# Difference between focal point and distance away from object? [duplicate]

I have a macro lens for the camera on my phone. I want to calculate the magnification of the lens because the one i purchased does not state it.

I've found this guide which states the mathematical relationships - http://www.wikihow.com/Calculate-Magnification

The only thing that i don't understand is the difference between the focal length and the distance between the object and the lens? I thought the focal length was the distance between object and lens.

Thanks Bobby

## marked as duplicate by TFuto, Michael C, inkista, mattdm, Philip KendallMay 4 '16 at 5:26

Remove the supplemental lens from the phone-camera. On a sunny day, hold the lens between a white sheet of paper and the sun. The lens will project a tiny image of the sun on the paper. You start with the lens touching the paper and slowly increase lens to paper distance. When the image of sun is a tiny spot of light, the distance lens to paper is measured with a millimeter ruler. This value is the focal length of the lens. Let’s make-believe this measurement is 200 millimeters. We annex “1/” the focal length. Thus for a 200mm the math is 1/200 = 0.005. Now we multiply by 1000 thus 0.005 x 1000 = 5. This is the power of the supplemental lens. By the way this value is called a diopter. Thus a 200mm = 5 diopter. The power is 5x. Your normal reading distance is about 500mm. If you hold a 5 diopter lens close to your eye, the revised paper to eye distance is 500 ÷ 5 = 100mm. In other words the 5 diopter lens allows you to read a paper at a distance of 100mm. The power is 5x.

1000mm = 1 diopter

500mm = 2 diopter

250mm = 4 diopter

200mm = 5 diopter

100mm = 10 diopter

50mm = 20 diopter

With a supplemental close-up lens mounted, the approximate distance from the supplemental to the object being photographed is the focal length of the supplemental lens.

Focal length is a measurement taken between the lens and a focused projected image cast by the lens when the lens is imaging a far distance object. The distance lens to image plane (imaging chip surface) is called the back focus. This distance will be same as the focal length if the camera is imaging a far distant object. The distance lens to object being photographed is called the object distance or subject distance.

There are two "magnifications" which are related to addon lenses: the maximum magnification achievable in combination with a given camera and relative (optical) magnification.

To know maximum magnification (ratio between object size and image size at focal plane) you need to photograph a ruler parralel to the frame as close as possible and then divide the viewable distance by sensor width (search for it online). This will give you absolute magnification. This ratio is typically 1:1 (object:image) for specialised objectives for interchangeable cameras and may be few times more for general purpose objectives. It does not characterize the recorded object size well though - 1:1 magnification with small sensor will give magnitude larger digital image of object than big sensor camera with same magnification would.

To know the relative magnification, you need to photograph a ruler from same point both with and without the addon lenses (ideally: with fixed focusing distance). Dividing ruler distance visible without the addon by distance visible with addon attached will give you roughly the optical magnification.

The only thing that i don't understand is the difference between the focal length and the distance between the object and the lens? I thought the focal length was the distance between object and lens.

Focal length noted on photographic objectives is more complex than optical focal length. Optical focal length is distance from the lens at which parallel rays converge. Photographic focal length (aka rear focal length) is a distance from rear nodal point to the sensor.

You may know optical focal length roughly if you project the sun (ideally: the bunch of parallel rays) into the sharp spot onto some non-burning material and measure the distance between exit lens and the sun image.

Focal length is the distance between the focus point on the lens and the plane of the image sensor. For example a 50mm lens set to focus at 10 ft has 50 mm between if's focus point and the sensor and 10 ft from focus point to subject.

• Focus distance is always measured from the film/sensor. Working distance is the measurement from the physical front point of the lens (which may or may not correspond to the lens' entrance pupil. – Michael C Apr 13 '16 at 12:39
• "50 mm between if's focus point and the sensor and 10 ft from focus point to subject." - what is "focus point"? – Euri Pinhollow Apr 13 '16 at 12:49
• The camera sports a converging lens. Light rays enter the lens and their path is altered. The revised path traces out a cone. The distance from lens to apex of the cone is the focal length. This measurement must be taken when the lens is imaging a far distant object. This object is said to be an infinite distance away (∞ Latin for as far as the eye can see). This happens when the object is about 3000 times the working lens diameter distant. At such a distance, light enters as parallel rays. Objects closer raced out an elongated cone of light. We call the elongated distance the “back focus”. – Alan Marcus Apr 13 '16 at 19:18