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Z Mount Lens


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6 replies to this topic

#1
Johnb

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I was just looking at a Z mount lens - 24-70mm f2.8 S

 

The first line of the spec says it is constant f2.8 and has a rounded 9 blade aperture.

 

Question,

If it is a constant f2.8 aperture, why has it got 9 rounded blades ??

 

This suggests to me that the aperture is adjustable.

 

If it is adjustable, can someone explain what is meant by constant ?

 

Thankyou.  

 

 



#2
Merco_61

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Constant, as in the max aperture not changing with the focal length.  



#3
Jerry_

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John,

If you consider that
« In optics, the f-number of an optical system such as a camera lens is the ratio of the system's focal length to the diameter of the entrance pupil ("clear aperture"). It is also known as the focal ratio, f-ratio, or f-stop, and is very important in photography »
(source: Wikipedia entry on f-number)
You will understand why many *zoom* lenses have variable *max* apertures.

The advantage of having a constant *max* aperture (as Peter already mentioned) is that the camera uses for autofocussing automatically the largest aperture that the lens allows, in order to let in most light while also having a reduced depth of field, ensuring that the focussing distance can be measured precisely. Therefore having a constant max aperture allows you to have at any focal length the same quality of autofocusing, even in less ideal light conditions.

The indication of « constant f2.8 » only applying to the *max* aperture, it does not mean that the lens will not offer you other f-stops. Which brings us to the blades which open/close for the choosen aperture.

As regards the (number of) blades and their shapes: imagine having a lens with 3 blades that are straight - your opening would have the shape of a triangle. Now go on with 4 and more blades and the opening will become closer to look like a circle, which would be the ideal shape, as it has no corners where light would be broken differently. However the more blades you have, the more complex (and expensive) your lens gets, so there is a compromise in the number of blades, also due to what is mechanically feasable.
The second aspect is the shape of the blades which will help to create here smoother pictures for the bokeh, due to the rounded shape.

#4
Johnb

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I think this is one of those topics which will one day just suddenly click into place. One minute I think I am getting it then it goes and I am back to square one. 

 

Anyway thanks for your explanations guys. I will keep working on it. 



#5
fallout666

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stays at F/2.8 unless you change it. if does not say something like this 4.8-6.3 or any other combo its stay at F/2.8 unless you change it. same for my F/4 i will be using this weekend. 



#6
Merco_61

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It is important to keep the terms constant and fixed aperture apart. Constant refers to the max aperture or full aperture being the same whatever the focal length is, fixed refers to the aperture in a catadioptric lens that can't be changed.

 

If you set the aperture to f/3.5 with your 18-300 @ 18 mm and zoom to 300 mm, the aperture will change to f/6.3. This is a variable aperture lens.

 

If you do the same with your 200-500, setting the aperture to f/5.6 @ 200 mm and zooming to 500 mm, the set aperture won't change as this is a constant aperture lens.

 

As you know, you can control the aperture by stopping down with both lenses.



#7
Johnb

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Thanks Peter and other guys.

 

I normally shoot in shutter priority but I am thinking that to see what you are describing in reality, I need to shoot in aperture priority. Just to observe what you are saying. That should make it clearer.