Point and Shoot world talk about the optical zoom whether it is 10X Optical zoom, 12X optical zoom or so forth but in SLR world we talk about focal length range of 18 mm - 35 mm or 70mm - 300mm and so forth. Lets see the correlation between optical zoom and these focal length ranges.
Optical zoom is calculated as a ratio of maximum focal length to minimum focal length. Which means if a lens has a range of 18mm - 55 mm then its zoom factor is 55/18 = 3X Optical zoom and similarly for 70mm - 200mm lens zoom factor is 200/70 = 3X (approx). Thus we see that optical zoom in both the case is approximately the same but shots captured from them will be different, explained later below.
In the specifications of Point and Shoot cameras it will be mentioned : Optical zoom is 12x and focal length is of range 36mm equivalent - 432 mm equivalent. This does not means that lens used in point and shoot has a focal range of 36mm to 432 mm but actually might be 6mm to 72 mm. So why do they mention 36mm equivalent - 432 mm equivalent lens. This means picture taken from these lenses will feel the same as taken from 36mm lens - 432 mm in 35mm cameras.
Because many people are familiar with focal lengths of lenses for 35mm cameras, the digital camera manufacturers choose to describe the focal length of their cameras by reference to the focal length that would produce a similar field of view on a 35mm camera.
By describing the lenses this way, the digital camera companies appeal to users' familiarity with 35mm camera equipment.
Lets see in little detail:
Change of field happens because the sensor size is different in different cameras. (smaller the sensor size cheaper the camera, more noise at low light).
While normal film cameras take 35mm film (it is a standard for the industry) there is much variety between manufacturers on image sensor sizes. The main reference point that people therefore use is the 35mm one which is considered ‘full frame’ size.
Details on this can be found here http://www.panoramafactory.com/equiv35/equiv35.html
The CCD arrays on digital cameras are typically much smaller than the imaging area of 35mm film. This means that the focal length that produces a "normal" field of view is much smaller in digital cameras.
Let me show this by an example. Here I am using two cameras to show the difference Sony Cybershot DSC H2 and Canon EOS 350D.
Specifications about sensors size and lenses are below:
Sony DSC H2
Sensor size 5.75 mm X 4.31 mm.
Lens Focal Range = 6.0 mm - 72.0 mm
Lens Focal Range (35 mm equivalent) = 36.0 mm - 432 mm
Canon EOS 350D
Sensor size 22.2 mm X 14.8 mm
Lens Focal Range = 18.0 mm - 55.0 mm ; 70.0 mm - 300.0 mm
Lens Focal Range (35 mm equivalent) = 28.8 mm - 88.0 mm ; 112.0 mm - 480.0 mm
Optical zoom is calculated as a ratio of maximum focal length to minimum focal length. Which means if a lens has a range of 18mm - 55 mm then its zoom factor is 55/18 = 3X Optical zoom and similarly for 70mm - 200mm lens zoom factor is 200/70 = 3X (approx). Thus we see that optical zoom in both the case is approximately the same but shots captured from them will be different, explained later below.
In the specifications of Point and Shoot cameras it will be mentioned : Optical zoom is 12x and focal length is of range 36mm equivalent - 432 mm equivalent. This does not means that lens used in point and shoot has a focal range of 36mm to 432 mm but actually might be 6mm to 72 mm. So why do they mention 36mm equivalent - 432 mm equivalent lens. This means picture taken from these lenses will feel the same as taken from 36mm lens - 432 mm in 35mm cameras.
Because many people are familiar with focal lengths of lenses for 35mm cameras, the digital camera manufacturers choose to describe the focal length of their cameras by reference to the focal length that would produce a similar field of view on a 35mm camera.
By describing the lenses this way, the digital camera companies appeal to users' familiarity with 35mm camera equipment.
Lets see in little detail:
Change of field happens because the sensor size is different in different cameras. (smaller the sensor size cheaper the camera, more noise at low light).
While normal film cameras take 35mm film (it is a standard for the industry) there is much variety between manufacturers on image sensor sizes. The main reference point that people therefore use is the 35mm one which is considered ‘full frame’ size.
Details on this can be found here http://www.panoramafactory.com/equiv35/equiv35.html
The CCD arrays on digital cameras are typically much smaller than the imaging area of 35mm film. This means that the focal length that produces a "normal" field of view is much smaller in digital cameras.
Let me show this by an example. Here I am using two cameras to show the difference Sony Cybershot DSC H2 and Canon EOS 350D.
Specifications about sensors size and lenses are below:
Sony DSC H2
Sensor size 5.75 mm X 4.31 mm.
Lens Focal Range = 6.0 mm - 72.0 mm
Lens Focal Range (35 mm equivalent) = 36.0 mm - 432 mm
Canon EOS 350D
Sensor size 22.2 mm X 14.8 mm
Lens Focal Range = 18.0 mm - 55.0 mm ; 70.0 mm - 300.0 mm
Lens Focal Range (35 mm equivalent) = 28.8 mm - 88.0 mm ; 112.0 mm - 480.0 mm
Top picture is from Sony at focal length ~ 70 mm and bottom one from Canon at focal length ~ 70mm. The picture shows the difference in angle of view due to different sensor size shot at 70mm focal length.
Now lets see how equivalent Focal helps
Now lets see how equivalent Focal helps
Top picture is from Sony at focal length of 70 mm which is equivalent to 432 mm in 35mm camera. Next picture shot from Canon at focal length 270mm which is equivalent to 432 mm in 35mm camera. These two pictures look similar even though at different focal length and gives an impression of picture taken from 35mm camera at 430mm focal length..
Because of the smaller format of DSLRs, telephoto lenses have a narrower angle of view than the same lens on a 35mm camera – this is the so called ‘crop’. Angle of View is simply that viewing angle which fills the frame (of any format). This determined by the film format in relation to the focal length of the lens – not the focal length on its own.
‘Crop’ is a fairly good term – the imaging area is physically smaller. Less of the image circle projected by the lens is used, therefore it is a crop. The image remains the same size at the film plane for a given lens and subject distance – it is in no way magnified. It does, however, take up a larger proportion of the (smaller) frame and so it is easy to see why some people call it a magnifying effect. This is also why a tele lens appears so much more powerful – the field or angle of view has been reduced. This is great for nature and sports photographers as the net result is more real pull than before with no trade off of maximum F Stop loss.
As an example: on a 35mm camera a macro lens focuses a 24mm long insect onto the film at same size – this is known as 1:1 ratio, or Life Size. 2.4cm on the film is about 2/3 of the frame in length so it will print to 2/3 of whatever (full frame) print size is made. On a DSLR with the same lens and at the same subject distance, the image at the film plane is still 2.4cm long at 1:1 or Life Size. However, 2.4 cm is about full frame in size and the subject will now occupy all the frame. For a given print size the subject will appear bigger in the frame. Thus it appears magnified compared to a 35mm camera.
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