digital photography terminology
focal length, angle of view and crop factors explained.

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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.

Related articles to this page

Digital photography exposure

Focal length, aperture and f-stops summarized

Shedding light on f- numbers

Depth of field explained

Which equipment will I need for my style of photography

Taking creative control of your photos

angle of view

Digital photography terminology
Focal length, Angle of view and Crop factors Summary

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:-

Focal length

  • Increasing focal length by a factor – decreases angle of view and increases image magnification by the same factor.
  • E.g. A 200mm lens will have half the angle of view of a 100mm lens and magnify the image twice as much.

Crop factor

  • The crop factor is the ratio of the sensor compared to a 36mm x 24mm full frame sensor (often called a 35mm sensor) and has the same effect as altering focal length.
  • E.g. Using a 200mm lens on a camera with an image sensor with a crop factor of 2 (as with for example a 4/3 dslr system cameras from Pentax) will give a magnification double that than would be obtained by a 200mm lens on a full frame sensor.

Digital photography terminology
Angle of view

A wide angle of view takes in more of the scene whereas a narrower angle restricts the image to that which is more directly in front.

We experience this effect when looking through a window.

Imagine standing with your nose pressed against the glass of the window – this would enable you to take in much of the scenery outside to the left and right as well as up and down – you would have a wide angle of view.

Now imagine stepping back several paces. This time when you look through the window the view to the sides, above and below would be restricted – you would have a narrower angle of view.

angle of view
angle of view

Digital photography terminology
Image magnification

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.

image magnification

digital photography terminology
Aspect ratios and viewfinder %

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.

crop factor

digital photography terminology
Crop factors

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 -

  • A full frame sensor of dimensions 36mm x 24mm has a diagonal of 43.2mm
  • An APS-C sensor of dimensions 22.3 mm x 14.9 mm has a diagonal of 26.8mm
  • The ratio between the two sensors is therefore 43.2/26.8 = 1.61 = the crop factor.

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.

crop factor

digital photography terminology
Other relevant factors

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:-

  • Perspective is related to actual subject to camera distance, so framing a subject by zooming in via longer focal length will not affect perspective, whereas moving closer to the subject and using the same focal length will.
  • The same is true if framing a subject with a lens of the same focal length mounted on cameras with different sensor sizes – the camera with the larger sensor requires you to be closer to the subject and therefore perspective will be different.
  • The actual depth of field is also affected by the crop factor ratio – so a smaller sensor will have a proportionally deeper depth of field. (One of the reasons why compact cameras with small sensors are unable to give the shallow depth of field of dslr cameras with larger sensors – an important limiting factor over creative control)
  • Whilst technically speaking longer focal lengths do not actually significantly affect depth of field – in practice because longer focal lengths compress perspective which means distant objects are magnified to a greater extent than near objects – this means that any depth of field blurring (bokeh) in the background is also magnified to a greater extent – thus giving the blurred background greater emphasis which in turn makes the depth of field appear shallower to the viewer.
  • Putting a lens of given focal length on a smaller sensor camera has disadvantages if you are looking for a wide angle to capture as much of the scene as possible – but conversely gives a benefit of greater magnification from the same lens - for example if your sensor has a crop factor of 1.6, then a 200mm lens will frame the same scene as a 320mm lens on the full frame camera – but a 200mm lens is likely to be significantly lighter and less expensive.

Digital photography terminology
Useful links from this page

go to JARGON BUSTER digital photography terms explained

Digital photography exposure

Focal length, aperture and f-stops summarized

Shedding light on f- numbers

Depth of field explained

Which equipment will I need for my style of photography

Taking creative control of your photos

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