Sirui Aurora 85 mm f/1.4
7. Distortion and field of view
Field of view
A rectilinear 85 mm lens should provide a 28.6-degree field of view on a full-frame sensor, and SIRUI gives exactly the same value in the specifications for the Aurora 85 mm f/1.4. We decided, however, to check the actual field of view in practice. To do so, we photographed a starry sky, using uncorrected JPEG files. We then transformed the pixel coordinate system (X, Y) into an equatorial coordinate system (right ascension and declination) describing the positions of stars in the sky. This allowed us to determine the lens’s field of view with very high accuracy and in the correct way — for rays arriving from infinity. The transformation was based on the positions of 133 stars evenly distributed across the entire image. The mean coordinate-grid fitting error was only 5 arcseconds.Our result was 28.54 degrees, with an error of 0.05 degrees, which is virtually identical to the value given in the specifications. Everything is therefore perfectly in order in this respect.
Distortion
Interestingly, even on the smaller APS-C/DX sensor, we can detect slight pincushion distortion, with our measured result being +1.13%. This is quite a surprise, as the last thing one would expect from an 85 mm-class lens is problems with geometric distortion.
For obvious reasons, the situation is worse on the larger full-frame sensor, where the distortion increases to a clearly noticeable +2.71%. The 1.4/85 lenses we have tested previously very rarely showed distortion exceeding +1%, so the SIRUI’s result approaching the +3% level has to be considered poor.
Please Support UsIf you enjoy our reviews and articles, and you want us to continue our work please, support our website by donating through PayPal. The funds are going to be used for paying our editorial team, renting servers, and equipping our testing studio; only that way we will be able to continue providing you interesting content for free. |
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
| Sony A7R IIIa, 85 mm, APS-C/DX | |||
 |
|||
| Sony A7R IIIa, 85 mm, FF | |||
 |
|||
