The Myth of the Ideal F-Stop for Maximum Sharpness in Camera Lenses
Understanding the ideal f-stop for achieving the sharpest images can be a complex topic, particularly when examining different lenses and camera configurations. While there are longstanding beliefs and rules, recent testing and research have revealed some unexpected results.
Overview and Historical Context
A decade ago, the conventional wisdom was that one or more stops down from the maximum aperture would yield the sharpest images. However, modern tests with high-quality lenses suggest a different approach. The deviation from traditional advice highlights the importance of considering lens design, quality, and distance when determining the ideal f-stop.
The State of Modern Lens Testing
For older lenses and lower-quality options, the traditional rule of stopping down remains applicable. However, for better-quality modern lenses, the sharpest images are often achieved at full aperture or only one stop down. This new understanding is crucial for photographers seeking the best results with their equipment.
General Rules for Sharpness
Like all photography advice, there are several generalized rules to consider when striving for the sharpest images:
1. Ideal Lenses
Optimal lenses are sharpest at their maximum aperture. This is why it's important to look for lenses designed for this purpose.
2. Center and Corner Sharpness
There's a significant difference in sharpness between the center and the borders, especially in corners. Older and faster lenses tend to have a more pronounced difference. However, many lenses are already sharp in the center wide open, while corners may still be soft.
3. Distance and Focus
Fast long telephoto lenses are generally very sharp wide open and only benefit minimally from stopping down, even for corner sharpness. Conversely, DSLR lenses are often less sharp at close distances and more so at infinity. Testing shows that the optimal distance for sharpness can vary, often requiring tests at 101 times the focal length.
4. Optical Quality and Design
MTF (Modulation Transfer Function) tests, which assess optical sharpness, often show differences that modern sensors can't capture. Therefore, it's important to consider real-world use rather than test results when choosing lenses.
5. Lenses for Specific Camera Systems
Different camera systems require different stopping-down practices. For example, Nikon DSLR lenses from the 1980s to the early 2000s needed to be stopped down to f/11, while modern DSLR lenses can be used at f/8. Nikon lenses designed for mirrorless cameras (Z) need to be stopped down by only two stops, and the Nikkor Z 58mm f/0.95 Noct is relatively sharp at full aperture and perfect at f/1.8. Canon R lenses are also highly regarded but may require an additional stop for perfect outer corners.
6. Megapixel Impact and Diffraction
The more megapixels a camera has, the more diffraction will become an issue. Diffraction reduces sharpness due to the light being forced through a smaller aperture opening. Therefore, full-frame cameras with 45 or 60 megapixels may lose sharpness when stopped down past f/5.6. This emphasizes the need for lenses to be sharp at larger apertures, especially in high-megapixel setups.
7. Practical Considerations for Everyday Use
Not all images require the utmost sharpness, particularly portraits. Modern lenses often produce more detail than necessary, leading some photographers to prefer slightly less sharpness wide open for enhanced aesthetic appeal.
8. High-Megapixel Lenses
Lenses designed for 45-megapixel full-frame cameras must perform optimally at f/5.6 or less to avoid diffraction deteriorating image quality.
Conclusion and Final Thoughts
The concept of the ideal f-stop for maximum sharpness is not a one-size-fits-all solution. It depends on the lens, its design, and the specific usage scenario. While the traditional wisdom still holds for some lenses, modern testing reveals that many lenses perform best at full aperture or just one stop down. By understanding these nuances, photographers can achieve the sharpest results.
Further Reading and Resources:
Reviewing Lenses with DXOMARK Test Charts and Lens Reviews LensRentals MTF Reviews