Field Curvature. Friend or Foe?

Some years ago I looked at the Samsung NX 16mm f/2.4 prime lens – a very appealing wide angle lens in terms of focal length and size – it’s tiny!

But I was put off by the review on Photozone.de which said:

sometimes the field curvature gets too extreme and, unfortunately, the NX 16mm f/2.4 has an issue here.

The test

In the end the lure of a wide angle prime was too great. I purchased one and, with some trepidation, took it on a test shoot. I’m a landscape photographer, so I chose a typical landscape scene, set the aperture to f/8, focussed at or near the hyperfocal distance of 7 feet and took some pictures. At home at put them on the computer and processed the RAW files in DXO. The result – perfectly sharp photos – sharp from corner to corner!

At the time I didn’t think too much about it. I just thought that maybe photozone had tested a bad sample or I had an unusually good sample or DXO was simply amazing. A friend of mine also got the same lens and had similar results. And he didn’t use DXO so maybe it was just that photozone had a bad copy.

Another lens – the EF 20mm f/2.8

Fast forward to this week and I’m looking at the old Canon EF 20mm f/2.8 USM lens as one to use on the new EOS RP. Again, I was put off by the review:

Things are rarely as simple as they appear and this is especially true for the EF 20mm f/2.8 USM. The resolution may be fine but the lens suffers from a rather massive amount of field curvature

This time, however, I got a second opinion.

I looked on Ken Rockwell’s site for his review and found something truly eye opening:

A little-known feature of the Canon 20 USM is “Intelligent Field Curvature (IFC),” meaning that the surface of best subject focus is not flat, and deliberately curved to fit real ultrawide subjects. The 20/2.8 USM curves its plane of best focus so that objects away from the center are brought into better focus if they are closer to the camera, exactly as subjects do in real life.┬áThis means that this lens doesn’t test that spectacularly on flat targets (which make boring photos), but in actual use, the 20/2.8’s images are much sharper since the plane of best focus is adjusted to focus on objects closer to the lens at the sides, tops and bottoms of the frame. In other words, a sweeping desert floor below you, or objects to your sides, will be in better focus than with a flat-field lens.

This info caused me to have one of those aha moments, when suddenly everything made perfect sense.The NX 20mm lens was so sharp because of the field curvature!

The explanation

Lens tests do not demonstrate how a lens performs in the real world. They show how well a lens performs when taking a picture of a flat test chart. For the lens to do well against a flat chart its plane of focus must be perfectly flat. Otherwise parts of the chart – the edges – will not be in proper focus and so won’t return a high score.

Well, how often do you use a wide angle lens to take a picture of a totally flat object? Very rarely for most of us. The only exception I can think of are astronomers, who use fast, wide, lenses to get sweeping panoramas of the night sky. Effectively, that’s a flat subject as everything is at infinity focus.

But most of us use a wide angle to get a deep picture – a prominent foreground object extending to a (hopefully) picturesque backdrop. To do that we use a small aperture (f/8 or f/11) and focus somewhere towards the lower half of the frame – the hyperfocal point. Then, the depth of field is deep enough to give an illusion of sharpness throughout the frame.It doesn’t take too much imagination to see how field curvature really helps here. The curved focus plane brings foreground objects into sharp focus even if the camera is focussed behind them – far sharper than a flat focal plane and hyperfocal focussing could ever do.True, the very top of the frame may not benefit from this – but typically it has blue skies or clouds in it and these won’t look bad even if they are a little out of focus.

So, for a landscape photographer at least, field curvature is your friend, provided the curve is bringing foreground objects into better focus. (If the curve goes the wrong way – making the edges of the frame blurred – then the curve works against you.)

Make use of this!

If you are a landscape photographer and you have a lens with favourable field curvature then you can use this to your advantage in two ways:

  • Just focus at the hyperfocal distance and feel smug because your foreground will be sharper than it would be with a perfectly flat field lens
  • If the most important part of the scene is further than the hyperfocal distance, focus on that part of the scene! The field curvature will bring the foreground into better focus anyway!

The reality is, having a wide angle lens with favourable field curvature is like having a tilt and shift lens without having to pay all that extra cash for it!

Lens tests

This is another reason I recommend not taking lens tests too seriously. They simply do not tell you how a lens will perform in the real world. If I had relied on the reviews I would never had purchased the NX 16mm and I would have lost out on the many wonderful images I took with it.Did I buy the EF 20mm? You bet I did!

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