Jump to content
Elmerviking

Constant aperture?

Recommended Posts

Advertisement (gone after registration)

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?

 

Share this post


Link to post
Share on other sites

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.

Share this post


Link to post
Share on other sites
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?

Share this post


Link to post
Share on other sites

Advertisement (gone after registration)

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.

Share this post


Link to post
Share on other sites

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'...

Share this post


Link to post
Share on other sites

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?

 

Share this post


Link to post
Share on other sites

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

Share this post


Link to post
Share on other sites
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!

Share this post


Link to post
Share on other sites

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 by jerryy

Share this post


Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Loading...

  • Posts

    • Thanks so much! My Q button was usable the day I bought my new Xt4 then whilst I was playing with the settings I disabled it somehow and didn't know what I'd done. You just made my day since I hadn't been able to work out how to get it back. I had noticed the lock on also but hadn't looked thru the manual yet to see what it was. 
    • Alguien me puede explicar como activar la sincronizacion en 2a cortina en mi X-T2? A pesar de dejar activado el rear de 2a cortina, cuando disparo con 1/1000 me sale como si estuviera haciendo la foto en sincro con la 1a cortina . Tengo un Godox 860II tengo que activar algo ahi?
    • Hello guest! Please register or sign in to view the hidden content. X-E2 + 27f2.8  
    • Thank you for your insights. I think even without my menu it just starts "from scratch" in the menus, which is especially annoying if you change the buried custom display settings. Another minor problem is the display of the highest ISO if ISO is in auto mode in the display. I find it distracting and not of much value, instead I would like to see a real time update of the chosen iso of the current lighting situation. If I half press the shutter it seems to work this way but it doesnt seem v
    • My R2  (Fuji)transmitter no longer displays ttl as a Mode choice  on my Fuji Xt2, x100f and x100v. I only see M or -- as options when pushing the mode button. My older R2 Pro model does work in all three modes, M, ttl and --.    Problem solved.  The newer R2Mark II transmitter has a Shutter option with a choice of either Curtain or Leaf.  When Leaf is selected (appropriate for x100) then the only mode choices are M/--. If you select Curtain then the choice become M/TTL/-- .  If you select C
×
×
  • Create New...