Focal Length Accuracy

Very interesting - I finally talked myself into getting a real macro lens - the Canon 100 f/2.8 USM. Couldn't wait to compare the results to my 24/105 which I'd been using for flower closeups.

Set up the tripod close to some small flowers with the 100mm Macro and shot - the two flowers pretty well filled up the frame.

Then leaving the tripod where it was, I put on the 24/105 and shot (the flowers didn't move either - they are on a flowering shrub in the park).

Oh yes, the macro was sharper - that was expected. What wasn't expected is that the flowers shot on the macro filled much more of the frame. Comparing them on the monitor, the diameter of the flowers shot by the macro are about 62 percent larger than the same flowers shot on the 24/105.

The 24/105 was zoomed right to 105 mm. If these numbers are accurate, should not the flowers on the 24/105 be larger than those on the macro images?

Apparently I not only gained IQ, I gained focal length.

No, hate to disappoint, but you gained magnification ratio. Everything you shoot with the macro lens will be 1:1, or in other words, life size. With you 24-105, everything will be smaller, say .25x instead of 1x. That's as much as I can explain, I'm sure someone else will do it better.

pagnamenta wrote in post #2866854
No, hate to disappoint, but you gained magnification ratio. Everything you shoot with the macro lens will be 1:1, or in other words, life size. With you 24-105, everything will be smaller, say .25x instead of 1x. That's as much as I can explain, I'm sure someone else will do it better.

The life size 1:1 ratio is only obtainable when the lens is at minimum focus distance and the subject is in focus.

The "film plane" to subject distance was the same in both cases. I didn't move the camera when taking the shots with the different lenses.

This lens can also be used as a normal 100 mm lens, it won't shoot everything at 1:1.

Anyone else?

Focal length claims by manufacturers are often way off, particularly with zooms and particularly at close range. I'm not at all surprised by what you've observed.

this can be explained by the fact that zooms don't have exact focal lengths as they are advertised. even when comparing zoom lenses, it is possible to have two lenses with the same advertised zoom range but have slightly different actual zoom range.

thus, the 24-105L @ 105mm could actually be like 90mm and thus your 100/2.8 seems longer.

edit: foxbat beat me to the button.

Jaetie and Foxbat:

Thanks for the replies - seems like the only reasonable explanation. It's what I had almost expected, but the difference was so large what with the 100mm being considerably longer than the 105mm.

Good to have two answers that agree - it's called confirmation - thanks.