs presented in the previous chapter: with a fast increase of MTFs from f/1.8 to f/2.8, a high plateau in the f/2.8-f/5.6 range and then a decline caused by diffraction.

Tamron SP 85 mm f/1.8 Di VC USD - Chromatic and spherical aberration


Here the situation looks normal only by f/1.8 and f/2.0, with the results are very similar to those presented in the previous chapter. Further stopping down doesn’t improve the resolution, on the contrary, it decreases it. It is a classic symptom of badly corrected spherical aberration. Unfortunately in this model Tamron repeated the mistake we’ve already witnessed while testing the Tamron 24-70 mm f/2.8 VC.

What’s interesting, though, the out-of-focus light circles don’t hint at serious spherical aberration correction problems. It proves that a proper assessment of that aberration shouldn’t be based only on their appearance. In the case of instruments where the spherical aberration makes itself felt only after a bit more pronounced stopping down the circles might look more or less normal.

Canon 5D III, f/1.8, in front of Canon 5D III, f/1.8, behind
Tamron SP 85 mm f/1.8 Di VC USD - Chromatic and spherical aberration Tamron SP 85 mm f/1.8 Di VC USD - Chromatic and spherical aberration