Tamron 100-400 vs 18-400 educate me on uses

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kgd New Member • Posts: 3
Tamron 100-400 vs 18-400 educate me on uses

Okay, I have went through a couple of videos and read a few threads about people asking to compare and contrast the 18-400mm and 100-400mm. I get the advice people provide, that these two lenses are for different purposes but there is rarely any concrete comparisons.

The 18-400mm is going to be a great walk around/travel do-it-all lens letting you capture wide to telezoom. There are compromises of course, slow (relative to an f4 or f2.8 zoom) and presumably poorer IQ. The 100-400mm is really a variation on the every popular 70-300, a little longer at both ends with good promise as a light weight outdoor sports lens, hiking tele or birder if you can't haul out a 150-600mm or big bad prime.  But honestly, if the 18-400 performs as well, or even nearly, as the 100-400mm why go the latter route?

Like the 18-400mm, the 100-400mm are considered slow lenses due to their higher value and variable apertures. The 18-400 mm has a minimum aperture of 3.5 while the 100-400 is 4.5 but where are the transition focal lengths?  i.e. at 100mm does the 18-400 mm have its max aperture at 4.5 or above? How about at 200 and 300 mm, do the lenses still compare in their max apertures? In know they are the same at 400mm.

Second, I've seen many commentators indicate that the 100-400mm is likely to be sharper in its range than the 18-400 mm because optical design limitations to make a super zoom will necessitate compromises.  But does that actually hold up?  I haven't seen anyone really look into that and reviews of each lens in isolation tend to suggest both are good (but relative to what?).

Last, the 100-400mm is full frame while the 18-400mm is designed for APC format.  This gives the 100-400 mm more flexibility in use across different bodies but is it perhaps heavier than need be to accommodate a full frame image circle.  Hypothetically, if the 100-400mm were designed for APC what kind of weight savings might one expect and how would that compare to the 18-400mm weight?

Thanks for your thoughts.

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