Good Day All, I was recently asked to shoot some pictures of some American Crocodiles in aid of a long term research project. By shooting their heads at a 90 degree angle, and using a range finder, the total body length can be calculated. But to use this information, the life-size magnification must be considered. I shot all pictures under the same settings to simplify the task, and I used a Canon Rebel XSi with their 55-250mm IS lens which was fully extended to 250mm. And so my question is what is the actual magnification(taking into consideration the smaller sensor) of this set up? Either a specific answer and/or a formula is/are appreciated. thank you for any help, ~Firk

Well, the basic lens equations are: 1/f=1/d1 + 1/d2 where f is the focal length of the lens, d1 is the distance to the object, and d2 is the distance to the image (i.e. the sensor) the magnification is given by M = h1/h2 = d1/d2 where h1 is the height of the object (probably size of the alligator) and h2 is the height of the image, i.e. that on your sensor. Now, if you measured the distance to the object, and can figure out the distance to the sensor from your lens. Then you can look at the size of the head on the sensor, and convert to an actual size, i.e. in inches. Then you could just use the magnification equation to get the actual size of the alligator head. I'm not exactly sure how this would all translate given a multi element lens, you technically have to compute an equivalent lens position for multi lenses. Frankly, I think the easier thing to do would be to just take test pics with your camera and lens on the same settings. Take several pics, from several distances, of things that you can then just measure directly with a tape measure. then you can compare your measured lengths to the size of the things in your picture and come up with an effective conversion factor without having to worry about the equations.

Remember that the XSi sensor is a 1.6x crop sensor, with physical dimensions of 22.5x15mm. So your focal length was not 250mm, but 400mm equivalent.

Eldrich's calculations are based on the 'true' focal length of the lens (250mm in this case) not a 35mm equivalent.

well I didn't really do any calculations...I was just stating the relevant equations. But yes, it would certainly have to be the true focal length, and the actual physical size of your sensor. But I think if he just follows what I said in my last paragraph(i.e. take a picture of a ruler at several distances, then check its size on the image, take a picture of a yard stick at several distances, measure its size etc, until you have an empirical law that gives you the conversion) then there would be no need to do calculations. In fact, i would probably do a bunch of testing if i did calculations anyway, just to check the calculations.