Ch. 14  The Camera

Focus

 

As we learned earlier in the course, the distance from a lens at which the image is focused depends on the distance of the object from the lens. Often, the distance to the object cannot be controlled, and since the distance of the detector (film or CCD) is also fixed in the camera, one changes the characteristics of the lens itself to achieve a good focus.

 

For cameras with a simple single-element lens, one can move the lens slightly closer or further from the detector plane. For lenses with multiple elements, the locations of more than one element will change, often by different amounts.

 

Exposure

 

To first order, the “response” of the light detector in a camera depends on the total amount of light it receives in the exposure. This exposure H0 is given by the product of the rate at which the light hits the detector (the illuminance EV) times the time of the exposure:

 

H0 = tEV

 

Shutter Setting

 

The exposure time t is set by opening and closing the shutter. Exposure times are given in seconds, and settings are generally in ratios of a factor of 2:

The exposure time t is set by opening and closing the shutter. Exposure times are given in seconds, and settings are generally in ratios of a factor of 2:

 

2 sec

1 sec

1/2 sec

1/4 sec

1/8 sec

etc.

 

Note that only the denominator is usually listed. “500” means 1/500 sec, not 500 sec!

 

 

 

Shutters are of 2 basic types:

 

Leaf Shutters: metal “petals” in a flat plane unfold outward.

Focal Plane Shutters: one half of a flat curtain opens, followed by a second curtain closing off the light path.

 

 

Many good 35 mm cameras with focal plane shutters have a fastest shutter speed of 1/2000 sec. The exposure can be increased indefinitely by locking the shutter in the open position. (Historically, this is called a “bulb” setting if you use the lock on the cable release, or “time” if the shutter has a built-in lock. Thus you may also see “B” and “T” on the camera shutter speed dial).

 

Aperture Setting

 

We also learned earlier that as the size of a lens (or other opening) increases, the light passing through will increase as the square of the diameter of the opening. In a camera, an artificial iris is closed down to control the illuminance reaching the detector. The effective f-ratio of the iris to the focal plane gives the “f-stop”. Here the nomenclature is tricky, because a factor of 2 or 1/2 gets squared in determining the illuminance and hence the exposure. Because the exposure time goes in factors of 2, the net throughput is also in factors of 2, so the effective diameters are done in factors of the square root of 2:

 

f/1.4

f/2

f/2.8

f/4

f/5.6

f/8

f/11

f/16

f/22

etc.

 

The numbers are “short-hand” in half of them. It’s easier to stamp or mold:

 

f/1.4

 

Than it is:

 

 f/1.41421356237…..

 

on the barrel of a camera lens!

 

In some cases, the “lowest” f-stop may be something other than the ones listed above. Wider openings = lower f-stops come with greater lens aberrations. Manufacturers will push the lowest f-stop to the practical limit. Many good ones reach f/1.2. Lenses of lower quality may not go below f/2.

 

At the other extreme, some cameras get tiny throughputs by having a f/64 setting (there have even been “clubs” like the f/64 Group that promoted this sort of thing. Why? Depth of Field! So let’s look at that now.

 

Depth of Field

 

Suppose a lens is situated some distance (larger than its focal length) from a light detector. There will be some distance from the lens that an object can be to be focused on the detector. That’s what the thin lens formula is all about! If the object is moved further away, the image will be focused somewhere other than at the detector, so that the image located at the detector itself will be out of focus and blurred. The circular region where a point object’s light hits the detector (or any imaginary plane) is the blur circle or circle of confusion.

 

Only a restricted range in object distances will the blur circles too small to notice. This range is called the depth of field.

 

 

 

The depth of field depends on the f-stop. The lower the f-stop the smaller the depth of field. The higher the f-stop, the greater the depth of field. In the limit where the f-stop becomes infinite, so too does the depth of field. Cameras with tiny openings called pinhole cameras, are used by some photographers to capitalize on this effect. (Note: as the aperture becomes a pinhole, lens aberrations also go away. Many simple pinhole cameras have no lens at all!).

 

 

Lenses

 

Camera lenses come in a wide variety of types: standard, wide-field, telephoto, etc. The type is set by the combination of the focal length of the lens and the film size. For “35 mm” film, with picture sizes of 24 mm x 36 mm, a 50 mm focal length lens gives an image “similar” to what the eye/brain system of a human “sees” well. Smaller focal length lenses give smaller focused images and more area around the object gets in the picture. Thus smaller focal length lenses are “wide-angle” lenses. Longer focal length lenses give bigger images and smaller fields of view and are “telephoto” lenses. (Sounds better than “narrow-field” lenses, right?)

 

 

 

 

Three separate lenses for the same camera: 50 mm, 150 mm and 250 mm.

 

Zoom lenses (variable focal length lenses) incorporate multiple moving lens sections to allow a wide range of focal lengths. This is the source of the “40x magnification” or “64x magnification” that both still and video camera retailers boast about in their commercials.

 

Camera Types

 

View

 

 

 

 

Here, the image is formed on a ground glass screen. The image is then focused. Then a filmholder (or plateholder) is inserted in front of the screen, its dark slide pulled, and the camera shutter opened. In use for 150 years now.

 

 

 

 

Front and back views if a standard 4x5 in (negative film size) view camera.

 

 

 

 

 

A view of the focus screen of a view camera. Notice that the image is inverted from the actual scene!

 

 

To take a picture, the filmholder is inserted, and its dark slide pulled out to allow light to hit the film.

 

Viewfinder

The viewer sets the focus using light not passing through the lens. Focus is achieved in the more expensive versions using triangulation by a rangefinder. (The device does just what surveyors do when getting distances over land). Those without a viewable focus adjustment are the basis of the popular “point-and-shoot” cameras.

 

 

 

 

Example of the use of a split-screen rangefinder. These can be used with very quiet leaf shutters, and avoids the noise of the flip-up mirror used in SLRs (bellow).

 

(Sorry, I don’t own one of these!)

 

 

Reflex

Here, light heading toward the film can be redirected toward a focus screen by a diagonal mirror. The mirror pop up out of the way just before the shutter opens, and drops back down after the shutter closes. The mirror flips the inverted image vertically, leaving image on the focus screen that is flipped left-right.

 

 

 

 

 

For easier operation, the image can be inverted and the direction of the viewer changed by using a internally-reflecting prism. This is the design used by the single lens reflex camera or SLR. SLRs are the most common type of quality 35 mm cameras around today.

 

 

 

 

 

 

A standard 35 mm SLR (vintage late 1970’s). Entirely manual, but with a through-the-lens light meter.

 

 

 

 

A related type of system is a twin-lens reflex. Like a viewfinder/reflex hybrid. Here, two independent lenses in parallel, one for the film and the other for viewing/focusing, are moved simultaneously for focusing on both the film and the focusing screen. A popular camera for beginning photography students, it has been around for years (your great-grandmother may have owned one of the fixed-focus types….). In many ways, there is no real reflex here, so the term twin-lens without the reflex is actually more correct for the camera shown.