Constant aperture?

[Elmerviking]

I have a Fuji xt30 with the xf 18-55 kit lens. The lens has a variable aperture, but if you choose f-stop smaller than 4.5, the aperture is constant throughout all focal lengths! How come? Does the aperture change while zooming even if it shows the same f-stop?

 

[Greybeard]

Not quite sure what you mean.

The maximum shutter changes based on the focal length so if you set a manual aperture of f/2.8 at 18mm, say - then aperture would change as you zoom out.

Any manual aperture smaller than f4 will remain the same through the zoom range unless you change it.

[Elmerviking]

3 hours ago, Greybeard said:

Not quite sure what you mean.

The maximum shutter changes based on the focal length so if you set a manual aperture of f/2.8 at 18mm, say - then aperture would change as you zoom out.

Any manual aperture smaller than f4 will remain the same through the zoom range unless you change it.

Exactly....isn’t it strange that the aperture (the opening of the lens) remains the same when you zoom in.? Shouldn’t the aperture change the same way it does at f 2.8 at 18 mm  when you zoom?

[Greybeard]

49 minutes ago, Elmerviking said:

Exactly....isn’t it strange that the aperture (the opening of the lens) remains the same when you zoom in.? Shouldn’t the aperture change the same way it does at f 2.8 at 18 mm  when you zoom?

I don't think so - the variable aperture is just a physical limit of the lens when wide open.

If you set a specific manual aperture you'd want it to stay the same (if possible) across the zoom range.

[Herco]

Aperture is the ratio between the focal length of the lens and the width of the iris (or aperture). So f2 at 18mm means an opening of 9mm. To reduce the size (and cost) of a lens, zoom lenses with longer focal lengths often have variable aperture. Your lens has a max opening of 6.4mm at 18mm (f2.8). To have the same aperture at 55mm the max. opening should be 19.6mm (55 / 2.8). That would result in a wider lens with larger glass elements and subsequently more costs and design issues. Hence the f4 at 55mm. It keeps the lens compact and cost efficient.

The downside of variable aperture is that the exposure changes while zooming in or out, which will need to be compensated by adjusting shutter speed or ISO. Professionals usually don't like that: e.g. a sports photographer being confronted with a longer shutter speed just because of zooming in. Another downside is that depth of field changes when zooming in or out which can lead to subjects being out of focus. Your camera will 'fix' that obviously in AF mode, but for most professionals it may ruin the effect they were looking for. That's why constant aperture zoom lenses are often referred to as 'professional grade lenses' or the 'holy trinity of f2.8 zoom lenses'...

[Elmerviking]

Well... f 8 at 18 mm means that the aperture is 18/8=2.25 mm. At 55 mm f8 is 55/8=6.87 mm!

So..how is this done? Does the aperture change when you zoom? 
Does it actually open (mechanically) from 2.25 mm to 6.87 mm?

 

[Herco]

In fact it does. Whether it is mechanically or electronically I can’t tell, but yeah the ‘pupil’ widens.

[Elmerviking]

8 minutes ago, Herco said:

In fact it does. Whether it is mechanically or electronically I can’t tell, but yeah the ‘pupil’ widens.

Thank you Herco! That is what I think also. I just wonder how it works when you use a manual lens, like Nikkor 80-200 f 4.5.

The aperture (“pupil”) obviously is constant when you zoom. I have to check that!

[jerryy]

Usually the constant aperture lenses have different kinds of elements and lens groups than do the variable aperture lenses, it is a different design idea. One other side benefit that may matter, is that the lenses often do not extend or retract while zooming.

But try not to get too fixated on the idea, there are plenty of high quality variable aperture lenses out there. Canon makes some “L” grade red banded variable aperture lenses as does Sigma and Tamron and ... Lots of those Leica mount Carl Zeiss lenses, which are the mutterings of legend, are variable aperture builds.

Edited September 1, 2020 by jerryy