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Microstock Photography Forum  General => Photography Equipment => Cameras / Lenses => Topic started 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 sensor’s 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)

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 :)

70mm for pretty people.
500mm + 2 x converter for ugly people.

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 12 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 85100mm on a full frame camera. Focal length doesn't change the perspective or looks of people, distance does.

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 sensor’s 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)

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

5D has larger sensor than 1D? Didnt know that! LOL! How?

1ds 36.0 x 24.0mm (fullframe sensor)
1d 28.1 x 18.7mm (APSH 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

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

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 noncamera eye. Give it a try!