The focal length, what is it ?

La focale, c’est quoi ?

Everything on the focal length and

to understand how it works


The focal length what is it ? Since the beginning of September I axis the articles more on the side of the technique, and the realization that on the writing story. You are a few to ask me for clarification on this or that technical term… it is true that each article leads to another… and I don’t write fast enough lol. In addition, there are so many things to say and explain that it will take me lots of time lol

In short in the article of Wednesday on the diving and the diving I’ve explained to you that the change of the focal length to accentuate the effects of these two techniques.

I’m not a great technician, I mean by that is that I have not done scientific studies , and that certain things, too technical, escape me (and I bumps also a little ;o))

Because the important thing is to understand the principle, what it is and how best to use it in your projects.

Then, a focal length what is it ?

On a film set, between each shot, the camera operator/ chief operator turns to the director and asks him the axis, and the focal length he wishes to use.

This simply means that the chief operator asked what goal he wants to use to film his next plan.

All focal lengths are expressed in MM : 10 mm, 12, mm, 14 mm, 18 mm, 25 mm, 32 mm, 50 mm or 85 mm

This list is not exhaustive, of course ;o)

Also called focal distance, i.e. the distance between the sensor of your digital camera and the optical center. For those who are still in the’film (film) this is the distance between the optical center and the film.

So the more you put a goal along, the more the focal length increases and the lens is short the longer the focal length decreases.

Short focal length or a long focal length ?

What are two definitions to know and I will rasure there is no magic. We have just seen that, depending on the length of the lens the distance lengthens or shortens.

This means that the longer your focal length, the more your subject will appear to be close.

And thus conversely, the more your focal length shortens, the more your angle of view is large.

So if you put a lens of 12 mm you will have a wider field than with a 25 mm

The higher the number on your lens will be small the more the field will be great. But attention, arrived at a certain figure the angle is so large that the image is distorted.

Moreover, the lens that distorts the more the image is what is called the fish-eye lens used a few years ago by Michel Muller in his mini-series ” Should not invite “

All these lenses are called fixed focal length, that is to say that you can’t zoom or de-zoom with. To take you away or bring you closer to your characters you need to change lens. But you have a better quality and a larger aperture than with a zoom.

But it’s more that some of the focal lengths each have a special feature. They are not made to shoot the same thing.

For example, a 40mm or a 50mm will be more for the portrait, a conversation, etc.. moreover, this focal length is closest to human vision.

A wide-angle such as 10 mm, 12 mm or 25 mm will be more to discover a place or make a group, put in situation.


The 85 mm and beyond will be more for bigger plans

Telephoto lens between 200 mm and 600 mm : it is the preferred objective of the paparazzi ;o) will be used for filming from very far away and have very big plans. We use the tele lens, for example, when one wants to shoot a character in the crowd, in the street and give an impression of grouillement or simulate a plane stolen (that is to say, without authorization). It is also used to re-create that film a camera or a picture taken by the cops stash etc…from the 200 mm of the bottom of the field, that is to say what is behind what you shoot will be as flattened.

The zoom is also from time to time used. It gives a ” colour “ different from the plan. When I speak of “color” it should be understood that aspect ;o)

How to control them ?

That is the question ! I think it is by forging that one becomes blacksmith. The more you will practice the different lenses, the more you’ll get eye.It is also important to think of the size of the sensor ! More the sensor of your camera will be little more objective than you will shrink the field. If you have a go-pro or a DSRL with the goal that you will choose will not have the same rendering ! Because the sensor of the two cameras are not the same !

In general when we talk about focal length and lens we talk about the size of a sensor “standard” that is to say, 24X36, full Frame, also called 35 MM Because a 50 mm or any other objective will be “more” tight with a dslr than with a DSRL 5D or D700 or a camera. It is therefore necessary that you know the size of your sensor and do the conversion compared to a 24×36 to know the good focal length to use.

For example : Un50 MM for a 24×36 will correspond to a 85 MM for an slr or a bridge.

I have worked with many filmmakers who have the eye (I mean, without having recourse to a scholar of the field) knew what focal length they wanted to use.

I remember that on a series of France2 , the director was preparing to shoot a scene where the characters were at the notary. Without looking, he asked for a 14mm. It would, therefore, turn with a focal length….short(bravo). Once the goal changed, he moved behind his combo (small monitor that sends the image seen by the cameraman in the camera) and a mechanical stage (that is to say that this is a rehearsal for technical and not for the actors).

The scene went off without a hitch. Once that has been finished, the developer asked to change the focal length and move to the 12 mm. I remember having looked a little surprised because the difference between a 14 mm and a 12 mm is absolutely tiny.

Once the change is made, I raise a new mechanical and there…it had nothing more to see ! Something had changed, the movements seemed more natural, it was also a bit more decor…details.

I will remember all my life this time as it has been for me like a revelation and a lesson in achievement. Yet there was no movement and abracadabrant, no explosion, no cascade, just an artist who has made up his plan as a painting , and where the slightest detail was important.

This is why, at the beginning of this article, I told you the biggest was to understand what focal length and how to use it rather than how it works (in its composition, technically, scientifically).


If you have an slr camera or a old camera with different lenses train you ;o)

I leave you a video of the cinema lessons that will explain you in pictures what is a focal length ; (o)


That’s it for this article on the focal length, of course there are other things to say, but it is a good place to start. At least now you know what is focal length and why it is used.

If you have questions on the focal length, leave me a message at the bottom of this article.

You can also read other articles by returning to the site map by clicking on this link.

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Tom W.


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