What does "life-size" mean? and how do you determine if you're taking a "life-size" image of the object you're photographing? I've seen this referred to as a 1:1 relationship.

If the object you're photographing is 1" long and the image on the camera screen is also 1" long then it is lifesize, i.e. the image on the film is exactally the same size as the real object. 1:1 is just the fraction representing 1 over 1. 1:2 =1/2 1:4 =1/4 etc. It's just convention.

So on 35mm film a 35mm object will cover all the way from left to right? Ive often wondered about this too.

To make it a little more concrete, if you are photographing a subject that is 5" tall and the subject fills the long dimension of a 4x5" negative, that was a life size or 1:1 exposure.

So on 35mm film a 35mm object will cover all the way from left to right? Ive often wondered about this too. Well, actually, no. A 35mm object will only cover 35mm of the frame, which is 36mm wide... 35mm film is named for the dimension across the roll (including the sprocket holes and edges.)

"So on 35mm film a 35mm object will cover all the way from left to right? Ive often wondered about this too." No. If the object being recorded is 36mm long and it fully fill the long dimension of 35mm film frame then it is 1:1. If an object is 24mm long and is 24mm long in either direction of a 35mm film frame then it is 1:1.

In printing it can also mean the objects size on the print is life size. Example 01: Like if one of BOB's buddies drags out a high school annual and we make a standup gag image of Bob S. for his birthday, or April fools, or a Photo convention roast party. Thus if Bob is 6ft tall; and the image in the annual is only 2 inches tall we force Bobs height to be 1:1 on the print. Thus Bobs gag image might be a cuttout 6ft tall and just his body; or maybe Becky Sue too the cheerleader! Example 02: enlarging a dress, quilting pattern, bird house, lawn furniture drawing/sketch out of a boob, magazine, pattern book so that the chap/gal can start quilting, using the jig saw with a PATTERN done 1:1

I don't want to belabor the point, but I teach a 3 cred. hour, 96 contact hour college course in CU-Macro. The relationship that is properly shown as a ratio is the image to object ratio. Life size is 1:1 (same as the fraction 1/1). Twice life size is 2:1 while one half life size is 1:2. To predict or calculate macro ratios (which will tell you the magnification), measure the film to lens aperture distance, divide by the focal length and subtract 1, for example, a 100 mm macro lens divided by a bellows draw of 200 mm would equal 2, minus 1 or an image size (magnification) of 1, or 1:1. To predict an image size, multiply the desired magnification by the focal length, add 1 and that will become the bellows extension. For example, with a 100 mm lens and a magnification of 2, add 1 or 3 and multiply the focal length by 3 (300mm - about 12") and this will be the bellows distance needed for a 2:1 magnification. BE+Bellows Extension, F=Focal Length, M=Magnification BE/F-1= M M+1xF = BE M+1 = f stops of exposure increase Lynn

Naturally, I forgot to say that we are talking about "prime" focus or long focus lenses. Zoom, true telephoto, or retro focus lenses can only be measured at the ground glass or by measuring the the actual image size for comparison. Wide angle LF lenses are very good, technically, for close up use since they are designed at close focus distances (please don't ask why). Lynn

In the above, there should be a separation between the BE calculation and the Exposure increase factor. Magnification + 1 equals f stops of exposure increase. Lynn again

Focus on a ruler or other device with inches marked out. Then place a another ruler across the ground glass. When an inch on the GG equals an inch on the ruler, you got 1:1.

As some others have pointed out, it may depend on whether you are comparing the size of the image on the film or on a print made from the film. In the latter case, you have to take account of the enlargement factor to get from the film to the print. Magnification is a technical term and usually refers to the ratio of image size to subject size. It is also the ratio of subject distance to image distance. 1:1 means that the image on the film is the same size as the subject. It occurs when the subject is twice the focal length of the lens from the lens. (Technically, when you say 'lens', you mean the front principal point of the lens, which in some cases may be some distance in front of the lens.) The exposure correction is obtained by adding one to the magnification. You take the f-number you would otherwise use and divide it by that number to find out what you should use. Alternately, you open up by an appropriate number of stops or fractions of a stop. To detrmine that, you use the formula 2 x log(factor)/log(2) Most people find it easier to use a table.