Lensbaby Burnside 35mm F2.8
Sharpness, or the ability to resolve small details, is an important measure of the performance of a lens. Sharp images let the viewer concentrate on the composition, color and light. On the other hand, soft images (except when looking that way on purpose) distract the viewer and decrease the perceived quality of the picture.
The Burnside is a Petzval design, purposefully dated. By choice, it presents strong field curvature which will decrease the edges and corners sharpness. We expect decent results in the center, less so in the corners.
There are many ways to evaluate sharpness. Some are quantitative, such as the number of lines per millimeter that can be resolved, while others are comparative, such as using a standardized scene to pit lenses against one another. The latter is the favored method at Pentax Forums.
Test Setup, Star Chart
In order to evaluate sharpness, we photograph a standard test chart (or "star chart") that can be used to compare lenses to one another. The general rule is that the distance from the lens to the test chart must be 100 times the focal length of the lens. For our lens, this means a distance of 3.5 meters.
The test is not designed to show how good a lens can be. Quite the contrary: it is a stress test designed to put the lens at its limits to show where it falls short. The test shows the apertures where the lens performs best, and makes it possible to compare with another lens.
Our test has the star chart put successively at the center, edge and corner of the field of view of the lens, testing all apertures each time.
We did not refocus but kept the center in focus even when testing the edges and corners. There are two reasons for this. The first is that we want to illustrate the effect of field curvature. The second is that, at this relatively close distance, the lens was not able to focus accurately except in the center; the focus throw simply was not long enough. Bear in mind that distant subjects will benefit from a deeper depth of field, which will improve edge-to-edge sharpness.
Resolution is obviously dependent on the sensor used. For this test, we used both of Pentax's current top-of-the-line sensor: the K-1's full frame 36 MP.
New Pentax DSLR bodies include the ability to disable anti-aliasing, a very useful feature allowing higher resolutions to be reached. However, the nature of our sharpness test and the pattern of the test chart mean that, with anti-aliasing totally disabled, moiré is likely to appear. In order to strike the best balance between sharpness and moiré reduction, the anti-aliasing simulators were activated and set at the "Low" setting.
Test Results at 35mm on full frame
The following images showcase the results at all apertures on full frame. Focusing was performed by using focus peaking in live view. You can click on images to see full resolution crops.
The center of the Burnside is adequate wide open. It improves marginally at F4 and remains mostly the same until F11. The smallest aperture of F16 is slightly worse but for the most part, results are comparable at all apertures except F2.8 The center never reaches excellent levels but is usable.
Edges and corners are a different story. Edges are almost completely blurred until F11 and corners are even worse.
We must stress that these results are not unexpected. They are part of the lens' identity and should not be overly worrisome. Users looking for modern edge-to-edge sharpness will need to look elsewhere, while those wishing for a creative tool should not be overly worried.
The Burnside 35mm's resolution figures are uncommon, to say the least. The center is usable at all apertures, but never excellent. This is not a lens for high-resolution photography. Edges and corners are mostly blurred, which contributes to the swirly bokeh and subject isolation but certainly won't yield useful resolution figures, especially below F11.
F2.8 to F8 will be best for subject isolation, while F11 and F16 should be used when some sharpness is desired in the corners.