 Halim Semihardjo |
relation of focal length,distance&magnification I'm confused with the relation of minimum focus distance and magnification.. from what I know, with the same focus disctance then a lens with twice focal length will have twice magnification like at 40 cm a 100mm lens will produce twice the magnification of 50mm and with the same focal length,then if we double the distance,the magnification will be half (100mm at 20 cm will have magnification twice than at 40cm) my question is,am I getting it right??..or the rules applies only to prime lenses..because from what I read..EF 100mm minimum focus distance is 31cm and produce 1:1 mag, and 180mm L produce 1:1 at 48 mm..how come??..if 100 can produce 1:1 at 31 cm,then a 180mm should be able to produce 1:1 at 31 * 180/100 = 55cm?? another thing that confuse me is zoom lenses like the EF 28-135mm IS which can only produce 1:5 magnification (at 135mm) ,although it has minimum focus distance of 50cm..which in my calculation,should have the magnification of 100mm at 32cm.. can somebody help me here..really confused...
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BetterPhoto Member |
So, what you are saying is that X-D=Y+pa to the Q factor of ZULU X 14 Squared to the Delta factor of 4= 100mm X 4 squared X 32 devided by .0635 equals distance.
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Eddie Lagos |
LOL!
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Jon Close |
The simple formula for Magnification is M = f / (s-f) where f is the lens's focal length, and s is the focus distance from the lens's principal node to the subject. However, it is of little practical use at very close focus distances. The close focus distance given in lens stats is usually measured from the film/image surface to the subject, not from the lens's pricipal node. Additionally, modern lens design is very complex. The location of the pricipal node of a lens can be anywhere, even in front of the lens, and can move with zoom and/or focus. Plus the lens's actual focal length may change with focus.
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Halim Semihardjo |
thanx Jon,the answer really help..
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Alan N. Marcus |
Focal length is measured, lens to film plane, when camera is focused at infinity. Longer focal lengths produce larger image sizes thus more magnification. The point of measurement at the lens is called a nodal. Some lens designs cause the nodal to fall forward or aft of center or even outside lens. The f number settings engraved on the lens are based on the lens set at infinity. As you focus closer and closer, the lens is racked further and further away from the film. At close focus distances the effective focal length becomes longer, magnification increases. At close focus the effective f number changes; the light energy at the film plane is reduced vs. the same aperture at the infinity position. Compensation of this effect is called “bellows factor”. I most cases the error is completely negated via electronic or mechanical means or film latitude. The manufacture generally prevents close focusing when this error exceeds 1/3 of a stop. Macro lenses utilize highly effective mechanical compensation by opening up the lens as it rack away from the film. Camera lenses are generally designed to take a 3 dimensional world and place it on a flat two dimensional film. Extreme close-up situations require a different lens design as this changes to fat to flat imaging. Macro lens designs to the rescue. To get a magnification of 1 or life size known as 1:1 the lens to subject – lens to film distance is slit down the middle. For example with a 100mm lens. The subject will be 200mm away from the lens.
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