Sony FE 16 mm f/1.8 G
6. Distortion and field of view
Field of view
A rectilinear lens with a focal length of 16 mm on a full frame sensor should provide you an angles of view of 107 deg and the producers provide exactly this value in their official specifications.The Sony FE 16 mm f/1.8G, after attaching to a camera, deactivates a distortion correction option setting it on automatic. It suggests very strongly that optics specialists passed on distortion correction and left it to the camera software. Such an approach is always a kind of compromise and it is an option possible to accept as long as you get someting extra in other place. Of course the lens also should offer you an appropriate amount of spare field of view for distortion correction and cropping of images. Of course, as always, we decided to check everything thoroughly.
In order to do so we took photos of starry sky and saved them as JPEG files, with automatic distortion correction, and RAW files, developped by a third-party software that doesn't correct distortion.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 could determine the field of view of the lens with utter precision and in the right way, so for rays of light coming from infinity.
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The transformation for the corrected JPEG file was based on the location of 203 stars spread evenly across the frame and the average mesh-fitting error amounted to 2 minutes and 5 seconds of arc. We got a result of 108.45 deg with an error that doesn't exceed 0.05 of a degree. As you see our result is slightly higher than official declarations and it reflects well the rectilinear focal lenght near 15.5 mm. In a moment we are going to show that this spare area is more than appropriate because even in corrected JPEG files there is still slight barrel distortion that can be additionally adjusted with different image processing programmes.
With uncorrected RAW files at our disposal we coud check how wide field of view offers you the optical system of the new Sony lens. This time our transformation was based on 209 stars and the average mesh-fitting error was higher and amounted to 4 minutes and 50 seconds of arc. The field of view is also noticeably wider, reaching 112.8 deg with a measuring error below 0.1 of a degree. Here you deal with an equivalent of just 14.4 mm focal length.
Distortion
First let's deal with distortion results on the smaller APS-C sensor. In case of a JPEG file, automatically corrected, we detected a very slight barrel distortion of -0.74%. After passing to an uncorrected RAW file you see that the optical system doesn't even try to fight off any defromations. You deal here with a very high level of -4.78%.
| Sony A7R IIIa, 16 mm, APS-C, JPEG | |||
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| Sony A7R IIIa, 16 mm, APS-C, RAW | |||
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As distortion problems are visible already on the smaller sensor, we wondered about the results on full frame. Photos of our testing chart, presented below, were saved both as corrected JPEG files and uncorrected RAW files and can explain the situation very well.
| Sony A7R IIIa, 16 mm, FF, JPEG | |||
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| Sony A7R IIIa, 16 mm, FF, RAW | |||
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While JPEG file doesn't provide you any reasons to worry, as it shows very moderate barrel distortion of -1.49%, the RAW files make us lose all the optimism. An average result across the whole frame reaches as high as -7.47% and it is no end of bad news. This value is kept in check by introducing moustache distortion. As a result, when you limit the measuring field to the 1:1 area the result increases to a huge value of -8.6%.
It's worth adding that these measurements we conducted with the help of out biggest testing chart but with such a wide angle of view as the one, provided by the tested lens, we still had to position ourselves only about 1 meter from the aim. In this case the increase of the size of a testing chart helps but only slightly. After all if you wanted to take photos from a distance of 5 meters, you would have to use a chart of significant dimensions – over 11 meters x almost 8 meters.
Ultra wide angle of view of the lens and, stemming from it, the significant depth of field, joined with a possibility of using the f/22 aperture allow you to avoid these problems and make a measurement for infinity. Even standing close to the chart you are able to set the focus (so the whole optical system) at infinity and, in case of the f/22 aperture, take more or less sharp photos of the chart that might be used for such measurements. We present them below.
| Sony A7R IIIa, 16 mm, FF, JPEG | |||
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| Sony A7R IIIa, 16 mm, FF, RAW | |||
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This time the results are a bit lower. In case of corrected JPEG files you deal with a level of -1.03% and for uncorrected RAW files this value increases to -7.17%. In this last case the result is also connected with the presence of a noticeable moustache distortion.
As you see, the spare field of view, even in JPEG files, is a sensible solution and we are very glad Sony decided to add it. As a result the JPEG files corrected in the camera are as bit as almost 108.5 deg and you can correct them once again with software installed in your PC. This way you get zero distortion and still you might enjoy the declared field of about 107 degrees.
You should remember, though, that there is a price to be paid for all of this. The original resolution, provided by the camera's detector is going to be reduced. All operations involving distortion correction and further cropping of images will decrease the effective resolution in a distinct way.
