In this episode of Mastering the Art of lens testing, we focus on 2 interrelated and difficult aberrations : Astigmatism and Field Curvature.
Astigmatism describes the inability of a photographic lens to image a point in the scene as a point on the sensor. It occurs when light rays entering the lens in the in one plane are focused at a different place than light rays entering in a plane at right angle. A point of light therefore becomes spread out into a line to some degree. And the rendering of an object detail depends on the orientation of that detail.
Astigmatism is only visible off-axis, unless the lens contains decentered elements. With modern lenses, it is rare to observe astigmatism at all.
There’s actually little point in trying to find astigmatism in a lens. The reason for testing is to ascribe or rule out astigmatism as a cause for unsharpness.
The classic test scene is the cart-wheel, as seen above. In an astigmatic lens, the spokes will be sharper than the rim or vice versa. Radial lines (the spokes) and sagital lines (the rim) are never equally sharp in an astigmatic lens. Since the points in the image direction get elongated in one direction, unsharpness will look like motion blur, either outwards from the center of the frame or at right angle to that.
So, try to find a flat subject such as a stained glass rose window and inspect your photograph for any unsharpness that’s limited to a specific direction.
Astigmatism usually does not affect the center of the image. The effect becomes stronger further away from the center of the photograph. Any deviation from this patter (asymmetric astigmatism, astigmatism at the center of the field) likely indicated a decentered lens.
Astigmatism cannot be treated in post-processing. You can reduce its effects by closing down the aperture.
Field curvature is one of the most difficult aberrations to correct for a designer. The essentially spherical surfaces of lens elements do not naturally create a flat image of the world. A naturally curved field is the main reason for ongoing research in curved sensors.
Imagine taking a photograph of that nefarious icon of lens testing, the brick wall. When the lens projects a spherical image of the flat wall, if you focus on the center of the wall, that center will be sharp, but as you move towards the edges, the zone of sharpness is progressively further away from the sensor. Therefore the edges and corners are increasingly unsharp.
Another way of looking at this is that the area of the world that is imaged as sharp on the flat sensor is itself a sphere, rather than a plane. This round surface can bend inwards (closer to the photographer at the edges) or outwards. Some configurations are actually helpful. For example, in a portrait with the subject at center, inward curvature ensures all the background is nicely blurred (out of focus) without having to buy an f/1.4 lens.
There are 2 ways to do this. A fun way and a useful way 😉
Useful first. Take a photograph of a flat subject (hey, the famous brick wall awaits) or landscape at infinity. A sharp center with blurred edges can indicate a lens that’s not sharp or a lens that has strong field of curvature. In the example below, the chair is in focus. The background is pin sharp are center and very blurry on the edges. This is not because the lens is not capable of producing sharp images in the corners but simply the edges and corners are out of focus.
To determine between the two, refocus so that the edges become sharp. If they do (the center will probably be blurred by now), then the lens has field curvature. If they never do sharpen up, the lens is simply not sharp. Be sure to do this at mid distances rather than infinity as, otherwise, you may not be able to refocus beyond infinity.
The fun way: if you lens can focus very close and has a wide maximum aperture, it can produce a very shallow depth of focus. If not, use an extension tube such as those found on macro lenses to reduce for min focusing distance. Take a photograph of a textured surface or a book to visually show the zone of focus. In the picture below (Zeiss OTUS 85) the sharpness area runs almost perfectly horizontally from the screw on the left to below the scratch plate.
This is not typical of what you will find in the field but it can help you visualise the shape of the curvature (inwards, flat, outwards). If flat, all is well. If not, always make sure you do not focus and recompose with the lens.
As with any unsharp photograph, there’s very little to do to fix a picture damage by field curvature. Stopping down helps prevent it though. It doesn’t actually fix field curvature, but the increased depth of field helps cover it up.
Anyone using focus stacking software may also have an antidote for field curvature. Taking photographs at various focus positions and combining all of them into a completely sharp on should produce a globally sharp picture. But I have not tested this and make no promises.
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