G'day Elvie.
The 70-300 DO is a first-generation DO lens and like the 400/4 DO Mark 1, it wasn't a success. Although it is small, very well made, and fairly light, it is not regarded as much good optically. It is very expensive indeed for its mediocre performance. If you are going to spend that sort of money on a lens in that focal length range, the excellent 70-300L is only 30% heavier and about the same price. Much better value. (I own a 70-300L and the only reason I don't use it as my go-to is that I also have a 100-400 II. Sometimes I think about selling it as unnecessary, given that I have its range covered by various other lenses, but I never quite go ahead because on those infrequent occasions when I do use it, I am always reminded of how sweet and usable a little thing it is.)
Then there are the four L Series 70/200s. You might consider the 70-200/4 IS which is an extra $200-odd and slightly lighter than the 70-300L, though I reckon you'd do better with the longer one, which is only one stop slower and 50% longer. The 70-200/2.8 models are very dear.
There are lots of cheaper 70(ish) to 300(ish) lenses. Most of them will be lighter than the DO but I don't know too much about them, other than the EF-S 55-250 USM which is around half the weight, $320ish instead of $1700ish, and easily superior optically. The trade-off is that it doesn't have the same tank-like build quality, and it is EF-S only. If you want a full frame lens in the lower price range, then you might look at the Canon 70-300 (non-L) which is said to be around about equal to the 55-250 and better built but costs nearly twice as much, or any of about 6 third-party lenses in this category. Avoid the Canon 75-300: it is much improved over the truly dreadful original, but so is a milk bottle.
First generation DO lenses are notorious for poor contrast, and the 70-300 DO is no exception. See here:
https://www.the-digital-picture.com/...mp=3&APIComp=1 I wouldn't be going there.
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A note re depth of field and width/length/field of view.
Depth of field is determined by (a) framing, and (b) aperture. Nothing else. (At least to a pretty good first approximation.) It makes no difference whether you use a 300mm lens and stand a long way away or a 50mm lens and stand close. If the subject is the same size in your viewfinder, the depth of field is the same. (Unless you use a different aperture.)
Any lens of a given focal length - 300mm let's say - behaves the same way on the same camera. At (e.g.) 85mm on an 80D, my friend's EF-S 15-85 (crop lens) and my EF 24-105 or my EF 85mm prime or my EF 70-300 (all three are full frame lenses) produce the same picture. There are differences of detail, of course: each lens has slightly different abilities, but these are
slight differences. I'd expect the prime to be marginally sharper and more contrasty than either of the small zooms, for example, and probably marginally sharper than even the outstanding 70-300, but I'm talking differences so small that most people couldn't see them without carefully comparing them side by side. For practical purposes, 85mm is 85mm is 85mm.