I described a method for producing a visual representation of the shape and depth of sharpness of a lens at different focal lengths and apertures here.
I have been working with this method and thinking about what it can contribute to our understanding of the characteristics of a lens.
In this post I compare two Canon RF 24-105mm zooms.
The first is the RF 24-105mm f4 L IS USM, currently selling for around AU$1410 retail in Sydney including GST.
The second is the RF 24-105mm f4-7.1 IS STM, selling for around AU$611.
I have selected just one aperture to show at each of the tested focal lengths. The distribution of sharpness is markedly affected by focal length, aperture and focus distance.
24mm f5.6
The L lens shows an obvious W shaped distribution of sharpness plus a mild degree of tilt, represented here by clockwise rotation of the sharp (dark) parts of the image.
The STM lens has a very different distribution of sharpness with more sharpness beyond the focus point than in front of it. You can also see sharpness is concentrated in a zone just behind the stick with less sharpness at the edges than is seen with the L lens.
Comment on the results at 24mm
In my empirical testing I had discovered that that particularly with landscapes the L lens gives the most even foreground-to-background sharpness when I place the focus box just above the center of the frame.
The STM lens works best with the opposite approach. With this lens I see the most even distribution of sharpness from foreground to background if I place the focus box just below the center of the frame.
The testing process using the [find edges] filter largely explains why these empirical findings work.
50mm f5.6
The L lens is well behaved here with sharpness distributed fairly evenly behind and in front of the focus point, extending out to the edges and with negligible tilt.
This copy of the STM version does not fare so well at this focal length.
Sharpness is distributed in a big U shape (oriented the opposite way from that seen at 24mm ) with more sharpness behind than in front of the focus point. Sharpness does extend to the edges but not in the focal plane. This lens is soft at the edges on the long side when a flat target is used.
I had noticed with this lens that several of my photos at this focal length were not sharp in the zone labelled B yet were quite sharp in the zone labelled A.
The pictures explain why.
105mm f8
The L lens looks pretty even here. Sharpness is centered on the focal point and distributed fairly evenly in front of and behind the focus point and also out to the edges. There is a small degree of rotation of the field.
The STM version at this focal length delivers sharpness which is well distributed front to back and side to side with just a hint of rotation.
In the field this is the lens’ best focal length for sharpness and the easiest to use due to the even distribution of sharpness.
Discussion
Use of the [find edges] filter in Photoshop on a fairly flat subject such as grass has helped me to understand several behaviours of my lenses which previously seemed to be a complete mystery.
Just why lenses behave so very differently as their focus distance, focal length and aperture changes remains an arcane secret of the lens designers’ art, craft and science, way beyond my comprehension.
In the case of the two lenses compared in this post my empirical findings have been that both versions are equally sharp in the center of the frame but the L is considerably sharper towards the edges. The L also gains more sharpness than the STM as the aperture is closed 1-2 stops from the maximum. The distribution of sharpness given by the L makes it easier to manage in practice.