does the combination of canon 7D and 500 4.0 IS + 1.4 ext + 25 mm extension tube work? can it keep the AF too ?. is there a math formulas to calculate the new focusing distance.
regarding the 500 4.0 IS + 1.4 ext + 25 mm extension tube, the minimum focus distance for a 500mm is 14.8 feet = 4511 mm, same with a TC attached.
The relevant formula for a simple lens is:
1/F = 1/a + 1/b
where F = focal lens, a= subject distance, b= lens principle plane to image plane distance.
For your case, F = 700 mm and without the extension tube, a= 4511 mm.
Then b=828.6 mm. We add a 25 mm extension tube then b = 828.6+25 = 853.6, then solve for the new "a" and we get a= 3890 mm = 12.8 feet.
It is a little more complicated in the compound lens with a TC attached, as the principle planes are not necessarily centered on the lens. But it at least gives you an idea of the effect.
In practice you will do better focusing manually, would recommend !! .. also can stack those rings and makes a difference for close focusing distance if you place the extension tube in front or behind the converter !!!
... btw send me a PM with your name so I can update the user name !!
Yes you can even add another 25mm tube. Think you can focus in closer with the 1.4 first then the extension tubes. I have focused in AF, but after 5.6 I think you will lose that option (if you put on the 2x TC). I still am waiting to get out and try the combo out again this year. Great for macro, especially dragonflies. The BG's are terrific.
Canon 25 mm extension tube
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