Flat-Field Lenses and Why They Matter When Shooting Close-Ups

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If you ask the average photographer what the difference between a macro lens and a “regular” lens is, they will tell you macro lenses enable you to get closer to your subject than regular, or conventional, lenses. While this is true, the ability to focus close is only part of the story. The other part of the story has to do with an optical characteristic called “curvature of field,” of which there are two types—flat-field and curved-field.

All Photographs © Allan Weitz 2020

Most consumer lenses are curved-field lenses, and as you might have noted from personal experience, most consumer lenses are quite good at taking extremely good photographs.

With few exceptions, most macro lenses are flat-field lenses. The difference is that when you take a straight-on picture of a subject with a flat surface, be it a document, a painting, or other flat, two-dimensional subjects, the center of the frame is sharper than the edges of the frame when taken with curved-field lenses, most noticeably at wider apertures. If you want to bring the edges into better focus, you have to stop the lens down. How many stops? That depends on the lens.

At wider apertures, curved-field lenses tend to be noticeably sharper at the center of the frame compared to the edges of the frame. If you want sharp edges, you must stop the lens down, which unless we are shooting under low light or want shallow depth of field is something we do regardless. The front elements of curved-field lenses are shaped more like a fishbowl compared to the flatter front elements on flat-field macro lenses.
At wide apertures, curved-field lenses tend to be noticeably sharper at the center of the frame than at the edges of the frame. For sharp edges, stop the lens down, which unless we are shooting in low light or want shallow depth of field, is something we do regardless. The front elements of curved-field lenses are shaped more like a fishbowl, compared to the flatter front elements on flat-field macro lenses.

Flat-field macro lenses are different. Macro lenses have flatter front elements compared to traditional curved-field lenses. They also deliver edge-to-edge sharpness, even illumination, and little, if any, distortion. Even wide open, the levels of sharpness at the edges of the frame are equal or close to the sharpness levels of the center of the frame. Stop down 2 to 3 stops and any differences in edge sharpness, vignetting, and distortion (if any) become null and void.

By the way, if you’ve ever printed photographs from negatives in a darkroom, the enlarging lenses are flat-field lenses. A curved-field lens would never be able to render the edges of the frame as sharp as the center of the frame. Light falloff and distortion would also become factors.

Curved-field lenses tend to display vignetting, most notably at wider apertures. Flat-field lenses are far more distortion-free compared to curved-field lenses. This is why macro lenses should be used for critical copy work and other forms of artwork and document reproduction. Conventional curved-field lenses can be used, but the results will never be critically accurate. Shown above is the front element of a Zeiss 100mm f/2.8 Makro-Planar.
Curved-field lenses tend to display vignetting, most notably at wide apertures. Flat-field lenses are far more distortion-free than curved-field lenses. This is why macro lenses should be used for critical copy work and other forms of artwork and document reproduction. Conventional curved-field lenses can be used, but the results will never be critically accurate. Shown above is the front element of a Zeiss 100mm f/2.8 Makro-Planar.

The differences between pictures taken with curved- and flat-field lenses can be best illustrated by taping a page of newsprint on the wall and focusing on the page from about 2' away at the widest aperture of your lens. If your camera is squared off to the newsprint, you’ll notice that while the center of the page is sharp, the edges get progressively softer toward the corners. This is because just as the front element of the lens is curved, so too is the plane of focus. And if you were to adjust your point of focus to sharpen the edges, the center of your frame will go soft. The only way to bring the center and edges of the frame into sharp focus is to stop down the lens.

Photographs of newsprint taken with a NIKKOR 50mm f/1.2 AI-S at f/2.8 (left), and at f/8 (right), show the difference in detail, distortion, and vignetting when using a conventional 50mm lens as a copy lens. Even stopped down, the edge detail never measures up to the results you’d get using a flat-field lens.
The same page of newsprint reads far clearer edge to edge when photographed with a 55mm f/2.8 Micro NIKKOR, even wide open. Once stopped down to f/8, the image becomes far superior to the same pages photographed with a standard 50mm lens.

The differences between flat-field and curved-field lenses become even more pronounced when you photograph flat artwork at close ranges.

The portrait of George Washington on a one-dollar bill reproduced at approximately 1.8x using a NIKKOR 50mm f/1.2 AI-S with 20mm of extension tubes. At f/2.8, the resolution is poor throughout most of the image area outside of the extreme central portion of the frame. Stopping down to f/16 doesn’t improve the image quality much.
Photographing a closeup of the same dollar bill using a 55mm f/2.8 Micro NIKKOR yields far better results, even wide open (left). Stopped down to f/8 (right), the image is sharp and crisp edge to edge.

I also tried digitizing a 35mm Kodachrome slide using my Franken-Scanner with the 55mm f/2.8 Micro NIKKOR AI-S I generally use for digitizing 35mm slides and a NIKKOR 50mm f/1.2 AI-S to see how the results compared. Even with the lens stopped down to f/16, the images reproduced with a conventional curved-field 50mm lens look as if they were shot with a Lensbaby. The image files captured with the 55mm Micro-NIKKOR were undistorted and sharp edge to edge, even at wide apertures.

Digitizing 35mm slides at 1:1 magnification with a standard 50mm lens at f/2.8 yields terrible results (left). Even stopped down to f/16 (right), the results look like they were taken with a Lensbaby.
Digitizing 35mm Kodachrome slides using a flat-field macro lens yields far better results compared to digitizing slides using a conventional curved-field lens.
Digitizing 35mm Kodachrome slides using a flat-field macro lens yields far better results than digitizing slides using a conventional curved-field lens.

Can flat-field macro lenses be used for portraiture, landscape photography, and other genres of photography? Absolutely. And having a lens that can capture sharp photographs from life-size to infinity is a handy tool to have in one’s camera bag, regardless of what you are photographing.

If there’s a so-called downside to using macro lenses for uses other than shooting close-ups, it would have to be that flat-field lenses tend to be “contrasty” compared to curved-field lenses, which can sometimes be problematic for portraits and beauty photography.

The only other notable difference between flat-field and curved-field lenses has to do with the way they render the plane of focus when used for conventional photography. Because macro lenses have flat focus planes, the depth of field in the photograph has an ever-so-different visual dynamic compared to the same image captured with a curved-field lens, and this holds true regardless of whether you’re shooting wide open or stopped down.

To test this statement, I photographed the hand rest of an old wood bench with a 105mm f/2.5 NIKKOR-P and a 100mm f/2.8 Zeiss Makro-Planar adapted for use on a Sony a7R III using Novoflex lens adapters. The focus point was the tip of the armrest. Putting aside the 5% magnification factor between the focal lengths of the two lenses, the differences between the background focus clarity and the degree of focus fall-off between the two lenses is noticeable. Which is preferable? The answer is subjective—which do you like better?

The arm of a weathered bench taken with a 105mm f/2.5 NIKKOR-P (left), and a 100mm f/2.8 Zeiss Makro-Planar (right), at f/2.8. The differences are subtle between flat and curved-field lenses when shooting from greater camera-to-subject distances, but they are there.
Even when stopped down to f/16, the differences between the curved-field 105mm lens (left), and the flat-field 200mm lens (right), can be noticed. Which rendition is preferable? That’s your choice. Aesthetics are subjective.

Have you ever captured and compared pictures taken with macro lenses and conventional lenses? What’s your take on the subject? Let us know in the Comments section, below.

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