All of the following are used in subjective testing except:

Subjective testing in the context of vision assessment involves patient responses to visual stimuli, allowing the examiner to gauge the patient's visual perception and preferences. The tools involved in subjective testing are designed to require the patient to provide feedback based on their experience.

The retinoscope, on the other hand, is primarily used in objective testing to estimate a person's refractive error by analyzing the reflection of light from the retina. This tool does not rely on the patient's subjective interpretation or experience; instead, it provides data based on the light reflexes observed by the practitioner.

In contrast, the other tools listed are all designed for subjective testing. The Maddox Rod is used to assess eye alignment and muscle function by creating a visual experience requiring the patient's input to determine any misalignment. The Jackson cross cylinder assists in refining the sphere and cylinder power through patient feedback regarding the clarity of images. The Duochrome test evaluates color contrast sensitivity and requires the patient to express which of two colored backgrounds provides clearer vision.

Therefore, the retinoscope’s lack of dependence on patient subjective responses clearly distinguishes it as a tool for objective testing, making it the correct answer in identifying which instrument is not used in subjective testing.