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Lens mount and sensor formats

Sony A-mount and E-mount systems

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

consequently the lenses as well.

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

( "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.

Image area with

35 mm full-frame image sensor

47° angle

of view

Lens

Same focusing

distance (50 mm)

Image sensor

plane

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

Alignment mark

Electrical contacts

Locking pin

Aperture lever

AF coupler

Flange back distance

Image sensor plane

Lens mount

Sony DT lenses

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.

Image area with

APS-C type sensor

32° angle

of view

Lens

Image sensor

plane

Aperture, f-numbers and depth of field

Aperture and exposure

The aperture in a lens—also known as the

affects depth of field, you'll need to be in control of

"diaphragm" or "iris"—is an ingenious piece

both aperture and shutter speed to create images

of mechanical engineering that provides a

that look the way you want them to.

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

Circular aperture (see page 16 for details)

"F-numbers" or "f-stops"

All lenses have a maximum and minimum

know that smaller f-numbers correspond to larger

aperture, expressed as "f-numbers," but it is the

apertures, and that F1.4 is about the largest

maximum aperture that is most commonly quoted

maximum aperture you're likely to encounter on

in lens specifications. Take the Sony SAL35F14G,

general-purpose lenses. Lenses with a maximum

for example. This is a 35mm F1.4 lens: 35mm is the

aperture of F1.4, F2, or F2.8 are generally

focal length and F1.4 is the maximum aperture.

considered to be "fast" or "bright."

But what exactly does "F1.4" mean? See the

"F-number math" box for some technical details,

The standard f-numbers you'll use with camera

but for a practical understanding it's enough to

lenses are, from larger to smaller apertures:

Focal length

F-number =

Aperture

Effective aperture

(size of the entrance pupil)

Focal length

Aperture and depth of field

"Depth of field" refers to the range between the nearest and farthest objects

in a scene that appear acceptably sharp. In extreme examples of narrow

depth of field, the in-focus depth might be just a few millimeters. At the

opposite extreme, some landscape photographs show very deep depth

of field with everything in sharp focus from just in front of the camera to

many kilometers away. Controlling depth of field is one of the most useful

techniques you have for creative photography.

Open (large)

Aperture

F2.8

F5.6

F11

F16

Shallow

Depth of field

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.

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.

Shorter focal lengths only require

Effective aperture

moderate effective apertures for

sufficient brightness

25 mm

100 mm

Longer focal lengths require

proportionately larger effective apertures

for the same "f-number" and brightness

50 mm

200 mm

Aperture and focal length values in the illustration are approximate.

Basically, larger apertures produce a narrower depth of field, so if you want to

shoot a portrait with a nicely defocused background you'll want a wider aperture

( lower F-number ) . There are times when other factors come into play. Lenses of

longer focal lengths are generally capable of producing narrower depth of field.

This is partly because an F1.4 aperture in an 85mm lens, for example, is physically

larger than an F1.4 aperture in a wide-angle 24mm lens. Additionally, the

distance between objects in the scene being photographed will have an effect

on the perceived depth of field as well.

Three keys to effective defocusing

Close (small)

There's actually more to shooting images with beautifully defocused

backgrounds than simply choosing a bright lens and opening

F22

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

75mm and 100mm are ideal for shooting portraits with nicely

Deep

blurred backgrounds.