Sony ALCSH0006, LCL60AM Sony A-mount and E-mount systems, Aperture and exposure, Number math

Lens mount and sensor formats

Aperture, f-numbers and depth of field

Sony A-mount and E-mount systems

Alignment mark

Aperture and exposure

Sony A series interchangeable-lens digital cameras are currently produced in two categories, each of which uses a different lens mount and different types of lenses.A-mount SLR (single lens reflex) type cameras have a more traditional shape and utilize moving mirrors or advanced translucent mirrors. Ultra-compact E-mount cameras don’t use reflex mirrors at all. Despite their remarkable compactness and portability, E-mount cameras feature APS-C format sensors and are capable of delivering image quality on a par with A-mount cameras.

In addition to overall size, the main difference between A-mount and E-mount lenses is their “flange back distance.”The flange back distance is the distance from the rear of the lens to the image (sensor) plane. Since many A-mount cameras have a reflex mirror between the rear of the lens and the sensor, precipitating the need to have a flange back distance that allows space for the mirror. E-mount cameras, on the other hand, are mirror-less and therefore can be designed with a much shorter flange back distance, allowing the body of the camera to be much smaller and

Electrical contacts

Locking pin

Aperture lever

AF coupler

Flange back distance

Image sensor plane

The aperture in a lens—also known as the “diaphragm” or “iris”—is an ingenious piece of mechanical engineering that provides a variable-size opening in the optical path often used to control the amount of light that passes through the lens.Aperture and shutter speed are the two primary means of controlling exposure. For a given shutter speed, dimmer lighting will require a larger aperture to allow more light to reach the image sensor plane, while brighter light will require a smaller aperture to achieve optimum exposure. Alternatively, you could keep the same aperture setting and change the shutter speed to achieve similar results.The size of the opening provided by the aperture also determines how “collimated” the light passing through the lens is. Since this directly

affects depth of field, you’ll need to be in control of both aperture and shutter speed to create images that look the way you want them to.

Circular aperture (see page 16 for details)

TECH TALK

F-number math

The f-number is the focal length of the lens divided by the effective diameter of the aperture. So in the case of the SAL3514G lens, when the aperture is set to its maximum of F1.4, the effective diameter of the aperture will be 35 ÷ 1.4 = 25mm. Note that as the focal length of the lens changes, the diameter of the aperture at a given f-number will change too. For example, an aperture of F1.4 in a 300mm telephoto lens would require an effective aperture diameter of 300 ÷ 1.4 ≈ 214mm! That would end up being a huge, bulky and very expensive lens, which is why you don’t see too many long telephoto lenses with very large maximum apertures. There’s really no need for the photographer to know what the actual aperture diameter is, but it’s helpful to understand the principle.

consequently the lenses as well.

Lens mount

“F-numbers” or “f-stops”

Sensor formats: 35mm full frame and APS-C

You may have heard the term “full-frame” in reference to cameras, but did you know it refers to the frame size of 35mm film? The image area of a frame of 35mm film is approximately 36mm x 24mm

Sony DT lenses

All lenses have a maximum and minimum aperture, expressed as “f-numbers,” but it is the maximum aperture that is most commonly quoted in lens specifications.Take the Sony SAL35F14G, for example.This is a 35mm F1.4 lens: 35mm is the focal length and F1.4 is the maximum aperture. But what exactly does “F1.4” mean? See the “F-number math” box for some technical details, but for a practical understanding it’s enough to

know that smaller f-numbers correspond to larger apertures, and that F1.4 is about the largest maximum aperture you’re likely to encounter on general-purpose lenses. Lenses with a maximum aperture of F1.4, F2, or F2.8 are generally considered to be “fast” or “bright.”

The standard f-numbers you’ll use with camera lenses are, from larger to smaller apertures:

1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22 and sometimes 32 (for you mathematicians those are all powers of the square root of 2).Those are the full stops, but you’ll also see fractional stops that correspond to a half or a third of the full stops. Increasing the size of the aperture by one full stop doubles the amount of light that is allowed to pass through the lens. Decreasing the size of the aperture by one stop halves the amount of light reaching the sensor.

(“35mm” is the width of the strip of film), and that’s the size of the image sensor in a 35mm full-frame format camera. Many interchangeable-lens digital cameras use slightly smaller “APS-C” format sensors that measure approximately 24mm x 16mm or less.There are a number of other sensor formats, including smaller sensors in digital point-and-shoot type cameras, but APS-C and 35mm full-frame formats are the two most commonly used in interchangeable-lens cameras.

It is important to understand that there are two “formats” for A-mount interchangeable lenses as well. Lenses with an image circle large enough to cover a 35mm full-frame sensor, and lenses with a smaller image circle that is sufficient for APS-C format sensors. Sony lenses that have “DT” in the model name are compatible with APS-C format SLR cameras only, while all other lenses will work with both APS-C and 35mm full-frame format cameras.

Lenses marked “DT” (Digital Technology) should only be used on APS-C format cameras because their image circle isn’t large enough to fully cover a 35mm full frame sensor. If you do use a DT lens on a full-frame camera, expect to see a darkening of the image towards the edges of the frame (vignetting). Although only E-mount lenses can be directly mounted on E-mount cameras, DT lenses can be mounted on these cameras via an optional adaptor.

F-number =	Focal length	Aperture
	Effective aperture

Effective aperture

(size of the entrance pupil)

Focal length

F4

Effective aperture

25 mm 100 mm

F4

50 mm

200 mm

Shorter focal lengths only require moderate effective apertures for sufficient brightness

Longer focal lengths require proportionately larger effective apertures for the same “f-number” and brightness

Image area with	Image area with
35 mm full-frame image sensor	APS-C type sensor

Aperture and depth of field

Aperture and focal length values in the illustration are approximate.

47° angle	32° angle
of view	of view

“Depth of field” refers to the range between the nearest and farthest objects	Basically, larger apertures produce a narrower depth of field, so if you want to
in a scene that appear acceptably sharp. In extreme examples of narrow	shoot a portrait with a nicely defocused background you’ll want a wider aperture
depth of field, the in-focus depth might be just a few millimeters. At the	(lower F-number). There are times when other factors come into play. Lenses of
opposite extreme, some landscape photographs show very deep depth	longer focal lengths are generally capable of producing narrower depth of field.
of field with everything in sharp focus from just in front of the camera to	This is partly because an F1.4 aperture in an 85mm lens, for example, is physically
many kilometers away. Controlling depth of field is one of the most useful	larger than an F1.4 aperture in a wide-angle 24mm lens. Additionally, the
techniques you have for creative photography.	distance between objects in the scene being photographed will have an effect
	on the perceived depth of field as well.

Lens

Image sensor plane

Same focusing

distance (50 mm)

Lens

Image sensor plane

Open (large)				Aperture			Close (small)
F2	F2.8	F4	F5.6	F8	F11	F16	F22

SHOOTING TIP

Three keys to effective defocusing

There’s actually more to shooting images with beautifully defocused backgrounds than simply choosing a bright lens and opening the aperture up all the way. That’s the first “key,” but sometimes a large aperture alone won’t produce the desired results. The second key is the distance between your subject and the background. If the background is very close to your subject it might fall within the depth of field, or be so close that the amount of defocusing isn’t sufficient. Whenever possible, keep plenty of distance between your subject and the background you want to defocus. The third key is the focal length of the lens you use. As mentioned above, it’s easier to get a narrow depth of field with longer focal lengths, so take advantage of that characteristic as well. Many photographers find that focal lengths between about

*The angle of view values in this example correspond to those of a 50mm lens.

Shallow Depth of field Deep

75mm and 100mm are ideal for shooting portraits with nicely blurred backgrounds.

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