Forgot Password
Pentax Camera Forums Home
 

Reply
Show Printable Version Search this Thread
07-31-2010, 12:27 PM   #16
Inactive Account




Join Date: Jun 2010
Posts: 301
i think the moral of this story is learn your equipment, and learn it well- 3 lenses you know how to use and know where to use them will make you a better photographer than a bag full of lenses for 'every occasion'

understand the limitations of your setup!

07-31-2010, 02:22 PM   #17
Veteran Member
Jodokast96's Avatar

Join Date: Oct 2007
Location: Erial, NJ USA
Photos: Gallery | Albums
Posts: 2,132
Amen Mark. Lowell, you keep throwing that chart around, but in real world usage, I've never seen this phantom underexposure you speak of.
08-01-2010, 07:24 PM   #18
Pentaxian
Lowell Goudge's Avatar

Join Date: Jan 2007
Location: Toronto
Photos: Gallery | Albums
Posts: 15,400
QuoteOriginally posted by Marc Sabatella Quote
True, and I could see how a TC might possibly exacerbate them (although I suppose it is also conceivable that the TC would actually *lessen* the problem).
but the TC would only make the problem go away if you are way down at F22 or something like that, on a lens with F5.6 or slower maximum aperture, so you are in the slow old MF range because an AF lens wont focus when it is that slow.

Actually the difference between a TC with contacts and stop down metering with a K lens is that the TC always meters wide open so the error is constant
08-01-2010, 07:26 PM   #19
Pentaxian
Lowell Goudge's Avatar

Join Date: Jan 2007
Location: Toronto
Photos: Gallery | Albums
Posts: 15,400
QuoteOriginally posted by Jodokast96 Quote
Amen Mark. Lowell, you keep throwing that chart around, but in real world usage, I've never seen this phantom underexposure you speak of.
you need to try doing a range of shots, at all apertures and you need to measure them. it is not critical in many applications, but if you are trying to meter, especially with spot metering, then you can be really screwed, Imagine being out by 2 stops. That takes a normal skin tone int the high key range,

I notice it mostly when I add a TC, and since I have put my split image in the K10D it has resolved most of the under exposure on fast lenses, and only now gives a problem when stopped down below F8, which I rarely do. usually I shoot at F4-5.6 on my fast lenses, or wide open

08-01-2010, 08:56 PM   #20
Pentaxian
Marc Sabatella's Avatar

Join Date: Sep 2006
Location: Denver, CO
Photos: Gallery
Posts: 10,686
Oh, I totally get where Lowell is coming from with these charts in general - it's just the specifics of the interaction with a TC that I'm questioning.
08-02-2010, 05:33 AM   #21
Pentaxian
Lowell Goudge's Avatar

Join Date: Jan 2007
Location: Toronto
Photos: Gallery | Albums
Posts: 15,400
QuoteOriginally posted by Marc Sabatella Quote
Oh, I totally get where Lowell is coming from with these charts in general - it's just the specifics of the interaction with a TC that I'm questioning.
Marc

My experience is limited to 3 TCs. I have both sigma TCs (1.4 x and2 x) and the SMC-F 1.7x and use on thesigma70-200f2.8 and vivitar 85mm F1.4

The sigma TCs are not aperture correcting and on the fast sigma zoom they lead to over exposure because the camera thinks the lens is F2.8 and not F4.0 or 5.6. The amount of overexposure is directly thedifference between the reported aperture and the true aperture. With the vivitar f1.4 and the pentax tc there is no error because the pentax tc modifies the reported aperture and corrects the aperture for the magnification of the tc
08-02-2010, 08:40 AM   #22
Pentaxian
Marc Sabatella's Avatar

Join Date: Sep 2006
Location: Denver, CO
Photos: Gallery
Posts: 10,686
QuoteOriginally posted by Lowell Goudge Quote
The sigma TCs are not aperture correcting and on the fast sigma zoom they lead to over exposure because the camera thinks the lens is F2.8 and not F4.0 or 5.6.
Sorry to keep harping on this, but this is the part I don't get. Why should that matter? I am trying to really think this through, because what you say is counter to my experience and everything I've read.

So the lens thinks it's f/2.8. It also seems only half the amount of light it would at f/2.8. So wide open, it should still meter perfectly - it is basing the shutter speed on the actual amount of light. It really doesn't pay attention to the aperture when wide open, because it doesn't need that info yet. It's only when you stop down that the cares what the aperture is. And I as I said, while the camera is wrong about the actual aperture at all times, it is right about about the *ratio* between the wide open aperture and the current aperture. I simply dont see any possible way from a technical perspective the calculation could possibley be affected. As I said, f/8 is three stops slower than f/2.8, so the camera will choose a shutter speed 8 times slower than wide open when the aperture is set to f/8. That's true when not using a TC (so the actual apertures really are f/8 and f/2.8 respectively) or when using a TC (so the actual apertures really are f/4 and f/11).
08-02-2010, 09:48 AM   #23
Pentaxian
Lowell Goudge's Avatar

Join Date: Jan 2007
Location: Toronto
Photos: Gallery | Albums
Posts: 15,400
Marc

Look at the curve for the K10d between f2.8 and f4 or f5.6 with the 50mm f1.4

The k10d specifically compensates for the metering error when KA mount lenses are attached because the aperture coding tells the camera the maximum aperture and the error is accounted for in software

When the TC is attached the camera compensates for the nonlinearity caused by the focusing screen but does so incorrectly because the sigma tc feeds the lens data right through. If the metering behaved properly like the *istD or the K10 eighths *istD screen there would be no issue

The issue is specific to K10 and K20 when using fast lenses

08-02-2010, 11:54 AM   #24
Pentaxian
Marc Sabatella's Avatar

Join Date: Sep 2006
Location: Denver, CO
Photos: Gallery
Posts: 10,686
Do you have a curve to post for a KA lens? The one posted earlier was, according to the label, for a K lens - no aperture coding at all.
08-02-2010, 02:20 PM   #25
Pentaxian
Lowell Goudge's Avatar

Join Date: Jan 2007
Location: Toronto
Photos: Gallery | Albums
Posts: 15,400
The post KA lens in manual meters perfectly in manual while in the A position but exactly the same in manual when out of the A position

It all comes down to the fact that the software has the exposure curve built in andcorrects for the error as a function of maximum aperture. That's the catch, when the maximum aperture is not reported correctly then it applies the wrong correction. If you look at the manual aperture f2.8 point it tends to under expose, so the camera for an A lens at f2.8 will add exposure, now add the tc making it f4 or 5.6 but the camera still thinks it has an f2.8 attached and adds exposure thinking it will under expose leading to an over exposure
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!
f/4.0, k-mount, lens, pentax lens, slr lens, tc
Thread Tools Search this Thread
Search this Thread:

Advanced Search


Similar Threads
Thread Thread Starter Forum Replies Last Post
Effects of adapting lens? smcook99 Pentax SLR Lens Discussion 25 04-10-2010 09:09 AM
Effects of Scottish Wind brkl Monthly Photo Contests 0 04-07-2009 01:19 PM
Freezing cold and its effects on camera. sylvanite Troubleshooting and Beginner Help 10 02-17-2009 12:21 PM
playing with light effects Kevin B. Post Your Photos! 5 01-16-2009 09:38 AM
Focusing screen and effects?? OrenMc Pentax DSLR Discussion 8 12-03-2008 08:56 PM



All times are GMT -7. The time now is 10:35 PM. | 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