Question about sharpness between lenses of different max apertures...

[splproductions]

Should a lens that has a max aperture of 1.4 be just as sharp wide open as a lens with a max of 2.8?

I noticed that wide open, my 24-70L is much sharper than my Sigma 50 1.4 wide open. But is that not a fair comparison? Would all lenses exhibit this phenomenon? (I.e., is it a property of the physics of light and not necessarily dependent upon the lens).

 

[analog.universe]

It is actually dependant on the lens. Very few lenses are at their best wide open, but some certainly do better than others. My 35mm 1.4 is as sharp wide open as my 24mm 1.4 is at f/2. By f/4 they're quite close however. My 50 2.0 is sharper wide open than either my 24 or 35 at 2.0. So, it's more a matter of lens design and the actual quality of the optics. There are some f/1.4 primes that are sharper throughout their entire range than any setting on certain 2.8 or slower zooms...

 

[chuasam]

Can you name me even a single lens that is at its best wide open?

 

[KmH]

Plus not all f/1.4 lenses will have equal sharpness at f/1.4.

It depends on the design of each lens and what approach each design takes for controlling optical aberrations in the lens. - Camera lens - Wikipedia, the free encyclopedia

Field curvature

 

[splproductions]

It is actually dependant on the lens. Very few lenses are at their best wide open, but some certainly do better than others. My 35mm 1.4 is as sharp wide open as my 24mm 1.4 is at f/2. By f/4 they're quite close however. My 50 2.0 is sharper wide open than either my 24 or 35 at 2.0. So, it's more a matter of lens design and the actual quality of the optics. There are some f/1.4 primes that are sharper throughout their entire range than any setting on certain 2.8 or slower zooms...

I do know that a lens isn't at it's best wide open, and that a lens will typically be its sharpest a couple stops down.

But just to clarify, I could have a f/4 lens and a f/1.4 lens, and the f/1.4 lens could be sharper wide open than the f/4 wide open. Is that correct? (We're just going to assume we're using high-quality lenses here).

 

[Big Mike]

But just to clarify, I could have a f/4 lens and a f/1.4 lens, and the f/1.4 lens could be sharper wide open than the f/4 wide open. Is that correct? (We're just going to assume we're using high-quality lenses here).

You can't really generalize it like that. You pretty much have to compare one specific lens to another.

Although, if we're going to be hypothetical...I'd say that the F4 wide open, will likely be sharper than the F1.4 wide open. I don't know how scientific it is, but I've heard plenty of photographers say that F8 is the sweet spot for most lenses/lens designs.

 

[Big Mike]

Can you name me even a single lens that is at its best wide open?

I would still assume that they get better when stopped down, but I've heard that large telephoto lenses are designed to be at or near their best with wide open apertures...as that is where they will most likely be used.

Think about it, if you have a 400mm F2.8 or F4, are you likely to be shooting at F8? Probably not. You'll be shooting wide open to get the fastest shutter speed you can. The designers know this and try to accommodate for it (or so I've heard).

 

[rgregory1965]

I have got lucky a few times with my Sigma 70-200 wide open...

Like this one http://www.thephotoforum.com/forum/just-fun/276182-still-playing-my-sigma-70-200-os.html

 

[analog.universe]

Can you name me even a single lens that is at its best wide open?

I would still assume that they get better when stopped down, but I've heard that large telephoto lenses are designed to be at or near their best with wide open apertures...as that is where they will most likely be used.

Think about it, if you have a 400mm F2.8 or F4, are you likely to be shooting at F8? Probably not. You'll be shooting wide open to get the fastest shutter speed you can. The designers know this and try to accommodate for it (or so I've heard).

The 400mm 2.8 was exactly the lens I was thinking of when I wrote that...

 

[MReid]

Some lenses are optimized for shooting wide open.....this usually means there is not much difference between wide open and the next couple f-stops....these lenses are Expensive!
Generally lenses are sharper a stop down, most noticeably in the corners.
As mentioned it varies between brands and models and price.
Mid price non-pro zoom lenses will normally not be sharp wide open....Mid price primes will have a better chance of being sharp wide open, but you definitely need to do your research to know which ones are or are not.

For me I do a lot of research to make sure a lens will be tack sharp wide open before I buy one, and then I immediately test it upon receipt to make sure I got a good copy.
There are also back and front focus issues that come into play.

 

[bratkinson]

Can you name me even a single lens that is at its best wide open?

I LOVE my Canon 135 f2.0L wide open! That's the only way I shoot with it!

Back to the base question...each lens and each manufacturer have lenses that are good at different stops, outstanding at others, and some are never very good. In 10 words or less, you generally get what you pay for.

 

[analog.universe]

Can you name me even a single lens that is at its best wide open?

I LOVE my Canon 135 f2.0L wide open! That's the only way I shoot with it!

As beautiful as that lens is wide open, in terms of actual sharpness, it follows the common curve with a peak at 5.6. The 400mm 2.8 discussed barely shows any change throughout the range until diffraction begins to set in at f/11-16. Meaning, the 400 doesn't really get sharper than it is when wide open, the 135 improves until 5.6.

 

[greybeard38 10]

Every lens I've ever used was at its' best 2-3 stops down. Some are better than others wide open but I've never seen any that were at their best wide open.

 

[chuasam]

While the Canon 400 f/2.8 is exceedingly sharp wide open at f/2.8. It's actually slightly sharper at f/4.0 See Canon Lens: Primes - Canon EF 400mm f/2.8L IS II USM (Tested) - SLRgear.com!

 

[analog.universe]

While the Canon 400 f/2.8 is exceedingly sharp wide open at f/2.8. It's actually slightly sharper at f/4.0 See Canon Lens: Primes - Canon EF 400mm f/2.8L IS II USM (Tested) - SLRgear.com!

It's interesting.. it looks sharper at 4.0 on APS-C and sharper at 2.8 on FF. Either way that lens is sick, since it practically pegs the scale through the first 3 or 4 stops.