Lens Sharpness and Diffraction Blur with the Canon R5

I recently upgraded from my 5D Mark III DSLR to a Canon R5 mirror-less camera, jumping from a 23 megapixel sensor to a 45 megapixel sensor. With the new camera I quickly realized that to make full use of its additional resolution,  it is necessary to adapt my shooting technique a bit, with shorter exposure times to freeze motion or camera shake, smaller apertures to achieve the desired depth of field, and consideration of shutter shock and diffraction blur among other things. Also obviously the demands on the optical qualities of the lenses increase.

The first of these elements I looked at is lens sharpness and diffraction blur, and in this blog post I’d like to present a small test, conducted with the Canon R5 and the EF 35mm F1.4L, the RF 50mm F1.2L, the RF 85mm F1.2L and the EF 135mm F2.0L. While a lot has been written on the topic already, and many tests can be found on the Web, it still was quite interesting for me to approach the topic from a practical side, run my own tests, with the specific camera and lens combination I use.

Disclaimer: in this post we will indulge in pixel-peeping to assess sharpness of images coming from a high-resolution sensor. For reasonably-sized screens and prints, viewed from normal viewing distances, the requirements when it comes to resolution and sharpness are much lower than the level we look at here. Only when cropping images or producing very detailed large prints, the additional resolution and higher demands to shooting technique come into play.

Let’s start with a short introduction into diffraction blur. Diffraction blur leads to image sharpness decreasing the smaller the aperture gets. The amount of blur is based on principles of physics, and present to the same amount at the same aperture in all lenses, regardless of their focal length and how well they are corrected. Increasing with smaller apertures, the diffraction blur develops in the opposite direction than other optical aberrations, which usually are better controlled the smaller the aperture gets. While the amount of diffraction blur is the same across all lenses for a specific aperture, the correction of other optical aberrations is lens-dependant, so each lens has its own sweet spot when it comes to sharpness, depending on where the impact of diffraction blur starts to outweigh the improvement of other optical aberrations. That sweet spot is called the Diffraction-Limited-Aperture (DLA) and is also depended on sensor resolution. If you like to know more, check out these articles: photographylife, scantips.

The test photos I took were all done with the Canon R5, from a tripod, with a cable trigger, electronic shutter, ISO 100 and were then scaled up to 300 %. The first series of test shoots focuses on the center of the image, the second on one of the corners. Since optical aberrations of lenses usually increase towards the edge of the frame, the DLA can shift to smaller apertures the further away from the center you look.

The results clearly show how the sharpness sweet spot is different for each lens. At the center of the frame, the RF 50 F1.2L and the RF 85 F1.2L are very quite sharp wide-open at F1.2 and produce their best results already at F2.8, because other aberrations are well under control at that point, and diffraction blur starts to weigh in. The other two lenses show more aberrations at their widest aperture and their sweet spot seems to be at F5.6 and F8.0, because till then those other aberrations outweigh the diffraction blur. At F16 the diffraction blur starts to have a significant impact and its strongest effect can be seen at F32, the smallest aperture of the EF 135mm F2.0L.

But have a look and see for yourself, here the results from the center of the frame.

The center of the frame - click to enlarge

On the edge of the frame the older EF lenses show a further drop of sharpness at their widest apertures, while the RF lenses already look quite good at F1.2 and F2.0. However stopped down, the edges quickly catch up for all lenses and I was surprised that the sweet spot of sharpness is quite similar to the center. The RF lenses are already at their sharpest at F2.8, in a tie with F4.0, and the EF lenses at F5.6 and F8.0.

The edge of the frame - click to enlarge

And here an overview of the test setups, with the overall image from each lens at its widest aperture. The focus was always on the word “were” in the title of the book, with the comparisons above showing the first letter “e” of that word.

The center of the frame

The edge of the frame

The book used in the test shots is “I never knew if you were the storm or the silence” from German model Luise Lanze and photographer Oliver Korbanek, a book I can highly recommend if you are interested in portrait work.

Conclusion

So, what conclusion to draw from this test?

First, I am very impressed with the improvements in lens design when comparing the newer RF lenses to the older EF lenses. Even the EF 135mm F2.0L, often titulated the “Lord of the Red Rings”, cannot really compete with the new RF 85mm F1.2. A big part of that seem to be the possibilities the new RF mount seems to offer in improving lens design, with its large diameter and short flange distance. To learn more about this, have a look at this white paper from Canon, specifically sections 8 and 9 on details of the RF mount and challenges managing lens aberrations.

Second, while lenses improve and sensor resolution increases, output formats have not changed to the same extend. While when pixel-peeping there seem to be huge differences in the observed results, for most practical applications all the tested lenses and apertures are still perfectly fine. So, when choosing the right aperture for your shot, other considerations, such as lighting conditions and depth of field, are usually more important than lens sharpness and the impact of diffraction blur. Only with the very narrow apertures from F16 to F32 I would be cautious and use them only when there is no other way to achieve the desired exposure.

But even if not needed in most situations, when having a high-resolution camera such as the Canon R5, it is good to know the many details to be considered to make the very most of its potential.

As always, I hope you found this post useful and feel free to contact me if you have any questions or comments.