Forgot Password
Pentax Camera Forums Home
 

Reply
Show Printable Version Search this Thread
04-09-2009, 01:02 PM   #1
New Member




Join Date: Apr 2009
Location: Richmond, VA
Posts: 14
FA 50 1.4 on K20d... 1.4-2 not a full stop?

I just recently switched to Pentax with the K20d, and got a fa 50 1.4.
After a couple days of shooting, I noticed that 1.4 didn't seem to be giving me a full stop more of light than 2.0. I did some "testing" with various static objects and in different lighting scenarios; in manual, keeping a fixed shutter speed but adjusting the aperture in thirds each shot 1.4>1.6>1.8>2.0. Comparing the histograms (although it's obvious in the picture) on the camera or in Lightroom is showing no change from 1.4>1.8. I thought the aperture to be defective, so I placed a return order with B&H.

Got the new lens, but it acts exactly the same. So it doesn't seem to be a defective lens, unless I just happened to get two in a row with the same defect. Could it have to do with being a film era lens on a digital crop sensor? Has anyone else noticed this?

04-09-2009, 01:59 PM   #2
Veteran Member
OregonJim's Avatar

Join Date: Mar 2009
Location: Willamette Valley, Oregon
Posts: 1,329
Someone mentioned in another thread that they measured a 50/1.4 to actually be 1.55 wide open. They got the same results with another lens of a different brand (but same basic optical design). I can't seem to get the search function to find that post at the moment.
04-10-2009, 07:06 AM   #3
New Member




Join Date: Apr 2009
Location: Richmond, VA
Posts: 14
Original Poster
FA 50 1.4 on K20d... 1.4-2 not a full stop?

I posted this in the "newbie" forum, but without much response. I'm a not so much a newbie to photography, but just switched over to Pentax...

I just recently switched to Pentax with the K20d, and got a fa 50 1.4.
After a couple days of shooting, I noticed that 1.4 didn't seem to be giving me a full stop more of light than 2.0. I did some "testing" with various static objects and in different lighting scenarios; in manual, keeping a fixed shutter speed but adjusting the aperture in thirds each shot 1.4>1.6>1.8>2.0. Comparing the histograms (although it's obvious in the picture) on the camera or in Lightroom is showing no change from 1.4>1.8. I thought the aperture to be defective, so I placed a return order with B&H.

Got the new lens, but it acts exactly the same. So it doesn't seem to be a defective lens, unless I just happened to get two in a row with the same defect. Could it have to do with being a film era lens on a digital crop sensor? Has anyone else noticed this?
04-10-2009, 07:46 AM   #4
Pentaxian




Join Date: Mar 2009
Location: Perth Australia
Photos: Albums
Posts: 1,511
in av mode my shutter speed changes automatically between 1.4 , 1.7 and 2.0 on the fa 50.

04-10-2009, 08:14 AM   #5
New Member




Join Date: Apr 2009
Location: Richmond, VA
Posts: 14
Original Poster
QuoteOriginally posted by WerTicus Quote
in av mode my shutter speed changes automatically between 1.4 , 1.7 and 2.0 on the fa 50.
Well, right, that's what it's supposed to do... and mine does too, in aperture OR shutter priority (automatically changing aperture), but in the 1.4-1.8 range(or 1.7 if in half stops) the auto exposure doesn't stay consistent between shots. If I'm in manual and keep the shutter speed the same between 1.4 to 1.8, the exposure is the same, but should be at least a half stop difference....

I guess I'm missing what you're trying to say.
04-10-2009, 09:25 AM   #6
Pentaxian
Just1MoreDave's Avatar

Join Date: Feb 2007
Location: Aurora, CO
Photos: Gallery | Albums
Posts: 4,815
I don't have the FA version. I do have the optically identical A version, which ought to act the same. I just did a quick test and could not duplicate your problem. In manual, I can change aperture from 1.4 -> 1.7 -> 2.0 -> 2.4 with a constant shutter speed, and get progressively darker photos. In Av, I can change aperture and let the camera adjust shutter speed, and get consistent histograms and exposures.

I can think of a couple of possible causes. I would look at the lens on the camera and verify that the aperture blades completely disappear when it's supposed to be wide open. I can't see aperture blades at f1.4 and definitely see them at f1.6 when I operate the DOF preview lever. That should test at least some of the camera-lens interaction. You can operate the aperture lever on the lens when it's off-camera to see if it's moving right. Test some other lenses to see if they have the same problem wide open vs. slightly stopped down.

Another idea is that you are testing partly outside the meter coupling range, and the meter is not accurate. I'm not sure that's entirely applicable but I would make sure to test in some reasonable light, not like one 40 watt bulb in an airplane hangar.

You could try to use the aperture ring instead of setting the aperture on the camera body. You would have to have the camera set to allow this, use M mode and the green button to meter, then see if changing the aperture ring changes the exposure. My lens's ring only has detents at f1.4 and f2, but it does appear to show a one stop difference between these settings.
04-10-2009, 10:11 AM   #7
New Member




Join Date: Apr 2009
Location: Richmond, VA
Posts: 14
Original Poster
QuoteOriginally posted by Just1MoreDave Quote
I don't have the FA version. I do have the optically identical A version, which ought to act the same. I just did a quick test and could not duplicate your problem. In manual, I can change aperture from 1.4 -> 1.7 -> 2.0 -> 2.4 with a constant shutter speed, and get progressively darker photos. In Av, I can change aperture and let the camera adjust shutter speed, and get consistent histograms and exposures.

I can think of a couple of possible causes. I would look at the lens on the camera and verify that the aperture blades completely disappear when it's supposed to be wide open. I can't see aperture blades at f1.4 and definitely see them at f1.6 when I operate the DOF preview lever. That should test at least some of the camera-lens interaction. You can operate the aperture lever on the lens when it's off-camera to see if it's moving right. Test some other lenses to see if they have the same problem wide open vs. slightly stopped down.

Another idea is that you are testing partly outside the meter coupling range, and the meter is not accurate. I'm not sure that's entirely applicable but I would make sure to test in some reasonable light, not like one 40 watt bulb in an airplane hangar.

You could try to use the aperture ring instead of setting the aperture on the camera body. You would have to have the camera set to allow this, use M mode and the green button to meter, then see if changing the aperture ring changes the exposure. My lens's ring only has detents at f1.4 and f2, but it does appear to show a one stop difference between these settings.
Hmmm, well, with the lens off, wide open I can't see any shutter blades, so that seems right.. Manual operation on my FA also only allows one click from 1.4 to 2, but when I tested like you said, manual operation using the green button to meter, it only meters 1/3 stop difference.

The only other lens I have right now is the 16-45 f4. Tested that in third stops from 4-5.6 and it works like it should.

This is really weird. I find it hard to imagine that no one else has ever seen this before and I happened to get the same problem on 2 consecutive lenses. Seems like there may be an issue with the K20D and fa 50 1.4 relationship. What body are you shooting with?
04-10-2009, 10:28 AM   #8
Veteran Member
Sean Nelson's Avatar

Join Date: Sep 2008
Location: Vancouver BC Canada
Posts: 353
Just so that you're aware, you WON'T be able to see any darkening IN THE VIEWFINDER when you stop down from f/1.4 to f/2. This is because the microprisms in the focusing screen reject the light from a cone wider than about f/2 to f/2.8. It's not a fault of the lens, it's just the way the focusing screen works.

This doesn't affect the light reaching the sensor, though - you should still see a difference in actual exposure between f/1.4 and any smaller aperture.

04-10-2009, 10:51 AM   #9
axl
Veteran Member




Join Date: Mar 2007
Location: Nove Zamky, Slovakia
Photos: Gallery | Albums
Posts: 7,181
I have to say I have noticed something similar with my FA31 and K10D. In Av mode if I point it at stationary subject with light source of constant intensity, the metering does NOT change the time value if I change the appreture from 1.8 to 2. But between f2 and f2.8 there is a full stop difference. So instead of 1.3 stop differnce I only have 1 stop difference between f1.8 and f2.8.
this and your experience with K20D and FA50 may suggest that there is something "weird" going on between KxxD bodies and FA lenses. Could it be the fucusing screen doing it's "magic" the same way it does when using older than A series lenses?

BR
04-10-2009, 11:31 AM   #10
Pentaxian
Just1MoreDave's Avatar

Join Date: Feb 2007
Location: Aurora, CO
Photos: Gallery | Albums
Posts: 4,815
QuoteOriginally posted by adhall82 Quote
This is really weird. I find it hard to imagine that no one else has ever seen this before and I happened to get the same problem on 2 consecutive lenses. Seems like there may be an issue with the K20D and fa 50 1.4 relationship. What body are you shooting with?
I'm using the *ist DS, so unfortunately a different body and different lens. There ought to be some K20D/FA 50 f1.4 users out there, though - not a rare setup.

You could take some photos which would emphasize depth of field differences. Something like the focus test chart or a ruler would work, close to the minimum focus distance, set up like a focus test. Turn off AF, use a tripod, focus somewhere in the middle. Take shots at f1.4, f1.7 and f2.0, and see if the depth of field changes for different apertures. I'm not sure how much variation to expect but it should be visible.
04-13-2009, 03:26 PM   #11
Veteran Member
AndrewG NY's Avatar

Join Date: Dec 2006
Location: Chappaqua, NY
Photos: Gallery
Posts: 677
I haven't tried to verify this but can't say I'm all that surprised. Are you measuring from the middle of the frame? Many lenses suffer considerable falloff wide open so it is likely that there might be considerable underexposure if the entire frame is evaluated.
04-13-2009, 04:01 PM   #12
Site Supporter
stevebrot's Avatar

Join Date: Mar 2007
Location: Vancouver (USA)
Photos: Gallery | Albums
Posts: 26,215
QuoteOriginally posted by AndrewG NY Quote
I haven't tried to verify this but can't say I'm all that surprised. Are you measuring from the middle of the frame? Many lenses suffer considerable falloff wide open so it is likely that there might be considerable underexposure if the entire frame is evaluated.
Andrew makes a valid point. A little vignette can significantly affect the histogram even if the subject was metered accurately. And, as Jim points out, many lenses have been sold with exaggerated maximum apertures. Usually the short-fall is not much, but it may exist none-the-less.

Steve
04-13-2009, 04:09 PM   #13
New Member




Join Date: Apr 2009
Location: Richmond, VA
Posts: 14
Original Poster
No, it's definitely an entire image under-exposure, not a vignetting... that I wouldn't mind so much. But thanks for the input. It basically just seems that it's effectively a 1.8, not a 1.4...
04-13-2009, 04:13 PM   #14
New Member




Join Date: Apr 2009
Location: Richmond, VA
Posts: 14
Original Poster
bump... any one else with k20d and 50 1.4?
04-13-2009, 04:26 PM   #15
Site Supporter
pschlute's Avatar

Join Date: Mar 2007
Location: Surrey, UK
Photos: Gallery
Posts: 715
QuoteOriginally posted by adhall82 Quote
bump... any one else with k20d and 50 1.4?
I dont have the camera or the lens. But can you post some photos with exif intact.

What you describe is a problem UNLESS:

1. you have enabled auto-exposure in the custom setting

2. You are using auto ISO
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, fa, k-mount, lens, pentax lens, slr lens
Thread Tools Search this Thread
Search this Thread:

Advanced Search


Similar Threads
Thread Thread Starter Forum Replies Last Post
Full frame K20D in 2009? Vote now! Yes or No. Adnan EROL Pentax News and Rumors 232 02-23-2010 03:22 PM
Super paragon lenses want stop down on K20D Dom Pentax SLR Lens Discussion 7 11-26-2009 03:45 AM
Fireengine K20D movie in full HD falconeye Post Your Photos! 8 01-28-2009 10:17 AM
Tonight's full moon, K20D & the Bigma switch79 Photographic Technique 10 05-21-2008 12:30 AM
Stop Down Metering with K20D and Tamron SP Lenses petez Pentax DSLR Discussion 39 04-05-2008 08:53 PM



All times are GMT -7. The time now is 09:41 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