Real fastest value at the exact focal length.

But, this is just a rule of thumb — it's not necessarily exactly two stops in every case.

For the question of what two stops mean, see What does f-stop mean?. In short, each stop is approximately the-square-root-of-two times the previous one. That means half the light is allowed in (which is why the seemingly-weird series of numbers was chosen). Two stops is doing that twice, which conveniently works out to simply being doubling. Two stops from f/2 is f/4, and and two stops from that is f/8. Or, starting at f/1.8, two stops is f/3.6.

The test results comparison tool used by Digital Photography Review is interesting for looking at this. Try this test of Pentax's 18-55mm zoom, which like most kit zooms goes from f/3.5 at the short end to f/5.6 zoomed out.

You can see that at 18mm, the sharpness improves as you go from f/3.5 to f/4 to f/5.6. At f/8, it's more even from center to corners, but doesn't get any sharper. And beyond that, it drops off.

Then, change the focal length to 55mm. You'll notice that with this lens, overall sharpness goes down from 18mm at f/5.6. At f/8, it picks up a little bit, and a little more at f/11, and then back down again at f/16.

So, that fits with the two-stops guideline pretty well. Same for the Canon kit lens, and Nikon too.

With the Pentax DA 15mm f/4 Limited test, they've got test results in third-stops, and there you can see that f/7.1 is no better than f/6.3, and f/8 is worse — so that's only 1⅓ stops. On the other hand the DA★ 55mm f/1.4 shows peak sharpness at f/4.5 — closed down by 3⅓ stops. This doesn't reflect a problem with the lens, just different priorities in design. The Nikon 50mm f/1.4G is similar, with peak sharpness at 3⅔ or 4 stops down. However, the entry level Nikkor f/1.8G fits the "two stops" guide quite well, peaking at around f/3.5.

It's also worth noting that this test is primarily concerned with sharpness, because that's easy to measure. Other image quality and rendering characteristics are also affected. Vignetting (light falloff in the corners) gets better the more you stop down, and it's usually gone after two stops. And bokeh quality is usually improved by stopping down, too — generally, lens bokeh is nicer/smoother stopped down a bit, but of course it's less visible since you have greater depth of field. Plus, shape of the aperture blades will become visible in specular highlights, which is a side-effect unrelated to stopping down per se — some modern lenses have rounded aperture blades to make that not be a concern.

What's the base f-stop when counting an ideal f-stop for lens?
As a bit of background - Windows has a facility for Touch/TabletPCs whereby it shifts the position of popups/menus depending on your "handedness" (to prevent the menu appearing under your hand). 

Essentially what this does is if you are set to "right handed" (which it seems to default to once you've connected a touch device), every popup you open is artificially kicked to the left - this causes massive layout issues where we try and line up a popup with the item it "popped up from" :

Tablet PC Settings set to Left handed - no artificial correction from Windows


Tablet PC Settings set to Right handed - Windows artificially adjusts our positioning


We can detect what this setting is set to with SystemParameters.MenuDropAlignment, and for trivial popups we can adjust this using the actual width of the popup - but for dynamic popups and when we throw right to left support into the mix, this just doesn't work.

Currently it's looking more likely that we are going to have to go through every popup, manually work out a value to adjust the kick, and add something like this to every one:

&lt;MultiDataTrigger&gt;
    &lt;MultiDataTrigger.Conditions&gt;
        &lt;Condition Binding="{Binding Source={x:Static SystemParameters.MenuDropAlignment}}" Value="True"/&gt;
        &lt;Condition Binding="{Binding RelativeSource={RelativeSource Mode=Self}, Path=FlowDirection}" Value="RightToLeft"/&gt;
    &lt;/MultiDataTrigger.Conditions&gt;
    &lt;MultiDataTrigger.Setters&gt;
        &lt;Setter Property="HorizontalOffset" Value="-50"/&gt; &lt;!-- Set to arbitrary value to test --&gt;
    &lt;/MultiDataTrigger.Setters&gt;
&lt;/MultiDataTrigger&gt;


So.. back to the question :-) Does anyone know of a way to stop a WPF popup looking at this setting at all? 

For those that don't have a touch device you can access the Tablet PC settings by running:

C:\Windows\explorer.exe shell:::{80F3F1D5-FECA-45F3-BC32-752C152E456E}

And see the mess it makes for yourself :-)
