Forgot Password
Pentax Camera Forums Home
 

Reply
Show Printable Version Search this Thread
06-07-2014, 05:44 PM   #16
Moderator
Not a Number's Avatar

Join Date: Mar 2012
Location: Venice, CA
Posts: 9,549
If the K-3 is like earlier Pentax then the green button should be set to "Tv shift" when in Manual or TAv mode. The default is P-Line shift. When set to Tv shift in Manual mode an M or K lens the green button will stop down the lens, measure light off the focus screen and the set the shutter speed from these measurements. P-Line wants to set both the aperture and shutter speed which cannot be done on non-A lenses.

See page 20 in the English language manual on E-Dial programming.

06-09-2014, 12:54 AM   #17
Junior Member




Join Date: Jun 2014
Posts: 41
Original Poster
ok Steve, I'll make some other tests in the conditions that you suggest, probably I have to improve my technique
I think the lenses I use are ok, both SMC Pentax A 50mm and Revuenon K 28mm give the same result, like you can see in my 'home made test' in previous post.
I think that K3 has a great metering system, I have not doubt on this...! (I started taking photographs in the 80s with a Pentax, then I had canon nikon, then I returned to Pentax and I was amazed by this camera)
So... I believe there is definitely a gap of exposure between shoot in 'A' mode or 'M' and I think this is quite normal:
nothing at f/2.8, but at f/8 I measure approximately 1/3 2/3 stop of difference, depending on the scene; is this what , since you are using a lot of old lenses, do you also find?

Not a Number, certainly I set green button in M mode in TVshift
06-09-2014, 08:58 AM   #18
Otis Memorial Pentaxian
Loyal Site Supporter
stevebrot's Avatar

Join Date: Mar 2007
Location: Vancouver (USA)
Photos: Gallery | Albums
Posts: 42,007
QuoteOriginally posted by Not a Number Quote
If the K-3 is like earlier Pentax then the green button should be set to "Tv shift" when in Manual or TAv mode. The default is P-Line shift. When set to Tv shift in Manual mode an M or K lens the green button will stop down the lens, measure light off the focus screen and the set the shutter speed from these measurements. P-Line wants to set both the aperture and shutter speed which cannot be done on non-A lenses.

See page 20 in the English language manual on E-Dial programming.
I remember reports of the behavior you are referring to and I don't believe that it applies to the K-3 or K10D* or any other of the Pentax flagship bodies. I get the same readings regardless of whether I use P-line, Tv shift, or Av shift. The camera "knows" when a non-A contact lens is attached. That is why it stops the lens down with green button in M-mode as opposed to attempting open-aperture metering. The stop-down is the key. In regards to general use of Tv shift in M-mode, I prefer that setting for my work when using A-contact lenses since I generally use a particularly aperture as a start point.


Steve

* bodies that I own
06-11-2014, 10:30 AM   #19
Otis Memorial Pentaxian
Loyal Site Supporter
stevebrot's Avatar

Join Date: Mar 2007
Location: Vancouver (USA)
Photos: Gallery | Albums
Posts: 42,007
I just reviewed your test data. The stop-down meter readings for your Pentax-A 50/2 (1/30s @ f/2.8 and 0.4s @ f/8) are in reasonable agreement with the wide-open (A-position) readings. While the time at f/8 is somewhat longer than the expected 0.25s, the overexposure (just under 1 stop) should have been relatively mild and not the reported gross overexposure.

It is possible that the light to the sensor may have been below the linear range (depending on ISO setting) for the sensor, though in that case, I would have expected significant underexposure.

If you have access to a hand-held meter with the ability to measure incident light, I would suggest that you do some comparison measurements of both stop-down and wide-open metering. If you have a photographer's gray card, a reading off of it (fill the frame) would be equivalent.


Steve

06-11-2014, 03:58 PM   #20
Junior Member




Join Date: Feb 2009
Location: Greenwich, SE London, UK
Posts: 35
QuoteOriginally posted by teoprome Quote
Hi, I read a lot about use manual lenses on Pentax cameras, so I have 'little' problem with my K3 and M lenses

When I use a manual lens (example a K mount lens with a lever on the back and no contact pins) in AV on my K3, it remains wide open and the measurement of exposure is correct, as expected: all ok.

In manual mode, I use this procedure:
set aperture on lens, measure exposure with green button (I can also see proper DOF preview), then release shutter

what I get is this:
with aperture 2.8, metering is correct, shot exposed fine
with other apertures, it seems that aperture values will not be considered on shot; exposure seems to be correct based on setted aperture, DOF preview seems to be correct, so when I take a shot the photo is overexposed...

example of time/f when stop down

measured time 1/15 at f/2.8
measured time (stopdown) 0.3'' at f/8

the value measured at f/8 is correct, 3 stops of difference... so shot is very overexposed

It's possible that the lens is stopped down correctly on DOF preview and the desidered aperture not read when the shutter is released?
I'm a little confused...

thank'you for all of your tips and experiences
Try this

When you attach the lens to the camera do not turn it to full lock mode, but stop just before. The idea of this is to stop the aperture lever on the lens being moved. Go into AV mode and see what happens.



Phil
06-11-2014, 06:43 PM   #21
Otis Memorial Pentaxian
Loyal Site Supporter
stevebrot's Avatar

Join Date: Mar 2007
Location: Vancouver (USA)
Photos: Gallery | Albums
Posts: 42,007
QuoteOriginally posted by f8orbust Quote
Try this

When you attach the lens to the camera do not turn it to full lock mode, but stop just before. The idea of this is to stop the aperture lever on the lens being moved. Go into AV mode and see what happens.



Phil
Ok...if done carefully this simulates a full-manual aperture at the risk of the lens falling off. How does this address the problem of the OP not being able to get expected exposure despite having reasonable stop-down meter readings?


Steve

---------- Post added 06-11-14 at 06:55 PM ----------

QuoteOriginally posted by teoprome Quote
So... I believe there is definitely a gap of exposure between shoot in 'A' mode or 'M' and I think this is quite normal:
nothing at f/2.8, but at f/8 I measure approximately 1/3 2/3 stop of difference, depending on the scene; is this what , since you are using a lot of old lenses, do you also find?
Sorry for the piecemeal commenting. A difference of 1/3 or 2/3 stop between stop-down and wide-open metering is not unexpected or a cause for alarm. The actual aperture diameter when stopped down may vary somewhat from what it is supposed to be due to mechanical issues. That is why, back-in-the-day, stop-down metering was pitched as a valuable feature of cameras like the Pentax Spotmatic. In theory, it is more accurate.

What would be a cause for alarm is if you got grossly wrong actual exposure. That is why I suggested using an external meter and testing against a blank wall or some other evenly lit, uncomplicated surface. The histogram for that case should be centered.


Steve
06-12-2014, 12:09 AM   #22
Junior Member




Join Date: Jun 2014
Posts: 41
Original Poster
Phil, I'm a little 'fearful to try this out to my new K3... I do not break anything?
I could break that damned lever on the lens definitively and then make some other test !!!

Steve, yes at f/8 the difference of exposure is reasonable, so the situation worsens significantly closing more; I'll make the test with gray card






thanks again, Teo

06-12-2014, 07:45 AM   #23
Junior Member




Join Date: Feb 2009
Location: Greenwich, SE London, UK
Posts: 35
Steve

I'm not suggesting Teo walks around taking pictures with an unattached lens. The idea is to see what exposure readings the camera provides compared to the lens being locked on. If the same, so be it. If different it might help to solve what might be going on here.

Regards

Phil
06-12-2014, 02:54 PM - 1 Like   #24
Otis Memorial Pentaxian
Loyal Site Supporter
stevebrot's Avatar

Join Date: Mar 2007
Location: Vancouver (USA)
Photos: Gallery | Albums
Posts: 42,007
QuoteOriginally posted by f8orbust Quote
I'm not suggesting Teo walks around taking pictures with an unattached lens.
That is good to know.

QuoteQuote:
The idea is to see what exposure readings the camera provides compared to the lens being locked on. If the same, so be it. If different it might help to solve what might be going on here.
I understand what you are suggesting, but suspect that a stop-down reading in Av mode may not be strictly equivalent to the same reading in M mode. I can't say for sure in regards to the K-3, but both the K-7 and K-5 series had different meter behavior depending on mode. The subject is complicated and would take several pages to explain thoroughly, but it is probably enough to say that no Pentax dSLR since the K10D has used the straight meter output as the basis for exposure settings. Proper metering in Av mode is no sure indication of how the camera would behave in M mode. That being said, I would suggest looking at the OPs description and testing. The meter is operating properly.

For the OP...I still find this whole thread fairly puzzling, particularly since you have not provided any images showing the reported gross overexposure. This is what I understand (with some reading between the lines) at this point:
  • You have evaluated stop-down metering in M-mode using two K-mount lenses, one a third-party non-A contact lens and the other a Pentax-A 50/2 with the aperture ring off the A position
  • Aperture displays F-- when A contact is not available (assumed)
  • Metered EV at f/2.8 is reasonable for both lenses (your test)
  • Metered EV at f/8 is reasonable for both lenses and essentially agrees with the EV at f/2.8 (your test)
  • Actual exposures done at f/8 are grossly overexposed in M-mode for both lenses (off A position for A 50/2) (your test)
  • Actual exposure done with the A 50/2 with lens in the A position in M-mode at f/8 is properly exposed (assumed)
  • None of the K-3 owners who have contributed to this thread have been able to reproduce this behavior on the K-3 with their Pentax-K or Pentax-M lenses Edit: I am going to do a more extensive attempt to reproduce.
From the above I think it is safe to conclude that the stop-down metering is essentially accurate and working properly on your camera, but that there is an exposure issue (shutter or aperture mechanism). It may be that the aperture actuator in your camera body is not moving full-travel at exposure time when a non-A contact lens is mounted. This would be evidence a defect or of damage to the camera. Edit: I am not so sure about the conclusions here. I am going to try a more rigorous attempt at reproducing the OPs experience.


Steve

Last edited by stevebrot; 06-12-2014 at 04:09 PM.
06-12-2014, 04:35 PM   #25
Otis Memorial Pentaxian
Loyal Site Supporter
stevebrot's Avatar

Join Date: Mar 2007
Location: Vancouver (USA)
Photos: Gallery | Albums
Posts: 42,007
Ok...I am going to eat a little crow here and then perhaps get to the bottom of this.

So from the top:
  1. Based on my field testing of stop-down metering on my K-3, I have been very enthusiastic about the results. Not only were the exposures acceptable, they were actually quite good!
  2. That being said, I never actually did a controlled test for accuracy compared to a reference (live view, another meter, open aperture, etc). I also had never evaluated linearity.
  3. This lapse in vigor occurred to me as I was compiling the list of bullet points in my comment above. My apologies to Teo.
  4. After amending my post I gathered up my K-3 and several lenses and did a few quick look-sees at various apertures and comparison to a hand-held meter, open-aperture readings from the K-3, and stop-down metering in live view
  5. Short story is that the viewfinder metering system in the K-3 looks to be quite the "piece of work"
  6. Two things I was able to establish for the lighting I had available at the time:
    • I was able to reproduce the OP's (Teo's) original observation. Exposure times are longer than expected at narrower apertures.
    • I was also able to determine that stop-down metering in both Av mode (using M42) and M mode M42 and K-mount) is non-linear with the trend being to longer exposure times as the aperture size decreases
I was not able to determine whether the non-linearity varied according to spectral composition or complexity of the subject (both possible given the nature of the new RGB sensor). I was also not able to test directly against a gray card or other non-complex target. I took test shots at various apertures and was surprised to note that the results in the crappy light available (rainy day outside) looked to be consistently good. Something funny is going on here.

When I get a better handle on this, I will post more complete results on a thread of its own.

My sincere apology to Teo for not being more diligent. You not going crazy.


Steve
06-13-2014, 01:56 AM   #26
Junior Member




Join Date: Jun 2014
Posts: 41
Original Poster
Hi Steve, I'll try not going crazy...

I apologize for not providing the images, so this behavior is so clear and constant for K3 that I thought it was encountered by many K3 users...

for your first points:
You have evaluated stop-down metering in M-mode using two K-mount lenses... yes
Aperture displays F--... yes
Metered EV at f/2.8 is reasonable for both lenses... yes
Metered EV at f/8 is reasonable for both lenses and essentially agrees with the EV at f/2.8.. yes
Actual exposures done at f/8 are grossly overexposed in M-mode for both lenses (off A position for A 50/2)... yes
Actual exposure done with the A 50/2 with lens in the A position in M-mode at f/8 is properly exposed (assumed).. in A mode all correct and very good
None of the K-3 owners who have contributed to this thread have been able to reproduce this behavior.... yes, and I'm very surprised by this

for seconds points
1.ok
2.ok
3.no problem, the important thing is to understand what happens, the cause, and how to solve (...and call Ricoh/Pentax to make something in next firmware upgrade... !)
4.ok,perfect
5.ok,perfect
6.
'I was able to reproduce the OP's (Teo's) original observation. Exposure times are longer than expected at narrower apertures...' very very well Steve
'I was also able to determine that stop-down metering in both Av mode (using M42) and M mode M42 and K-mount) is non-linear with the trend being to longer exposure times as the aperture size decreases... ' very very well Steve

'I took test shots at various apertures and was surprised to note that the results in the crappy light available (rainy day outside) looked to be consistently good. Something funny is going on here'
yes, also for me in my last external tests, in 'medium-low gray' sky lights, no problems...

I am very glad that the problem has been reproduced (and that my camera is not broken...); if my son will leave me a bit of time I do other tests

thank's Steve

Last edited by teoprome; 06-13-2014 at 02:09 AM.
06-13-2014, 07:34 PM   #27
Otis Memorial Pentaxian
Loyal Site Supporter
stevebrot's Avatar

Join Date: Mar 2007
Location: Vancouver (USA)
Photos: Gallery | Albums
Posts: 42,007
I did some testing this afternoon and while far from comprehensive, it is a starting place. See:

https://www.pentaxforums.com/forums/172-pentax-k-3/265657-k-3-stop-down-meter...rformance.html


Steve
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!
alarm, aperture, cameras, dof, exposure, f/8, k-mount, k3, lens, lens on k3, lenses, manual lens, meter, mode, pentax, pentax lens, preview, revuenon, shot, shutter, slr lens, steve, stop-down, time
Thread Tools Search this Thread
Search this Thread:

Advanced Search


Similar Threads
Thread Thread Starter Forum Replies Last Post
SMC Pentax-M 135mm F3.5 on K3 gryhnd Pentax SLR Lens Discussion 4 03-19-2014 11:20 AM
would any one mask the ricoh on k3 pentax user Pentax K-3 & K-3 II 91 01-25-2014 07:07 PM
Any freezing problems with K30?(like those with K5) or any other problems? simbon4o Pentax K-30 & K-50 21 01-03-2013 01:54 PM
MZ-M problems, any ideas? pero Film SLRs and Compact Film Cameras 4 05-31-2011 03:26 PM
What problems can I expect using an M-class lens or older on a K100d? Grimlock Pentax SLR Lens Discussion 10 09-10-2007 09:31 PM



All times are GMT -7. The time now is 06:03 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