Lens aperture query..

[JohnN]

Hi,

Despite playing around with my camera for way too long I've a basic question for you guys.

If you have a lens that will do f1.8, but you take an image at f4, would the image be the same if taken by a f4 lens at f4?

Assuming of course that the lenses perform as well as each other?

Ta.

 

[Davec223]

Yes and no, the lens that can do f1.8 will be sharper generally than the f4 lens shot wide open. If you are using the same focal length on each lens the dof will be the same at f4.

 

[dinners]

It's not an exact science but in theory an f1.8 lens can be sharper at f4 than an f4 as an f1.8 is 'stopped down' from it's max aperture.

Also an f1.8 lens may focus faster (when used at f4) than a f4 lens as lenses focus at their max aperture.

Apologies if that makes no sense.

 

[JohnN]

Okay, so using a lens example my Sigma 17-50 2.8 at f4 should in theory be sharper than the 24-105L f4?

And should focus quicker... actually that bits very interesting, again all things being equal the 2.8 should focus quicker than an f4 irrespective of f number?

 

[Scarecrow]

The speed of focus will also depend on the focusing motor, as some types are quicker than others, so it's not a given that a 2.8 lens will focus faster than a 4.0, unless they are using the same type of motor. Even then there may be variations between different manufacturers.

 

[dinners]

Okay, so using a lens example my Sigma 17-50 2.8 at f4 should in theory be sharper than the 24-105L f4?

And should focus quicker... actually that bits very interesting, again all things being equal the 2.8 should focus quicker than an f4 irrespective of f number?

I worded it carefully (aperture being the qustion) however some lenses will obviously beat others hands down regardless.

All other things being equal - the theory stands.

 

[draiman]

Yeah. Focus speed has nothing to do with aperture. That's more to do with focus accuracy as the camera uses the lens wide open to focus. Take the 85Lii which is f1.2 but the focus from 0 - infinity take a good deal longer than pretty much most lenses.

 

[Nawty]

That's more to do with focus accuracy as the camera uses the lens wide open to focus.

I thought most of them were actually limited to f/5.6? Pro bodies being the general exceptions.

Basically, all other things being equal (which they aren't) the same focal length and the same aperture will give the same results.

Things that make a difference are lens quality and aperture blade design (which affects the bokeh). Whether it is wide open will make a difference but which is better will depend on the lens quality.

 

[DizMatt]

Okay, so using a lens example my Sigma 17-50 2.8 at f4 should in theory be sharper than the 24-105L f4?

And should focus quicker... actually that bits very interesting, again all things being equal the 2.8 should focus quicker than an f4 irrespective of f number?

a sensible question and lots of good answers here
I would also add that you wouldn't put money on the sigma in that equation being sharper than the Canon L glass as that particular lens is rather good.
quality of glass is a factor of course.

there is also a curve ball here, not a straight line.
for example, if a lens runs from f4 to f22, there might be a sweet spot at f8 or f9 which is the sharpest it can be.
the reason for compromising that and running at f4 would be to raise the shutter speed so it's faster or if you wanted a specific depth of field (DOF) effect.