rsolomon Posted October 28, 2007 Share #1 Posted October 28, 2007 Advertisement (gone after registration) i have been playing around with some formulas to determine how different focal length lenses (say 100 mm and a 50 mm and a 135mm) at the same fstop actually allow the same volume of light to reach the film plane. i have come to a place that doesn;t make sense in my formula below... the place i am stuck is the relative disitance to the film plane... it seems in my formula the longer the lens, the shorter the distance between the focal point of light in the lens to the film plane....i know this is relative to 100 somehow thanks for example: on 100 mm lens at f4: diameter is 25 mm: (100 / 4) total volume of light is 2500: (25*100)*(100/100) reaching the film plane is 2500 * the relative distance from 100: 2500 * 1 = 2500 for example: on 50 mm lens at f4: diameter is 12.5 mm: (50 / 4) total volume of light is 1250: (12.5*50)*(100/50) reaching the film plane is 2500 * the relative distance from 100: 1250 *2 =2500 for example on a 135 mm lens at f4 diameter is 33.75 mm: (135 / 4) volume of light is 3375: (33.75*135)*(100/135) reaching the film plane is 2500 * the relative distance from 100: 3375 *.7407 =2500 Link to post Share on other sites More sharing options...
Advertisement Posted October 28, 2007 Posted October 28, 2007 Hi rsolomon, Take a look here technical question on apeture and film plane distance. I'm sure you'll find what you were looking for!
stumac Posted October 28, 2007 Share #2 Posted October 28, 2007 Hi Richard, My optical theory is about 50 years old and a little rusty, by volume of light do you mean the time integral of the area of the aperture. By definition the aperture of a lens is the ratio of the focal length to the diameter, so any lenses with the same aperture no will let the same amount of light through. I think your calculations show this. If you are looking to quantify focus shift with change of aperture, this is a different ball game. Regards, Stuart Link to post Share on other sites More sharing options...
rsolomon Posted October 28, 2007 Author Share #3 Posted October 28, 2007 so as the formulas show more light units are needed for longer lenses to take in the light required for the same f stop...a 50 mm 1250 units of light.... a 135 mm 3375 units of light.. my question is the distance from where the light rays are focused inside the lens to the film plane . i have this question because i thought the longer the lens the further the distance from where the light rays focus (in the lens) to the film plane (in the camera) ... and this makes sense .... however in my formula is show a shorter distance ... yest it seems to work so im looking to see where my formual is off thanks thanks Link to post Share on other sites More sharing options...
marknorton Posted October 29, 2007 Share #4 Posted October 29, 2007 Err... that "formula" will yield 2500 no matter what value you put in. Do you want to try another couple of million values to be sure? If the focal length is f... "Diameter" = f/4 "Volume of Light" = (f/4)*f*(100/f) = 25f "Reaching the film plane" = 25f*(100/f) = 2500 Your result is independent of f... As Stuart says, the lens aperture already takes the focal length into account so that the illumination of the sensor by different lenses with the same aperture will be the same. The illumination depends on the area of the aperture, not its diameter (to a first approximation) which is why the aperture number sequence (1, 1.4, 2, 2.8, 4, 5.6, 8...) increases by root 2 each time. Open the aperture by 1 f-stop and you increase the diameter by 1.414 and the area by 2 for double the exposure. Link to post Share on other sites More sharing options...
masjah Posted October 29, 2007 Share #5 Posted October 29, 2007 Just to follow on a little from what Mark has said, the aperture area is proportional to the square of the lens diameter (well, roughly speaking, the lens diameter). On the other hand, the illumination of the film falls of as the inverse square of the focal length. So the ratio of these two quantities will give a consistent indicator of the amount of light on the film whatever the focal length of the lens. This ratio (focal length/diameter) is the f number. Link to post Share on other sites More sharing options...
rsolomon Posted October 29, 2007 Author Share #6 Posted October 29, 2007 thanks guys,,,, okay, i was focused on the diameter of the apeture verse the volume of the apeture. so is there a way to calulate the volume of the apeture for different lenses ? say 50mm lens @ f5.6 ? or 100 mm @f5.6 ... im trying to figure out if more light is needed for a longer lens to produce the same "light to film plane..and why ? Link to post Share on other sites More sharing options...
stumac Posted October 29, 2007 Share #7 Posted October 29, 2007 Advertisement (gone after registration) I am confused by your term "volume of aperture", for a compound asymmetrical lens as most modern ones are, do you mean the "area of aperture X distance between front and rear nodal points" Regards, Stuart Link to post Share on other sites More sharing options...
rsolomon Posted October 29, 2007 Author Share #8 Posted October 29, 2007 i'm probably using the wrong term. Im trying to understand 1. "how much light" comes in based on the lens and the apeture, 2. if # 1 has math behind it, is there a relationship to the length of the lens and the distance "focused light rays" are from the film plane thanks for sticking with me... Link to post Share on other sites More sharing options...
stumac Posted October 30, 2007 Share #9 Posted October 30, 2007 A couple of thoughts to ponder, The amount of light passing through any lens regardless of focal length will be the same for the same f no by definition, ie 50 mm at f2 = 200 mm at f2. The amount depends on the luminosity of the scene, ie the brightness. The focal point may change with change in aperture, I have not seen evidence of this using long focus process lenses with very small depth of field The distance from the lens center to the film plane where the image is formed for a simple bi convex lens is the focal length. For compound lenses this not always the case, telephoto the image distance is less than the effective focal length, wide angle the image plane distance may be more than the effective focal length, retro focus design. For a good understandable discussion on lens design, I recommend the chapters in the Ilford Manual of Photography if you can find a copy, long out of print I think. Regards, Stuart Link to post Share on other sites More sharing options...
giordano Posted October 30, 2007 Share #10 Posted October 30, 2007 For a good understandable discussion on lens design, I recommend the chapters in the Ilford Manual of Photography if you can find a copy, long out of print I think. Fortunately not. For the last few decades it's been published by Focal Press as the _Manual of Photography_ , and Amazon shows the 9th edition as in print. Link to post Share on other sites More sharing options...
rsolomon Posted October 31, 2007 Author Share #11 Posted October 31, 2007 thanks ! im headed there now Link to post Share on other sites More sharing options...
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