'We want to make lenses that can be used forever': Sony engineer discusses G Master lenses
'We want to make lenses that can be used forever,' says a senior engineer behind Sony's new G master lenses. At the launch of the ‘G Master’ range of high end lenses, we spoke to Motoyuki Ohtake, Distinguished Engineer in Sony’s Lens Design Department about the process and the philosophy behind the latest lenses.
The development process series involved re-thinking several parts of the design and manufacturing process, he says.
|Motoyuji Ohtake, Distinguished Engineer, Opto Design Department, Core Technology Division, Digital Imaging Business Group at Sony.|
To understand how the lenses came about, he explained the usual process of lens development. ‘Sometimes we propose a new lens but often it comes from the product planning department [the marketing department that assesses potential requirements and demands]. We then make a series of rough designs, some are big, with high optical performance, others are more compact but maybe not so optically strong. We discuss which design to proceed with, based on what we think is the optimal balance or cost, performance and size to make the perfect product.’
After deciding which of the initial designs to pursue, there’s a great deal of collaboration between teams, he explains: ‘we work with the mechanical team, the lens motor team, the lens control team, the lens element team and maybe the equipment team who will have to prepare the manufacturing process.’ Each of these team feeds its expertise into the design. ‘Maybe the optical team proposes a new lens design and the motor team tells us which motor is best. Or warn us if the focus will be too slow. They feed back about the mechanical aspects,’ he says.
The G Master series required many of these teams to re-think their parts of the process, from design to manufacture.
Re-thinking basic assumptions
‘For the G Master lenses we decided we would assess the spatial frequency at 50 lines per mm,’ says Ohtake: ‘Usually lens makers, including ourselves, evaluate lenses at 10 and 30 lpmm (or 10, 20 and 40 for Carl Zeiss-branded optics).’
‘At the start of the process we all agreed we should change the spacial frequency [to a more challenging target],’ he says: ’but which is best to get good performance? We could design for 100 lpmm but the lens would become very bulky and long - which might not be a very practical lens. A balance of the size and the optical performance was very important.’
The target of 50 lpmm wasn’t dictated by the company’s 40MP camera or 4K video, he says. ’All our FE lenses were designed for at least 40MP. Because we have an image sensor team within Sony, we get to see the sensor roadmap, so we’ve been designing for this all along with FE. With the G Master we’d like to make lenses that can be used forever.’
A focus on bokeh
But it’s not just the more stringent frequency assessment that was developed for the G Master lenses, Ohtake explains: ‘We had to discuss what good bokeh means. We have some designers from Minolta who understand that the spirit of the ‘G’ lenses was good bokeh in the background but we had no way to evaluate that.
‘We looked at what is considered good bokeh and how it affects not just the background rendering but also the transition from perfectly sharp to out-of-focus regions. We developed a way to evaluate bokeh and were able to make a simulation. This meant we didn’t have to build a lens to see how it performed, we could now computer model it before taking a design too far.’
This is a significant change, Sony says, as it means bokeh can be one of the primary design considerations, rather than being something that can only be adjusted later in the process, once the main aspects of the design have been settled upon.
Another piece of the puzzle - shape and smoothness
This analysis of the factors that affect bokeh showed that both the precision of the lens molding and the smoothness of the lens surface could have an effect.
‘Traditionally it was very hard to achieve both: current technology gives a roughness on the scale of 20-30nm on the aspheric surface. Improving this usually involved polishing, which can then lead to the lens element being slightly unevenly shaped.’
‘We developed a new way of making the lens element and a new molding process, including a new machine. Now we can get roughness down to around 10nm and get a more accurate shape to the aspherical surface.’
|Ohtake wouldn't budge when we asked which his favorite lens was, but immediately reached for the 85mm F1.4 when we took this group shot.|
The first three G Master lenses use three different AF motor technologies between them - emphasizing Ohtake’s point that different technologies work better in different contexts.
The 24-70mm F2.8 uses a Direct Drive SSM system (piezoelectric element). This is very fast, very quiet and very precise. We used a linear motor for the 24-70mm F4 but this lens has a heavier focus element, so direct drive was a better choice.
The focus element in the 85mm F1.4 was even heavier, however. ‘For the 85mm we use a ring type focus motor. This is very good for heavy lens elements and our lens software team developed a good algorithm so that it works well with contrast-detection autofocus' (a traditional weakness for ring-type designs).
Finally, the 70-200mm uses a combination of a linear actuator and a ring-type focus motor. ‘The focus group had become too heavy so we separated the two focusing lenses. One is very heavy, so we used a ring type motor for that one, then used a linear motor for the other. The ring type is used to quickly achieve approximate focus and the linear motor is used for the high precision aspect.’
Still correct to optically correct
Discussing the idea that bokeh and sharpness have previously been in conflict, we asked Ohtake about other trade-offs. We’ve been told that the ability to correct lateral chromatic aberration in software makes lens design easier, since you don’t have to correct it optically, which can quickly complicate the lens design and detract from other parameters.
Not for G Master lenses, he explains. ‘Light doesn’t separate nicely into red, green and blue' (the color channels that most cameras capture, and which can be adjusted, relative to one another, to correct lateral CA). It’s a continuum with each wavelength being displaced slightly differently. ‘To get the really high contrast we wanted in G Master, we had to suppress it in the lens.’
The future of APS-C
We also asked Ohtake about Sony's APS-C lenses for E-mount. His team likes designing APS-C lenses, he says: ‘The focus elements are light, so it’s easier to design. We have all these focus motor technologies in-house and we’d like to try them in APS-C lenses if that’s what the Product Planning team says is required.’