Mount size

With Sony’s E mount 10mm smaller than that of new mir­ror­less cam­eras from Canon and Nikon, is there a jus­ti­fi­ca­tion for a large lens mount?

Amateur Photographer - - Tech Talk -

One of the most com­mented on fea­tures of the new Nikon Z-series cam­eras is the large lens mount di­am­e­ter. In fact, it’s not as large as the ru­mour mill would have had it be­fore the full de­tails were known. At 55mm, it pos­sesses a bare one mil­lime­tre larger in throat di­am­e­ter than the Canon EF lens mount, which is also the di­am­e­ter cho­sen by Canon for its new RF mir­ror­less mount. Nonethe­less, it raises the ques­tion, what is the ad­van­tage of the larger lens mount, given that the com­pet­i­tive Sony E mount is a full 10mm smaller and ap­pears to func­tion per­fectly sat­is­fac­to­rily.

If you do a web search on the ben­e­fits of a large di­am­e­ter mount, most of the pages found will talk about the abil­ity to sup­port large aper­ture lenses, yet the ven­er­a­ble Le­ica M mount has a clear di­am­e­ter of only 44mm, and has had avail­able lenses as fast as f/0.95. There is a sim­ple prin­ci­ple that dic­tates how fast a lens a mount can serve. Con­ven­tion­ally, the f-num­ber of a lens is de­fined to be ra­tio of the fo­cal length to the aper­ture. Thus a 50mm lens with an f-num­ber of 0.95 would have an aper­ture of 50mm/0.95, or 52.6mm. The same ra­tio ap­plies to the position of the exit pupil and its di­am­e­ter. If the exit pupil is pro­ject­ing a cone of light to a point of fo­cus at the cen­tre of the frame, then if the lens mount is not to ob­struct this cone, some sim­ple ge­om­e­try shows that the ra­tio of the dis­tance of the lens mount from the sen­sor (its reg­is­ter) and its di­am­e­ter must also be the same. This tells us that the Le­ica M’s 44mm di­am­e­ter and reg­is­ter of 27.8mm could sup­port a lens with an f-num­ber of 0.63, which should be fast enough. The new Nikon Z mount, with its 16mm reg­is­ter and 56mm di­am­e­ter could ac­com­mo­date an f/0.28 lens, which is phys­i­cally im­pos­si­bly fast for a well­cor­rected lens.

How­ever, pro­ject­ing light to the cen­tre of the frame is not the whole story. Light also needs to be pro­jected into the cor­ners of the frame. Whilst an exit pupil po­si­tioned right in the lens mount can pro­ject to the whole of the sen­sor, in the cor­ners of the frame, the light cone strikes the sen­sor at a very oblique an­gle. This worked quite well with film, but is prob­lem­atic for sil­i­con sen­sors fit­ted with mi­crolenses, which are used to enhance sen­sor ef­fi­ciency.

A par­tial so­lu­tion to this is to off­set the mi­crolenses in the cor­ners and edges of the sen­sor to­wards the cen­tre, so that they can more eas­ily ac­cept light pro­jected from that di­rec­tion. An op­ti­cally bet­ter so­lu­tion is to de­sign lenses which have an exit pupil fur­ther from the sen­sor, al­low­ing the light cone to strike the sen­sor with more per­pen­dic­u­lar­ity. How­ever, this re­quires a larger sen­sor throat. The di­a­gram, left, shows how this works.

The black line on the right rep­re­sents the 43mm di­ag­o­nal of the sen­sor, while the diverg­ing dot­ted lines show how the width of an f/0.95 exit pupil varies with its dis­tance for the fo­cal plane. The green exit pupil is placed at the Le­ica M mount’s 28mm flange dis­tance, the red at 50mm and the blue at 100mm. The blue ‘L’-shaped lines rep­re­sent the flange of the Z mount, and from this we can see how it can ac­com­mo­date an f/0.95 exit pupil at 100mm. Mean­while the green ‘L’-shaped lines rep­re­sent the Le­ica mount, and clearly only the 28mm exit pupil can prop­erly il­lu­mi­nate the cor­ners. Thus, the new Nikon mount al­lows for more dig­i­tal-friendly fast lenses.

The large-di­am­e­ter Nikon Z mount al­lows the de­sign of dig­i­tal-frendly fast lenses

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