# Need an answer for this question

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#### KingKongChar

##### Guest
How to calculate the 35mm equivalent from a given focal length from a brochure, for example, is there a formula?

#### thoa_rs

##### Senior Member
If they gave you the focal length in the brochure that is already the 35mm.

#### mpenza

##### Senior Member
give an example of such brochures? usually the manufacturer will give the 35mm equivalent directly in the brochures.

#### roygoh

##### Senior Member
Otherwise you will need the actual size of the CCD to calculate the 35mm equivalent focal length.

I believe the effective 35mm focal length is given by:

Width of 35mm film
-------------------------- X actual focal length
Width of CCD

The width instead of height is used because the horizontal angle of view is generally the main point of interest when dicussing focal length of lenses.

If you calculate the equivalent 35mm focal length using the height of film and CCD sensor you will get a different result because the aspect ratio for film (3:2) is different from that of consumer/prosumer digicams (4:3).

Hope this helps.

- Roy

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#### KingKongChar

##### Guest
Thanks Roy for the detailed explanation. Well, let me give an example, say for Canon G3, in the specs, it is given that the focal length for its lens is 7.2 (W) - 28.8 (T) mm , I believe this is definitely not the 35mm equivalent. So is there a formula to convert these type of value to 35mm equi. ? If so, is there one formula for any digicam? I tried to do a backward convertion for one cam and apply on another cam but not applicable as the values too far off.

Roy, is your formula applicable to film or digicam, because it has both the film and CCD values?

#### roygoh

##### Senior Member
Originally posted by KingKongChar
Thanks Roy for the detailed explanation. Well, let me give an example, say for Canon G3, in the specs, it is given that the focal length for its lens is 7.2 (W) - 28.8 (T) mm , I believe this is definitely not the 35mm equivalent. So is there a formula to convert these type of value to 35mm equi. ? If so, is there one formula for any digicam? I tried to do a backward convertion for one cam and apply on another cam but not applicable as the values too far off.

Roy, is your formula applicable to film or digicam, because it has both the film and CCD values?
The focal length of the G3 lens is from 7.2mm to 28.8mm.

The CCD width is 7.2mm (taken from www.dpreview.com).

The angle of coverage is thus 2*ATAN(half of CCD width / focal length), which is from 53.13 degrees (wide end) to 14.25 degrees (tele end).

On 35mm format, the focal length that will give a horizontal angle of view of 53.13 degrees is given by:

half of film width / tan(angle of view / 2)

35mm film is 35mm wide . So half the film width is 17.5mm.

Then the focal length that will give 53.13 degrees agle of view is 36mm, if you plug the figures into the above formular.

If you calculate for the focal lenght that will give 14.25 degrees angle of view, then you will get a focal length of 140mm.

The above calculation is long winded in order to explain the concept behind finding the equivalent 35mm focal length that will give the same angle of coverage as a digicam with a known CCD size.

The entire calculation can be compressed into the equation that I have given in my earlier post (using similar triangle calcuation) to get to the answer quicker.

Also, when dealing with consumer/prosumer digicams with 4:3 aspect ration, the 35mm equivalent focal length is calculated based on the horizontal angle of view (film aspect ration is 3:2).

- Roy

#### Zerstorer

##### Senior Member
A rough guide would be to multiply it by 4.5x. That's the approximate FLM for most digicams.

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