Forgot Password
Pentax Camera Forums Home
 

Reply
Show Printable Version Search this Thread
09-29-2010, 09:37 PM   #1
Senior Member




Join Date: Mar 2010
Photos: Gallery
Posts: 214
If someone can figure this out, you're a genius...issue w/ wrong exposure for lens

So I took two test shots with my camera and I shot the first one at F1.4, ISO 1600, 1/125...the second shot is F2.8, ISO 6400, 1/125. Both of the shots should have the same amount of light but why is the one shot at F2.8 ISO 6400 brighter? The depth of field is shallower though. I bought this lens used and was testing it out.

F1.4, ISO 1600, 1/125
http://www.flickr.com/photos/48244452@N06/5037207905/

F2.8, ISO 6400, 1/125
http://www.flickr.com/photos/48244452@N06/5037207975/in/photostream/

09-29-2010, 10:05 PM   #2
Senior Member




Join Date: Nov 2009
Location: Southwest
Photos: Albums
Posts: 155
To me the depth of field looks quite a bit more shallow at F1.4 ISO 1600 - as would be expected.

The real question is what lens are you using? Is it A, M, screw mount?

Not all lenses meter properly on the digital bodies, so the exposure can vary somewhat depending on aperture.
09-29-2010, 10:17 PM   #3
Loyal Site Supporter
Canada_Rockies's Avatar

Join Date: Nov 2007
Location: Sparwood, BC, Canada
Photos: Gallery | Albums
Posts: 9,134
QuoteOriginally posted by crossover37 Quote
So I took two test shots with my camera and I shot the first one at F1.4, ISO 1600, 1/125...the second shot is F2.8, ISO 6400, 1/125. Both of the shots should have the same amount of light but why is the one shot at F2.8 ISO 6400 brighter? The depth of field is shallower though. I bought this lens used and was testing it out.

F1.4, ISO 1600, 1/125
IMGP8942 | Flickr - Photo Sharing!

F2.8, ISO 6400, 1/125
IMGP8943 | Flickr - Photo Sharing!
If your lens is missing an A position on the aperture ring, or if you turned the aperture ring off the A position, the lens will only stop down in M mode. In Av mode (the only other mode the camera will use with a lens without the A position) the lens does not stop down, but rather shoots always wide open. This would account for the brighter image at 6400 - two stops more light than at 1600.

There are well documented metering errors with anything from the K10d (which I own) and later camera bodies when the lens is not set to an automatic aperture. The lens will expose in M mode correctly at only one aperture, and will overexpose in one direction and underexpose in the other.

The problem is specifically with the focusing screen which is designed, in the later cameras, to give a brighter image. Replacing the focusing screen with the L series screen (I personally have the LL-60 with grid lines) results in accurate metering. I was worried that it would cause erroneous exposure with the DA lenses, but they expose the same. I use five lenses routinely, my three DA series and my two M series lenses. Replacing the focusing screen with the LL-60 gave me correct metering with all five lenses - and anything else I put on the front from time to time.
09-29-2010, 10:20 PM   #4
Pentaxian




Join Date: Apr 2008
Location: Central Ohio (formerly SF Bay Area)
Posts: 1,464
I think we can take metering out of the equation entirely -- these are manual settings, and an FA 50mm lens was used (it's in the EXIF). And all else being equal, I agree with the OP that the exposure should be basically the same!

So I'm stumped.

09-29-2010, 10:25 PM   #5
Senior Member




Join Date: Jan 2010
Location: HCMC
Posts: 272
Hi,

Just a hunch.... I think the DOF on image IMG...43 is shallow and focused or metered on the carpet, hence brighter (based on the pictures where you can actually see the center of the focused area). IMG...42 is focused on the white container that's why it was darker.
09-29-2010, 10:44 PM   #6
Senior Member




Join Date: Nov 2009
Location: Southwest
Photos: Albums
Posts: 155
QuoteOriginally posted by Quicksand Quote
I think we can take metering out of the equation entirely -- these are manual settings, and an FA 50mm lens was used (it's in the EXIF). And all else being equal, I agree with the OP that the exposure should be basically the same!

So I'm stumped.
I just spent 10 minutes trying to figure out how to view EXIF from flickr these days. Right-click in browser doesn't work......only to see it conveniently has an EXIF page. The more you know....

QuoteOriginally posted by HoBykoYan Quote
Hi,

Just a hunch.... I think the DOF on image IMG...43 is shallow and focused or metered on the carpet, hence brighter (based on the pictures where you can actually see the center of the focused area). IMG...42 is focused on the white container that's why it was darker.
Seeing that it is an FA lens and where the focus is, I agree.
09-29-2010, 10:46 PM   #7
Senior Member




Join Date: Mar 2010
Photos: Gallery
Posts: 214
Original Poster
Thanks for replying....The lens is a FA SMC 50mm F1.4 auto focus. It's on the A position and i'm using center weighted metering, same position so the exposure should be the same. I tried the same thing with my Tamron 28-75mm and my exposures came out equal when I set my aperture to f2.8 and f5.6 and adjusted my ISO accordingly leaving my shutter speed constant. I'm still confused...
09-29-2010, 10:47 PM   #8
Senior Member




Join Date: Mar 2010
Photos: Gallery
Posts: 214
Original Poster
Originally posted by Quicksand
I think we can take metering out of the equation entirely -- these are manual settings, and an FA 50mm lens was used (it's in the EXIF). And all else being equal, I agree with the OP that the exposure should be basically the same!

So I'm stumped.
I just spent 10 minutes trying to figure out how to view EXIF from flickr these days. Right-click in browser doesn't work......only to see it conveniently has an EXIF page. The more you know....

Originally posted by HoBykoYan
Hi,

Just a hunch.... I think the DOF on image IMG...43 is shallow and focused or metered on the carpet, hence brighter (based on the pictures where you can actually see the center of the focused area). IMG...42 is focused on the white container that's why it was darker.
Seeing that it is an FA lens and where the focus is, I agree.

Click on Pentax K-x on the top right, it will bring up the EXIF info

09-29-2010, 10:53 PM   #9
New Member




Join Date: Sep 2010
Posts: 15
QuoteOriginally posted by HoBykoYan Quote
Hi,

Just a hunch.... I think the DOF on image IMG...43 is shallow and focused or metered on the carpet, hence brighter (based on the pictures where you can actually see the center of the focused area). IMG...42 is focused on the white container that's why it was darker.
Correct me if I'm wrong because I'm no expert, but metering has nothing to do with this mystery since both shots were taken in manual exposure mode.

I can't figure out why the two shots have different exposure.
09-29-2010, 10:53 PM   #10
Loyal Site Supporter
Canada_Rockies's Avatar

Join Date: Nov 2007
Location: Sparwood, BC, Canada
Photos: Gallery | Albums
Posts: 9,134
QuoteOriginally posted by lowspark86 Quote
I just spent 10 minutes trying to figure out how to view EXIF from flickr these days. Right-click in browser doesn't work......only to see it conveniently has an EXIF page. The more you know....
You need to open the "original" size to see EXIF on Flickr. Click all sizes, then original.
09-29-2010, 10:54 PM   #11
Pentaxian
twitch's Avatar

Join Date: Nov 2009
Posts: 4,571
QuoteOriginally posted by HoBykoYan Quote
Hi,

Just a hunch.... I think the DOF on image IMG...43 is shallow and focused or metered on the carpet, hence brighter (based on the pictures where you can actually see the center of the focused area). IMG...42 is focused on the white container that's why it was darker.
I disagree. That could fool the camera metering into thinking it needed to expose for a different EV, but that wont matter when the exposure is controlled to be the same as was done in this case.
09-29-2010, 10:56 PM   #12
Loyal Site Supporter
Canada_Rockies's Avatar

Join Date: Nov 2007
Location: Sparwood, BC, Canada
Photos: Gallery | Albums
Posts: 9,134
I reiterate - the lens must be set on the A position. If you don't do that, the lens will not stop down. I see absolutely no other way to get this "interesting" result. Use the camera to set the f/stop, not the aperture ring.
09-29-2010, 10:59 PM   #13
Senior Member




Join Date: Mar 2010
Photos: Gallery
Posts: 214
Original Poster
I am using the camera to set the aperture. It is on the A position and the type of metering has nothing to do with it because I am taking the same shot and manually adjusting my settings by 2 stops.
09-29-2010, 11:04 PM   #14
Inactive Account




Join Date: Aug 2008
Location: Michigan, USA
Photos: Gallery | Albums
Posts: 7,485
I don't know about on the Kx (or even my K7) but this lens has a long history of sometimes tripping on digital SLRs, especially indoors, in low light. The only thing I can think of (without really thinking about it) is either the f2.8 shot didn't stop down completely or the f1.4 shot isn't actually wide open. You say you did the same thing with your Tamron zoom and the exposures were basically the same. I'm assuming you did it exactly the same way. Held shutter speed constant and adjusted Aperture and ISO. That's what makes me somewhat think it's the lens/camera combo.

What I would do as a next part of the experiment is to Spot meter the bottle and manually set your exposure accordingly. Once at f1.4 and again at f2.8. Set both so the meter reads 0EV (the middle) and take the photos. Actually since this is a White bottle, you should (if you spot meter) open the exposure by 1-2 stops. Spot metering is a lot harder to fool than CW metering.

09-29-2010, 11:07 PM   #15
Loyal Site Supporter
Canada_Rockies's Avatar

Join Date: Nov 2007
Location: Sparwood, BC, Canada
Photos: Gallery | Albums
Posts: 9,134
QuoteOriginally posted by crossover37 Quote
I am using the camera to set the aperture. It is on the A position and the type of metering has nothing to do with it because I am taking the same shot and manually adjusting my settings by 2 stops.
Are you adjusting your settings with the camera in M mode with your ISO set to a single value, not automatic? If you are not, the camera might be screwing with your settings. Other than that, I'm just as stumped as everyone else.
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!
f1.4, f2.8, iso, k-mount, lens, pentax lens, shot, shots, slr lens
Thread Tools Search this Thread
Search this Thread:

Advanced Search


Similar Threads
Thread Thread Starter Forum Replies Last Post
Super Genius experiment: Ricoh lens stuck on K100DS aseno Pentax SLR Lens Discussion 13 04-11-2010 08:34 PM
SDM issue - what am I doing wrong? Simon Pentax SLR Lens Discussion 11 08-12-2009 03:11 AM
Flash exposure issue slawek Flashes, Lighting, and Studio 20 07-10-2009 09:38 PM
K20d, still hotpixel issue with jpegs or something wrong with my camera? Karn Pentax DSLR Discussion 9 08-09-2008 03:33 PM
Test of the K10D exposure issue with M lenses Dana G Pentax DSLR Discussion 14 03-22-2007 11:20 AM



All times are GMT -7. The time now is 04:18 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