[pofenloch]

Thanks in advance for your help. Sorry if this is a basic question. I keep reading and hearing that you should be taking pictures with a f stop around f 11 . This is where the lens is sharpest. Why buy a fast lens with a f 2.8 if you are going to shoot at an aperture at f 11? Thanks again .

[Adam]

Thanks in advance for your help. Sorry if this is a basic question. I keep reading and hearing that you should be taking pictures with a f stop around f 11 . This is where the lens is sharpest. Why buy a fast lens with a f 2.8 if you are going to shoot at an aperture at f 11? Thanks again .

Because sometimes you want to sacrifice a bit of sharpness for better bokeh, faster shutter speed, or a lower ISO. Fast lenses are usually pretty sharp close to wide open, especially primes.

[angerdan]

1. I keep reading and hearing that you should be taking pictures with a f stop around f 11.
2. This is where the lens is sharpest.
3. Why buy a fast lens with a f 2.8 if you are going to shoot at an aperture at f 11?

1. Increasing the f-stop number (greater aperture) increases depth of field. Sometimes (like in product photography) this is neccesary or wanted.
2. every lens has its sharpest point at another f-stop. Most lenses delivers highest sharpness 2-3 f-stops about fastest aperture.
3. Shooting at f11 with an f2.8 lens decreases sharpness significantly, so it's mostly wether usefull than recommended to stop down 4 f-stops.

[SpecialK]

I keep reading and hearing that you should be taking pictures with a f stop around f 11 .

That is wrong. Typically lenses are sharpest about 2 stops down, so f5.6-8. Someone posted a chart some time ago using the DA Limiteds, the 55 1.4 and maybe another one or two. All of them dropped off resolution after f8.

It used to be "f8 and be there", but the chart indicated 5.6 is the sweet spot.

[DeadJohn]

...Why buy a fast lens with a f 2.8 if you are going to shoot at an aperture at f 11?...

This advice is often used for landscapes, where you want foreground, background, and corners to all look sharp. Stopping down to smaller apertures increases depth of field and helps hide some lens weaknesses.

If you have a good lens and shooting a distant mountain with nothing in the foreground, f2.8 might work great.

If you have a less good lens, or a scene with things near and far, f11 might be needed to get sharpness across the frame. If you stop down too much, diffraction physics kick in and reduce sharpness.

Here's an example where I used f/16 to increase depth of field. I focused on the yellow road marking a little into the scene, and f/16 allowed things in front of and behind that focus point to be acceptably sharp:

[pres589]

'f8 and be there' is about in-focus depth of field and not sharpness at the point of focus. You can basically stop changing focus if you set the lens to f8 and just a bit short of infinity.

Most consumer lenses will offer fairly uniform, high performance, across the frame at f8.

[SpecialK]

'f8 and be there' is about in-focus depth of field and not sharpness at the point of focus. You can basically stop changing focus if you set the lens to f8 and just a bit short of infinity.

Most consumer lenses will offer fairly uniform, high performance, across the frame at f8.

Yes, but that relies a lot on camera-subject distance, too.

[Scorpio71GR]

For some lenses f11 is the sharpest point, my Tokina 400mm prime is an example. This lens has a maximum apeture of f5.6, two stops down at f11 produces the best IQ. I know this because I did tests to determine it. All lenses are different even identical lenses. Some lenses will be sharp wide open and others will not. Shooting a lens at the maximum apeture may only produce a sharp center but poor borders. Stopping the lens down can increase border sharpness. Typically for a f2.8 lens maximum IQ will be between f5.6-f8. F8 is normally the smallest apeture you can goto before diffraction starts. F11 is usually the maximum apeture before diffraction starts to degrade the IQ. F11 will usually be comparable to f4 on a f2.8 lens. Sometimes overall depth of field is more important than ultimate sharpness. I have some lenses I shoot at f11 and others I usually shoot at f6.3. Different lenses, different settings, applying a one for all rule to all lenses is nonsense.

[normhead]

Thanks in advance for your help. Sorry if this is a basic question. I keep reading and hearing that you should be taking pictures with a f stop around f 11 . This is where the lens is sharpest. Why buy a fast lens with a f 2.8 if you are going to shoot at an aperture at f 11? Thanks again .

There are lots of reasons for buying 2.8 lenses.

Most AF systems are calibrated for ƒ2.8 so that's their most accurate setting. Your lens auto focuses with the lens wide open, then stops down when you take the image. In low light ƒ2.8 is huge for accuracy of AF. An ƒ2.8 lens remains functional for auto focus long after an ƒ5.6 lens has given up. That is a big advantage, I take mushroom images on the forest floor under a canopy. Even at the highest point of the sun in the sky I can have difficulty getting an ƒ5.6 lens to focus. Sometimes I have difficulty getting an ƒ 2.8 lens to focus....

Next up, lens charts.... this is a lens chart, it shows where the lens is test chart sharpest.... the selected lens is the FA 100 macro 2.8

Where the bars are highest, the lens is sharpest at the point of focus. the sharpest part of this lens is ƒ5.6 or ƒ8 look how where ƒ22 is, it looks abysmal right? Why would you use a ƒ22 when you could use ƒ5.6 where it's a lot sharper?

Well I just happen to have an example to show you why.
The higher your F number, the wider your depth of field.(The distance between the nearest and furthest points in acceptable focus.
Here's a mushroom shot at ƒ5.6 the sharpest part of the lens.


Here's one shot at ƒ22, where the lens has already suffered from serious loss in sharpness due to diffraction. Notice that while technically the lens is sharper at ƒ5.6 more of the image is sharp at ƒ22.



Notice the second image may not be as sharp technically, but even at it's reduced level of sharpness, the lens is capable of reproducing the detail in the scene, but more of the scene is in focus. So there's a choice to be made. Do you want more of the scene in focus, or do you want those areas that are sharp, to be the sharpest. This is where test charts really fall down. The most important part of an expensive lens is how it reproduces out of focus areas, but a 2D test chart can't show you that.

Which brings up another topic referring to sharpness. Usually when people are talking about sharpness they talk about test chart sharpness. However, the proper photographic metric is "acceptable focus". Acceptable focus is simply focus that is discernible to the human eye. The corollary is that once you achieve sharpness that's acceptable, additional focus really doesn't add anything. Looking at my mushrooms, we can see from the test charts that the middle of the bigger mushroom which was my focus point is much sharper at ƒ5.6 than the ƒ22 image. The problem is our eye can't see the difference. You heard it here first, those who insist on the absolute sharpest test chart image are talking about things that can be seen with a test chart, but not necessarily by the human eye in a real photograph. If the sharpness of your lens out resolves the acuity of your eye, you don't sacrifice anything giving up resolution. Providing resolution beyond what you can actually see isn't giving up anything at all.

Here's another image taken at ƒ2.8


Not the sharpest part of the lens according to the charts, but pretty cool eh? One razor thin little part in focus and the rest out of focus. With an ƒ2.8 lens, I always have a choice. I can do the ƒ22 image, or I can do the 5.6 image or the ƒ2.8 images. In my experience it's rare you want the ƒ2.8 image, but it sure is something special when it works for you.

When I started out 12 years ago, I had an 18-55, a Sigma 70-300. The first lens bought was an FA 50 ƒ1.7 to give me som low light capacity. Currently I've added, a 300 2.8, a 200 2.8, a 100 2.8, a 50 2.8 a 40 2.8. My zooms are the 18-135 for APS-c the 28-105 variable aperture lenses, and my 60-205 ƒ4. You end up buying 2.8 or faster glass just to expand your horizons a bit. And once you do that, you discover it actually is more functional in terms of AF and keeping you shooting later into the evening.

There are a lot of issues here for sure. Sharpness in a lab means test chart sharpness, which is largely irrelevant to the photographer, because most subjects don't require a lens to be better than average in sharpness.(get ready for howls of indignation from test chart sharpness freaks.)
There is acceptable focus, the term preferred by photographers (not lab technicians) which acknowledges that a certain degree of softness from most lenses is not discernible by the human eye. (That is a fact that optiofiles will vehemently argue, claiming that they have to have the absolute sharpest lenses and they can see a difference, when every photographer knows that learning to manipulate the parts of the image that are in acceptable focus is much more valuable than obtaining absolute sharpness.)
And then there is the percentage of the image that is in focus. The ƒ2.8 floating mushroom above is very sharp where it's sharp, but most of the image is intentionally soft. The ƒ22 image is imperceptibly soft, but much more of the image is in focus. Essentially what you go with is not really based on lens sharpness, but on what photographers call "acceptable focus." Also important is using depth of field to create subject isolation where the more traditional sources of subject isolation ( contrast, colour and texture) are not working for you.

So two things can be meant by sharpness. Absolute test chart sharpness, which is photgrpahically a pretty useless concept unless you are copying books or museum paintings or something 2D. Or it can mean how much of the image is in acceptable focus, which is where your ƒ11 image qualifies as sharp. It's sharp enough your eye can't tell it's not sharp but not sharp in absolute terms

From a photographic perspective, test chart sharpness is only relevant in labs. The term acceptable focus defines what you can actually see. But I suspect people who say ƒ11 is the sharpest aperture are really talking about how much of the photo is in acceptable focus, not test chart sharpness.

If you're new to photography, it's quite likely that your biggest source of confusion will be people who discuss test chart sharpness as if it wasn't often negated by your understanding acceptable focus. When some discuss sharpness, they are talking about test chart reproduction of the sharpest part of the lens. Others are talking about how much of the image is in acceptable focus, and much of what is in acceptable focus is much softer than the sharpest part of a sharp lens.

It answers the question, "how can I get sharp pictures with this lens that everyone says is not sharp enough for them." I do have a couple of M lenses made 40 years ago that are not sharp. These days, kit lenses are much better than those lenses. These days, sharpness is pretty much a selling point that goes beyond photographic necessity when discussing photography.

Let the howling begin.

[Penview52]

...f is the ratio of diameter of hole through which the light enters to the front of lens diameter.
large aperture...small f number, small aperture ... large f number. ( properties of lens )
I believed that f number is that - is not the basis of sharpness of image, whether you have shallow or deeper depth of field, f 2.8 or f 11 there is a zone of acceptable sharpness within a focus applicable to all lenses.
Set to auto mode and the camera calculates the best f value rather than setting to aperture mode then you have to rely on yourself the best f value.

[pres589]

Auto aka Green mode may not go for the most sharp f-stop. I believe that Program mode, where available, is better for that.

One thing that test charts can tell you is how much the edges fall off versus center sharpness. Another thing is how useful wide open a lens is. Recently I took a bunch of test shots with my three 28mm primes as well as my 18-135 for comparison sake. My Albanar Macro is garbage wide open with nothing in focus that I could see at all. It was shocking. At f8 it was night and day different. It's almost like it's really an f4 lens with too many positions on the ring.

[jbinpg]

@Penview52: Not quite. Here is the official definition of f-stop.

"The f-stop of a lens is determined by dividing the focal length of the lens by the diameter of the size of entrance pupil, i.e., the lens opening."

[Penview52]

f stop is the diameter of aperture opening unit less ratio between aperture and lens diameter. If you have two circles , the other small circle is inside the big circle, they can have ratio based on the extending of lens known as focal length. The small circle is where the regulated light pass through that's where it gives light/sharpness of the image. The big circle is the lens diameter that all light can pass through .
Going back to the original topic, poster pofenloch - just buy the f 2.8 lens to give you peace of mind. My point is that sharpness of image is not based solely on f number, other factors to be considered.
I do not disagree to your opinion jbinpg, we only have different views of understanding. It does not end at dividing the focal length by the diameter of the aperture, that is value of N.

[clackers]

f stop is the diameter of aperture opening unit less ratio between aperture and lens diameter. If you have two circles , the other small circle is inside the big circle, they can have ratio based on the extending of lens known as focal length. The small circle is where the regulated light pass through that's where it gives light/sharpness of the image. The big circle is the lens diameter that all light can pass through .
Going back to the original topic, poster pofenloch - just buy the f 2.8 lens to give you peace of mind. My point is that sharpness of image is not based solely on f number, other factors to be considered.
I do not disagree to your opinion jbinpg, we only have different views of understanding. It does not end at dividing the focal length by the diameter of the aperture, that is value of N.

Jbinpng's definition of aperture is correct, Penview - not yours.

[Penview52]

Jbinpng's definition of aperture is correct, Penview - not yours.

Ok I believed that ratio of focal length divided by aperture diameter is the scale but it does not ends there. This is not clear and what is the implication of this scale? I believed that it ends with ratio of light intake between aperture and lens diameter, also this f stop varies and light intake varies also. You are all correct but what i expect is little explanation.