Photographers expect a good and uniform resolution performance from their lenses. With lensTESTER, the resolution can be evaluated for the image center and four corner areas. Normally the center achieves the best resolution score. Depending on the focal length and aperture setting, the results of the boundary areas turn out to be a little bit weaker. If you use a wide-angle lens and choose an open aperture, for example, the resolution often tends to drop significantly towards the image corners.
It is assumed that the resolution decrease to the edges behaves fairly evenly. However, what if the sides have mutually distinct differences, or the resolution is particularly weak? A message will inform you if an uneven distribution of the resolution occurs after analyzing, for example.
A faulty lens, an inexact test setup or user mistake could also result in deviating results.
Possible user mistakes:
- shaky tripods
- imprecise alignment of the camera to the test chart
- unbalanced illumination
- manual shutter release (motion blur due to release with the finger)
Please do not be too hasty in judging the lens to be the cause, we recommend checking the shooting conditions. This way you can exclude possible error sources step by step.
1) Tripod with a stable stand
For test shootings, always use a tripod with a stable stand. A loose holding device can cause shaking which could possibly lead to blur. If all stars show a blur it is most likely motion blur due to a shaky tripod.
2) Parallel alignment
Aim the camera very precisely, so that the sensor is aligned parallel to the test chart. If the camera is positioned too far to the left or right, the center of the sensor will not target the central Siemens star. This displacement means that the resolution to one of the sides could be much weaker than on the opposite side.
3) Homogenous illumination of the test chart
Even a non-uniform illumination can lead to varying resolution results. If the left side of the lensTESTER is illuminated more than the right, for example, the unbalanced light distribution will lead to deviations in the test results.
Make sure that the test chart is uniformly illuminated, so all areas (center, left, right, top, bottom) have a homogeneous brightness. A light meter can be helpful.
4) Recommended settings
If the camera is aligned parallel to a test chart, which is homogenously illuminated, deviating results can also occur through the type of releasing used. Using a tripod already provides protection against camera shaking. Nevertheless, other factors can cause unwanted blur:
- Image stabilization
- Manual releasing
- Mirror impact
Deactivate image stabilization
Using a tripod makes use of the existing image stabilization unnecessary because movements by the photographer do not need to be corrected. It is important to deactivate the image stabilization (camera/lens). If it is activated while using a tripod, the stabilization could try to correct falsely perceived movements. Thus, blur could occur in your test image.
Activate the mirror lock-up
When using a digital SLR camera, keep the mirror impact in mind. Due to the movement of the oscillating mirror while releasing, little shakes/vibrations may take place, which could cause slight blurs in the test images. For this reason, the mirror lock-up should be activated, if possible. Thus, the mirror is raised up before releasing and does not cause any light shakings.
Self-timer or remote release use
Even the photographer may produce unwanted blur. Although a tripod is used, releasing with the finger can cause minimal shaking. Thus, a self-timer (at least 5 seconds) or a remote release should be used.
An imprecise set focus can lead to deviations in the results. Select the manual focus mode and use the live View Mode for assistance. If possible, scale up the live view and start focusing. Focus precisely on the central Siemens star.
To avoid discrepancies that could appear through the manual focus setting, we recommend to shoot a focus range. Take about 10 pictures of the test chart and refocus for every single shoot. This allows for choosing the sharpest image for later analysis.
Tip: aperture series
An aperture series could be helpful to determine a possible incorrect camera alignment, when the results from a test image show a stronger reduction in resolution to one side of the image. By closing the aperture of the lens, depth of focus rises and the results should become more homogeneous. However, if a camera is not placed parallel to the test chart, a resolution decrease to a certain side is visible, independent of the aperture setting. If the results become more homogeneous with an increasing aperture setting, a faulty lens or a user mistake could be the reason. In this case, we recommend rearranging the test set up and camera settings.