focal length & aperture

[joel28]

Why does the aperture/f-stop change when focal length is changed?

 

[dxqcanada2]

Do you mean in most Zoom lenses ?
If so ... it is based on the design of the lens, as the lens elements move and increase the focal length the effective aperture becomes smaller ... there are many zoom lenses with a fixed max aperture throughout the range by designing the lens to compensate for this.
The lens design is more complicated, therefore more costly ... so you do not see the cheap zoom lenses with a fixed (not variable) max aperture.

 

[joel28]

got it, Thanks!

 

[Tuffythepug]

Change of focal length means that the rear element of the lens is moving closer or further away from the film plane. Further away means less light reaches the film plane at any given shutter speed. Closer means that more light reaches the film plane at any given shutter speed. So the aperture has to adjust wider or smaller accordingly to give proper exposure. You could change the shutter speed and retain the aperture setting if you are trying to achieve greater or lesser depth of field.

oh, I see that dxqcanada has already addressed this. carry on.

 

[joel28]

Now it makes even more sense! Thanks!

 

[KmH]

Lenses that automatically change the maximum lens aperture when the focus length is increased are known as 'variable aperture' zoom lenses.
The other, more expensive type of zoom lens that does not change the maximum lens aperture when the focal length of the lens is increased are known as 'constant aperture' zoom lenses. Constant aperture zoom lenses also usually cost more because they have higher quality optics, and a larger diameter (more expensive glass) to accomodate the maximum aperture at the long end of the lenses focal length adjustment range.

El cheapo, 'fixed aperture' zoom lenses also don't change lens aperture when zoomed, because they only have 1 non-adjustable aperture. Many confuse fixed aperture with constant aperture

It's interesting to look at the math.

Assuming an 18-55 mm f/3.5 to f/5.6 zoom lens:
At 18 mm and f/3.5 the diameter of the lens aperture is - 18 mm (the f in f/3.5) divided by 3.5 = 5.14 mm which is considered a fairly large lens aperture.
At 55 mm and f/5.6 the diameter of the lens aperture is - 55 mm (the f in f/3.5) divided by 5.6 = 9.82 mm which is considered a smaller lens aperture, even though it's actually has a wider diameter than 18 mm @ f/3.5.

Assuming a constant aperture 24-70 mm f/2.8 lens:
Set to 24 mm @ f/2.8 we get a lens aperture diameter of 8.57 mm and at 70 mm @ f/2.8 we get a lens aperture diameter of 25 mm.