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09-04-2010, 08:17 AM   #1
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Pentax-M 100mm exposure calibration, Av values.

Since the M lenses meter wrong, i wanted to make a simple correction chart, to know how to correct at each F stop.
I did all the measurements (camera Tv reading at each lens Av setting). But not all of the Av ring clicks have Av values written for them.

They go like this:
F: 2.8 ? ? 5.6 ? 8 ? 11 ? 16 22
I'm guessing they might be:
F: 2.8 3.5 4 5.6 6.3 8 9 11 13 16 22

If referring to wikipedia, then full stop values are:
f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22
Then the values between might be full, half or third stops. E.g. 3.2 instead of 3.5.
(F-number - Wikipedia, the free encyclopedia)

Thats why i'd be very happy if someone knew the actual values for Pentax-M 100mm f/2.8 (NOT macro).

After i gather all necessary data i will update this thread with the calibration graph/table.
Currently i'll be using only the known values.

P.S. How come i get different Tv readings (1/30, 1/25, 1/20) in static, controlled setting, when simply pushing green button several times.


Last edited by ytterbium; 09-04-2010 at 09:04 AM.
09-04-2010, 09:58 AM   #2
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The first whole stop above the minimum aperture is typically not labeled on M lenses AFAIK (there's just a dot there instead), so the first click should be F4.

This is what the stops should be for that particular lens:
2.8 4 4.5 5.6 6.7 8 9.5 11 13.5 16 22

Adam
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09-04-2010, 10:32 AM   #3
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QuoteOriginally posted by Adam Quote
The first whole stop above the minimum aperture is typically not labeled on M lenses AFAIK (there's just a dot there instead), so the first click should be F4.

This is what the stops should be for that particular lens:
2.8 4 4.5 5.6 6.7 8 9.5 11 13.5 16 22
Indeed. Now i'm in trouble. They are rounded.
The actual full stop values are:
2.8284 4.0000 5.6569 8.0000 11.3137 16.0000 22.6274
This means i will get some deviation just because of this.
09-04-2010, 10:43 AM   #4
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QuoteOriginally posted by ytterbium Quote
Since the M lenses meter wrong
Actually, it's not the lens that is generally at fault, but certain cameras. I mention this because my camera (K200D) is better than some in this respect, and I do have that lens. I actually get pretty consistent exposures just by Green buttoning it all the way down - assuming I'm shooting in good light. Indoors, I wouldn't normally shoot stopped way down, but when I just tested it, I do end up with exposures that are noticeably and increasingly dark after f/11 or so. I suspect I'm below the EV limits of the metering system by then.

Anyhow, I can confirm that based on extensive real world use as well as boring testing, the first stop definitely appears to be around a full click stop slower than f/2.8. The camera usually meters at half the shutter speed, and the resulting picture is almost exactly the same brightness. And on the occasions when the camera meters only a half stop slower shutter speed, the picture is definitely darker. Someone once told me that the fact that the stop is labeled with a dot is supposed to help indicate that this is a "regular" stop and not an "in-between" stop. The M28/2.8 is labeled similarly.

I can also confirm that as far as I can tell, the in-between stops after f/5.6 seem to function as true half stops as well as anyone might reasonably expect these things to be calibrated. I have my camera set to half stops, and each click pretty reliably takes me down one half stop on my exposure - again, both in real world use and in boring testing. Not surprising, since I've told the camera not to use third stops, but if they truly were third stops, I'd expect the resulting shots at the in-between clicks to be noticeably off the exposures at the full stops, and that isn't the case.

I'd also suggest that the last couple of stops might not be marked correctly on the lens, or else my aperture blades might be a bit slow. Even in good light. the Green button tends to meter these as a half stops rather than a full stop, and the resulting pictures are not half a stop too dark. In poor light, as I mentioned, they *are* too dark, but it seems the camera sometimes doesn't meter f/16 any differently from f/11, and I attribute that to exceeding the metering lower limit. Outdoors, I've also tried using the DOF preview instead of the Green button to see if slow aperture blades are too blame for any inconsistency I see here, and it seems possible, but my tests are inconclusive - sometimes the meter changes by full stops while doing a DOF preview sometimes not (see below). And of course, when I take the actual picture, I'm subject to any slow aperture blade issues.

So I'm on the fence about whether f/16 is accurate or not, but the actual pictures seems to suggest that f/22 is only about a stop and a half past f/11, so somewhere, something might be slightly off. I can't really prove what, though.

QuoteQuote:
P.S. How come i get different Tv readings (1/30, 1/25, 1/20) in static, controlled setting, when simply pushing green button several times.
Assuming you are completely controlling the light so passing clouds or atmospheric disturbances aren't involved, and the camera is on a tripod, so there is no variation in framing, etc, then I always put this down to a camera meter being an essentially analog mechanism and just not all that precise. Especially if the actual metering reading is on the cusp between two different "clicks" in the camera, I just kind of expect to see values occasionally fluctuate. Seems to be the case with all lenses. This does, of course, make it hard to do the sort of testing we are discussing. It also makes me not normally care to do it at all - you can computer all you want, but given that the meter reads themselves are not perfectly consistent. there's still going to be a trial and error component to it. So given that my camera pretty much always meters within half stop of correct (except at f/16 and beyond in low light), I just blow it off.


Last edited by Marc Sabatella; 09-05-2010 at 04:39 PM.
09-04-2010, 03:50 PM   #5
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Test results

Here are my initial results. It is very likely that there is something wrong with actual and true Av calculation and matching. It would be much simpler to use spreadsheet and just divide/multiply by 2 the shutter speeds by hand, but i wanted to make a script just in case i need to test another lens.

Some numbers.
Metered values:
Av=2.8 4 4.5 5.6 6.7 8 9.5 11 13.5 16 22;
@ISO100, 1/Tv [s]
Tv=100 50 30 20 13 8 6 4 3 2.5 1.6667;
@ISO400, 1/Tv [s]
Tv=400 200 125 80 50 30 25 15 13 10 6;

Calculated values:
Av=2.83 4.00 5.66 8.00 11.31 16.00 22.63
@ISO100, 1/Tv [s], Tv=
100.00 50.00 25.00 12.50 6.25 3.13 1.56
@ISO100, 1/Tv [s], Tv=
400.00 200.00 100.00 50.00 25.00 12.50 6.25

Side by side @ iso400:

AvM=2.80 4.00 4.50 5.60 6.70 8.00 9.50 11.00 13.50 16.00 22.00;
AvC=2.83 4.00 ... 5.66 ... 8.00 ... 11.31 ... 16.00 22.63;
TvM=400 200 125 80 50 30 25 15 13 10 6;
TvC=400 200 ... 100 ... 50 ... 25 ... 12.5 6.25;

Graphs:

Conclusions:
When looking at the graphs, several serious limitations must be noted. First, i did an incorrect assumption that exposure (and Av, Tv) values at f/2.8 (or any given f-stop) are correct. This means that the data doesn't represent the full exposure (Tv) error, but only the amount influenced by Av. E.g. if at f/2.8 you need to add +1EV for correct exposure, then at some other f-value, it has to be Tv+1EV+/-Error, to get the correct Tv.

Even if i expected the calculated Av/Tv curve to be a straight line, when looking at the pictures the middle one is indeed exposed brighter to the others. This confirms that the calculated line should be lower than measured values around f/8.

The biggest difference can be observed around f8..f11. It is nearly a half stop between 1/50 and 1/30 at iso400. In addition the value 1/30 used is fastest measured. Previously i told that in this same situation i could get even 1/20 with repetitive green button presses. This is more than one stop of a difference.

To improve the results i should make a reliable and precise Av value matching, possibly including histogram calculations from RAW files to determine change in amount of light. This could even reveal the true light transmittance of the lens (T-values).
For better comparability then a correct relative graph could be drawn - a horizontal curve showing only the error instead of current, climbing one.

Surprisingly, the simple test proved to be a little more complex than expected. Mostly because of actual and rounded f-stop difference, prohibiting direct comparison and simple interpolation. E.g. you cant assume that Tv changes from 200 to 100, with av changing from f2.8 to f4. Besides it is not known if tv=200 is actually at 2.8, 2.83 or some other value rounded to 2.8.

Now i need to figure out a term for what a measurbator has when he finishes he's measurbation.

Test scene:
Simple, bright towel. No windows, one incandescent and two halogen lights. Camera on tripod.



1. ISO400, 1/400, f/2.8


2. ISO400, 1/30, f/8


3. ISO400, 1/10, f16 (note - one Av stop, should give 1/15 for one Tv stop)

Rough code i wrote for GNU Octave/Matlab:
Code:
%Compares calculated (correct) exposure value curve with actual readings
%from camera meter

clear all;
iso=[100; 400];

av=[2.8 4 4.5 5.6 6.7 8 9.5 11 13.5 16 22];
index=1:1:length(av);

known=[1 1 1 1 1 1 1 1 1 1 1];

tv=1./[100 50 30 20 13 8 6 4 3 2.5 1.6667;...
400 200 125 80 50 30 25 15 13 10 6];

av_base=2.8; %value with correct tv, must match some known av that is full stop
tv_base=[tv(1,av==av_base);tv(2,av==av_base)];

av_known=av(known~=0);
tv_known=[tv(1,known~=0); tv(2,known~=0)];

av_true=sqrt(2).^(3:1:9);
index_tr=1:1:length(av_true);
av_base_idx=index_tr(abs(av_true-av_base)==min(abs(av_true-av_base)));

switch av_base_idx
case 1,
tv_pwr=[0, 1:1:index_tr(end)-1];
case index_tr(end),
tv_pwr=[-(index_tr(end)-1):1:1, 0];
otherwise
tv_pwr=[-(av_base_idx-1):1:-1, 0, 1:1:(index_tr(end)-av_base_idx)];
end;
tv_true=[tv_base(1,:).*(2.^tv_pwr); tv_base(2,:).*(2.^tv_pwr)];

plot(av_true,tv_true); hold on;
plot(av_known,tv_known,'*');
xlabel('f - number');
ylabel('Shutter speed [s]');
title('Correct and actual shutter speed compared:');
set(gcf,'color',[1 1 1]);
legend(['Calc. speed ISO',num2str(iso(1,:))],...
['Calc. speed ISO',num2str(iso(2,:))],...
['Measured speed ISO',num2str(iso(1,:))],...
['Measured speed ISO',num2str(iso(2,:))]);
(can this code be collapsed somehow with it's own scrollbar?)

Last edited by ytterbium; 09-04-2010 at 04:17 PM.
09-04-2010, 06:05 PM   #6
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You guys have forgot more than I'll ever hope to know about this stuff,
But I still feel temperature rise above ambient is a critical factor related to
sensors and metering, more so with cmos than ccd, but still, it affects both.
Has anyone ever seen anything related to that?
Agree with Mark, regulated light source would be critical to stability of data
Something I dont understand, on average, with all manual lens I have,
2/3's stop -EV gets me close in most, average circumstances
(excluding full open/closed apeture)
my m300/4 @ f/4 does just fine with above
Tam.360B on other hand ,is dead on the money, at f/2.8 with no compensation.
closed up, even half stop, same applies, need around -2/3EV
all are average,obviously, exceptions occur and Tam. does have "A" info at hand.

Last edited by BillM; 09-04-2010 at 06:18 PM.
09-05-2010, 02:52 AM   #7
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QuoteOriginally posted by BillM Quote
...obviously, exceptions occur and Tam. does have "A" info at hand.
"A"-type lenses are known to meter correctly.
09-05-2010, 05:21 AM   #8
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I have 2 observations here

First my experience with pentax lenses is that the first and last aperture clicks are full stops and all the middle are half stops

Second camera metering. On old. Lenses is inaccurate at best

Look for my post on the K10 metering

09-05-2010, 05:58 AM   #9
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Hi ytterbium, sure does on my A70-210, but Tamron of mine is a little erratic.
Dont mean to divert thread in that regard.

would like to ask if you feel shooting burst mode at each apeture setting
and taking average would add stability to data in regards to each term

really ramps up shutter clicks though

If heat were to manifest itself, that would be the kind of situation it would show.
Lowell always has something good to say, going to look at his thread.
regards,
Bill
09-05-2010, 06:16 AM   #10
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I doubt that temperature should affect the reading a lot.
Since silicone is pretty sensitive to temperature changes, most schematics and structures have some sort of mechanism to avoid heat influence, if required.
Otherwise you'd get readings all over the place instead of EV portions.

The changes i observed i would account to the following factors:
-Eye position relative to the viewfinder (e.g. in sunny days be sure to push your eye hard to the viewfinder, so light doesn't enter it and affect exposure).
-Changing light reflected from the walls (glossy bathroom tiles).
-Minor factor but - slight movement of the fabric photographed, caused by wind of my motion.

Also i've always wondered if lights in viewfinder don't affect readings. Theoretically, they should be strobed and reading taken while all of them are off.
09-05-2010, 06:28 AM   #11
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I agree Y, trying to help smooth those 'lumps' you have in plots

perhaps a shot excluding spectrum of LED's(assume) would show in histrogram
Kind of a "dog chasing tail"... how to establish white balance

Last edited by BillM; 09-05-2010 at 06:36 AM.
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