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09-03-2015, 10:54 AM   #1
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Imprecision - variable aperture zooms / MF focal length

Two relatively minor things are bothering me on the K-3 - that I haven't seen discussed. First, maximum aperture settings on variable aperture zooms are not exact and can skip 2/3rds of a stop when stopping down. Second, unlike the older Q which allows for entering the exact focal length of a manual lens on startup, you have to choose among what the menu offers (therefore I can't get 14mm in EXIF or as my SR setting).

This hasn't been noticed much in the past - as I shoot a lot with primes or constant aperture zooms. However, I recently acquired the Sigma 12-24 f/4.5-5.6 which will be a high-demand lens when FF comes out. I also have the 18-55 WR for occasional use in inclement weather, and the old F 70-210 f/4-5.6. Normally, I shoot at one-third EV stop down settings (rarely use flash at 1/180th requiring half stops). With all of these lenses, you tend to see the lenses stay at the claimed at f/4.5 all the way as you zoom up to f/5.6 - skipping the f/5 setting, and the same as you zoom toward open. Sometimes you can get the f/5 to register (especially on the 18-55), sometimes not (almost never can get it with the other two lenses - and on the Sigma most of its range is around f/5 as maximum). Moving the aperture setting on a lens that should have f/5 available as maximum will result in skipping between f/4.5 and f/5.6; if you start the lens at the widest focal length you can set f/5 but when you reach the range of the zoom that is f/4.5 it jumps down to that setting and will not allow f/5. Frustrating! By the way, I notice that half stops are also using f/4.5 at the supposed midpoint between f/4 and f/5.6 - which most certainly it is not. Also, testing out the situation fully, I checked on the other end of the variable spectrum to minimum aperture (not that I ever use that setting) and found the behavior to be similar - often in the middle of the range the minimum aperture - such as f/25 is skipped entirely and the aperture shown moves from f/22 to 29.

At first blush, this shouldn't be a big deal, but I do feel that most zoom lenses tend to improve greatly when stopped down by only one-third of a stop. A few years ago, I would have chalked this up to my general view that variable aperture zooms aren't worth the worry (although the 70-210 shot in the middle of its range is impressive even wide open). Now, the best available walk-around zoom lenses - IMHO - are the 16-85 and the Sigma 17-70 (C) - both of which happen to be variable aperture. The skipping aperture settings is enough to discourage me from moving to one of those lenses from the venerable 16-45 where I always have access to every third of a stop. I get the strong sense that constant aperture lenses tend to yield more consistently accurate exposures than the variable aperture lenses, as well.

The issue with setting the exact focal length is more inconvenience than anything that will have a direct impact on image quality. Acquiring the Sigma 12-24 means that it will be more difficult to discern shots taken with it vs. the Samyang 14mm (which is usually stopped down to at least f/5.6 for IQ purposes) in the EXIF. I set it at 12mm on startup, as I'd rather undercompensate SR than overcompensate - it likely is closer to a 13mm anyways. I do know how to rewrite the EXIF using EXIF Pilot, but there just isn't enough incentive for me to go through that process. Obviously, Pentax already has the ability to program to exact focal length and precision SR because the Q allows for it. Apparently the Q requires greater precision than the larger format models (sarcasm).

For those who are bothered by either of these issues (especially the variable aperture problem), I'd be interested in your reaction and experiences. I certainly understand that many users don't see either of these items as a concern, but I wasn't intending to post the thread as a poll or opportunity to be defensive. I view the K-3 as a great camera, but these two concerns would prompt me to lower its rating by a total of 1 point on a 100 point scale.


Last edited by ScooterMaxi Jim; 09-03-2015 at 11:00 AM.
09-03-2015, 11:13 AM   #2
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I think it is lens dependent since I have noticed my DA20-40 have 4 threshold max aperture setting from 20-40 range, 2.8, 3.2, 3.5, 4.0. I am not sure about the new 16-85 lens though.
09-03-2015, 11:15 AM   #3
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QuoteOriginally posted by ScooterMaxi Jim Quote
At first blush, this shouldn't be a big deal,
That was my first reaction, but as I read through I wonder what real world impact this is having? I've noticed that on some lenses (the 16-85 in particular) the k-3II seems to underexpose a lot. Is this related? The 16-85 is the first modern variable aperture lens I have used extensively. Having moved to it from the 16-50 where I did not see any exposure issues, I'm wondering if this is related to your findings or something else.
09-03-2015, 11:41 AM   #4
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QuoteOriginally posted by aleonx3 Quote
I think it is lens dependent since I have noticed my DA20-40 have 4 threshold max aperture setting from 20-40 range, 2.8, 3.2, 3.5, 4.0. I am not sure about the new 16-85 lens though.
The chip in the lens has an influence, but there might be more than one factor involved. As mentioned, even on the Sigma you can get the f/5 reading to show in the middle of the range as expected, but you have to shift back and forth a few times to convince the camera to pick up the right signal. In other words, the lens is sending the information (at least some of the time), but the body isn't always responding. Even on the still-current 18-55, it is skipping the f/5 setting about as often as it is picking it up. Shifting the lens zooming to a longer focal length once f/5 is set will often result in the camera going back to f/4.5 - which is the opposite of what you would expect.

09-03-2015, 11:41 AM   #5
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QuoteOriginally posted by aleonx3 Quote
I think it is lens dependent
Yes...the lens tells the camera what its current max aperture is. Some reviewers have started including the increment points and values as part of their review. Borderline and corner case conditions aside, the camera is pretty dumb about this sort of thing. After all, what source of data does it have except for what comes across the data contact? As for the fine points of set vs. actual exposure-time aperture on a variable aperture zoom, I believe that most of us would be happy if there were 1/3 stop agreement between the two regardless of what the reported maximum might be. Neither accuracy nor precision are particularly good.*

As for the SR focal length input...I have never touched a Q, but I do believe that all Pentax dSLR with SR limit manual focal length input to those actually offered by Pentax at some point in history.

For record, I have not been particularly bothered by either of these nor do I anticipate thinking about them much even after contributing to this thread.


Steve

* It should be noted that relative aperture may also change with close focus (depends on lens design), though I don't believe that there is any lens/camera adjustment beyond simply measuring attenuated light and compensating that way.

Last edited by stevebrot; 09-03-2015 at 11:53 AM.
09-03-2015, 11:44 AM   #6
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QuoteOriginally posted by ScooterMaxi Jim Quote
you have to choose among what the menu offers
FL information is for SR system. K3's SR system has to move a much bigger unit quickly and precisely than on Q's to compensate the camera shake. I guess the mathematic models to calculate the movement is so complicated and might be easier to preset some FLs.
Just guess.
09-03-2015, 12:08 PM   #7
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QuoteOriginally posted by stevebrot Quote

As for the SR focal length input...I have never touched a Q, but I do believe that all Pentax dSLR with SR limit manual focal length input to those actually offered by Pentax at some point in history.

Steve
That would be unfortunate parochialism if it were so, but Pentax offered 16, 17, 105mm (possibly more) lenses that do not correspond to the settings in camera. Correspondingly, the body settings include some numbers not offered among Pentax primes, but you need to input some sort of number on an MF zoom for SR calibration purposes. I tend to use 65 for the A-35-105 because I'm usually shooting it toward the long end and don't see 65mm commonly listed among the shots I take with my other lenses (such as the 28-70, for instance).

---------- Post added 09-03-2015 at 02:21 PM ----------

QuoteOriginally posted by grahame Quote
FL information is for SR system. K3's SR system has to move a much bigger unit quickly and precisely than on Q's to compensate the camera shake. I guess the mathematic models to calculate the movement is so complicated and might be easier to preset some FLs.
Just guess.
That might be the case. Still, if you could set the FL precisely - and the SR would default to the closest math model computation available - you would still be able to see the right focal length in the EXIF.
09-03-2015, 12:25 PM   #8
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QuoteOriginally posted by ScooterMaxi Jim Quote
you would still be able to see the right focal length in the EXIF.
That is right. Hope pentax can hear and change in the firmware.

09-03-2015, 12:51 PM   #9
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Have you tried setting camera to half EV steps?
09-03-2015, 01:20 PM   #10
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QuoteOriginally posted by jbinpg Quote
Have you tried setting camera to half EV steps?
Yes, I have a User setting for half steps (relating to use of universal flash in bright ambient light conditions). So, tested that out and made note in OP (which I admit is lengthy). Pentax provides you with f/4.5 as a half stop (it should be no less than 4.7 in reality); so all you are getting is less precise steps. It seems to me that is a sign of fudge factoring...
09-03-2015, 04:13 PM   #11
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Technically, variable aperture lenses shouldn't lead to incorrect exposure, since the camera will still meter wide-open (thus accounting for the exact loss of light as you zoom), and it still has precise control over much the lens will be stopped down by. There is rounding involved when it comes to the reported maximum aperture, so what you see in the EXIF may not be entirely accurate, however.

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09-03-2015, 04:49 PM   #12
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QuoteOriginally posted by Adam Quote
Technically, variable aperture lenses shouldn't lead to incorrect exposure, since the camera will still meter wide-open (thus accounting for the exact loss of light as you zoom), and it still has precise control over much the lens will be stopped down by. There is rounding involved when it comes to the reported maximum aperture, so what you see in the EXIF may not be entirely accurate, however.
Adam, of course the lens is rounding. The variable nature of the lens allows for infinite open aperture values. My point is this: the Pentax communication protocol - even at one-third EV stops - often misses the signal that the lens is somewhere close to f/5. So a lens that happens to be on the scale of variability at f/4.7 (selected arbitrarily) should not default to a selection between f/4.5 and f/5.6 - when f/5 is most likely going to yield MTF - as opposed to f/4.5 or f/5.6. As a result, I believe the metering system also gets a false message and will tend to expose incorrectly. However, best MTF is my main concern - as minor exposure shifting in RAW shooting isn't a big deal (most often).

Last edited by ScooterMaxi Jim; 09-03-2015 at 09:56 PM. Reason: I meant to say "as opposed to f/4.5" on the last reference, not f/4.7
09-03-2015, 05:40 PM   #13
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QuoteOriginally posted by ScooterMaxi Jim Quote
However, best MTF is my main concern - as minor exposure shifting in RAW shooting isn't a big deal (most often).
That is the part that puzzles me. I have never heard of a "1/3 stop below full open" rule.


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09-03-2015, 10:14 PM   #14
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QuoteOriginally posted by stevebrot Quote
That is the part that puzzles me. I have never heard of a "1/3 stop below full open" rule.

Steve
I'm not aware of it as a "rule" either. At the same time, most of the tests I've seen show a typical zoom performing considerably better at one stop down than IQ at wide open (not counting lenses on very small sensors). My strong impression from the many zooms I've shot is that the largest share of IQ improvement comes already by just one-third stop down. Whether this is true or not, I would expect the various f/3.5-5.6, f/4.5-6, or f/4-5.6 zooms to allow me to set f/5 and not have to chose between f/4.5 and 5.6 for no apparent good reason when every other 1/3 stop throughout the aperture range (short of minimum aperture closed all the way down) is available.
09-04-2015, 08:51 AM   #15
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QuoteOriginally posted by ScooterMaxi Jim Quote
Adam, of course the lens is rounding. The variable nature of the lens allows for infinite open aperture values. My point is this: the Pentax communication protocol - even at one-third EV stops - often misses the signal that the lens is somewhere close to f/5. So a lens that happens to be on the scale of variability at f/4.7 (selected arbitrarily) should not default to a selection between f/4.5 and f/5.6 - when f/5 is most likely going to yield MTF - as opposed to f/4.5 or f/5.6. As a result, I believe the metering system also gets a false message and will tend to expose incorrectly. However, best MTF is my main concern - as minor exposure shifting in RAW shooting isn't a big deal (most often).
Yep, lots of oddities. With my K-3 set to 1/3 stop increments, max aperture reported in viewfinder as I zoom in:

DA 18-135: 3.5 --> 4.0 --> 4.5 --> 5.6 (no 5.0)
Sigma 17-70 C: 2.8 --> 3.5 --> 4.0 (no 3.2)

Any coincidence these two lenses don't seem to have as consistent metering as my constant aperture zoom? By consistent, I mean likelihood of Apple Aperture changing the exposure when I hit the Auto Exposure button. In other words, I'm not using a subjective measure but letting the computer decide.
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