Forgot Password
Pentax Camera Forums Home
 

Reply
Show Printable Version Search this Thread
05-24-2015, 05:00 AM   #1
Forum Member




Join Date: May 2011
Location: greater Philadelphia area
Posts: 80
Exposure accuracy of "A" lenses on older film bodies ("K" series)

(Pardon me if this has been discussed before; I've looked at other threads and didn't find anything quite like this question.)

Exposure inaccuracies when using K and M lenses on at least some DSLRs are well documented. From what I've read, these exposure/metering problems arise because between the M and A series lenses Pentax changed the way f-stops were calculated (something like going from linear to non-linear measures, or using diameter of the opening vs. area of the opening--sorry, I've forgotten those details at this point). Here's my question: do the same kind of exposure inaccuracy problems arise going the other way, that is, does using A-series or later (F, FA, DFA) lenses on early K-mount film bodies (KX, K2, ME, etc.) throw off the camera's metering? I tend to assume there's no problem, given that in the late 1980s Pentax was selling A-series lenses on the ME and MX bodies. But does anyone have any first hand experience with this? (Why am I interested? I use M and K lenses on my K film bodies, and was considering purchasing an A lens.) Thanks for any insight.

05-24-2015, 05:24 AM   #2
Pentaxian
Lowell Goudge's Avatar

Join Date: Jan 2007
Location: Toronto
Photos: Gallery | Albums
Posts: 15,400
No.

The issue is not going the other way because before the A series cameras the aperture lever was a switch. It always stopped down the lens to the selected aperture. The aperture coupling was not removed until I think the *ist series cameras. Much later

Edit note.

The issue on DSLRs is with respect to the metering and the scatter of light off the focusing screen. from testing I did when the issue was first reported, it is a function of the native aperture of the lens, or the aperture when doing actual stopped down metering on KSeries lenses.

The camera bodies in A series lenses know the maximum aperture and compensate for this issue, when metering in a mode. This was best exhibited by the error introduced when adding a teleconverter with A contacts to an a series lens. Exposure was perfectly flat, just shifted by a constant error because the TC modified the native aperture of the lens without changing the A contact information.

Last edited by Lowell Goudge; 05-24-2015 at 05:43 AM.
05-24-2015, 05:33 AM   #3
Pentaxian




Join Date: Oct 2011
Location: Savannah, GA
Posts: 5,005
QuoteOriginally posted by Lowell Goudge Quote
No.

The issue is not going the other way because before the A series cameras the aperture lever was a switch. It always stopped down the lens to the selected aperture. The aperture coupling was not removed until I think the *ist series cameras. Much later
That's what I would figure, but why would it be a problem the other way around on a modern camera if you are using it the same way? If I am using a K or M lens on a DSLR, that means I'll be in M mode metering with the green button, which actually stops down to take a reading (so that should be accurate). It is actually using 'A' lenses normally where it is guesstimating exposures that I'd expect to see some inaccuracies, or using K/M lenses on older cameras with non-crippled mounts where it was essentially doing the same thing mechanically instead of electronically.
05-24-2015, 05:39 AM   #4
Pentaxian
Lowell Goudge's Avatar

Join Date: Jan 2007
Location: Toronto
Photos: Gallery | Albums
Posts: 15,400
QuoteOriginally posted by vonBaloney Quote
That's what I would figure, but why would it be a problem the other way around on a modern camera if you are using it the same way? If I am using a K or M lens on a DSLR, that means I'll be in M mode metering with the green button, which actually stops down to take a reading (so that should be accurate). It is actually using 'A' lenses normally where it is guesstimating exposures that I'd expect to see some inaccuracies, or using K/M lenses on older cameras with non-crippled mounts where it was essentially doing the same thing mechanically instead of electronically.
See my edit note to my post. I spent a long time looking into this, and found for example, the K10 screen, which is identical size to then*istD had horrible non linearity in K lens metering, but when I put the *istD screen into the K10 it was almost perfect (like the *istD)

I think it is a function of the way light hits the screen, and the impact of the scatter off the screen, as it hits the metering segments, that causes the issue.

05-24-2015, 05:57 AM   #5
Forum Member




Join Date: May 2011
Location: greater Philadelphia area
Posts: 80
Original Poster
Thanks for all the very helpful information!
05-24-2015, 06:28 AM   #6
Site Supporter
Site Supporter
dakight's Avatar

Join Date: Sep 2014
Location: Phoenix, AZ
Photos: Albums
Posts: 1,181
Issues like this are exactly why I almost always use a 3 exposure bracket. Yeah, it uses more storage and takes a little longer to process but I almost always have at least one of the three that has a useable exposure. It's not only inaccuracies in the metering but also different light conditions can and often do confuse the meter. So a set of bracketed exposures with + or - 1 or 2 EV almost always yields something at least close to correct.
05-24-2015, 09:30 AM   #7
Pentaxian
Lowell Goudge's Avatar

Join Date: Jan 2007
Location: Toronto
Photos: Gallery | Albums
Posts: 15,400
I test all my lens and body combos for exposure. Most follow reasonable rules , sick as my Tamron 28-75-2.8 which is spot on wide open but for some reason, the aperture lever is not calibrated correctly and exposure drifts up gradually as you stop down, t being +1 at F32. Knowing this makes me conscious of the errors so I check thenhistogram and set EV comp when using this lens
05-24-2015, 10:02 AM   #8
Site Supporter
Site Supporter




Join Date: Jan 2010
Location: Michigan
Photos: Gallery
Posts: 2,200
I only have one -A lens among my collection of -M.
I rarely use the SMC Pentax-A 1:1.4 50mm , but took it to Korea with the Pentax MX one time, and did not notice any exposure problems.
This film was Kodak iso 800 home processed in C41 Digibase
Note that the Pentax MX has a limited meter coupling range with faster and slower films and users should be aware of that, as explained in page 22 of the manual.
As I recall the second photo had the lens at f/2 or so and was metered with vf centre on the bright part of the road before framing.
https://app.box.com/s/omlyj78yl9m884dq7kdv
https://app.box.com/s/oo6slnkpofcjoh4zsouf

05-24-2015, 10:17 AM   #9
Site Supporter
Site Supporter




Join Date: Dec 2012
Location: Essex County, Ontario
Posts: 478
QuoteOriginally posted by Lowell Goudge Quote
See my edit note to my post. I spent a long time looking into this, and found for example, the K10 screen, which is identical size to then*istD had horrible non linearity in K lens metering, but when I put the *istD screen into the K10 it was almost perfect (like the *istD)

I think it is a function of the way light hits the screen, and the impact of the scatter off the screen, as it hits the metering segments, that causes the issue.
Great work. Possibly know if K20 is similar? Recently got one and haven`t yet tried older lenses on it. Anybody know of any exposure trends to watch for?
05-24-2015, 10:23 AM   #10
Moderator PEG Judges
Loyal Site Supporter
Kerrowdown's Avatar

Join Date: Aug 2008
Location: Highlands of Scotland.
Photos: Gallery
Posts: 33,139
QuoteOriginally posted by From1980 Quote
Possibly know if K20 is similar? Recently got one and haven`t yet tried older lenses on it
I'm still using K20D's with exclusively A series primes... pleased to report no real issues of problems with exposures at all.
05-24-2015, 10:25 AM   #11
Site Supporter
Site Supporter




Join Date: Dec 2012
Location: Essex County, Ontario
Posts: 478
Thanks for the experience.
05-24-2015, 10:49 AM   #12
Pentaxian
Lowell Goudge's Avatar

Join Date: Jan 2007
Location: Toronto
Photos: Gallery | Albums
Posts: 15,400
QuoteOriginally posted by From1980 Quote
Great work. Possibly know if K20 is similar? Recently got one and haven`t yet tried older lenses on it. Anybody know of any exposure trends to watch for?
K20 meters just like K10. Badly on non A lenses. Somewhere I posted how all the bodies performed from *istD through K5. It is a chart showing grey scale vs f stop. Note 45 grey scale =1 stop between 25 and 230 greyscale. The ends are non linear but the middle of the range has about 5 uniform stops
05-24-2015, 08:32 PM   #13
Loyal Site Supporter
Loyal Site Supporter
Canada_Rockies's Avatar

Join Date: Nov 2007
Location: Sparwood, BC, Canada
Photos: Gallery | Albums
Posts: 9,624
QuoteOriginally posted by Lowell Goudge Quote
K20 meters just like K10. Badly on non A lenses. Somewhere I posted how all the bodies performed from *istD through K5. It is a chart showing grey scale vs f stop. Note 45 grey scale =1 stop between 25 and 230 greyscale. The ends are non linear but the middle of the range has about 5 uniform stops
When I was looking into this discussion in the dark ages, I solved two of my problems at the same time with my K10D by installing an LL-60 screen from the *ist D series. The other problem is leaning buildings and trees. I seem to lean to the right all the time. I'm not sure if it is because I've been married almost 50 years and She Who Must Be Obeyed usually is on the left where I don't hit her with the K10D on its wrist strap or some other reason, but I do lean. The 1/3 lines in the screen help me tell which way is up
05-24-2015, 08:39 PM   #14
Pentaxian




Join Date: Oct 2011
Location: Savannah, GA
Posts: 5,005
QuoteOriginally posted by Canada_Rockies Quote
When I was looking into this discussion in the dark ages, I solved two of my problems at the same time with my K10D by installing an LL-60 screen from the *ist D series. The other problem is leaning buildings and trees. I seem to lean to the right all the time. I'm not sure if it is because I've been married almost 50 years and She Who Must Be Obeyed usually is on the left where I don't hit her with the K10D on its wrist strap or some other reason, but I do lean. The 1/3 lines in the screen help me tell which way is up
Some of the K10Ds actually had a unlevel horizon problem, might not be you.
05-24-2015, 09:43 PM   #15
Loyal Site Supporter
Loyal Site Supporter
Canada_Rockies's Avatar

Join Date: Nov 2007
Location: Sparwood, BC, Canada
Photos: Gallery | Albums
Posts: 9,624
Never thought of that! The grid lines help quite a bit, so I think I'm a little bit out of warranty to claim the tilt. I bought the K10D in November 2007.
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!
a-series, bodies, exposure, exposure accuracy, film, film bodies, k-mount, lenses, pentax, pentax lens, series, slr lens
Thread Tools Search this Thread
Search this Thread:

Advanced Search


Similar Threads
Thread Thread Starter Forum Replies Last Post
Dear, "i...i" and "r....t" on ebay, bidding on the 50-135mm lenses mee Pentax SLR Lens Discussion 64 02-07-2015 07:25 PM
"Click" in between "C" and "MF" mode on focus selector? K-50 maxwolfie Pentax K-30 & K-50 3 01-29-2015 04:19 AM
Old "film" glass on digital bodies... is there like a black list? rburgoss Pentax SLR Lens Discussion 16 09-18-2014 05:04 PM
Optical differences between Pentax "K", "M", and "A" lenses 6BQ5 Pentax SLR Lens Discussion 31 01-10-2014 01:02 PM
Going from a "film" to a "digital" back to a "film" lens? 6BQ5 Pentax SLR Lens Discussion 9 06-18-2013 03:40 PM



All times are GMT -7. The time now is 09:50 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