Originally posted by Pål Jensen You don't get extra DOF but less DOF. In addition, the APS lens wil give more DOF, larger maximum magnification( for similar lenses) and larger range of DOF. However, hardly anyone buy long telephoto lenses for its absolute shallowness of DOF. They buy lenses for speed; ie the ability or lack thereof of shooting in lower light with faster shuterspeeds. F:2.8 lens is an F:2.8 lens regardless of format and mm variations in DOF wide open is way down on most photographers list of priorities when making images.
Yes, less DOF, meaning more options as you can always stop down. The slower shutter when stopped down is counterbalanced by the one stop gain in ISO, so the speed point is rather moot even though it's a slower lens. FF generally results in more options, at the cost of weight and size. Weight and size might be an issue with many people,
but may not be for many, especially when shooting for pay, as the hesitance factor is nonexistent, you have to shoot, it's not a matter of "my gear is too heavy to carry". hence I said that DA limited line is the one that needs to be pushed, as it shows where the APS-C benefit really is.
If you think no one is shooting telephoto wide open, why does 70-200/2.8 lenses even exist (and is arguable the most popular flavor of that FL)? Many people shooting portraits with the 70-200/2.8 shoots alot of their images at 2.8. I disagree that DOF is way down the list of priorities, it is certainly behind composition and light, but in 95% of my shooting it's definitely the highest out of three in the exposure triangle. Isn't that why Av and M is the most popular mode, and not Tv or Sv? I know that's oversimplifying things, but I disagree that DOF is "way down the list"
Like Pal Jensen said it is kind of a sidegrade as you do lose some things like MFD, but in my case anyway, the benefits outweigh the costs.
Equivalency is brought up by FF users because it's a real benefit. As I mentioned in another thread, Tamron 28-75 in FF basically can do the job of any prime from 18-50mm that you use at f/2 in APSC (including 50mm, 35mm and 28mm!) meaning less lens changes and less missed shots. Of course like mentioned, this is all just specs, not taking flare resistance, microcontrast, size, subject shyness due to huge lens, MFD etc etc into account, and therefore maybe a just a sidegrade for your shooting. For mine it was an upgrade.