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DSLR lens features: focal length / coverage and aperture / focusing and anti-shake

DSLR Tips Lens buying guide

 
  Most popular Sigma lenses  
 
  Most popular Tamron lenses  
 

The DSLR factor

One crucial point to note is the above focal lengths and rules only apply to DSLRs with full-frame sensors, or older 35mm film models. The vast majority of DSLRs however have physically smaller sensors which crop the field of view and effectively multiply the focal length by between 1.5 and two times. So-called ‘cropped’ models from Nikon, Sony and Pentax effectively multiply focal lengths by 1.5 times. Cropped models from Canon effectively multiply by 1.6x, while any DSLR based on the Four Thirds standard by Olympus or Panasonic, effectively multiply by two times.

 
Canon EOS 5D - full frame sensor Canon EOS 400D / Rebel XTi - cropped frame sensor
 












This is why digital photographers commonly talk about ‘effective’ focal lengths, where the actual lens focal length is multiplied by the DSLR’s ‘crop’ value. So a standard DSLR kit zoom lens with a focal length of 18-55mm on, say, a Nikon cropped body, would actually give an equivalent coverage of 27-82mm.

This means if you want 50mm standard coverage on a Nikon cropped DSLR, you’ll actually need to use a lens with a 33mm focal length (35mm models are closest). If you want 28mm wide angle coverage, you’ll need to use a lens with an 18mm focal length, and so on. So always remember to multiply the actual lens focal length by the crop factor of your particular DSLR – that way, you’ll know what you’re getting.

Note: since cropped-frame DSLRs aren’t using the full area of normal lenses, many manufacturers additionally offer models which are only corrected for this smaller frame. Canon, Nikon, Sony and Pentax refer to these types of lenses as EF-S, DX, DT and DA respectively. These aren’t suitable for full-frame DSLRs though, so if you’re thinking of upgrading to full-frame in the future, try to avoid these models.

Lens coverage

To illustrate the views you can expect at different focal lengths we took the following images from the same spot with different lenses. Remember the focal lengths quoted here are effective for a full-frame body, so to match the coverage with a typical DSLR, you’ll need to divide them by the crop factor of your particular model – see previous page for an explanation.

So if you have a Nikon, Sony or Pentax cropped body, divide the following focal lengths by 1.5 times. If you have a Canon cropped body, divide them by 1.6 times, and if you have a Four Thirds body, divide them by two times. So if you like the coverage of the 28mm example below and want it with a cropped Nikon DSLR, you’d divide it by 1.5 times to give you just over 18mm. Conversely, if you want to see what a Nikkor DX 18-200mm lens would give you in practice, just multiply it by 1.5 times to give 27-300mm.

Lens coverage by focal length from same position (equivalent to full / 35mm frame)
     
17mm equivalent
20mm equivalent
24mm equivalent
28mm equivalent
             
     
35mm equivalent
50mm equivalent
70mm equivalent
100mm equivalent
             
     
135mm equivalent
200mm equivalent
300mm equivalent
400mm equivalent


Aperture

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The second most important lens specification is the aperture – this refers to its light gathering capabilities. The bigger the aperture, the more light it can capture, and the better it can work in dimmer conditions. Lenses with bigger apertures also allow you to take photos with a smaller depth of field which allows you to better blur backgrounds. Big apertures are undoubtedly nice to have, but they involve physically bigger pieces of glass in the lens which makes them bigger, heavier and much more expensive.

Standard portrait
 

The aperture of DSLR lenses is commonly described as a focal ratio, or f-number. This is the ratio between the focal length of the lens over the diameter of the opening in the lens. So as the opening gets bigger on larger aperture lenses, the f-number gets smaller. An iris built-into every lens actually lets you reduce the size of the opening to control exposure and depth of field, but the important figure is the maximum aperture – or the smallest f-number.


On fixed focal length lenses, there’ll be one number – for example, 50mm f1.8. On zoom lenses, there’ll typically be two numbers, one for each end of the range – for example 18-55mm f3.5-5.6 – which means f3.5 at 18mm and f5.6 at 55mm. Note some premium zoom lenses have a fixed aperture throughout their range – for example 17-55mm f2.8 which is f2.8 regardless of the zoom setting.

F-numbers of 1.4, 2.8 and 4 may sound similar, but they actually represent a significant difference in light gathering power. For example, an f1.4 lens can gather twice as much light as an f2.0 model, or four times more than an f2.8 model. Similarly, an f2.8 lens can gather twice as much light as an f4 model, or four times more light than an f5.6 model.

A lens which gathers twice as much light lets you use a shutter speed that’s twice as quick, or the same shutter speed when it’s twice as dark. A lens which gathers four times more light lets you use a shutter speed that’s four times quicker, or the same shutter speed when it’s four times darker. Clearly lenses with big apertures, and therefore smaller f-numbers, are ideal when you’re taking photos in low light or of quick action.

Again though, the price you pay is a bigger, heavier and more expensive lens, especially if it’s a zoom. The exception to the rule are standard 50mm lenses (which thanks to the crop factor on most DSLRs act like a short telephoto of 75mm to 100mm).

These can be surprisingly affordable, and with most models offering f1.8 apertures, they’ll actually gather over eight times more light than a typical 18-55mm kit lens when it’s zoomed-in to the same focal length. Their small f-numbers also mean you can easily blur the background. That’s why standard 50mm lenses make a perfect introduction to low light and portrait photography.

Now to our final page: focusing and anti-shake.


DSLR lens features: focal length / coverage and aperture / focusing and anti-shake


All words, images, videos and layout, copyright 2007-2014 Gordon Laing. May not be used without permission.

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