Forgot Password
Pentax Camera Forums Home
 

Reply
Show Printable Version Search this Thread
03-12-2014, 02:44 PM   #1
Pentaxian




Join Date: Jan 2012
Location: Slovenia
Photos: Gallery
Posts: 2,180
How to explain this

I have a K-5 II and an 18-135. Recently, I've been fooling around in the video mode, and came across this:
Set Aperture Control to Fixed, set lens to 18mm and f/5.6 (therefore it's closed down a bit) and start recording.

Zoom to 135mm. The indicated aperture is f/5.6, but as you zoom, the aperture blades don't actually move, and the aperture stays closed a bit. That means, of course, that the lens is not wide open at 135mm, and is not actually at f/5.6.

Now, how the flip do I explain this in simpler terms? I've been trying to get this across to another photographer (who is experienced and knows the concept of aperture), but have had 0 success, even though other people understood it from my explanation.

03-12-2014, 02:54 PM   #2
Administrator
Site Webmaster
Adam's Avatar

Join Date: Sep 2006
Location: Arizona
Photos: Gallery | Albums
Posts: 43,226
A lot of lenses are designed like this, and they do it to keep the aperture consistent. It's perfectly normal.

Adam
PentaxForums.com Webmaster (Site Usage Guide | Site Help | My Photography)



PentaxForums.com's high server and development costs are user-supported. You can help cover those costs by donating. Or, buy your photo gear from our affiliates, Adorama, B&H Photo, or Topaz Labs, and get FREE Marketplace access - click here to see how! Trusted Pentax retailers:

03-12-2014, 02:54 PM   #3
Pentaxian
fgaudet's Avatar

Join Date: Oct 2013
Location: Ontario
Posts: 726
Might be a stupid question but how can you see if the aperture blades are moving or not?
03-12-2014, 03:09 PM   #4
Pentaxian




Join Date: Jan 2012
Location: Slovenia
Photos: Gallery
Posts: 2,180
Original Poster
QuoteOriginally posted by fgaudet Quote
Might be a stupid question but how can you see if the aperture blades are moving or not?
You hear it When not filming, the aperture opens up as you zoom (with a soft click + stutter on LCD image). When recording, nothing happens.

@Adam So the lens is designed to be at f/9 when you set it to 5.6? I'm talking about the 18-135 3.5-5.6 WR, not a constant aperture lens. If you set it to 5.6 at 18mm (closed down 1.5 stops) , it should open up as you zoom to 135mm (where 5.6 is wide open).

03-12-2014, 03:14 PM   #5
Pentaxian
JinDesu's Avatar

Join Date: Jun 2011
Location: New York City
Photos: Gallery
Posts: 5,626
I think the reasoning is this:

The aperture sets the opening for the light to pass through - and is a certain size.
A variable aperture zoom lens changes the opening for the light to pass through as well, based on positioning of the elements.

When you set the lens at F5.6 and the aperture blades close to that size, and then you proceed to zoom - the zooming elements are reducing their respective entry hole to the same size as the hole generated by the aperture blades. Basically, as you zoom in - you are restricting the opening by the lens elements, but as long as the opening is equal to or larger than the opening set by the aperture blades, your aperture reading is defined by the aperture blade settings. Whereas if you have your aperture opened to F3.5 and zoom in, regardless of the fact that your aperture blades create a larger opening, the lens elements restrict the light themselves.

Does this make sense?
03-12-2014, 03:16 PM   #6
Site Supporter




Join Date: Mar 2012
Location: Perrineville, NJ
Posts: 1,365
Let's say you were zooming from 67.5 mm to 135 mm, so that the focal length was being doubled.

In that case, the aperture as viewed from the front of the lens should appear to grow in diameter by a factor of 1.41, such that the area doubled. This is as viewed from the front of the lens: Since the front element moves relative to the aperture blades -- like the effect you get when you hover a magnifying glass up and down over an object -- the lens blades do not necessarily need to slide open more to maintain f/5.6.

It's not really about the aperture, it's about the entrance pupil:
Entrance pupil - Wikipedia, the free encyclopedia
03-12-2014, 03:20 PM   #7
Pentaxian
fgaudet's Avatar

Join Date: Oct 2013
Location: Ontario
Posts: 726
As you zoom in, is your exposure getting darker or stays pretty much the same?
03-12-2014, 03:40 PM   #8
Administrator
Site Webmaster
Adam's Avatar

Join Date: Sep 2006
Location: Arizona
Photos: Gallery | Albums
Posts: 43,226
QuoteOriginally posted by Giklab Quote
@Adam So the lens is designed to be at f/9 when you set it to 5.6? I'm talking about the 18-135 3.5-5.6 WR, not a constant aperture lens. If you set it to 5.6 at 18mm (closed down 1.5 stops) , it should open up as you zoom to 135mm (where 5.6 is wide open).
Not necessarily; if the lens is actually slightly faster at 135mm, then the aperture blades would need to stay just shy of wide-open in order to keep the f-stop at the right value. I don't have my 18-135mm in front of me at the moment to check, but one of the Nikon zoom lenses I recently used does the exact same thing at the long end.

With that said, as others are asking, if your exposure isn't consistent, then there might be something wrong with the lens. But otherwise what you are discovering is normal.


Adam
PentaxForums.com Webmaster (Site Usage Guide | Site Help | My Photography)



PentaxForums.com's high server and development costs are user-supported. You can help cover those costs by donating. Or, buy your photo gear from our affiliates, Adorama, B&H Photo, or Topaz Labs, and get FREE Marketplace access - click here to see how! Trusted Pentax retailers:

03-12-2014, 11:50 PM   #9
Pentaxian




Join Date: Jan 2012
Location: Slovenia
Photos: Gallery
Posts: 2,180
Original Poster
QuoteOriginally posted by fgaudet Quote
As you zoom in, is your exposure getting darker or stays pretty much the same?
NAILED IT! Thanks a lot. The exposure gets darker, but only while actually recording, if I'm just using the live view, the aperture opens up as I zoom.
@Adam Nah, this lens is something like f/4.5-7. No need to restrict it more . [[I'm just saying that since the video mode locks the aperture while recording, it stays closed when it should open as you zoom.]]
@Tanzer and @Jindesu see brackets above. You two are both correct, it just doesn't apply here (it's a software limitation/bug of the camera).

Edit: Semi-related, on our forum, the discussion has turned to how the K-5 would have stuttered just as the K-3 does if we could control aperture during video and how Nikon doesn't allow aperture control except in high end bodies, because of 'extra mechanisms' needed for correct operation of the mechanical aperture lever.

Last edited by Giklab; 03-13-2014 at 12:02 AM.
03-13-2014, 06:01 AM   #10
Site Supporter




Join Date: Mar 2012
Location: Perrineville, NJ
Posts: 1,365
Interesting finding. Thanks for clarifying and sorry for the confusion.

Do you have a link to the other thread, regarding the K-3 aperture stuttering?
03-13-2014, 01:13 PM   #11
Pentaxian




Join Date: Jan 2012
Location: Slovenia
Photos: Gallery
Posts: 2,180
Original Poster
QuoteOriginally posted by Tanzer Quote
Do you have a link to the other thread, regarding the K-3 aperture stuttering?
It's not on this forum (access might be denied), it's a part of our general K-3 thread and it's in Slovene, so I'll just sum up. A guy bought a K-3 and noticed that Live View and the video itself stuttered if the aperture was changed. When I mentioned the OP issue in another thread the similarity of the behaviour of the K-5 II and K-3 stood out, and someone mentioned that maybe the K-5's video mode is limited because of this stuttering (possibly the effect of the mechanical aperture connection).
03-13-2014, 02:17 PM   #12
Site Supporter




Join Date: Mar 2012
Location: Perrineville, NJ
Posts: 1,365
Great summary, thanks for sharing this.
03-13-2014, 02:18 PM   #13
Veteran Member




Join Date: May 2012
Posts: 698
AFAIK all Pentax DSLR's before the K-3 cannot move the aperture blades while the shutter is open - a limitation imposed by the mechanical design.

So it cannot adjust the aperture diaphragm to maintain a constant F number. I think it should have been possible though for the actual aperture to be properly indicated as the lens is zoomed in and out.

I've tried the same with my K-30 and it seems to behave exactly as you described in the opening post as far as not moving the aperture blades is concerned. But it does change the F number shown on the LCD accordingly with the change in zoom to indicate the real aperture of the lens. So it seems to me just a minor software error in the K-5ii which they could easily fix if someone notified them about it.

The K-3 behaves correctly, maintaining constant real aperture as the lens is zoomed by changing the diaphragm opening in sync with the zoom change. The exception to that is of course when the aperture is at its end stop and cannot open/close further, in which case the displayed F number then changes accordingly.

The one thing which is not 'perfect' is that the blades open/close in steps rather than smoothly. I think the steps are about 1/3 of a stop but not sure.

QuoteOriginally posted by fgaudet Quote
Might be a stupid question but how can you see if the aperture blades are moving or not?
Easy answer - just look through the lens, possibly shining a light into it - it is very easy to see the aperture blades and whether or not they move.

On the K-3 you can even hear them move.
03-14-2014, 09:50 AM   #14
Pentaxian




Join Date: Jan 2012
Location: Slovenia
Photos: Gallery
Posts: 2,180
Original Poster
Yeah, we should report this, but to who? I've sent an email to Pentax UK and another Pentax Customer Service address about some other bug, and while the UK rep had responded, nothing has been done yet to correct the issue (interestingly, that bug is also not present in the K-3).
03-21-2014, 07:38 AM   #15
Veteran Member




Join Date: Jul 2010
Location: Santa Cruz, CA
Photos: Albums
Posts: 377
I know I'm a bit late, but how I would explain this is by saying that increasing the focal length equally stops down the aperture.
Reply

Bookmarks
  • Submit Thread to Facebook Facebook
  • Submit Thread to Twitter Twitter
  • Submit Thread to Digg Digg
Tags - Make this thread easier to find by adding keywords to it!
135mm, aperture, bit, camera, dslr, f/5.6, k-5, k-5 ii, k-5 iis, k5, lens, pentax k-5
Thread Tools Search this Thread
Search this Thread:

Advanced Search


Similar Threads
Thread Thread Starter Forum Replies Last Post
Nature How am I going to explain this. AussieTrev Post Your Photos! 10 11-20-2013 01:42 AM
How to explain thing to people who don't speak photography. ASheffield Photographic Industry and Professionals 15 10-06-2013 09:59 AM
Can someone explain how to use Registax 6 with jpegs? ChopperCharles Pentax Q 6 12-31-2012 10:09 AM
Please explain how to use the K5 AF improvements. Deiberson Pentax K-5 34 05-12-2011 10:20 AM
Please explain this to me... Raptorman Pentax SLR Lens Discussion 3 12-02-2009 02:45 PM



All times are GMT -7. The time now is 05:18 AM. | See also: NikonForums.com, CanonForums.com part of our network of photo forums!
  • Red (Default)
  • Green
  • Gray
  • Dark
  • Dark Yellow
  • Dark Blue
  • Old Red
  • Old Green
  • Old Gray
  • Dial-Up Style
Hello! It's great to see you back on the forum! Have you considered joining the community?
register
Creating a FREE ACCOUNT takes under a minute, removes ads, and lets you post! [Dismiss]
Top