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Microstock Photography Forum - General => Photography Equipment => Cameras / Lenses => Topic started by: litifeta on June 11, 2008, 17:32

Title: How to calculate the correct lens length for portraits
Post by: litifeta on June 11, 2008, 17:32
I have seen this on a Pro forum. The equation is to (square the sensor dimensions then add) square root the total. Then double the result.

(eg)

Canon 30D sensorís dimensions are 22.7 X 15

22.7 squared = 515.29
15   squared = 225

515.29 + 225= 740.29

The square root of 740.29= 27.2
Double 27.2 = 54.4

So according to this the most accurate I need is the lens at 54mm. Anything less will make the model fatter, anything more will make the model thinner.

For those of you without a calculator, type in your figure you want the square root of in excel as such =sqrt(740.29) then hit ENTER. (Where 740.29 is your total of the squared sensor dimensions)

Title: Re: How to calculate the correct lens length for portraits
Post by: Sean Locke Photography on June 11, 2008, 19:27
That looks like a bunch of random equations.  Why not just use what looks good.  Or take the subjects age, multiply by the day of the week, and divide by their waist size :)
Title: Re: How to calculate the correct lens length for portraits
Post by: massman on June 12, 2008, 03:20
70mm for pretty people.

500mm + 2 x converter for ugly people.
Title: Re: How to calculate the correct lens length for portraits
Post by: Perry on June 12, 2008, 04:03
I think that's a bit too theoretical. If you want a natural looking portrait you should use a lens that allows you to stay at a natural viewing distance from the model. When you look at a persons face you stand 1-2 meters away (1 meter=intimate, 2 meters="normal"), and when you want to see the whole person you stand 3+ meters away. Then you just have to pick the lens that allows the correct cropping. Usually that's around 85-100mm on a full frame camera. Focal length doesn't change the perspective or looks of people, distance does.
Title: Re: How to calculate the correct lens length for portraits
Post by: Peter on June 12, 2008, 05:01
I have seen this on a Pro forum. The equation is to (square the sensor dimensions then add) square root the total. Then double the result.

(eg)

Canon 30D sensorís dimensions are 22.7 X 15

22.7 squared = 515.29
15   squared = 225

515.29 + 225= 740.29

The square root of 740.29= 27.2
Double 27.2 = 54.4

So according to this the most accurate I need is the lens at 54mm. Anything less will make the model fatter, anything more will make the model thinner.

For those of you without a calculator, type in your figure you want the square root of in excel as such =sqrt(740.29) then hit ENTER. (Where 740.29 is your total of the squared sensor dimensions)



there are only 2 types of sensores. small ones (22.7x15) and full frame (35x25 or so...), this formula is useless. It is either one or the other. Only eos 1D and 5D have full frame sensor, and all others (in canon world) have the same size sensor (22.7x15)
Title: Re: How to calculate the correct lens length for portraits
Post by: Alatriste on June 12, 2008, 05:55
1d mark3 CMOS 28.10 x 18.70 mm
1ds mark3 CMOS 22.2 x 18.7 mm
5d CMOS 35,8 x 23,9 mm
40d, 400d... CMOS 22.2x14.8mm
Title: Re: How to calculate the correct lens length for portraits
Post by: Peter on June 12, 2008, 06:01
5D has larger sensor than 1D? Didnt know that! LOL! How?
Title: Re: How to calculate the correct lens length for portraits
Post by: Alatriste on June 12, 2008, 06:14
1ds 36.0 x 24.0mm (full-frame sensor)
1d 28.1 x 18.7mm (APS-H size sensor)
5d 35.8 x 23.9mm
40d     22.2 x 14.8mm CMOS

Now it was taken from the canon site so it must be right  :D

Title: Re: How to calculate the correct lens length for portraits
Post by: chellyar on June 12, 2008, 07:46
That forumla is just getting double the diagonal size of the sensor..  (Pythagoras' theorem)

The 'Standard' lens for any format is the same focal length as the diagonal size of the sensor, hence 50mm being the 'standard' for full frame 35mm, the diagonal is actually 43mm, but lets not split hairs...

In theory double the standard length is meant to be a good flattering perspective for a head&shoulders portrait.  I seem to remember that coming up in the one photography course I ever did, ages ago.  Hence 85mm lenses with big apertures for portraiture.

All of that said, just use what looks nice....   ;D
Title: Re: How to calculate the correct lens length for portraits
Post by: sharply_done on June 12, 2008, 09:38
Forget the formula - the simplest way to determine the "normal" focal length for your camera is to hold it vertically while using both eyes to look at your subject. Zoom in or out until the image from the camera synchronizes with your non-camera eye. Give it a try!