Sony DSLR-A200K Brochure and Specifications | Page 5

Sony Cameras Brochure and Specifications - DSLR-A200K.
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Aperture and exposure

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. 

Aperture, f-numbers and depth of field

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.

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.

Shallow

Depth of field

Deep

Open (large)

Aperture

Close (small)

Circular aperture (see page 16 for details)

Aperture

F-number =

Effective aperture

Focal length

Focal length

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

Effective aperture 

(size of the entrance pupil)

Effective aperture

  Aperture and focal length values in the il ustration are approximate.

F4

F4

F2

F2.8

F4

F5.6

F8

F11

F16

F22

25 mm

50 mm

200 mm

100 mm

“F-numbers”  or “f-stops”

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. 

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 wil  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 
real y no need for the photographer to know what 
the actual aperture diameter is, but it’s helpful to 
understand the principle.

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 
wel . Many photographers find that focal lengths between about 
75mm and 100mm are ideal for shooting portraits with nicely 
blurred backgrounds.

TECH TALK

SHOOTING TIP

8

9

 

 

 

 

Image area with 

35 mm full-frame image sensor

Image area with 

APS-C type sensor

Same focusing 

distance (50 mm)

Lens

Lens

Image sensor 

plane

Image sensor 
plane

47° angle 

of view

32° angle 

of view

Lens mount and sensor formats

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.

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.

*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. 

 

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