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Some common digital photography terminology you will come across is “focal length”, “angle of view” and “crop factors”.
This article begins with a summary of the relevance of these terms, how they are interrelated and their significance to your digital photography.
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Whereas the topic of EXPOSURE covers controlling the intensity of light falling upon the image sensor, the factors listed above determine what image is actually framed and therefore captured by the camera.
When you view a scene with your eyes, the image you see is an interpretation of all the incoming data collected by your retinas.
The image recorded by your camera's image sensor is different in that it is an actual record of everything in front of the camera that is transmitting or reflecting light that can be captured via the lens and image sensor.
The proportion of the scene that is actually framed onto the image sensor is determined by the angle of view, focal length of the lens and image sensor size of the camera.
These factors are proportionately related as follows:-
Digital photography terminology
Increasing the focal length, narrows the angle of view such that a smaller area of the subject is projected onto the image sensor, effectively magnifying the resultant image.
The degree of magnification, focal length and angle view all increase or decrease by the same ratio -
i.e. If focal length is doubled – angle of view is halved - image magnification is doubled.
Camera apertures and lens elements are effectively circular. The potentially focused image at the focal plane is therefore also circular. However, image sensors and the media upon which images are finally displayed, whether computer monitors, tv screens or prints all tend to be rectangles.
The ratio of the shorter to longer side of a rectangle is known as its aspect ratio.
Image sensors come in differing dimensions and aspect ratios.
The common aspect ratios are 3:2 and 4:3
So, the image that is actually recorded is only the rectangular portion from within the image circle that falls upon the rectangular image sensor.
Ideally the image you can see through your rectangular viewfinder will match exactly the image you are recording on your sensor. This isn't always the case however with some cameras' viewfinders only showing around 95% of what is actually going to be recorded - it's important to be aware of this as you may be capturing unwanted elements in your image that you will have to crop out in post processing.
When mounting a lens at a specific focal length onto a camera with a smaller image sensor, part of the image circle will fall outside of the sensor and thus not be recorded.
Thus the final image is framed as it would have been if a larger sensor had been used but with that camera physically closer to the subject.
Thus this cropped image has an equivalent angle of view and an equivalent focal length to those that would have been required if using the lens on a full frame sensor.
It should be stressed that the actual focal length and angle by which the light is being converged towards the focal plane remains the same.
This reduction in the effective field of view – means that the final captured image when viewed will have the same magnification as if the equivalent focal length and its associated angle of view on the larger sensor had been used.
The factor by which the image is magnified as a result of the smaller sensor is therefore equal to the ratio between the larger and smaller sensor size. This is most easily calculated by comparing the ratio of the sensors' diagonal measurements.
For example -
As image area captured, effective field of view, focal length and image magnification all increase or decrease by the same ratios – the crop factor can be used to calculate that the “cropped image” effectively frames the same proportion of the scene that a full frame sensor would with a focal length 1.61 times longer, (or angle of view 1.61 times narrower) and therefore the image captured appears 1.61 time larger.
A detailed explanation of how and why the following are affected by focal length and crop factor is outside the scope of this article, but in summary they are:-
Physically moving closer to an object to frame it in your viewfinder as opposed to doing this by magnifying it via a longer focal length (or increasing the effective focal length by using a smaller sensor) do NOT give the exact same result. The major differences are:-
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