What is the focal length of a lens ?

The focal length is the distance from the sensor surface to the optical center of the lens.

It is expressed in millimetres.

We found it on our targets.

She’ll determine the angle of view.

The importance of choosing the right focus Before purchasing a lens, it is essential to know the focal length you wish to use.

Remember that the angle of view of human vision corresponds to a focal length of about 50mm on a 24x36mm sensor.

The shorter the focal length, 10mm, 24mm, 28mm for example: The further away the elements in the image appear.

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  • The more they give perspective to the image.
  • The wider the angle of view.
  • The more they distort, especially on portraits.
  • The shorter the focusing distance.

The longer the focal length, 85mm, 200mm, 400mm for example: The narrower the angle of view.

  • The closer the elements in the image appear.
  • The more the prospects are squashed.
  • The less depth there is in the picture.
  • The greater the focusing distance.

The zooms

They have the particularity of changing focal length, i.e. you can go from 70mm to 300mm for example in a jiffy without moving.

A normal zoom like for example a 17-55mm or 18-105mm is called transtandard because it offers the possibility to cover a lot of subjects.

The advantage is undeniable, if you want to take a picture of a landscape, you choose a short focal length on the lens by simply turning the zoom ring.

If, on the other hand, you want to make a portrait right afterwards, you turn the ring to the most suitable focal length and that’s it.

  • Ultra wide-angle lenses start at 10mm.
  • Telephoto lenses start at 50mm up to about 200mm, like a 70-20mm F/2.8.
  • Large telephoto lenses, usually used for sports or wildlife photography, start at 200mm up to 400mm.

Fixed focal lengths

They have only one focal length and do not have zoom rings.

You’ll have to move around to do the framing.

Here are the main fixed lens focal lengths that exist.

There are others, of course.

16mm, 24mm, 35mm, 50mm, 85mm, 100mm, 200mm, 300mm, 400mm, 500mm, 600mm, 800mm, 1000mm, 1200mm The advantages Optical quality, often better than a zoom lens Faster autofocusing The price of a 50mm f1.8 is ridiculously low.

Disadvantages A fixed focal length requires you to move around to change the framing, but this cannot be considered a disadvantage as you get used to it very quickly.

A change of lens will be necessary depending on the subject, so here again it is important to think before taking a picture.

These fixed focal lengths are very popular in all areas.

Fixed focal lengths

They have only one focal length and do not have zoom rings.

You’ll have to move around to do the framing.

Here are the main fixed lens focal lengths that exist.

There are others, of course.

16mm, 24mm, 35mm, 50mm, 85mm, 100mm, 200mm, 300mm, 400mm, 500mm, 600mm, 800mm, 1000mm, 1200mm

The advantages

  • Optical quality, often better than a zoom lens
  • Faster autofocusing
  • The price of a 50mm f1.8 is ridiculously low.

Disadvantages

  • A fixed focal length requires you to move around to change the framing, but this cannot be considered a disadvantage as you get used to it very quickly.
  • A change of lens will be necessary depending on the subject, so here again it is important to think before taking a picture.

These fixed focal lengths are very popular in all areas.

Nikon Nikkor AF-S 50mm f/1.8G

  • Lens This optic will make you love the picture.
  • Ideal focal length for portrait photography.
  • Very nice blurry backgrounds (bokeh).

Affordable Canon EF 50mm F/1.8 STM lens

  • Just like the Nikon version, this 50mm offers excellent optical quality.

Focal length multipliers

It is an interesting accessory that fits between the camera body and the lens.

It will allow us to change the focal length of a lens by multiplying it by a coefficient.

There are several multiplying coefficients, the x1.4, the focal length doublers and triplers.

Here is a selection I recommend.

So with a 50mm lens and a 1.4 x teleconverter mounted on an APS-C SLR camera body, its focal length becomes a 75mm.

A doubler on a 200mm, naturally becomes a 400mm.

Isn’t that great? Yes, but there’s the flip side…

a loss of quality and a loss of light.

On a focal length doubler, you will lose two f-stop values, so a lens initially opening at F/2.8 will automatically change to F/5.6 with the multiplier.

It will therefore be necessary to compensate for this loss of brightness, either by increasing the sensitivity to regain a suitable speed or by using a tripod, especially when using long focal lengths.

You will also lose quality depending on the model you choose.

In general, low-end multipliers are to be avoided as they strongly degrade quality, especially if mounted on low-end lenses.

If you want to multiply your focal length without too much loss, you have to pay for it because this kind of material is often expensive and it is the only way to get a quality photo in the end.

I strongly advise you to use bright fixed optics (i.e. between f/1.2 to f/4 maximum) with a focal length of 50mm minimum)

The zooms are not forgotten, but only if they open at least at f2.8.

Avoid mounting a multiplier on a 55-200mm type zoom lens or similar, as they are dimly lit at the departure and of average quality.

Ressources

To check the best lense qualities :  www.dpreview.com

 

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