[v5planet]

Hey guys,

Recently I've been giving a lot of thought to our mirrorless future. One thing I had heard is that removing the mirror and thereby allowing lenses to sit closer to the sensor will result in us being to engineer smaller lenses, even if sensor size is held constant.

Can anyone explain to me why this would be the case, or point me at a resource that does so?

[Adam]

The reason the lenses can be made smaller is that the focal length is naturally shorter. Thus, you don't need to do any fancy optical tricks to obtain, for example, an ultra-wide prime lens. On DSLRs, a true focal length (distance from the focal point to the sensor) of something like 8mm or 10mm is physically impossible, so the lens needs very big elements and complex optical formulas in order to get the same FOV.

[RioRico]

You can get an idea of the scales involved if you look at non-reflex film cameras: rangefinders (RFs) and simple viewfinders (VFs).

I have had cheap 135/FF VFs about 1.25 x 2 x 3in / 30 x 50 x 75mm, the thickness mainly being for the 135 cartridge. These aren't folders; their 35mm lenses can just sit very close to the film, needing no mirror clearance. Right now I'm holding a cheap "focus-free" Vivitar IC-400. With its film transport and big bright VF, it's 1.25 x 2.75 x 4.25in / 30 x 70 x 110mm. Yes, a full-frame camera just over an inch thick! I'll guess the lens is about 28mm.

I've a Halina 6-4 medium-format 6x6cm VF (non-folder) that is barely larger, with body 30mm thick and total thickness with lens being 70mm, so I figure it's a 65mm lens. I've cut a 4.5x6cm mask for it, so it's a pocketable 645 that weighs just 420g naked. Pocket Hassy!

I've a couple copies of the Argus-C3 (The Brick!). This interchangeable-lens 135/FF RF's body isn't really thin, but its lenses are minuscule. The Argus Cintar 50mm and Enna Sandmar 35mm are 20mm deep; the Enna Tele-Sandmar 100mm, which I use a lot on my K20D, is just 45mm deep.

With thin cameras, short lenses can extend almost to the sensor frame, and longer slower lenses can be built small. Size DOES matter.

[v5planet]

The reason the lenses can be made smaller is that the focal length is naturally shorter. Thus, you don't need to do any fancy optical tricks to obtain, for example, an ultra-wide prime lens. On DSLRs, a true focal length (distance from the focal point to the sensor) of something like 8mm or 10mm is physically impossible, so the lens needs very big elements and complex optical formulas in order to get the same FOV.

Without knowing precisely how the optics of modern lenses work, I will say that makes intuitive sense. Does this benefit evaporate once you leave the wide end of the focal range? i.e. is there a benefit (or perhaps even a penalty??) to lenses on the telephoto end?

[Adam]

I suppose that extreme telephoto lenses would physically need to be slightly longer, but the difference would most likely be negligible as all you would need is an air gap. Plus, since many interchangeable cameras tend to use sensors smaller than APS-C, the lenses will have crop factors and won't need to be as big anyway.