shutters, lenses and aperture

[David Gardner]

The latest in a series of stupid beginner questions:

Is there any advantage to putting a lens of one maximum aperture on a shutter th at has a larger maximum aperture? My understanding is that the maximum aperture of a lens is set by its design, and that a shutter with a larger maximum apertur e is capable of delivering that aperture only to a lens designed for the larger aperture. OTOH, someone told me that I'd get a brighter view if I put my f8 lens on a f4.7 shutter. So what's the truth?

Thanks in advance. David

[Bob Salomon]

Shutters come in sizes that vary in thread size. 0, 1 and 3 are the current sizes.

To screw aa 0 size lens into a larger thread size would require custom machining but that would only add expense, not added light.

The f stop of a lens is a ratio derived from the maximum useable diameter of the lens and its focal length.

Putting a lens in a larger size shutter would not have any effect on the diameter of the lens or the focal length.

[Jerry Gardner]

David,

The maximum aperture of a lens is determined by the optics, not the shutter. The aperture blades in the shutter are fully retracted when the lens is "wide open".

If you replace a shutter, you typically need to scribe the aperture scale on the shutter, which is blank when it comes from the factory.

[Robert A. Zeichner]

The maximum speed achieved in a lens is determined by the diameter of the widest aperture deliverable by the diaphragm designed for that lens divided into the focal length of the lens. This means that a 100mm lens with a maximum aperture diameter of 25mm would be an f4 lens. (100 / 25 = 4). Obviously an external diaphragm would have to be calibrated to achieve the same result, but other issues also come into play if it's located in a plane other than the one that particular formula of lens requires.

It is possible and fairly common for LF users to take a barrel lens (one without a shutter, but with a calibrated diaphragm) and mount it in front of a shutter. In fact, I have a number of different graphic arts lenses adapted to fit one size of shutter. The advantage is that you can test that one shutter and be confident that the shutter speeds on all your lenses are identical. You can also save a bunch of money by not having to buy a shutter for every lens. The downside is that if the shutter fails, you lose the use of every lens that fits it. It is also important to understand how mounting a lens further in front of a shutter can cause vignetting. This is something that Steve Grimes describes in detail on his web site. If the combination doesn't cause this problem, you can put together an impressive array of lenses for a fraction of the cost of individually shuttered optics. When I use this type of arangement, I tape the diaphragm of the shutter in the open position so it doesn't creep closed and foul up my exposure. I always use the diaphragm in the barrel lens itself. And my results have been excellent. I hope this helps you.

[Steve Grimes.]

The truth is that an f-8 lens will deliver only f-8 even if its in a shutter whose iris goes larger. (Sort of like a 50 horsepower car will only go 75mph and fitting it with a speedometer that goes to 125mph won't make any difference.)

SKG