Sigma A 50 mm f/1.2 DG DN
6. Distortion and field of view
Field of view
A rectilinear 50 mm lens on a full frame sensor should provide you an angle of view of 46.8 deg and such a value exactly is given by the producers in official specifications. We decided to check how big this field really is. In order to do so we took photos of starry sky; then we transformed the pixel layout (X,Y) from the photo into the equatorial coordinate system (right ascension and declination), which locates a star on a celestial sphere. That way we were able to measure the field very precisely as it should be done, for rays of light coming from infinity. Our transformation was based on positions of 132 stars spread evenly across the frame and average mesh-fitting error amounted to a tad over one minute of arc.Our final result amounts to 48.07 deg with a measuring error not exceeding 0.1 of a degree. The real field of view of the tested instrument is a tad wider than the declared one and it corresponds to the 48-49 mm focal length.
Distortion
In case of the smaller APS-C/DX sensor you won't have any problems with geometric deformations of images as the distortion level reaches just -0.33%.
What's interesting, you you take average measurements on the whole area of the full frame our result decreases to a practically zero level of _0.07%. One glance at photos of our chart and you know why. Sigma optics constructors decided to make distortion control easier by introducing slight moustache distortion that, near the very edge of the image, bends straight lines in the opposite direction.
If you limit your measurements to the area within 1:1 markings your result will increase to -0.50%.
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. |
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Although we are not big fans of moustache distortion all results, presented in this chapter, are low and don't give you any reason to complain. In the Sigma's defence you might add that the constructors of the more expensive Sony 1.2/50 GM adopted a similar approach.
| Sony A7R IIIa, 50 mm, APS-C | |||
 |
|||
| Sony A7R IIIa, 50 mm, FF | |||
 |
|||
