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The Canon EF 8–15 mm is a full frame lens so its resolution test should be based on RAW files from the full frame Canon EOS 1D MkIII reflex camera. We tested the lens on that camera accordingly. What’s interesting, though, you can find out more about its real achievements looking at the results of the test conducted on a smaller sensor of the EOS 50D. No matter how paradoxically it sounds, it is easy to explain why. In order to make our test chart include most of the field of vision on full frame we must get the lens very close to it, working in fact near the minimum focusing distance available. It makes the images from the resolution measurement fields very distorted and shifted. The resolution measurement in the field centre is not possible because the fields, conventionally used for it, are shifted from the centre almost by half of the distance from the upper and bottom frame edge. What’s more, the measurement fields on the edge of the frame are so far away from the centre that they are practically positioned on the border of depth of field and certainly far beyond the curvature of the field. The measurements you get there provide results very different from real achievements of the tested lens.

The situation on the smaller sensor looks far better, though. Although the resolution measurement fields in the frame centre, at the shortest focal lengths are also moved away from the centre, the range of this shift is so slight that we can still assume we measure the resolution in the centre, not somewhere else. The measurement fields on the edge don’t “run away” so much; apart from that, because of the distortions, they are situated on the detector in almost the same place as those on full frame.

Everything described above is clearly visible in two photos, published below which were taken at the least extreme focal length so, in this case, at 15 mm. What’s more, you can see that in the case of the full frame test we have a problem with the appropriate illumination of the scene because no matter how we position the lamps their light is always reflected by the chart. In the case of the test conducted on the 50D the chart can be illuminated more or less evenly.

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EOS 1Ds MkIII	EOS 50D
15 mm	15 mm

Let’s check how the lens performs in the frame centre when it comes to RAW files from the EOS 50D. We would like to remind here that in the case of that detector the decency level is set near 34 lpmm and the best fixed-focal lenses can reach results as high as 51-53 lpmm.

The performance you can see on the graph can be only described by one word: revelation. At shorter focal lengths the lens, even wide open, reaches a level of 50 lpmm or higher and by f/5.6 it gets to a record-breaking values of 53-54 lpmm. At 15 mm it fares a bit worse but only if you compare it to splendid results at the shorter end. On the absolute scale the results on the level of 47 lpmm, which we got even at the maximum relative aperture, are still very good after all. The performance in the frame centre is simply beyond reproach.

Let’s check how the lens fares on the edge of the frame. We should add that, in reality, it is the edge of the APS-C frame but to tell you the truth the image from the area further away from the centre is so distorted that the measurement fields for the APS-C and full frame are situated close to each other on the full frame sensor.

Here you can have some slight reservations when it comes to the performance at the maximum relative aperture. On slight stopping down the lens exceeds the decency level, set near 34-35 lpmm. In actual fact the situation is even better in real life photos because when you use a fisheye lens you rarely take pictures from the distance of 15-20 cm with a good resolution on the edge of the frame in mind. When the photographed object is positioned further away and when we care more about a nice landscape frame, the influence of the depth of field and field curvature is marginalized and the edges, even by f/4.0 and on full frame, still look decent or even good.