How does lens size matter?

[Gautam Valluri]

Hello Everyone,

I was just wondering if the lens size matters in 16mm?

For example, how is an image shot with a 25mm Kern-Switar C-mount lens (which is tiny) differ from an image shot on a PL-mount 25mm Zeiss Superspeed MK III (which is considerably bigger)?

Besides the obvious sharpness from the quality of glass, I'm wondering if it affects the FOV in anyway by having a lens with a larger diameter?

Any thoughts, diagrams or examples would be welcome.

Thanks!

[Aapo Lettinen]

Lenses have different geometry on the edges of the image circle and that can affect the effective field of view quite a lot. on wide angles the difference can be as big as 4 or 5mm or more equivalent focal lenght difference (which can translate to even couple of dozen degrees fov in worst case) but it varies from lens to lens so only possibility is to test to figure out how much difference there is. But almost always lenses differ at least 2 or 3mm or so.

A larger format lens should generally have slightly smaller field of view in most cases because it is likely to draw the center area of its image circle relatively flat whereas the smaller format lens uses all of its image circle on the format and thus the distortion on the edges is likely to compress just tiny bit more fov to the frame.

edge sharpness may be better on the larger format lens, but overall sharpness depends on the quality and type of the lens so it is not possible to say without testing how the center sharpness would behave.

contrast is often worse in larger format lenses compared to small format ones if the coatings are approximately same. this is because there is lots of unused glass surface causing extra reflections and spreading out-of-image light all over the place. Using hard mattes can help with this to a point but generally speaking smaller format lenses should have better contrast and brighter colours if everything else is the same as there is less extra glass area to mess up the contrast

[Simon Wyss]

The absolute size is reflected in the price. Optical glass costs more than window float pane and bigger mechanical parts are pricier, too. Medium relative aperture lenses have smaller elements compared to speedy ones which again leads to cheaper products. Mass produced amateur optics are cheaper than professional volumes but not necessarily worse. If you can handle an Animar 26 mm, f/1.9 you get beautiful images.

As a keen cinematographer you know what lens you want, a triplet, a four-glass lens, a six-elements design, opening of f/4.5, opening f/1.8, and so on. Depending on the type of camera in question, a heavy tripod Mitchell 16 Professional, a hand-held GIC 16 or a Beaulieu R 16, you gather luxurious glass or compact and lightweight lenses, respectively.

One thing is important, the mechanical quality of diaphragms. Regardless of size some makes hide second-class stuff. It comes down to single products and series whether the diaphragm is robust or weakish. Unfortunately.

[Helge Abrahamson]

In my experience and as a rule of thumb, the larger the front lens element, the better depth separation (nothing to do with shallow focus depth). This is caused by the lens looking "behind" foreground objects, kinda the same way as a 3D camera setup... The light enters the front lens element from a bigger separated angle, making background, behind foreground objects, visible - causing better depth separation.

P.s. if above makes absolutely no sense, it's probably because of my poor english... Sorry...

[Dom Jaeger]

Yeah, interesting question.

Element size relates to the design, which can be quite varied in terms of what the designer wanted to achieve. Every lens is a different set of compromises, a different path to a particular outcome.

The primary factors in lens design choices are the angle of view and the maximum aperture. You’ll find both wide angle lenses and telephotos tend to have larger front elements than standard focal lengths, but for different reasons.

A wide angle lens needs a large front element to allow the disparate angle light rays to all see the entrance pupil, which is where the lens FOV originates. The deeper the entrance pupil the larger the front element required. One consequence of a smaller front element can be “cat’s eye” bokeh where the bokeh balls become cut off wedges towards the edge due to the entrance pupil being only partially visible from the outer angles.

A telephoto lens needs a large front element because it needs to be at least as large as the entrance pupil, which in a longer focal length can be quite sizeable if the lens has a relatively fast aperture. 

Whether a design uses a positive (magnifying) or negative (diminishing) front optical group, or internal focussing elements, and the spacing of these groups, all determines the size and depth of the entrance pupil, which in turn affects the front element size requirements. 

It’s easy to assume that a smaller front element in a lens of the same focal length is a sign of lesser quality, but that isn’t always the case. It’s really just a design choice.