How sharp is my smartphone really?

Camera test with a Siemens star

Smartphones take impressively good photos these days. Night mode, AI sharpening, multiple lenses, huge megapixel counts—on paper, it sounds like pocket-sized optical magic. But how well does a smartphone camera really capture fine details? And what happens when you're not just photographing a landscape, but a test pattern designed to really put the camera to the test?

This is exactly where the Siemens star comes into play. This classic test pattern reveals where an optical system can still clearly resolve fine details and where lines merge, flicker, or are suddenly artificially sharpened by image processing. With a small LED backlight, it becomes a compact test setup that allows for a very clear comparison of smartphone optics, lenses, and optical systems.

The idea behind the experiment

In this application example, a smartphone is aligned with the backlit Siemens star on the e9u-BGLT-20x20-B. Various camera modules, distances, and settings are then compared. The goal is not a fully calibrated MTF measurement, but rather a practical and clearly visible comparison of imaging performance.

It gets particularly interesting when you look beyond the center of the image. Many smartphone optics produce very sharp images in the center but show significantly more blur, distortion, or chromatic aberrations towards the edges. The camera's built-in image processing also plays a major role: noise reduction, sharpening, and HDR can visibly alter the final result.

Materials needed

• e9u-BGLT-20x20-B LED background lighting with battery pack and Siemens star
• Smartphone with one or more cameras
• Small tripod, cell phone mount, or sturdy experimental setup
• AAA batteries for the battery pack
• Optional: Camera app with manual controls for focus, shutter speed, ISO, and white balance
• Optional: Ruler or spacer for repeatable positioning
• Optional: PC or tablet for viewing image crops

Experimental setup

The test target is placed on the lab bench or in a small holder. The smartphone is mounted so that the optical axis of the camera is as perpendicular as possible to the plane of the Siemens star. A stable mount is important, because even slight camera shake or variations in distance can make the comparison difficult.

The distance should be chosen so that the Siemens star is clearly visible in the image but does not fill the entire image frame. For edge tests, you can deliberately position the target at different locations within the image field or slightly move the smartphone without changing the distance.

Experimental procedure

1. Insert three AAA batteries into the battery pack and connect it to the e9u-BGLT-20x20-B. If the battery pack is equipped with a switch, switch on the illumination.
2. Place your smartphone steadily in front of the Siemens star.
3. If possible, disable automatic features such as night mode, beauty filters, or strong automatic HDR.
4. Adjust the exposure so that the bright areas of the Siemens star are not overexposed.
5. Take your first photo with the main camera.
6. Optionally, take additional photos using the ultra-wide-angle, telephoto, or macro camera.
7. For edge tests, capture the Siemens star successively at the center and at different positions within the image field. Then compare equally sized image crops at the same magnification.
8. Save images and compare image crops at the same magnification.

What can be observed?

The Siemens Star highlights several features of the camera system:

• Sharpness and focus plane at the center of the image
• Edge blur and field curvature
• Decentering or tilt in the optical setup
• Distortion, especially with wide-angle cameras
• Moiré and aliasing effects caused by the interaction between the test pattern and the pixel grid
• The effect of sharpening, noise reduction, and JPEG compression
• Differences between the main camera, ultra-wide-angle, telephoto, and macro modules

Interpretation: Megapixels aren't everything

A smartphone with a high megapixel count does not automatically deliver the best level of detail. What matters is the entire imaging chain: lens, focus, sensor, pixel size, signal processing, and software. The Siemens star test reveals the effects of this imaging chain with striking clarity. Fine lines may become blurred, be reconstructed incorrectly or appear sharper after digital sharpening than they were actually resolved by the optical system.

Image crops from the center and the edges are particularly interesting. If the center has sharp resolution but the edges become noticeably softer, this is typical behavior for many small lenses. If one side of the image is noticeably worse than the other, this may indicate tilt, decentration or a setup in which the camera and test target are not exactly parallel.

Qualitative and semi-quantitative comparison

For a simple qualitative assessment, it is often sufficient to simply look at the live image or at saved photos. For a semi-quantitative comparison, however, the viewing distance, camera settings, lighting, and frame should be kept constant.

• Equal distance between the camera and the test target
• Consistent exposure and the lowest possible ISO setting
• No digital zoom for camera comparisons
• Same output size or same pixel view when comparing
• Focus conditions that are as identical as possible
• When RAW data is available: Comparison of RAW and in-camera JPEG

Important: This test is not a substitute for a standard-compliant laboratory measurement of the modulation transfer function. However, it provides a very clear and practical assessment, which is often exactly the right starting point for development, training, and quick comparisons.

Common pitfalls

Extensions to the experiment

• Comparison of different smartphones under the same conditions
• Comparison of the default camera app and the manual camera app
• Comparison of JPEG and RAW images
• Investigation of macro lenses, close-up lenses, or microscope adapters
• Testing webcams, industrial cameras, or USB cameras
• Images taken at various distances and a resulting discussion of magnification and resolution
• Comparison of the center of the image with the four corners to assess field performance

What does this mean for teaching?

This experiment connects a familiar everyday device with real optical phenomena. Almost everyone is familiar with smartphone photos, but few have consciously observed just how closely the lens, sensor, and software work together. The Siemens Star brings this interplay directly into the picture: resolution, contrast, focus, aberrations, and digital image processing become visible without the need to first set up a large optical test bench.

This makes the experiment well-suited for hands-on labs, project weeks, training sessions, and short demonstrations. A simple photo can quickly spark a discussion about optical imaging, sensor resolution, and image processing—and that's exactly where things start to get exciting.

Product used

For the experiment described here, the e9u-BGLT-20x20-B is used, a compact LED backlight with an integrated Siemens star on a glass substrate. Technical specifications, connection details, safety instructions, and cleaning instructions are described on the product page.

[Linkplatzhalter: Produktseite e9u-BGLT-20x20-B]

Safety notice

The test target is intended for use by qualified personnel in dry indoor environments. Do not stare directly at the illuminated area for extended periods. If the glass substrate is damaged, the product must not be used any further. The maximum supply voltage of 5.5 V DC must not be exceeded.

Conclusion

The Siemens Star quickly reveals what's really happening inside a camera. Suddenly, it becomes clear where the focus lies, how much image quality degrades at the edges, when fine details disappear, and just how much software alters the result. This turns a small, backlit test target into an astonishingly versatile tool for optical experiments—from quick smartphone comparisons to aligning small laboratory setups.