why can't you focus when zooming all the way in?

[Dacaur]

MFD = Minimum focus distance. Measured from the sensor, it's the minimum distance at which the lense can focus. It's different for every lense. My 35mm f2.8 macro has a 5.1" MFD, which is about an inch or so from the front of the lens. 50mm 1.8 is aroun 13". My 18-135 nano usm is 15.4".

 

[Original katomi]

Refer back to post 7
Canon home uk lists the minimum focusing distance for you lens and many others

 

[pocketshaver]

for what its worth, whats the lens diameter on this thing? Ive found that my LARGE diameter lenses, say the one that uses a 62mm thread on the filter has a different distance requirement from the 58mm and 52mm diameter ones.

Took me a tad time to realize that one needs me to be about 3 feet away to USE MACRO function on it, while the other needs me about 12 inches away, the 52mm 100-300 macro zoom, but when I turn it down to 100 and toss on a x1 macro diopter I need to be 3 inches away form an object.

 

[petrochemist]

for what its worth, whats the lens diameter on this thing? Ive found that my LARGE diameter lenses, say the one that uses a 62mm thread on the filter has a different distance requirement from the 58mm and 52mm diameter ones.

Took me a tad time to realize that one needs me to be about 3 feet away to USE MACRO function on it, while the other needs me about 12 inches away, the 52mm 100-300 macro zoom, but when I turn it down to 100 and toss on a x1 macro diopter I need to be 3 inches away form an object.

Lens diameter is not directly related to minimum focusing distance, but longer focal length lenses will often have longer MFDs and they also need wider objectives for reasonable speeds.
A +1 diopter in line with any lens will make it focus at 1m when set to infinity & closer at other focus settings, a +2 diopter will half these distances. Despite some manufacturers labeling there is no such thing as a x1 diopter - diopters are an alternative way of sating focal length and are simple postive (for converging lenses) or negative (for diverging lenses) numbers. For the record a lenses diopter is defined as 1/(focal length in meters).