Forgot Password
Pentax Camera Forums Home
 

Reply
Show Printable Version Search this Thread
03-04-2010, 09:05 PM   #1
Pentaxian
Swift1's Avatar

Join Date: Sep 2009
Location: Oregon
Photos: Gallery | Albums
Posts: 3,807
Exposure Problems with Variable Aperture Lenses.

I have 2 manual focus 70-210 zoom lenses. One is a Vivitar S1 70-210 2.8-4 and the other is a Tokina SD 70-210 4-5.6. Both lenses have an "A" setting. Using either lens at the A position and at max aperture at the long end (f/4 for the Vivitar and f/5.6 for the Tokina) on my K10D, I get correct exposures at 70mm but get 1+ stop overexposure at 210mm. Am I missing something? I would assume that f/4@70mm should have the same EV as f/4@210mm assuming the lighting hasn't changed.
I also tried both lenses on an ME that I know has accurate metering. Using a white wall for metering, I set the Vivitar to 70mm and f/4, metered then zoomed to 210mm (still at f/4) and got a shutter speed that was 2x what it was @ 70mm.

Anyone else experience this?

Any help is much appreciated,

Swift1

03-04-2010, 11:52 PM   #2
Pentaxian
Steinback's Avatar

Join Date: Jan 2008
Location: GTA, ON, Canada
Photos: Gallery | Albums
Posts: 1,672
Overexposure at the long end would make sense. The A contacts on the Series 1 are probably telling the camera body that it is using open aperture metering at a constant f/2.8. In reality at the long end the camera is seeing f/4 so it sees the scene 1 stop darker than it should and compensates by letting in 1 stop more light. You can probably use the exposure compensation feature to offset the change in f-stop as you zoom.
03-05-2010, 03:45 PM   #3
Pentaxian
Lowell Goudge's Avatar

Join Date: Jan 2007
Location: Toronto
Photos: Gallery | Albums
Posts: 15,312
welcome to the wacky world of metering issues.

can you shed some light on what camera you used?

There is an issue which is most noticible with K10D and K20D cameras where teh focusing screen has an impact on metering, and the camera makes errors with MF lenses. the point here is, when the lens is wide open, does the aperture registered by the camera change or not?, i.e. on the series 1 does it change from F2.8 to F4 when you zoom?

In the old days, the metering woudl take care of this, but with today's metering, it needs to know the true aperture to meter correctly

see the attached graph. This is how the metering reacts on several different cameras as a function of aperture (manual aperture lenses)

you can see a big shift when you change from F2.8 to F4. UNless the lens has the correct electronics to change the aperture registered when you zoom, you could have problems

03-06-2010, 12:24 PM   #4
Pentaxian
Swift1's Avatar

Join Date: Sep 2009
Location: Oregon
Photos: Gallery | Albums
Posts: 3,807
Original Poster
Thanks Lowell and Steinback.

Lowell, I am using a K10D with a Katzeye screen.
After playing around with the Series 1 a bit more here is what I have worked out.
With the lens set the "A" position,
I get correct exposure @70mm and 210mm if I @f/2.8 for both focal lengths. The camera is thinking the lens is at 2.8@210mm when really it is f/4.

I get correct exposures at tall other apertures if I set EV comp to 0 @ 70mm and -1 @210mm.

Is this normally the case when using manual focus variable aperture zooms?

Sorry if this is a silly question,

Swift1

03-07-2010, 07:21 PM   #5
Pentaxian
Just1MoreDave's Avatar

Join Date: Feb 2007
Location: Aurora, CO
Photos: Gallery | Albums
Posts: 4,814
The KA mount does not really support variable aperture zooms. As you have seen, you can set f2.8 at 210mm on the Vivitar. Auto-focus lenses can send more information to the camera so it can see variable aperture lenses for what they are.

I think you have a combination of this problem, metering inconsistencies and maybe something else - lens issue, maybe. I can't come up with a theory that covers every symptom you describe. The most unusual problem is with the ME, which should be the simplest system.
03-08-2010, 04:59 AM   #6
Pentaxian
Lowell Goudge's Avatar

Join Date: Jan 2007
Location: Toronto
Photos: Gallery | Albums
Posts: 15,312
QuoteOriginally posted by Swift1 Quote
Thanks Lowell and Steinback.

Lowell, I am using a K10D with a Katzeye screen.
After playing around with the Series 1 a bit more here is what I have worked out.
With the lens set the "A" position,
I get correct exposure @70mm and 210mm if I @f/2.8 for both focal lengths. The camera is thinking the lens is at 2.8@210mm when really it is f/4.

I get correct exposures at tall other apertures if I set EV comp to 0 @ 70mm and -1 @210mm.

Is this normally the case when using manual focus variable aperture zooms?

Sorry if this is a silly question,

Swift1
Although I don't know the katz eye performance, (no one has done a set of shots from wide open to full stop down with a manual lens for me) the performance you describe is exactly the issue with the base focusing screen.

if you look at the curve for the K10D with its own screen, you will see it has a greyscale value of about 85 at F2.8 and 135 at F4. a change of 45 greyscale in this range of exposure is 1 stop. therefore if you are at 70mm, everything is OK because pentax know the performance of the focusing screen and correct the meter for it, but whe you are at 210mm the lens is at a real aperture of F4 but the camera thinks it is at F2.8 and applies the same correction, causing a shift of 50 greyscale or 1 stop (over exposed)

this is exactly how my K10D works when I put a 1.4X TC om ny sigma 70-200 F2.8 as well.it requires about 1 stop under expose correction, and when I go to a 2x C I need to add -1.3 stops
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!
aperture, f/4, k-mount, lenses, pentax lens, slr lens, tokina, vivitar
Thread Tools Search this Thread
Search this Thread:

Advanced Search


Similar Threads
Thread Thread Starter Forum Replies Last Post
About zoom lenses with variable aperture Vranx Pentax SLR Lens Discussion 15 07-24-2010 07:43 PM
Constant Aperture vs Variable Aperture Netsoft Pentax SLR Lens Discussion 13 01-11-2010 07:29 AM
Infinitely Variable Aperture Super-Takumar? straightshooter Pentax SLR Lens Discussion 15 09-12-2009 01:27 PM
For Sale - Sold: Gossen Pro Exposure Meter + Variable Angle Attachment Bob Tuttle Sold Items 12 11-29-2008 05:42 AM
Variable vs. fixed aperture zooms: am I missing something here? Finn Pentax SLR Lens Discussion 11 05-15-2007 05:05 AM



All times are GMT -7. The time now is 01:52 PM. | See also: NikonForums.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