Why is a lens soft wide open?

[hamlet]

So i'm wondering why the area that is within the dof softer for example: on my 50mm at f1.4 than say f2? Why does stopping down to f2 make the dof sharper?


And here is another example of a pro lens that costs an arm and a leg: Zeiss 55mm f/1.4 Otus Distagon

The sweet spot for this lens is definitely f/4 – f/5.6, so when I wanted maximum sharpness I favored f/5.6.

 

[tirediron]

It's the physics of optics; I'm sure someone like Helen could actually explain it; me, I simply know that it's a fact and accept it. Regardless of the lens, or the gigabucks put into it's development, using a smaller (usually mid-range) aperture will always get you sharper focus. Of course in higher-end lenses the difference between wide-open and stopped partway down is much less than on cheaper lenses, but it's there.

 

[paigew]

my 50 1.4 isn't soft wide open. It is just more challenging to nail focus due to the shallow dof.

Not wide open...but this was shot at 1.8 which is generally wider than I go with that lens...I usually keep it at 2.2
image-30.jpg by paige_w, on Flickr

 

[Solarflare]

DoF means Depth of Field, which means the size of the area in which a lens will appear sharp.

Mathematically, a lens will always ony be sharp in one area, the socalled "focus plane". But before and behind that plane, there is an area in which the photo still will appear sharp enough. This is the DoF.

If the lens itself (not the DoF) isnt sharp, the DoF wont be too clearly defined. Basically everything will be out of focus, so to speak. This has nothing to do with DoF. It means the lens cannot focus light coming from an object on the focus plane into a single point very well, thus everything will be blurred, as if its out of focus.

It is typical that a lens will get sharper if you stop it down. Thats because there are less ways light can travel from the subject on the focus plane to the sensor plane.

Wellmade glas of course can already be very sharp when wide open.

My own glas is like this:

AF-S 70-200mm f4 VR - stopping down does NOTHING to sharpness.
AF-S 50mm f1.8 - can be used wide open, but stopping down slightly improves sharpness
AF-S 28mm f1.8 - pretty much the same as 50mm.
AF-S 16-35mm f4 VR - stopping down helps definitely. Wide open still tolerable though. Either way, my softest lens.

 

[hamlet]

Yes, yes, we know the dof will get sharper when you stop down. Wish we had a learned person here, a grand master or photographic wizard with arcane knowledge in these matters.

 

[KmH]

The DoF does not get sharper when a lens is stopped down. The DoF gets deeper.
You can have a lens that delivers soft focus even if the DoF is deep.

A simple lens (crown) gets thinner from the middle out to the edges.
As the glass gets thinner the point of focus moves and there are multiple points of focus - from the middle and thickest part of the lens out towards the edge.
All the light in the scene passes through all parts of the lens so light from the edge focuses at a different distance than light that goes through the middle of the lens. So unless corrected, used with a wide open aperture none of the scene is sharply focused.

To correct the situation a second lens (flint) is added and the 2 lens pair is known as a doublet Doublet (lens) - Wikipedia, the free encyclopedia
Worse yet is that the primary colors also focus at different distances causing chromatic aberration.

Part of what makes a lens expensive is adding lens elements so both focus, chromatic aberration, spherical aeration, coma, and other optical aberrations are minimized.

Apochromatic lens groups are used to aid correcting chromatic aberration - Apochromat - Wikipedia, the free encyclopedia

 

[pixmedic]

Its magic.
Natures mysteries.

 

[hamlet]

Very good, thank you.

 

[480sparky42 41]

It's soft because Mishelle says so.

 

[TCampbell]

This would be easier to explain with a picture. Lacking a picture... I'll do my best.

Picture a nice curved glass element (not the whole lens... just one element).
Now picture just behind this, you have an adjustable aperture that you can make either large or small.

Imagine that it is set to a large opening.

For each ONE pixel on your sensor, light is actually coming through and focusing from across the entire surface of this lens. The same is true for the pixel next to that one... and the next one, etc. In other words if you could shrink yourself down to microscopic size and stand on just ONE of those pixels, you would see light coming through from the entire opening in the lens... not just one pin-point location.

This means any property of optics (optical flaws, chromatic & spherical aberration, etc.) has more opportunity to cause a bit of light to land in some location OTHER than the spot where you ideally would have wanted it when you have a larger opening. Hence... "less-sharp" focus in areas meant to have "sharp" focus. At the same time... a pleasing advantage is that you can have much stronger out-of-focus blur in those areas which were not meant to be in sharp focus.

When you shrink the size of the aperture/opening, light is confined to a smaller space. Since the opening shrinks around the central axis of the lens where the glass tends to be less curved, the effect of things such as spherical and chromatic aberration is reduced... and this results in a sharper and higher-contrast image.

Back in the early days of optics when the first telescopes only had a single lens at the front and a single lens at the back, the only way to get somewhat sharp images was to use a very high focal ratio. In order for a telescope to be larger it had to much much longer. They grew to ridiculous lengths.

There are problems with going over-board... which can introduce diffraction problems (due to the wave-nature of light.)

At some point, someone realized that one could reverse (or at least substantially reverse) the optical degradation of the single element by stacking a second element behind it... but with a concave front-side and either a flat or nearly flat convex curve on the back side. (you can read a bit about it here: Achromatic lens - Wikipedia, the free encyclopedia )

But this started a realization to correct for the problems of the lens... eventually apochromatic triplets were introduced and today we have lenses with many many more elements.

Ultimately the important thing is that one can "reduce" the problems... but generally not get rid of them entirely. So even today and using the very best lenses money can buy... you can improve the optical quality by reducing the aperture size.

Something to keep in mind is that we tend to not use all the pixels on our cameras for most uses of our photos. My own 27" monitor is 2560 x 1440 pixels (about 3.7 megapixels). My camera, on the other hand, is 5760 x 3840 (about 22 megapixels). That means unless I'm making a fairly nice sized print... you aren't going to get a chance to inspect the image closely enough to see the minor flaws. This gives you some wiggle room for lens quality.

 

[gsgary]

Don't have that problem with my Voitlander 50mmF1.5 ASPH

 

[sk66]

Have you ever seen a prism break light up into a rainbow? Well, something similar happens when light passes thru a lens element. A better lens does a better job of keeping all of the wavelengths together. Stopping down reduces the opening preventing a lot of the "errant" light from passing thru which improves sharpness.

But stopping down also introduces diffraction; and every smaller aperture produces a point larger than the previous aperture. Eliminating lens aberrations/errors and increasing diffraction by stopping down is a tradeoff and there's usually a "sweet spot" for every lens (~ 2 stops from wide open). A better lens requires less stopping down and a truly "diffraction limited" lens would require no stopping down (I don't know of one though).

 

[shaylou]

my 50 1.4 isn't soft wide open. It is just more challenging to nail focus due to the shallow dof.

Not wide open...but this was shot at 1.8 which is generally wider than I go with that lens...I usually keep it at 2.2
image-30.jpg by paige_w, on Flickr

It's just my opinion but this is a little soft. Personally when I shot that lens I had to stop it down more than that to get it tack sharp.

 

[shaylou]

I have the 70-200 II, 24-70II and the 100L 2.8 and they all 3 are sharper stopped down. I have never shot a lens that was not.

 

[Derrel]

Some lenses perform better at wide-open aperture than others. According to Thom Hogan, he mentions that Nikon's exotic super-teles are optimized for wide-open shooting, since soooo many users end up shooting a 300/2.8 or 400/2.8 or 500 or 600mm wide-open.

I know the old 400mm f/3.5 ED-IF I have is very good wide-open, IMPROVES to peak at about f/4.5, but actually is decidedly worse at f/5.6...just as Bjorn Rorslett describes it...

But "normal, regular, non-exotics", like say 50mm f/1.4 lenses, or 35mm f/1.4 lenses, usually have "issues", meaning optical defects, that manifest themselves when the lens is shot wide-open. Typically, loss of evenness of illumination across the entire field, i.e. "vignetting" or "light fall-of"; veiling glare, or a subtle, overall loss of contrast across the frame; sharp central areas, but much lower-resolving edges and corners; and so on. A lot of normal, regular, non-exotic lenses suffer from coma, which causes the lens to render out of focus points of light as blurry blobs, and not as POINTS of light...

When you start looking at night-time shots that have point light sources, like street lights for a really perfect example, you see that wide-open, a large percentage of lenses suffer pretty badly from coma wide-open, but that stopped down a couple stops, the performance improves.

Here is a REALLY GOOD review of the Sigma 50mm f/1.4 ART lens, compared against the much less-costly Canon 50mm f/1.4 EF lens; It also shows some night-time shots made with the Sony 50mm f/1.4 ZA lens. The new Sigma 50mm f/1.4 is somewhat of an "exotic" 50mm lens, being priced at say, roughly eight times the cost of a Canon 50mm f/1.8 for example, and roughly double the price of a Canon or Nikon 50mm f/1.4;

Gear for Image :: Sigma 50mm f1.4 DG HSM Art Review

HERE is the Canon 50mm f/1.4 EF, shot stopped down to f/2.0. Look at the bad coma: 2674B9495346EBEA1E56ED

HERE is the Sigma 50mm f/1.4 ART stopped down to f/2.0--pretty good!: 2674B9495346EBEA1E56ED

HERE is the Sony 50mm 1.4 ZA at f/2.0: Holy Purple Spaceships, Captain Kirk!:244FF3445348212122CCFE

Why is a lens soft wide open? Well, if it is, it's affordable!!!! We do not always need wide-open apertures these days, so many lenses have traded less-than-perfect wide-open performance, and some optical defects, as a way to keep costs, size, and complexity manageable. if you want a lens that's really good wide-open, if you PAY a load of money, you can get one that's very good wide-open.