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I am building a camera using a very large front glass element for the lens (diameter is 70mm).

At first, I thought I should try to find a shutter that is 70mm in diameter as well, one that can function like older cameras (set the speed and aperture, push a lever, release shutter).

But now I wonder if I really have to use a shutter that is 70mm in diameter or larger, or if I can I use a smaller one? I am assuming that if I use a shutter of a smaller diameter than the lens, the properties of the lens will change (for example, it will become less fast), because of the smaller aperture ring behind it?

That said, are there shutters that are 70mm in diameter?

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  • I don't understand why you need a 70mm shutter. Lots of cameras have 70mm elements with smaller shutters. – xiota Apr 19 at 23:29
  • Well the lens is also 70mm in diameter at the back – MicroMachine Apr 20 at 2:04
  • Can you provide a lens diagram? Also what format are you targeting? – xiota Apr 20 at 2:34
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I am assuming that if I use a shutter of a smaller diameter than the lens, the properties of the lens will change (for example, it will become less fast), because of the smaller aperture ring behind it?

That is not necessarily the case. The physical size of the aperture is (almost?) never the same size as the objective lens element; because its physical size is not what matters. What matters is whether the aperture obstructs the path that the light must follow while being focused at the image plane. That is the aperture's apparent/effective size as seen/magnified by the objective element (which is called the entrance pupil).

I.e. the aperture is (almost?) always a physical restriction; but what actually matters is if it is an optical restriction or not.

If the aperture diameter appears to be the same size as the objective element (due to the magnification of it), there will be no optical restriction to the light travel, and the lens's f-ratio/maximum aperture will be determined by the diameter of the objective element itself (it cannot be any larger). And if the aperture appears smaller than the objective element, then the lens's f-ratio will be determined by its apparent diameter because it is smaller and it is an optical restriction.

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A hat is the stylish photographer’s traditional front of lens shutter (and lens shade) though a press fit lens cap with a handhold in the center provides better control. Of course low sensitivity film/sensor and narrow apertures are needed when photographing in bright light due to the slow shutter speed.

Packard shutters are a pneumatic alternative that can provide a faster speed of about 1/8 second and can be front mounted.

Do it yourself options might include a rotating shutter similar to the design of movie cameras or a gravity powered knife shutter with an adjustable slit. It is more a question of desire, will to conduct wonky experiments, and engineering skills than anything else.

Even a leaf shutter can be pretty simple. Take a look at a Holga or Diana. The shutter is a couple of plastic blades and an unimpressive spring.

From a design standpoint, the smaller the maximum aperture the smaller the shutter can be. The same is true for shorter focal lengths.

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  • Does your last sentence mean that even if I'm using a f/0.75 lens with a 70mm diameter, and placing the small Holga shutter behind it (after removing the front lens element of the Holga), the image captured will still be f/0.75? I like the hat idea but we're talking about a fast lens here, even a Holga shutter might overexpose the image. – MicroMachine Apr 18 at 19:48
  • @MicroMachine No. It means that a shutter for a 30mm f8 lens can be smaller than a shutter for a 300mm f8 lens. Depending on the entire design of the lens system. – Bob Macaroni McStevens Apr 18 at 19:59
  • I came here to type almost exactly this. Glad you had it up already, upvoting is much faster. – Zeiss Ikon Apr 19 at 14:02
  • Focal length doesn't necessarily affect the physical size of a leaf shutter (aperture)... the change in magnification can allow using the same physical size for the same f-ratio. – Steven Kersting Apr 20 at 12:10
  • @StevenKersting If there’s magnification then the shutter is not before the the lens. – Bob Macaroni McStevens Apr 20 at 13:48
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Your shutter only needs to be big enough to cover the sensor, not the front element of the lens. There are many lenses out there that have much larger front elements, but the image circle by the time it passes through the lens mount is a fraction of the size of the mount, typically a bit larger than the sensor, to reduce vignetting. That said, medium and large format cameras obviously have to have larger shutters to deal with their larger sensors...

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  • Certainly true for focal plane shutters (part of camera), not true for leaf shutters (usually but not always part of lens) – BobT Apr 18 at 15:27
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    Leaf shutters are just like physical aperture diaphragms: the actual size does not matter. It is the apparent size as viewed through the front of the lens (i.e. the entrance pupil) that matters. Any magnification between the front of the lens and the diaphragm will affect the size of the diaphragm as seen through the front of the lens, whether it is a shutter diaphragm or an aperture diaphragm or both in one unit. – Michael C Apr 18 at 15:34

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