Nikon D7100 Camera Lens

[kallisti]

Nice illustration.

But I think the discussion may have become too focussed on what happens at a single focal length.

Another option left out of the discussion on the zoom would in fact be to zoom in to a longer focal length from a greater distance to get approximately the same view as the 50mm closer up. At the greater focal length, even though the zoom can't open up as much to get the bokeh, it will have less depth of field/more bokeh for for a given f-stop than it would at 50mm, and if it's long enough, possibly similar to the 50mm opened wide? This would have an advantage in situations where getting too close to a subject would distract the subject (if a person or animal) and mess up the shot. The quality of the bokeh may suffer, but how much may depend on the details of the shot. Sometimes not as nice, sometimes fine.

And of course, if one would like to but just can't get physically close enough in time to get a shot, the zoom will allow one to get many shots that the 50 would just miss.

I agree with you somewhat. For many images, your points stand. In this particular example (referring to my last post), you are off--I shot it between 8 and 9 feet from the door and the extent of my deck wouldn't allow me to step back and shoot it with a longer focal length without falling off.

More importantly, the focal length you choose impacts the relationship of the elements within your composition. The following isn't a "classic" example, but it may serve:

D3100 and 18-200 lens. 18mm, f/4.5, 1/6 sec, ISO 3200.

Similar composition overall to my last post regarding the framing, but using the wider angle means I had to get closer which radically changed the elements in the composition. Would have stepped back to shoot at a longer focal length, but not possible because of physical constraints. Note also that because this was shot at a wider angle, the depth of field expanded in the process.

The 18-200 is a great lens, especially for someone just starting out. Voting with my wallet, I just bought one for my niece! She is a beginning photographer who has been using a kit lens and making good use of it. Photography doesn't seem to be a passing fad for her and I think she will benefit from the exposure to a wider range of focal lengths. At some point she may want/need to learn about what a fast prime can offer. But that isn't the tool she needs right now and may not be something she ever needs/wants.

As I stated, all gear involves trade-offs

 

[kallisti]

Don't want to beat a dead horse, but shot this same composition again as far back on my deck as I could get. To get roughly the same field of view as the previous examples, I used a 90mm lens on a FF body. Roughly 13 to 14 feet away from the door based on the numbers on the lens barrel. Shot this on a mini-tripod and was leaning over my deck to try to get the focus right. Because of the positioning, had to use Live View to focus.

Leica M (240) and 90mm f/2.5 lens. 90mm, f/4.0, 0.7 sec, ISO 800.

Because of the longer focal length, the depth of field is shallower even though the aperture is roughly the same compared to previous shots with the 18-200 lens. So by using a longer focal length, it is possible to approach the degree of background blur compared to a faster lens with a shorter focal length. If I was able to use a longer focal length by stepping back further (not possible in this example), I could equal or even exceed the degree of background blur compared to the 50mm lens shot at f/1.4.

However, note how the composition changes comparing this shot to the 50mm shots from 2 posts ago or the 18mm (27mm in FF terms if keeping the focal lengths consistent between posts) in the previous post. All of these have similar compositions when looking at the door framing the image. However, the elements within the frame change dramatically with the different focal lengths. The wide-angle shot has extraneous details from my kitchen included. The 50mm shots have details from things hanging on the fridge. The 90mm shot is the most compressed and has the fewest extraneous details. Note how the chair in the background changes in size between the different focal lengths.

Even though you can achieve "similar" fields of view with different focal lengths and different subject distances, the relationship of the elements within the composition change as well. They aren't "equal" in any sense. Sometimes this may not matter. Sometimes it does however.