CatMarley Loc: North Carolina

F2.8 gathers twice as much light as F4. It also has a narrower depth of field which means that F2.8 will blur the background more than F.4. However with a 400mm lens the depth of field of both lenses is quite shallow, and is about 2 inches at 20 feet. This changes with a DX camera: since the FOV is smaller, there is an even shallower DOF (and therefore more blurring) with the 2.8 than with the 4.0.

Mercer Loc: Houston, TX, USA

As I understand it, one stop remains one stop, which does not vary regardless of the lens' focal length, because f stop expresses the arithmetic relationship between the area of the lens' opening and distance from this opening to the camera's focal plane. You can confirm this by noting that a photographic light meter does not mention anything but f stop (and film speed).

You might be thinking of depth of field, which can vary according to the lens' focal length and/or f stop.

Ron Dial Loc: Cuenca, Ecuador

If you are shooting JPEG's, then 1 stop can make quite a difference. Makes the mid-tones go mushy.

DVZ Loc: Littleton CO

Here is an informative article regarding many aspects of fast vs slow glass including background bokeh. It's written by Steve Perry.
https://backcountrygallery.com/amazing-high-iso-the-end-of-fast-glass/

lamiaceae Loc: San Luis Obispo County, CA

Fast, LOL f/3.5 to 4.5. I guess I am spoiled as I am not a sports or wildlife photographer so I am mainly using short(er) fast lenses, f/1.4, 1.7, 1.8. 2.0, 2.8. Though I do know "slow", my view camera lenses are f/5.6 and f/8.

snapshot18

Bill_de :

One stop makes difference at 2mm or any in between. If one stop MORE would make a perfectly exposed image, one stop less won't make the image unusable- just a bit darker. I'm sure the 400mm f2.8 is many times more costly than the other you mentioned; but, will even 50% of your images REQUIRE the extra expense? Only you can decide.

Gene51 Loc: Yonkers, NY, now in LSD (LowerSlowerDelaware)

You are 100% correct. Something about the focus performance on an F2.8 lens, even if you choose a smaller aperture, that makes the deal for me - especially in really terrible light. But being able to use shallow DoF has it's merits in some cases.

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Thomas902 Loc: Washington DC

"how much the 1 stop difference affects the look of the background?" Bill; who are your clients? And what is it about their expectations that predicate concern for out of focus area(s) of the deliverables?

I shoot league soccer with the AF-S 200-400mm f/4 VR and it is certainly of no concern to my clients as to the "aesthetics" of out of focus areas... First and foremost is the decisive moment captured in high acuity... little else matters... That said, the one stop can (and does) make a difference in focus speed... I abandon my 200-500mm f/5.6 not because of acuity but for want of focus speed... it simply doesn't work well for action sports, while the AF-S 200-400mm f/4 VR locks instantly... it is not the camera's focus system but rather the amount of light delivered to the AF sensors that is of concern... also the 200-500mm f/5.6 focus motor simply isn't up to the task of action sports...

As for as aesthetics? Are you shooting fashion editorials? where this actually is a serious issue for getting published?

Best advice? Assist commercial shooters in your market to get your head around what pays and what is not relevant to the mix... lots and lots of marketing hype regurgitated in these forums... (in my humble opinion)

Final note: Never a wise move to be the first kid on the block with a new toy... ask any D800 or 200-500mm f/5.6 user... unless you love being a beta tester... enough said...

All the best on your journey Bill...

Fotoartist Loc: Detroit, Michigan

Even though I have a couple of expensive 2.8 lenses, I'm down on them because of the weight (and expense), and especially since camera focusing systems get better and better and don't need as much light. Background busyness is an issue but most times I would rather have depth of field in the subject. Besides there are easy ways in Photoshop to blur a background but there is not much you can do to add depth of field to a subject in post.

CatMarley Loc: North Carolina

You have had a lot of opinions related to exposure, which I realize was not your question. You were interested in bokeh - the blurring of the background. And the salient point was using a 400 mm lens. The reality is that at 400 mm, there will not be a whole lot of difference in the usable depth of field, it is going to be very shallow wide open - so much so that the difference may not be perceptible with an FX sensor. It may be perceptible with a dx sensor because of the even shallower DOF.

carl hervol Loc: jacksonville florida

Why not a picture at different aperture that is the best way to see for your self .

Bill_de Loc: US

I don't have a long 2.8 lens so although I can read the charts I can't really "see" what the difference is. I'm not sure if going from 5.6 to 4.0 has the same visual impact as going from 4.0 to 2.8.

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Fotoartist Loc: Detroit, Michigan

I can't help you because while I have owned a 300 2.8 I rarely shoot it without adding an extender which changes the f stop to f4 or 5.6. I'd always sacrifice those f stops for longer focal lengths in nature. I do have a 70-200 2.8 which I do take advantage of the 2.8 end but thats because smoothness of backgrounds gets harder to achieve at those focal lengths and is prized more.

GED Loc: North central Pa

Bill here are two shots from a 400mm/f2.8 lens(Nikon) the first is at f4 the second is at f2.8. Distance of camera to flower in focus 11 ft the flower out of focus was 8 inches behind the one in focus and the background was 6ft behind the second flower. There is a difference which you can see, the biggest advantages to me are the brighter viewfinder for focusing and easier on the eyes and having that extra stop for low light levels when you have the best light of the day most times. Hopes this helps you

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Bill_de Loc: US

Thank you very much!

The one stop certainly makes a discernable difference. While I will wait to see a few more reviews on the new zoom, I am now strongly leaning toward this prime.