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I'm kinda new here and maybe I misunderstood OPs question. OP usually shoots at f/8 and is asking about the effects of shooting at f/4. I find f/4 lacks depth of field for real estate interiors.

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

Your question is asking for a broad generalization, and it is hard to respond to. You'd have to compare two specific lenses, or better yet, two specific copies of the lenses you are inquiring about. There are some outstanding zooms, primes in both F2.8 and F4.

On the other hand, if you are doing RE work, why are you not considering Canon's excellent tilt/shift lenses, and a no-parallax head for stitched panos. The amount of extension distortion and volume anamorphosis as you get shorter in focal length can provide misleading results. I don't know what market you are in and what you charge, but if you are fussy and want to step up your game, a longer lens and stitched pano would be my suggestion. Compare the two images below. One was taken with a 14mm lens (14-24) the other was a 4 image stitched pano taken with a 24mm tilt shift lens.

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LFingar Loc: Claverack, NY

Nice photos, but, the pano didn't do the ceiling fan justice. The vertical lines in the pano are definitely straighter though.

josquin1 Loc: Massachusetts

The new RF 16mm though does not have IS and if you're out shooting landscapes you might want that as the R does not either. The 16-35 f4 does have it so it could be a better solution for your R. Just saying.

TriX Loc: Raleigh, NC

The question that I always struggle with when i’m considering a new wide angle lens (currently thinking about a Canon EF 35 f2 USM with IS), is when do I really need IS? If I’m outdoors shooting landscapes, there’s plenty of light for a reasonable shutter speed, even with a small aperture, and if I’m inside, shooting real estate interiors, I’m using a flash. Maybe at night or for astrophotography? So far, I’ve never needed IS with my 17-40 f4L. I’d be interested to hear when others find IS helpful with a wide angle.

Canisdirus

Don't get hung up with IS in regard to wide angle lenses...it's not nearly as important than when using long lenses.

copladocus

I was going to suggest the addition of a TS lens but you beat me to the punch. I have used one, Canon TS-E 24mm f/3.5L II Tilt-Shift Lens (borrowed) a number of times and they produce fantastic results in real estate photos. I have seen way too many exterior and interior photos with visually skewed angles bad enough to make one think it was an MC Escher drawing. If I get into doing real estate on a regular basis that TS lens will be my first purchase. That said, you can also reset wall angles in both LightRoom and PhotoShop but the results are less than what you could get with out of the camera with the TS lens.

R.G. Loc: Scotland

I think your question harks back to a time when f/2.8 often meant 1) a recent design, 2) it was squarely aimed at professionals and 3) it was designed with high resolution digital cameras in mind. If they were compared to older and less bright glass then yes, you could assume that most of the time f/2.8 meant better IQ.

However, comparing contemporary glass the differences have less to do with aperture and more to do with the market they're aimed at. One example that springs to mind is the difference (or lack of it) between the Sony 24-105 f/4 and the 24-70 f/2.8. I don't own either but according to various reports the IQ is close enough to be indistinguishable to all intents and purposes.

Are primes better than zooms? Assuming you're comparing lenses that are similar in other important ways then yes, in very general terms primes are better than zooms. Primes can have a simpler design which makes them easier to optimise to a high level. The differences are becoming less as lens design advances but the advantages that primes have means they will continue to have better IQ (in very general terms, and when comparing like with like) for the foreseeable future.

If you're not a professional, how meaningful those differences are depends on your own personal assessment. If you're obsessive and a pixel peeper, they will be important. On the other hand good PP software and good PP skills could render the differences irrelevant.

twosummers Loc: Melbourne Australia or Lincolnshire England

Hi my friends and thank you all for your responses - all very enlightening. In a nutshell my question could be paraphrased as - Canon have 2 RF wide angle zooms - one is f2.8 and one is f4. They are now available at about the same price point. So if I generally shoot at f8 is the f2.8 (fast) lens likely to produce better results than the f4 also at f8?

As always, keep safe

R.G. Loc: Scotland

You're comparing two examples of high end glass so the answer is "No". In fact the f/4 lens may be slightly less likely to suffer from lens flare (not that either example will be especially prone to it) as well as almost certainly being smaller and lighter.

Canisdirus

Take the f/2.8 if the price is about the same.

amfoto1 Loc: San Jose, Calif. USA

Canon now has three full frame ultrawide lenses for their R-series cameras:

RF 15-35mm f/2.8L IS USM is the largest, heaviest and most expensive. It's about 3.5" in diameter (82mm filters), 5" long, weighs 840 grams and costs $2400 US.

RF 14-35mm f/4L IS USM is a little bit smaller, weighing and costing a bit less too. It's about 3.3" in diameter (77mm filters), 4" long, weighs 540 grams and costs $1700 US.

RF 16mm f/2.8 STM is the smallest, lightest and least expensive. It's new and claimed to be 1.6" diameter, 2.7" long. I think this is backwards... we'll see when they reach stores. It uses a 43mm filter, weighs 165 grams and costs $300 US.

Note that the 16mm lens doesn't have image stabilization. It also uses slower STM (stepper motor) focus drive, while the other two lenses use USM (ultrasonic motor). In real world use the difference in focus speed is probably negligible, since wide lenses only need to move their focus groups a very short distance to go from infinity to closest focus. Finally, because this lens isn't an L-series, Canon doesn't include the matched lens hood with it (EW-65C sold separately for $30 US). The other two lenses are L-series and include matching hoods. (BTW: the 65mm diameter of it's lens hood and photos of the 16mm lens itself suggest I'm right, that the stated dimensions are mixed up.)

In the past prime lenses like the 16mm were preferred by many photographers for better image quality than zooms could provide. However, today's zooms are far better than zooms used to be, so there's likely to be little difference.

f/2.8 vs f/4 max aperture probably won't matter to you. Someone shooting night skies or other challenging lighting conditions might want as large an aperture as possible, but this is less a concern with the electronic viewfinder of a mirrorless camera, than it was with the optical viewfinders of DSLRs. In fact, it comes to zooms and when both are available the slower (f/4) lens is always the smaller, lighter option... and often has better image quality. (Among Canon's EF lenses, the EF 16-35mm f/4 has image quality superior to their first EF 16-35mm f/2.8 as well as the II version of it. Only the III version is comparable to or perhaps even slightly better than the f/4 version... which also is the only one of the four lenses to have IS.)

Which of these lenses will give the best image quality is hard to say right now. The RF 15-35mm f/2.8L IS USM has been around for a couple years and is thoroughly reviewed.... while the RF 14-35mm f/4L IS USM was only announced a couple months ago and is in very few peoples' hands, so far. The RF 16mm f/2.8 STM is even newer, only being formally announced this week and unseen by almost anyone. Still, you probably can get some reviewers impressions of all three lenses online. I know Gordon Liang has posted a review the 16mm lens on Youtube, for example.

EDIT: The first reviews of the RF 14-35mm f/4L IS USM are starting to show up online and generally state it's an excellent lens with great image quality. There is some vignetting in the corners and right now only the most recent version of Canon Digital Photo Pro software has a profile built in to correct that (and any barrel distortion). That reportedly works quite well and no doubt there will soon be a similar profile in Lightroom, Capture One, DXO and other image post-processing software. Look for similar reviews of the RF 15-35mm f/2.8L IS USM, if you wish. Maybe it has better IQ, but it's hard to image there's very noticeable difference.

The RF 14-35mm f/4L IS USM is the closest focusing of the three. It's able to do 0.38X magnification, a bit more than 1/3 life size... while the RF 16mm can do 0.26X and the RF 15-35mm f/2.8L IS USM is the least capable in this respect, only able to do 0.21X or about 1/5 life size.

There is noticeable difference between 14mm, 15mm and 16mm focal lengths. One or two mm makes no difference with telephotos, but can be significant with ultrawide lenses. However, the stated focal length of pretty much all lenses is something rounded off.... so without side-by-side comparison it's hard to say how these three compare. You say 16mm is "wide enough" for your purposes, but the prime lens will be less versatile than a zoom, although to some extent, this depends upon what other lenses you have in your kit. If you have a 24-105mm or 24-70mm, there's a really big jump from 16mm to 24mm! It's a big gap that you might find need to fill.

EDIT: You say "both lenses cost the same"... but they won't. f/4 lenses are virtually always less expensive than f/2.8 counterparts. That's true of the RF lenses, too. The RF 14-35mm f/4L IS USM sells for $1700, while the RF 15-35mm f/2.8L IS USM costs $2400. $700 more for one additional stop... is it worth that to you?

It might be worth considering a third party lens, where there are some additional wide angle options. There are Samyang & Rokinon (same company) 14mm f/2.8 lenses in RF mount with autofocus (note that these have a convex front element that precludes using standard screw-in filters). And if you don't mind manual focus (and manual aperture), there are Venus Laowa 14mm f/4... who also offer both 12mm f/2.8 and 15mm f/2 "Zero-D". These last two are special designs to limit distortion, which might be a benefit for architecture photography.

Finally, possibly the most ideal lenses for architectural photography are some of the wide Tilt-Shift that Canon offers. The widest currently is the TS-E 17mm f/4L in EF mount, so would require an adapter to use on an R-series camera. This also is a manual focus lens (but the aperture is electronically controlled from the camera). There is also a future tilt-shift 14mm f/4 lens said to be coming in the RF mount (TS-R?).... suggested to be the first of this type of lens to have autofocus. But who knows when that might be available. And it will likely be quite expensive. There also are TS-E 24mm f/3.5L II (supposedly there will be a future TS-R 24mm, too), TS-E 50mm f/2.8L Macro, (1:2), TS-E 90mm f/2.8L Macro (1:2) and TS-E 135mm f/4L Macro (1:2). There also were older TS-E 45mm f/2.8 and TS-E 90mm f/2.8 that are excellent lenses (movements are less versatile than the new versions).