Focal Length vs Aperture Size
The focal length of the lens affects the maximum aperture size available.
In simple terms, the f-number of the lens increases as the lens is "zoomed in", resulting in a smaller relative aperture. While the physical aperture size does not change, the relative aperture size gets smaller as the focal length increases.
As the f-number = focal length / diameter of the lens,
increasing the focal length (zooming in) results in a larger f-number as the diameter of the lens remains constant.
Referring to Image 1 above, the specification of the lens states 18 - 55mm 1:3.5 - 5.6 This means that the aperture size is f/3.5 at 18mm, and f/5.6 at 55mm. The rest of the images shows the change in aperture size as the lens is "zoomed in". The camera was set to Aperture Priority with the widest aperture setting selected.
As the camera "zooms in", notice the aperture size increasing from f/3.5 to f/5.6
How important is this?
Hell it IS important. It signifies how good your lens construction is, especially at telephoto range.
Remember the rule of thumb for shutter speed requires at least 1/focal length to minimize blurring due to handshake.
For a standard Nikon AF 100 - 300mm f3.5 - 5.6 lens. At 300mm telephoto, the required shutter speed would be 1/300s. The fast shutter speed would mean that the camera needs to compensate it by either opening up the aperture or by increasing the ISO. With the above point mentioned, the widest available setting is f/5.6 , which could pose a problem in low light conditions. The only option left is to boost the ISO at the expense of noise, or use a tripod.
That is why lens which are capable of having small f-number (f/2.8, f/1.8, f/1.4 etc) are known as "fast" lens. Prime lens (fixed focus) lens are superior to zoom lens for the same reason, although they lack the versatility of zoom lens.
Still dont understand?
The bottom line. When the lighting condition is poor, try not to zoom in. Take most of your shots wide angled, so as to maximise the aperture size and shutter speed. If you noticed most of my shots are night are wide angled for the 2 reasons.
At 18mm, the minimum shutter speed required is 1/20s (easily achievable), and the widest aperture at f/3.5 allows sufficient light to enter for a reasonable image to be taken. ISO can be kept at 400 or 800 depending on the situation.
Otherwise, use a 50mm f/1.8 prime lens.
In simple terms, the f-number of the lens increases as the lens is "zoomed in", resulting in a smaller relative aperture. While the physical aperture size does not change, the relative aperture size gets smaller as the focal length increases.
As the f-number = focal length / diameter of the lens,
increasing the focal length (zooming in) results in a larger f-number as the diameter of the lens remains constant.
Referring to Image 1 above, the specification of the lens states 18 - 55mm 1:3.5 - 5.6 This means that the aperture size is f/3.5 at 18mm, and f/5.6 at 55mm. The rest of the images shows the change in aperture size as the lens is "zoomed in". The camera was set to Aperture Priority with the widest aperture setting selected.
As the camera "zooms in", notice the aperture size increasing from f/3.5 to f/5.6
How important is this?
Hell it IS important. It signifies how good your lens construction is, especially at telephoto range.
Remember the rule of thumb for shutter speed requires at least 1/focal length to minimize blurring due to handshake.
For a standard Nikon AF 100 - 300mm f3.5 - 5.6 lens. At 300mm telephoto, the required shutter speed would be 1/300s. The fast shutter speed would mean that the camera needs to compensate it by either opening up the aperture or by increasing the ISO. With the above point mentioned, the widest available setting is f/5.6 , which could pose a problem in low light conditions. The only option left is to boost the ISO at the expense of noise, or use a tripod.
That is why lens which are capable of having small f-number (f/2.8, f/1.8, f/1.4 etc) are known as "fast" lens. Prime lens (fixed focus) lens are superior to zoom lens for the same reason, although they lack the versatility of zoom lens.
Still dont understand?
The bottom line. When the lighting condition is poor, try not to zoom in. Take most of your shots wide angled, so as to maximise the aperture size and shutter speed. If you noticed most of my shots are night are wide angled for the 2 reasons.
At 18mm, the minimum shutter speed required is 1/20s (easily achievable), and the widest aperture at f/3.5 allows sufficient light to enter for a reasonable image to be taken. ISO can be kept at 400 or 800 depending on the situation.
Otherwise, use a 50mm f/1.8 prime lens.
Labels: basic, photography
posted by white5tar at 1:56 AM
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