Apature and sharpness

SIB

SIB

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Steven
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I'm not sure if this belongs in here or in basics so please feel free to move if more appropriate there.

Anywhoo.....I've been looking at lens reviews recently and I'm intrigued at reading stuff like "Images are soft at F2.8 but stop down to F4 or F8 and it's pin sharp". This is not a direct quote but the kind of things i'm reading.

So why have I spent all this money getting an F2.8 apature lens when it's going to produce soft images unless i stop it down to F4 (for example)? Taking this through logically, does a lens whose maximum apature is F4 produce soft images at F4 and these not get sharp until F8? Or will it be sharp at F4?

I appreciate not all lenses are created equally but is it becuase you're at the maximum apature of the lens that makes the image soft or is it that an F2.8 zoom lens will always produce softer images at this apature unless stopped down?

Hope this makes sense? It does in my head!!
 
A good f/2.8 lens is sharp at f/2.8

Several lens I can think of are sharp at f/2.8 - Tamron 17-50 f/2.8, Tamron 28-75 f/2.8, Nikkor 24 / 28-70 f/2.8, Carl Zeiss 24-70 f/2.8, Nikkor 14-24 f/2.8, Nikkor 300mm f/2.8, Nikkor 70-200 f/2.8 VR etc etc - I could probably list a dozen more.

A decent f/2.8 lens will be sharp at f/2.8
 
Morning Steven :)

Most lenses have what is known as a sweet spot. The range of apertures that give maximum image quality. They are generally in the middle of the range. The reason for this is that lenses are made up of a number of elements i.e. individual pieces of glass. And they move too. That's what zooms do, they move and the elements inside move back and forwards. So if you have 6 elements and several of them are moving about then there is a very slight drop off at each end of the aperture scale. Usually it's not THAT noticeable but we seem to have bred a colony of pixel peepers for whom nothing but absolute optimal sharpness is good enough.

Personally I shoot wide open a lot of the time due to shooting in low light and wanting shallow DOF so I take it all with a HUGE pinch of salt and get on with taking pictures. And I've yet to have a client complain because an image was a little soft at the very outer edges ;)
 
A very good question actually. And I think I get where you are coming from.

The way I see it,
a zoom lens will need to be stopped down to get the best from it.
So if you have a 2.8 lens, as you say stop it down to F4 and it gets sharper.
If you have a F5.6 lens, stop it down to F8 for the sharpness to kick in.

F4 is considerably faster than F8, and will make the world of difference in low light!

Take a look at THIS thread for as it is a bit relevant to yours.

Just seen Andy's post above.
Like he says, if you pay the ££££'s for a high quality lens, it will still be sharp wide open (such as the Canon 70-200mm F4 L)
 
You can compare various lenses here:

http://www.the-digital-picture.com/...IComp=0&APIComp=0&LensComp=400&CameraComp=474

You will probably be surprised at how bad some usually well regarded lenses are especially in the corners, but remember in real life you aren't shooting charts and the best real life way to see how a lens performs is a large picture at 100% crop.

But these charts give you a very good start.

And obviously the camera you are using will also make a difference.
 
Many thanks for the prompt and detailed replies, one of the reasons i like this place so much is people taking the time to help :).

So as a general rule of thumb, if i'm not needing a really quick shutter speed or there's plenty of light i would get a slightly sharper picture stopping my Canon 70-200 F2.8 L IS down to say F4 or F5.6 but probably not that i would really notice "real world" (which i have to say i haven't which is why i was asking in the first place!!)?
 
SIB said:
So as a general rule of thumb, if i'm not needing a really quick shutter speed or there's plenty of light i would get a slightly sharper picture stopping my Canon 70-200 F2.8 L IS down to say F4 or F5.6 but probably not that i would really notice "real world" (which i have to say i haven't which is why i was asking in the first place!!)?
Click to expand...

Slightly sharper maybe but to be quite honest, going by the charts, I would say that you would barely notice the difference - bearing in mind that you would have to be much more careful with focussing because of the reduced DOF at 2.8 - which is much more likely to give you an OOF image then the quality of the glass which is recognised by almost everyone as superb.
 
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