If I have a lenses (or in my case extension tubes) that would allow the lens to magnify 1:1, does a 1.5x crop factor body make it really 1.5:1 ?
I'll just take this out into an actual answer, and the answer is 'NO', the crop factor doesn't make it a 1.5:1 ratio. What it does change is the ratio of information per pixel which would be an valid new designation.
Why? Because the 1:1 ratio is a designation of how large the lens renders subjects on the focal plane whatever that plane is, it is an optical designation. A 2cm square subject will be rendered as 2cm squared on either a FF sensor or a 1.5x crop sensor. To suggest otherwise takes any meaning away from the designation since every body you put it on would effectively give it a different meaning. A 1:1 ratio is still a 1:1 ratio on a 10D at 6MP or a 5DmkII at 22MP or a 1.5x crop body at 18MP.
It'd be like saying with film, if a different film emulsion had better resolving power it'd changed the magnification power of the lens, or that printing it on a larger print did.
I guess the answer is yes and no. Technically speaking, the image circle projected by the lens is 1:1, and your sensor is capturing a smaller part of the center of that circle...cropping it. This fits with the formula for magnification:
M = (di - f) / f
Where di is the distance from the lens to the sensor, and f is the focal length. Crop factor or sensor size is not taken into account when calculating the magnification of a lens. From this perspective, the purely optical perspective...the answer is no.
Now, when you factor megapixels and native print size into things, the answer is probably "sort of". If you have an APS-C and FF sensor with the same number of megapixels, the final unscaled "magnification" of the image in print would appear larger with the APS-C for two reasons. First, it packs more megapixels into less space, and second, that greater number of megapixels represents a smaller portion of the image (narrower field of view), which increases the apparent magnification. You could make a larger unscaled print of a smaller part of your subject with the APS-C than you could with the FF.
Okay, after talking with Shizam in the chat room, I think I'm going to take a stab at this questions.
Really, this is a question of semantics. The most commonly used definition of 1:1 means that the size of the object is the size of the image on the sensor. With a smaller sensor, the object will thus be smaller. But it's still a 1:1 really by the definition that I've applied. It will appear like a 1:1.6 image would if a full framed camera were to take the same image. If the pixel size on a full framed camera were the same, then the image would appear exactly the same. If the pixels were larger on the full framed (Most likely), then the image resolution will be slightly higher on the crop sensor, but still look the same as if it were cropped. Really what happens when you are using a crop sensor is that you are throwing away the outer edges of a full framed camera, but aside from that, everything else remains the same.
The minimum focal distance will remain exactly the same on the cropped sensor, BTW.