[slip]

If a lens has F2.8 (for example) at wide open and is soft, should it be called a 2.8? or is it "the lower the F stop, the more we can charge"
just wondering as I think if a lens is to soft wide open then maybe it shouldn't be advertised that way?

just wondering what others think

thanks

randy

[VoiceOfReason]

How much of the softness would be due to shallow DOF? What about if it is soft in the corners, but the center is reasonably sharp? Those could be the resons behind it. Besides, aperature size is a hard number, so even if IQ is crap until 5.6 and it has an actual max aperature size of 2.8 then it's still a 2.8.

[wasser]

If not the actual f-stop then what?

The f-stop is mathematically calculated and should, therefore, always be given precisely. The quality of the image at any given aperture is subjective and, as such, should be left to the opinion of an informed buyer.

[VoiceOfReason]

Here's somethign that explains aperature (regardless of IQ of the lens) Aperture - Wikipedia, the free encyclopedia

[blackcloudbrew]

It should be called and f2.8 lens, period. That's because it's physical aperture is f2.8, how it performs is another measure for sure but whether it's good or bad doesn't change that fact that it's an f2.8 lens.

Look at it this way, I've read that most lenses are typically sharpest about 2 stops from their maximum aperture, so based on that would you want to rename most lenses? (Hope that wasn't too general of an observation.)

[Na Horuk]

Maybe actual max. light transmission?
But that is the reason why we have reviews. Focal lengths are also not always quite as the lens says, and macro zooms are rarely really macro. All sorts of things are important and the marketers will use anything to try to make their product more appealing. We, the buyers, are forced to find information and then act upon it. Or we can just blindly buy, which will work out all right 75% of the time..

[JinDesu]

How soft is too soft for you?

[Jonathan Mac]

But it will still be f/2.8 when it comes to viewfinder brightness. You could make all lenses f/1.4, but they would be so big and heavy and expensive for terrible IQ without stopping down that no-one would buy them. I think a lens needs to be useable wide open, but not necessarily for high res landscape shots and everyone's definition of useable is different.

[audiobomber]

Light transmission is a much more honest way of reporting brightness than aperture. You still need stops for photography, but Light Transmission should be more widely reported in reviews.

Regarding the original question, whether a lens should be called f/2.8 even though it's soft at that aperture, yes, it should. If the aperture opens sufficiently, it's a 2.8 lens. It's up to the user to decide whether performance is acceptable wide open.

What really bugs me though, is when people call a variable aperture lens by its widest aperture. The usual example I see is Sigma 17-70 f2.8. That is plain wrong, the 17-70 is not a 2.8 lens, it is 2.8-4 or 2.8-4.5.

[eddie1960]

Light transmission is a much more honest way of reporting brightness than aperture. You still need stops for photography, but Light Transmission should be more widely reported in reviews.

Regarding the original question, whether a lens should be called f/2.8 even though it's soft at that aperture, yes, it should. If the aperture opens sufficiently, it's a 2.8 lens. It's up to the user to decide whether performance is acceptable wide open.

What really bugs me though, is when people call a variable aperture lens by its widest aperture. The usual example I see is Sigma 17-70 f2.8. That is plain wrong, the 17-70 is not a 2.8 lens, it is 2.8-4 or 2.8-4.5.

T stops haven't been used anywhere except cinematography in decades. they are far more accurate of course. It is definitely needed to keep light uniform with lens changes in a filmed scene. for photography F stops are "close enough" so to speak, since you aren't trying to control a switch between shots for exact light quality.
and I agree if it is a variable aperture call it that. the sigma is only 2.8 at 17mm, not so useful when you want 2.8 for a portrait at 70

[westmill]

You said it yourself... a spade is a spade... F2.8 is f2.8. What you are describing is simply a good or bad F2.8 lol

[audiobomber]

T stops haven't been used anywhere except cinematography in decades.

DXOMark measures and reports T-stops in their lens reviews, but I can't think of anyone else that does this.

[eddie1960]

DXOMark measures and reports T-stops in their lens reviews, but I can't think of anyone else that does this.

the new Canon lenses targeted at the cine world all have T stops as well, as do the Zeiss Cine compact prime lenses (sadly not on K mount just EF, PL, F MFT,E mounts - they have interchangeable mounts available so a production can swap them between cameras)

[DaveHolmes]

Generally... Stick a hood on that badboy and it'll look a bit sharper wideopen than it did without one...

My FA50 appears to take sharper (or at least contrastier) pictures wide open now it has a screw-on metal hood... My FA28 is also rockin' chinese-bay metal and I frequently consider replacing the plastic hood of the FA35 with a £3.99 (GBP & including postage) chinese screw-on...

[Lowell Goudge]

As people talk hoods, one thing to consider in the ASP-C world is that you can gain a ton, by extending the hoods far beyond the stock hoods for most legacy lenses because the hoods were either deisgned for multiple lenses, and they are designed for full frame field of view. I use the hood of my SMC tak 200/4 on my supertak 85 for example,

Steep down 1-2 focal length steps with all your hoods because the light that covers a full frame (film for example) just goes into baffles in the mirror box on a DSLR