# Canon EF-S 18-55mm f/3.5-5.6 IS II Critical Aperture?



## Bobby Ironsights (Dec 6, 2014)

Hi, Sorry, I meant the Canon EF-S 18-55mm f/3.5-5.6 IS STM, does anyone know it's critical aperture? 
Can't find the answer googling.


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## qleak (Dec 7, 2014)

Bobby Ironsights said:


> Hi, Sorry, I meant the Canon EF-S 18-55mm f/3.5-5.6 IS STM, does anyone know it's critical aperture?
> Can't find the answer googling.



As with most zoom lenses the answer depends on the selected focal length. On average it looks like f8 is pretty good with that lens.

Check out the lens measurements on dxomark.

You'll want to click sharpness instead of the dxomark score


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## Big Mike (Dec 8, 2014)

What do you mean by 'critical aperture'?


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## Bobby Ironsights (Dec 8, 2014)

Big Mike said:


> What do you mean by 'critical aperture'?


The critical aperture is the aperture at which a lens is sharpest and freest of aberrations.

Basically it's your go-to aperture if light is sufficient. It used to be more important in the old days, lenses are getting pretty good throughout their range these days, but there's still a noticeable difference in side by side comparisons.

Surprisingly, f/8 has been traditionally the sharpest aperture since about the late thirties, leading to the old chestnut, "_f/8 and be there_" In ye old wooden camera days the sharpest aperture might be a pinprick f/32 or f/64.

And right after I wrote that, I noticed you're a professional photographer and teacher. I'll stop now.


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## Big Mike (Dec 9, 2014)

Bobby Ironsights said:


> Big Mike said:
> 
> 
> > What do you mean by 'critical aperture'?
> ...



That's what I thought you meant, I've just never heard it called 'critical aperture' before.  Which may explain why you've had a hard time finding results on Google etc.  

I usually start out assuming that most lenses for the APS-C or 35mm format will be sharpest at or around F8.  I do often mention 'F8 and be there' to my students when mentioning this.  I also teach that many lenses will show a very noticeable difference once stopped down about one stop for their max aperture.  I then go on to show that image quality starts to drop off (due to diffraction), around F11 and smaller.  I use this as one reason that a typical prime lens is better than a typical kit lens.  With a maximum aperture of F5.6, stopping down one stop brings you to F8 and then you might start loosing quality as you get smaller.  So F8 really is the rather small sweet spot.  
A prime lens, on the other hand, with a max aperture of F1.4, is starting it's sweet spot at F2, is really great by F2.8 and continues that up to F11 or F16.  So it has a much larger sweet spot in terms of image quality due to aperture choice.  

Of course, you could take a bunch of test shots with your lens and see for yourself.  Or you could review your entire catalog of images and filter by lens and aperture and see how they compare in 'real world' scenarios.


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