RF14-35 F4 not sharp at 35mm wide open

I have a nice CANON RF 14-35 F4 USM lens that performs flawlessly exept  for one issue I have with it.  Just want to check with owners of this lens if this “flaw” is also present in yours.

when I take a close up (this lens has a nice 20cm close up range) the picture is obvious much more blurry at 35mm F4 compared to all other focal points wide open .  This improves at F5/6 dramatically but I am a little disappointed as the 35 mm F4 would be the most used wide open setting from all focal point ( some blurry background on a portrait)

I played around a bit with text on a Coca Cola can at short distance ( 20-40 cm in front of the focal plane) and the difference is striking in the negative sense when F4 is used at 35mm vs all possible settings from this lens.   Please I hope some of you owning this lens ar willing to share your experience or are willing to do some testing as well and let me know.

I know that it is extremely difficult to make a flawless zoom especially on the wide angle spectrum but I hope  that  in one way or an other this issue can be remedied/ corrected.

I am grateful for your response

OBSERF8 wrote:

I played around a bit with text on a Coca Cola can at short distance ( 20-40 cm in front of the focal plane) and the difference is striking in the negative sense when F4 is used at 35mm vs all possible settings from this lens. Please I hope some of you owning this lens ar willing to share your experience or are willing to do some testing as well and let me know.

Text on a curved surface up close at a large aperture is pretty much a recipe for failure. Did you try text on a flat object? Did you try increasing your working distance and seeing when the image gets sharp? Lastly, that lens isn't remarkably sharp at 35mm (widely reported in reliable reviews), so don't expect "tack sharp" images from it, especially around the edges, at 35mm.

If you're skilled at post-processing and have an application capable of it, you should be able to correct at least some of the softness. For example, I use Capture One Pro and find that I can correct both peripheral softness and vignetting using a circular gradient and appropriate adjustment sliders.

Thanks Abbott for your fine response. Yes I am aware of the rounded shape influence of the cola can and indeed tried the same test on flat surface with same result.  I would say that from about 70 cm ( 2-3 inches) the sharpness gets back to par with all other settings.
As the full open F4 at 35mm differs so clearly especially in the center from the other focal points wide open at close distance I just wonder if this was my  particular copy or a general property of the lens.  Hope to get some more response from people willing to test this or have the experience about it.   For close up shots from a leave , insect or many other subjects I tend to want to use 35mm F4 and must always think about stopping down . Not a big issue but I would have liked particularly the 35mm F4 super sharp wide open. 
Best regards,
OBSERF8

From what I've seen from my 14-35 and read from reliable reviews, the 14-35 isn't super sharp at any FL. However it does extremely well for a lens with its FL range.

In my earlier response, I told you how I correct lens shortcomings using Capture One Pro. I just did a cursory test and the lens looks as expected at its closest FL on my R5: fairly sharp at the center, softening as the edge is approached. And as I said in the other response, it's correctable.

If you're looking for extremely sharp results from this lens on something that close, then I'd suggest that you explore focus stacking, stopping down (use deconvolution if you go smaller than f/8), using an extension tube with something like the 24-105, or something other than this lens.

Did you read reviews for this lens before buying it?

Maybe someone else will be more help for you than I am...

Abbott Schindler wrote:

From what I've seen from my 14-35 and read from reliable reviews, the 14-35 isn't super sharp at any FL. However it does extremely well for a lens with its FL range.

In my earlier response, I told you how I correct lens shortcomings using Capture One Pro. I just did a cursory test and the lens looks as expected at its closest FL on my R5: fairly sharp at the center, softening as the edge is approached. And as I said in the other response, it's correctable.

If you're looking for extremely sharp results from this lens on something that close, then I'd suggest that you explore focus stacking, stopping down (use deconvolution if you go smaller than f/8), using an extension tube with something like the 24-105, or something other than this lens.

Did you read reviews for this lens before buying it?

Maybe someone else will be more help for you than I am...

Thanks again,

the matter I raise in this thread is absolutely no deal braker for me as for my purpose the lens is 98% perfect ( street photography) .  The softness in the center at F4 at 35mm will not improve with focus stacking as it not a focus matter in the first place.   Anyhow thanks for thinking along with me and let’s see if anyone else comes up with a 20-30cm testing .