# Thread: how to be sure 35mm is really 35mm??

1. ## how to be sure 35mm is really 35mm??

i have recently measure how wide angle my camera (C4040) can take on the widest focus length of 35mm, which i measured to be 42 degrees. i measure by putting 2 rulers in front of the lens, so that in my viewfinder can only barely see the edge of both the rulers.

however, i read a book about focal length and found that 35mm can take more than 42 degrees...( about 50+degrees, i forgot already)

at the nearest zoom, the angle is about 21 degrees, which is also not as i have read........

so my question is, how can we be confirm that C4040 is really 35mm to 105mm (i.e. 3x zoom) ..... can we judge by the angle of coverage?

2. How did you perform your calculations?

Btw, the angle of view for a 35mm focal length lens on 35mm film is 54 degrees, while that for a 105mm lens is 19 degrees. Seems pretty close to your calculations to me (note that the rate of change of angle of view is steeper towards the wide end). Don't forget also that the 35-105mm zoom lens specification is only an approximate equivalent; the actual physical focal lengths are much smaller, and the angle obtained also depends on the physical size of the part of the CCD that captures the image, so some allowance is reasonable with regards to these figures.

3. Originally posted by Midnight
How did you perform your calculations?

Btw, the angle of view for a 35mm focal length lens on 35mm film is 54 degrees, while that for a 105mm lens is 19 degrees. Seems pretty close to your calculations to me (note that the rate of change of angle of view is steeper towards the wide end). Don't forget also that the 35-105mm zoom lens specification is only an approximate equivalent; the actual physical focal lengths are much smaller, and the angle obtained also depends on the physical size of the part of the CCD that captures the image, so some allowance is reasonable with regards to these figures.
You're on the right track here, however take shots of the rules and then you can measure exactly the width on the image and from there determine the exact angular coverage. Oh, don't forget the angular coverage of a lens isn't from left to right, it's measured along the diagonal where only a single value is given.

4. the way i measure is basically putting 2 rulers forming a V toward the centre of the lens.......

by doing that, what you can see on the LCD is the edge of the rulers.....then i draw the lines formed by the 2 rulers and i then measure the angle using a protractor......

at 35mm C4040 widest, i measured it at 42 degree

at 105mm C4040 narrowest, i measured at 21 degrees

from midnight's description of:

54degrees for 35mm
19degrees for 105mm

then olympus C4040 of claimed 3x optical zoom is not true...

what think you??

5. Hmm, interesting, never tried this myself, but I don't think it is accurate to assume that the angle of view subtended by the two rulers from the centre of just the front element of the lens is equal to the angle of view yielded by the entire lens structure itself. Any of the experts here can comment?

6. anyone got ideas??

is it reasonable to judge the camera focal length by measuring the angle of view???

7. Actually you should just put the ruler in front of the camera, and take a picture of the ruler markings, as Ian suggested.

The you measure the perpendicular distance from the ruler to the plane of the CCD. From there you can work out the angle of view.

eg

distance from ruler to camera CCD = 10 cm
take the picture, the ruler shows markings = 10 cm

That means your angle of view = 45 degrees.

8. hi guys maybe can correlate ur findings with this..
FOV Calculator

9. Ian, actually i don't understand why you suggested that the angle of view should be taken diagonally, not from left to right. is it by defination?

streetshooter, can you explain a little how you do the calculation.....

denizenx, thanks for your link...... seems to me like optical zoom specification by olympus C4040 is not accurate, less than 3x optical zoom.

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