Focal length and aperture: how to choose your zoom lens

A zoom lens has undeniable qualities. However, there are some points that a zoom cannot compete with. We will see how to choose a fixed focal length for your image.

The focal length, which length to choose?

When choosing a lens, the first criterion to observe is the focal length of the latter. There are a multitude of them, from different manufacturers, ranging from ultra wide-angle to super telephoto. Be careful if you use an APS-C or micro 4/3 sensor box, the conversion factor may mislead you. To do this, multiply the actual focal length by the conversion factor below:

Canon APS-C sensor : x1.6

Nikon and Sony APS-C sensor : x1.5

Micro 4/3 sensor : x2

For example, a Canon EF 50mm f / 1.8 STM mounted on an APS-C reflex camera like the Canon EOS 90D will give an angle of view corresponding to what an 80mm would give on a full-frame camera.

In order to choose the focal length that suits you best, you must first define your activity. Are you going to focus on portraiture instead? Of the landscape ? We can roughly segment the uses according to the focal length as follows:

8 mm to 20 mm  : Architecture, landscape

20 mm to 40 mm  : street-photography, life-style

50 mm to 135 mm  : portrait

135 mm and more  : animals, sky, details...

If you have a standard zoom lens, which is usually supplied as a kit with a case, it may be a good idea to review your photos. Using specialized software (such as Bridge or Lightroom), select your favorite images then observe the focal length used for each. The focal length that comes up the most therefore corresponds to the one that best meets your expectations.

A big opening?

One of the advantages of the fixed focal length over the zoom is its ability to open wide . Most of the time, depending on the manufacturer, the same focal length can be available in several versions depending on the lens aperture value. For example, we find 50 mm opening at f / 1.8, f / 1.4 and even at f / 1  ! It should be noted that, in many cases, optical quality and build quality go hand in hand with a large aperture .

A large aperture will capture more light, very useful for low light photos. The background blur will also be more pronounced, which is often what portrait painters are looking for .

On the other hand, the price can quickly increase, as can the weight and the bulk. To see according to your budget and your use. If you use your lens for travel or carry it often, weight can be a deciding factor. The " pancake  " objectives  lend themselves perfectly to this game. On the other hand, if you are into studio photography, quality is the most important parameter. Your turn to judge.

If you are absolutely looking for a zoom, the possibilities in terms of aperture are limited. At the same price, a zoom will have a much smaller aperture ( f / 2.8 maximum against generally f / 1.4 for an average fixed focal length). There are two types of aperture for zooms:

The zoom constant openness are the most expensive and able to maintain the same aperture throughout the focal range covered. We can for example cite the excellent Canon RF 70-200 f / 2.8. Whether you're shooting at 70mm or 200mm, you'll be able to keep the diaphragm open at f / 2.8. 

The slippery aperture zoom lenses are the most common and least expensive. The aperture decreases as the focal length increases. 18-55mm f / 3.5 - 5.6 lenses are very common in kit cases. When the adjustment ring is placed on the minimum focal length, here 18 mm, the aperture is f / 3.5. Once placed at its maximum position of 55mm, the aperture is only f / 5.6. 

The brand of the lens, manufacturer or third party?

Whether you have a Hybrid or Reflex, Canon or Nikon, Olympus, Sony or Fujifilm camera, all camera manufacturers also have a suitable optical fleet. To be sure to benefit from optimal quality and certain compatibility, it is more convenient to buy the lenses of the brand of your camera. But there are third-party brands that also offer lenses. It may be interesting to look into it a bit.

Among them, we can count Sigma,  DZOFILM and Samyang . These brands, specializing in optics, have the advantage of offering high quality pebbles, often even higher than what device manufacturers can offer, for a more affordable price. These brands therefore deserve our attention.

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simon101 Joined: October 21st, 2019