Egami on the Advantages of APS-C Format Lenses

[Sator-Photography]

As I read Japanese, I've been reading the Egami blog a lot of late.

 

One of the things that might have been overlook on his blog is this entry:

 

http://egami.blog.so-net.ne.jp/2016-06-13

 

フジノンの画質評価が高い理由の一つは、相対的に巨大な光学系をAPS-Cに与えているからですね。 35mmフルサイズでそれをやると1.5倍の大きさになります。 フォーマットサイズが変わっても使う人間が変わらないので、小さなフォーマットは相対的に大きな光学系を採用し易いのです。

What he writes is that APS-C format lenses are "enormous" relative to the sensor size. Egami says that this is one of the reasons that Fuji X system optics have a reputation for having such image quality. If you tried to do that with a full frame lens it wouldn't work out as it would end up 1.5 times larger. The reason he gives is that even if you make the sensor format bigger the user doesn't change, meaning that smaller format lenses are easier to design optics for because of their size relative to the sensor.

I guess what he means is that small format lenses have better light gathering ability relative to the sensor. So while it is true that larger format sensors have better light gathering ability, the downside is that it is harder to make the lens larger relative to the sensor to take full advantage of it. By comparison, smaller sensor formats permit the engineer to make the lenses larger to compensate for the loss of light gathering ability from a smaller sensor.

What that means is that if you were to make full frame or medium format lenses of comparative size relative to the sensor like Fuji XF lenses, they would end up unacceptably large, unwieldy, and expensive. Of course, that's the reason why you will never have a medium format f/1.4 Otus lens. Even if it is perfectly technically feasible to scale up the Otus 85mm f/1.4 to medium format proportions, it would just be so elephantine that it would just be impractical to make or sell, let alone to use.

This probably partly explains a problem with larger formats where the MTF plots at maximum apertures like f/1.2 or f/1.4 generally look rather bad. It's almost like you are shooting with a soft focus lens, and the aperture needs to be stopped down as much as one stop (or even more) to make the sharpness more acceptable. Not so with the best APS-C lenses which give you sharp images shooting wide open. In fact that is the reason Fuji give for choosing the APS-C format for their X series.

That's why if you shoot with a Canon 5DsR, then you can forget about spending thousands on a second hand copy of the film era 50mm f/1.0 lens. The 50MP sensor will just show up the softness of the lens, and the f/1.0 aperture would be unusable. It's bad enough shooting wide open with the Canon 50mm and 85mm f/1.2 lenses...unless you want a soft focus look. In my own experience, I have found the Fuji XF 56mm f/1.2 APD sharper shooting wide open than even my Zeiss Otus 85mm f/1.4.

At the moment people may point out that smaller format sensors have limitations in terms of light gathering ability resulting in a worse signal/noise ratio, high ISO performance, as well as lower maximum resolution. However, that is rapidly improving. In a few generations, APS-C sensors will achieve 36-50MP resolutions. Then to justify their existence, larger format sensors will have to be around 80-120MP, which in addition to the problem of brutally exposing lens softness when shooting wide open, the high resolutions will make for an unforgiving shooting experience.

I suppose larger format lenses could be made bigger and better to compensate for this, but then they will be expensive and unwieldy. In fact, that is likely what is going to have to happen once full frame sensor reach 80-120MP, because it will brutally expose softness shooting wide open. In future, full frame lenses might end up being medium format sized. It leaves me a little worried about the future of larger formats especially medium format. However, the future of smaller formats like the Fuji X system looks rather bright.

[milandro]

this is, I am afraid, the consequence of digital sensor being formed by a grid of sensitive cells, they can only work well with rays coming as perpendicular as possible towards the grid of sensitive cells.

 

Unlike a film which has the capability to gather photons at an angle and still being gathered by the silver on the film, a sensor works pretty much like when you stand in front of a open fully open blade curtain.

 

If the blades are completely open and you ( the sensor) stand exactly in front of the open blade, you practically don’t see the blades. Move more to the left , or right, a portion of the image will be blocked by the blades.

 

Lenses made for traditional photography were not made for this. Even if they cover the format of a FF sensor, they will not perform well because of this and of course, perform better if you reduce the pupil by means of a smaller aperture than if you use it fully open. 

 

When you use a FF lens onto a smaller sensor, you are mostly using the center and more perpendicular rays, so, despite the inaptitude of a traditional lens towards its use on a digital sensor, you can still get pretty good results but still not in the same league of lenses made for the purpose.

 

The lenses for a APS-C camera need being larger because despite the size of the optical elements they have to use a more perpendicular projection that a lens for traditional analog photography would hence larger lens even if the image circle is relatively small.