Originally posted by Lowell Goudge there is an interesting article somewhere about this.
I'd be interested to read that if you can remember how to find it.
Originally posted by Lowell Goudge it is true of virtually all zooms but more so of super xooms. due to the internal focusing (almost a requirement for AF) the lens designs are such that focusing changes the focal length also and that true focal length is only at infinity.
I checked the 55-300 against my D FA 100 mm and FA 50mm and it was off by a small amount. Nothing at all like what the internal focussing 18-250 did to the stated FL.
Originally posted by Lowell Goudge I looked at several lens reports and no one reports on the effective focal length as a function of distance.
There must be a test somewhere yor all the measurebaters out there to report on this
I agree, those figures should be reported. It's ridiculous that they aren't, because FL is as relevant as any of the characteristics that
are measured. I'm interested for example in how the Bigma compares in FL to the new 150-500mm. I read a few reviews and didn't even manage to find out whether the Bigma is an internal focus design. From its spec I imagine it is? How is actual focal length not seen as important in lens tests? Here's the testing I did:
Picasa Web Albums - Dan - FL compare