Slightly confused about lenses with continous f2.8 aperture

ziggy©

ziggy©

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Ok, i am looking at the 17-55mm f/2.8 lens and im a bit curious of the continous aperture of f/2.8.

My understanding of this is that the aperture of the lens will always be f/2.8 no matter what zoom range you are in.

Is this not a disadvantage? What if i want to take a landscape shot and want maximum DOF? If every shot will be at f/2.8 wouldnt everything i take have low dof or have i misunderstood something?
 
you can still stop it down to f4 f8 f16 etcetc, f2.8 is just the maximum (lowest number) aperture, and it can do that at both 17mm and 55mm, unlike kit lenses.
 
F/2.8 is the maximum aperture size that you can have at any given focal length. The lens can still be stopped down from that.
 
you've misunderstood. All constant f2.8 means is that the maximum aperture will remain at 2.8 regardless of focal length and won't change as you zoom. You can stop down to smaller apertures
 
Half of what you think is correct.

The max aperture of the lens will be f2.8 regardless of focal length but you can adjust the aperture down to eg; f5.6 at any time or focal length.
 
Someone jump in here if im wrong but....
As far as im aware your aperture can be 2.8 at 17mm all the way upto 55mm where as most lenses will be say 2.8 at one end 3.5 at the other.
You always have the option of setting higher for more DoF... you just have the option of getting more light in across the range.
 
Yep you misunderstand Ziggy. Don't worry.

The f2.8 constant just means that is is not variable. Now a variable aperture lens is one where it is say f4-5.6. That means as you zoom the aperture changes and you have no choice but to go with it it can be a MAXIMUM of f4 at 70mm and Max of f5.6 at 200mm (if it's a 70-200)

Now a fixed focal length can be any aperture YOU CHOOSE between f2.8 (It's widest aperture) and whatever is it's smallest aperture f16....f22.

So it's your choice where you set it.

Panic over :)
 
It would mean that the lens has the advantage of being used at f2.8 throughout the range of the zoom, it up to you what you select the aperture to be.

When using aperture priority mode, and chosing f2.8 then aperture would be unchanged in the range, and the shutter speed would change where needed to get the exposure correct.
 
Oh i get it now.

I thought the term "Continous" or "Constant" meant that the aperture would be fixed at f/2.8 at all zoom ranges
In that case its not a problem then.. :)

Thanks
 
ziggy©;2521402 said:
Oh i get it now.

I thought the term "Continous" or "Constant" meant that the aperture would be fixed at f/2.8 at all zoom ranges
In that case its not a problem then.. :)

Thanks
Click to expand...

We all have to learn somewhere.

Chris
 
One of advantages of the continuous maximum aperture is that the viewfinder brightness and AF performance will be the same right across the zoom range of the lens.

Remember that the lens doesn't stop down to the selected F stop until the shot is taken (or the DOF preview button is pressed).
 
what it means is that 2.8 is the maximum aperture you can have throughout the zoom range. you can still have all the way up to like f32 etc. the advantage is that you can use it more in low light
 
Sorry if this sounds rude, but its a lot of money to splash if you haven't got to grips with the basics of exposure

Maybe better to sort out what you want and then weigh in on the most suitable lens.

Could be most of your shots are at a longer focal length or perhaps your thing would be wide angle orientated

As I said its not me trying to be a clever clogs, just make sure its right for you before spending big (unless you are rich obviously)
 
Hi Ziggy,
I bought the very same lens a couple of weeks ago from this forum second hand. Its now on my 7D and its a wonderful lens - very sharp and the focus is deadly. I read all the reviews etc, and they are correct.
The 2.8 really helps with low light and the IS is superb - seems better than my 17-85 IS. I shot a perfectly acceptable 1.3 sec exposure as a test and was rather surprised at the quality. The constant 2.8 across all zoom give you much more choice.
If you have the cash - go for it.
Ged
 
That's the reason they are so expensive and the longer the focal length, the bigger the end of the lens has to be to keep that constant aperture when you need it.
mega ££££'s for anything f4 over 500mm
 
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