Focus demands Resolution and Contrast
What determines sharpness and blurring in an image? Bertram Hönlinger, optics expert at ZEISS, talks about the performance of modern camera lenses and the limits of what is technically feasible.
Mr. Hönlinger, in photography the topic of “sharpness” is touched upon time and again. What exactly does it mean?
Bertram Hönlinger: One of my colleagues once put it like this: “Focus is an out-of-focus term.” By that he meant that there is no clear-cut definition of focus in photography. We see the concept much more on the subjective level.
But you are an optics expert. So I am sure you have a very specific image in your mind of what focus means.
Focus is closely related to depth of field. As soon as I focus on a point within a shot, say the eyes of a model when I am taking a portrait, a focal plane is created that ideally runs parallel to the camera’s sensor. One point on this focal plane will appear as a single point on the sensor – or almost will. It is a different story with the details in my motive points which lie either in front of or behind this focal plane: the rays of light reflected by them do not meet at a point in the camera sensor, but are in fact spread across an area around the target point.
This is known as the circle of confusion …
Correct. The size of the circles of confusion determines whether or not we see an image region in focus – the smaller the circles, the better defined the image. For a long time, it was standard practice for a circle of confusion of max. 0.03 millimeters to still be perceived as being in focus. But as cameras have ever-higher resolutions, it now makes sense to have values between 0.01 and 0.015 millimeters.
But there are still distinct differences in terms of focus – for example, when I work with different aperture settings. Why is this the case?
Let’s assume that I have learned my trade as a photographer and always perfectly focus my shot. I could still get different results with different camera settings, i.e., that one image looks a lot more in focus than the other, even though I focused properly. Two factors explain the difference: one is my camera system’s resolution capacity. If I can recognize finer details in an image, such as skin textures in a portrait, the image appears clear to me – but only if the second factor also comes into play: the contrast – or, more specifically, the micro contrast. The details will then be visible, but with just a low contrast between the light and dark structures I will see an image as not being completely sharp, despite the high resolution. Contrast and resolution capacity vary with different aperture settings, as well as with different lenses and cameras.
How can I use focus as a creative tool?
Playing with focus and blurring is a key design element. It allows me to make my main subject stand out against an out-of-focus fore- or background. In this way, I can draw the viewer’s attention to what is truly important and create the impression of spatial depth.
And how can I create an image with exceptional definition?
First I need a high-quality, perfectly corrected lens, ideally with a consistently high level of performance in terms of contrast rendition and resolution — in the center of the image and at the edges focused to infinity or to the closest focusing distance, and this at f/1.4 or at f/22. Of course, there is no such thing as the perfect lens. So all we can do is try to come as close to perfection as possible. Each lens therefore has an aperture value at which it achieves the best possible image quality, i.e., the best contrast rendition and resolution. With DSLR lenses, this aperture value is primarily between apertures f/4 and f/11, and often at f/5.6. This is the aperture setting I should select if I want to achieve optimal performance.
What can the photographer do to achieve all this?
It goes without saying that the photographer plays an important role in creating focus. The best equipment will not help me if I do not focus precisely. The depth-of-field scale can be a great help: when taking photos, you can use it to predict what will be in focus before you take the shot. With manual lenses, the scale is engraved on the lens; some of the newer models such as the ZEISS Batis lenses have a digital display, which is far more precise. You can also find a focus table in our product data sheets on the ZEISS website. Here is a tip: Take photos of the same subject with slightly different focusing and then compare the shots on the screen.
“The camera’s resolution capacity and the reproduction of micro contrasts makes all the difference when it comes to image definition.”
What is your opinion of manual focusing?
There are many application areas where pure manual focusing is clearly superior to autofocus lenses. An example is landscape photography. The long angle of rotation of the focusing ring of manual focus ZEISS lenses allows a very precise focus setting, even near infinity. So you can focus on and concentrate on the composition rather than the correct focus setting. Manual focusing is also best for macro shots because you can handle the extremely shallow depth-of-field.
When designing a lens, is it easier to correct typical aberrations with a manual focus prime lens than with an autofocus lens?
By all means. With manual focus lenses optics designers do not have to make as many compromises as they do with autofocus lenses. That is why manual focus lenses usually produce a more consistent performance across the entire image field.
To what extent can software compensate for optical aberrations?
During post-processing on the computer, I can improve the subjective apparent sharpness. There are many different approaches here; for example, I can increase the contrast on the edges, adapt the dynamic range, or work with filters. I always recommend experimenting with different settings – the technique you choose ultimately comes down to personal preference. But one thing is certain: no image editing software can make up for a missing detail rendition on a lens-camera set-up. After the fact, I cannot simply conjure up textures or details that my sensor has not captured.
When you think back 15 to 20 years ago, how have manual lenses changed in their technology?
You cannot really compare lenses today with 15 years ago. First of all, the sensors have improved tremendously and the cameras have a higher resolution. Therefore, modern lenses also need to have a more sophisticated construction. But these are general differences. For ZEISS manual-focus lenses specifically, the main difference to 15-20 years ago is that the lenses have been significantly improved through the use of new lens designs. For instance, the high-performing ZEISS Otus 1.4/55 uses a Distagon lens design. Traditionally, lenses in this focal length from ZEISS use a Planar lens design. In recent years, our optic designers found a way to use the more sophisticated Distagon lens also for standard focal lengths. This construction allows us to achieve better image results.
Are manual-focus lenses more robust than autofocus lenses?
They tend to be. With a manual focus lens optic designers have to rely on a different focus mechanism. ZEISS lenses use a so-called helicoid focus. Recall that with autofocus lenses it is important that the parts are light so the motor can work fast. With manual focus lenses, we have a lot more design freedom in that regard. That means we can use more robust, long-lasting materials such as metal for the internal lens movements.
What are the optical performance limits today for taking photographs with large sensors?
Due to ever-higher-resolution cameras, there is a need for optics that achieve great things with a large aperture. Modern lenses are thus much more complex than they used to be; they feature better correction and more lens elements. The occurrence of chromatic aberrations can be reduced by using special types of glass materials and aspheric lenses can correct further spherical aberrations. Modern lenses are therefore bigger, bulkier, and more expensive – but they’re also better.
Looking at the future of optical imaging – where do the limits lie?
There’s actually quite a bit we can do. There are no limits in sight in terms of the performance of our camera lenses. For instance, we build optics for industrial applications with resolutions in the nano range. But the question we really should be asking is this: What makes sense in photography? The high resolutions on sensors are already oversized for many areas of application. I can even photograph big poster campaigns problem-free with a 12-megapixel camera, so in fact only a handful of applications really max out the capabilities of the newer sensor models. Building lenses that boast even greater performance than is currently available in fact only makes sense for special applications, for instance in aerial photography.
Resolution is key when it comes to definition, but it’s certainly not everything. The subject was photographed twice with the same camera but with different lenses: on the left with the ZEISS Classic 1.4/50 and on the right with the ZEISS Otus 1.4/55, both at full aperture. Even though the resolution is identical and the details are easy to see on both photos, the image on the right looks much more well-defined. This is due to the high micro contrast achieved with the ZEISS Otus at full aperture.
ABOUT BERTRAM HÖNLINGER
Bertram Hönlinger is a trained photographer and a qualified photo engineer. He joined ZEISS in 2006 after holding various positions in the photography industry. In his spare time, he enjoys taking landscape and still life photos.