Canon EF 50mm f/1.8 STM problems

Autofocus Issues

1. Slow Autofocus: Users have reported that the autofocus can be slow and may struggle to lock on, particularly at wide apertures like f/1.8. This can lead to missed focus, especially in dynamic shooting situations where subjects are moving[4].

2. Focus Accuracy: Some users have experienced issues with the lens consistently missing focus, particularly in low-light conditions or when shooting at close distances. This can be exacerbated by the shallow depth of field at f/1.8, making precise focus critical[4][2].

3. User Error vs. Lens Defect: There is a debate among users about whether focus issues stem from the lens itself or from user inexperience. New DSLR users, in particular, may find it challenging to achieve sharp images at wide apertures due to the narrow focus range[4][2].

Build Quality Concerns

1. Fragility: The lens is primarily made of plastic, which can make it susceptible to damage from drops or impacts. Some users have reported that earlier versions (like the f/1.8 II) are particularly fragile, with cases of the lens breaking from minor falls[1]. The STM version is noted to have improved build quality, but it still may not be as robust as higher-end lenses[1].

2. Mechanical Issues: Users have noted that the focusing mechanism can become misaligned or jammed, especially if the lens has been dropped or mishandled. In some cases, users have had to resort to repairs or replacements when the lens fails to focus properly[1][2].

Optical Performance

1. Sharpness and Aberrations: While the lens is generally praised for its optical quality, some users have mentioned that it can be soft wide open and may exhibit chromatic aberrations and vignetting, particularly at f/1.8. However, many still find the image quality acceptable for the price[2][3].

2. Bokeh Quality: The STM version features a 7-blade aperture, which improves the quality of the background blur (bokeh) compared to previous models that had a 5-blade design. This enhancement is appreciated by users who prioritize aesthetic background rendering[1].

Conclusion

Overall, the Canon EF 50mm f/1.8 STM lens is a popular choice due to its affordability and decent optical performance. However, potential buyers should be aware of possible autofocus issues, the fragility of the lens, and the need for careful handling. For those who prioritize robust build quality and reliable autofocus, considering a higher-end lens may be advisable.