Half stop meter error on Minolta and Olympus manual focus cameras

Recently I was pulling out some of my 35mm SLR gear to take on a trip. I noticed that both the Minolta lenses (MC and MD) and also the Olympus OM lenses have approximately half a stop less rotation of the aperture ring when going to the maximum aperture. Some quick tests showed that the lightmetering is indeed about 1/2 a stop out at the maximum aperture compared to smaller apertures. I did not find anything in some online searching (but maybe I used poor search words). Does anyone know why this was done?

Urmonas said:

Some quick tests showed that the lightmetering is indeed about 1/2 a stop out at the maximum aperture compared to smaller apertures. I did not find anything in some online searching (but maybe I used poor search words). Does anyone know why this was done?

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I am not aware of any meter purposely designed and calibrated to be "off" so it must be that they need adjustment or your quick test wasn't done correctly or your reference meter is off or any combination of these.

Are the lenses f/1.8 or f/1.7 lenses?

If so, the maximum aperture is more than a full stop larger than f/2.8, and that might explain what you perceive to be a 1/2 stop discrepancy.

Looks like I need to clarify my description. The issue is with the spacings on the aperture ring. The position for maximum aperture is 1/2 stop away from where it should be. For example with a f/1.7 lens the rotation from f/2.8 to f/1.7 is 1 stop worth (same as distance from f/2.8 to f/4 etc.) There is no position for f/2 as it is taken by f/1.7. For a f/1.4 lens the rotation to get from f/2 to f/1.4 is about 1/2 a stop worth (same rotation as half stop detents at other apertures).

As my simple test I measured the exposure of an evenly lit wall. At smaller apertures the exposures are consistent for example f/4 at 1/125 = f/8 at 1/30 but at maximum aperture the exposure is off by half a stop f/1.7 at 1/500 instead of f/1.7 at 1/800.

I have only seen this on Minolta MC/MD lenses and Olympus OM lenses at least in the limited range of cameras I own. Now both Minolta and Olympus are reputable brands so there must be a reason for this half stop at maximum aperture position difference.

f stops are not linear, they are based on the square root of 2. Each hole stop either cuts the light in half or doubles it depending on the direction you go on the scale if the scale is in whole stops.
Whole stops are f1, f1.4, f2, f2.8, f4, f5.6. f1.7 is the half stop between f1.4 and f2.
https://en.wikipedia.org/wiki/F-number

What you may be perceiving as error may simply be due to the cam action of the aperture ring and font printing on the lens.

Lens focal length divided by entrance pupil diameter equals the f stop. Entrance pupil diameter is lens focal length divided by f stop.
Any 50mm lens at f 1.7 has an entrance pupil of 29.4117 millimeters, at f2 it has an entrance pupil of 25 millimeters. The entrance pupil diameter is determined by looking through the front of the lens at the aperture opening.
A 4.4 millimeter difference in diameter is 2.2 millimeters in radius when viewed through the front of the lens and is likely to be mechanically so close that the aperture ring milling to make the half stop distinct was cost prohibitive.

you are right there is usually half a stop missing on Minolta MD / MC lenses. I don't know why but this is the way they did it.

Looks like I need to clarify my description. The issue is with the spacings on the aperture ring. The position for maximum aperture is 1/2 stop away from where it should be. For example with a f/1.7 lens the rotation from f/2.8 to f/1.7 is 1 stop worth (same as distance from f/2.8 to f/4 etc.) There is no position for f/2 as it is taken by f/1.7.

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Sounds normal. The difference between f2.8 and f1.7 is 1.5 stops

For a f/1.4 lens the rotation to get from f/2 to f/1.4 is about 1/2 a stop worth (same rotation as half stop detents at other apertures).

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The difference between f2 and f1.4 is one stop. There are no half stop detents on Olympus lenses.

Which OM body are you using? If the OM 1, are you using the proper battery. Your meter should read correctly on intermediate stops.

OK I am clearly doing a very bad job of explaining things here. Let me try from a different angle.

For open aperture metering to work the meter circuit must know how far below "fully open" the aperture is set to. This is done using some form of variable resistor which is set by the aperture ring on the lens. To have this work across all lenses the amount of rotation of the aperture ring for 1 stop must be the same. Now Minolta MC/MD and Olympus OM lenses do not meet this requirement, specifically at the maximum aperture the amount of rotation is about 1/2 a stop less than it should be. I checked the metering circuit against itself by starting at a f/16 and opening up the aperture one detent position at a time. Each full stop of aperture change should be compensated by a full step change in shutter speed, so going from f/16 to f/11 requires the shutter speed to change from say 1/30 to 1/60. This is fine for all aperture settings EXCEPT the largest aperture where there is 1/2 stop error in metering compared to smaller apertures. The error is logically expected given that the aperture ring did not rotate the full amount to correctly tell the metering circuit what the aperture setting actually is. This means that at full aperture the exposure will be 1/2 stop underexposed. Now this is an issue if shooting slide film where 1/2 stop error is visible. There must be a reason why this was done. Does anyone know why?

I have never heard of a camera that would give half stop shutter speed readouts. On some cameras the meter scale will show the amount of over or under exposure in third stop increments in Manual mode.
Some electronically controlled shutters will give fractional stop shutter speeds in auto exposure modes but not in manual mode. I think that at a half stop the next higher shutter speed will be displayed.
You need to check the manuals for the bodies you have or independent test results to find out how the shutter works in auto exposure modes and the meter readout limits.
It is possible the aperture resistor is misreading at the wide end.

Which Olympus body are you using?

For Olympus at least, there are no resistors in the lenses. There is a mechanical transfer of the setting of the aperture ring, via the levers at the back of the lenses, and in the throat of the camera body.

FWIW, I attempted to replicate your observations. I don't have a f/1.8 lens for my OM equipment, but I do have a 50mm f/3.5 macro lens.

On two of my bodies (OM-4T and OM-2n) the meters indicate approximately a 1.5 stop difference in exposure between the maximum aperture of f/3.5 and the next full, click stopped aperture of f/5.6.

MattKing said:

For Olympus at least, there are no resistors in the lenses. There is a mechanical transfer of the setting of the aperture ring, via the levers at the back of the lenses, and in the throat of the camera body.

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The aperture lever in the camera body connects to a variable resistor inside the body. The voltage produced by the position of the lever electronically tells the meter what the aperture is adjusted to.

Urmonas said:

This is fine for all aperture settings EXCEPT the largest aperture where there is 1/2 stop error in metering compared to smaller apertures.

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How did you determine there is a one-half stop error on the Olympus?

Yes, your question is a bit hard to follow. I'll start out with a few observations. I've shot pretty much only slide film for 20 years thru numerous Zuiko lenses and various OM bodies (mainly 1, 4, and 4T, but also some thru my 2). I don't often shoot with the lens fully open, but have on occasion. Metering is invariably on manual, so there is no opportunity for the electronic shutter to "fudge" a bit. I have never had an improperly exposed shot on any Oly camera unless I just screwed something up.

But that does bring me to a question I've wondered about and have posted here in the past (without getting a satisfactory answer), which may relate to your question. Shutter speeds on modern cameras do not coincide to the film ISO, but we seen to "pretend" they do. For 100 ISO film the "standard" shutter speed is 125, for 200 ISO is is 250, for 800 ISO it is 1000. Everyone just ignores the ~1/3 stop mismatch and goes on their merry way. Cameras from before about 1960 had different shutter increments, e.g. 25, 50, 100, 200. So your 100 ISO film could be set at a shutter speed of 100. Could it be that you are seeing the "rounding error" that comes from the fast shutter speeds that go along with the fully open aperture? An OM-1 goes up to 1000, an OM-4 goes to 2000, but there is no 800 or 1600 shutter speed that would be the actual, even number increments of 100 ISO film.

Obviously the OP is unaware of the allowable error in mechanical shutters. If he were then a half stop difference in metering would not be a source of concern.

thuggins said:

But that does bring me to a question I've wondered about and have posted here in the past (without getting a satisfactory answer), which may relate to your question. Shutter speeds on modern cameras do not coincide to the film ISO, but we seen to "pretend" they do. For 100 ISO film the "standard" shutter speed is 125, for 200 ISO is is 250, for 800 ISO it is 1000. Everyone just ignores the ~1/3 stop mismatch and goes on their merry way. Cameras from before about 1960 had different shutter increments, e.g. 25, 50, 100, 200. So your 100 ISO film could be set at a shutter speed of 100. Could it be that you are seeing the "rounding error" that comes from the fast shutter speeds that go along with the fully open aperture? An OM-1 goes up to 1000, an OM-4 goes to 2000, but there is no 800 or 1600 shutter speed that would be the actual, even number increments of 100 ISO film.

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ISO is a measure of a film's sensitivity to light. There is no such correlation between shutter speed and ISO, hence no standard shutter speed for a given ISO, no 1/3 stop mismatch, and no rounding errors.

thuggins said:

Shutter speeds on modern cameras do not coincide to the film ISO, but we seen to "pretend" they do.

Click to expand...

Film speed (ASA) equals the reciprocal of the "correct" shutter speed for exposure in normal daylight at f/16, i.e. ASA 100 = 1/100 sec. Whether or not such a shutter speed is available on a given camera is totally unrelated to the film speed.

The exposure error resulting from using 1/125 sec shutter for ASA 100 film in daylight is trivial.
Also, a shutter speed marked 1/125 may actually deliver 1/100*.

At higher speeds, mechanical shutters have a speed tolerance of +/- 30%.
Apertures have no specified accuracy that I know of.
These are some of the reasons why two people using identical equipment may get different results.

- Leigh

*NB: There are a couple of reasons for this.
1) Operator convenience.
Shutter speeds should progress by a factor of 2.
But depending on whether you start at 1 second or 1/1000 second, the numbers are different:
1, /2, /4, /8, /16, /32, /64, /128, /256, /512, /1024, or
/1000, /500, /250, /125, /62.5, /31.25, /15.625, /7.8, /3.9, /1.95, /1
A combination of the two series was deemed to be most "user friendly".

2) Advertising/marketing hype.
Some cameras could do flash sync only at a speed slower than the advertised/marked speed.
E.g. marking a sync speed as 1/125 when it was actually 1/100 could be good advertising.

Discussion
OM system lenses did NOT have detents at 1/2 EV points!
Note that, depending upon data source, f/1.8 is called -0.66EV smaller than f/1.4 and it is also called -0.75EV smaller than f/1.4...and the Leica User Group shows f/1.8 falling IN BETWEEN the two increments!
I tend to believe the Leica U.G. chart (which only shows values between f/2 and f/41.2), as it shows the derivations...

For those of you that are interested and have the skills,

A Wikipedia article, in showing f/1.8 on BOTH quarter-f/stop AND third-f/stop increments, tends to be somewhat in agreement with Leica scale (because of rounding to the nearest tenth)! In any event, the 'debate' about what increment f/1.8 belongs is somewhat moot, as most lens manufacturers RATE their lenses with max f/stops which DO NOT REFLECT REALITY, as more precise lens tests by Modern Photography and Popular Photography in the 1960's and 1970's an 1980's prove (measurements showed f/1.86 and ff/1.87 on different OM 50mm f/1.8 lenses)...This adds uncertainty to any engraved max value of numeric f/stop!

OM-4 Observations
I just did this observation on my OM-4 with 50mm f/1.8 lens (with the camera on ISO 400 and pointed so that an 18% grey card filled the viewfinder) in Auto mode the shutter speed scale in the viewfinder has increments of 0.33EV

1. at f/2.8, shutter speed indicated 1/15 + 0.66EV
2. at f/4, shutter speed indicated 1/8 + 0.66EV
3. at f/1.8, shutter speed indicated 1/30 + 0.66EV to 1/60 (bouncing between the two)

I very carefully repeated my observations several times to ensure that I was making consistent observations about segments being displayed on the OM-4.

OM-1n Observations
So then I took my recently calibrated OM-1n, and repeated the observations with the same three aperture settings, and observed this



Note that the 1EV displacement of the needle from f/2.8 to f/4 resulted in the needle covering fully the white space between the edge of the white opening and the needle, but the (between-2/3-and-3/4 EV) displacement from f/2.8 to f/1.8 left a visible gap in the window. The OM-1 viewfinder marks do indicate 1/3 EV increments (1/3EV when the needle just touches the black 'tab', 2/3 EV when the needle just covers the tip of the black 'tab', 3/3 EV when the needle just touches the edge of the white window opening) The needle position for f/1.8 was NOT equivalent to 1/3 EV nor 2/3EV nor 3/3EV positions of the needle.

I very carefully repeated my observations several times to ensure that I was making consistent observations about needle position on the OM-1.

Conclusions
From these observations I contend that the OP observations have no bearing...both OM-1 and OM-4 behaviors seem to reflect the in-between 0.66EV and 0.75EV true aperture value of f/1.8, and the increment of the detents for f/2.8 and f/1.8 is NOT a 'a half stop out' as is contended.

faberryman said:

ISO is a measure of a film's sensitivity to light. There is no such correlation between shutter speed and ISO, hence no standard shutter speed for a given ISO, no 1/3 stop mismatch, and no rounding errors.

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Apparently you have never heard of this rule called "Sunny 16"? The entire premise of Sunny 16 lies in the fact that every film speed has a "standard" or "natural" shutter speed, and that speed is in theory the inverse of the ISO.

Leigh B said:

The exposure error resulting from using 1/125 sec shutter for ASA 100 film in daylight is trivial.
Also, a shutter speed marked 1/125 may actually deliver 1/100*.

At higher speeds, mechanical shutters have a speed tolerance of +/- 30%.
Apertures have no specified accuracy that I know of.
These are some of the reasons why two people using identical equipment may get different results.

Click to expand...

This would indicate that you don't shoot transparency film. The difference between 100 and 125 is 1/3 of a stop and 1/3 stop is certainly noticeable on a slide. Light meter manufacturers didn't consider the difference trivial either, as meters from the era when speeds were undergoing the transition show both sets of shutter speeds. +/- 30% is essentially a full stop of error, which would make the camera useless for slide film. None of my Oly's have that kind of error.

Leigh B said:

*NB: There are a couple of reasons for this.
1) Operator convenience.
Shutter speeds should progress by a factor of 2.
But depending on whether you start at 1 second or 1/1000 second, the numbers are different:
1, /2, /4, /8, /16, /32, /64, /128, /256, /512, /1024, or
/1000, /500, /250, /125, /62.5, /31.25, /15.625, /7.8, /3.9, /1.95, /1
A combination of the two series was deemed to be most "user friendly".

Click to expand...

This is the best explanation I've seen for the change, and especially makes sense as the shutter speed changes coincided with the popularity of that silly Light Value system in the 1950s. By making making shutter speeds work like f stops (changing the amount of light by a factor of 2) you could treat them as interchangeable. It also allows finer control of exposure as the new system gives an extra shutter speed in the mid range (8, 16, 30, 60 vs. 10, 25, 50).

wiltw said:

OM-1n Observations
So then I took my recently calibrated OM-1n, and repeated the observations with the same three aperture settings, and observed this

Note that the 1EV displacement of the needle from f/2.8 to f/4 resulted in the needle covering fully the white space between the edge of the white opening and the needle, but the (between-2/3-and-3/4 EV) displacement from f/2.8 to f/1.8 left a visible gap in the window. The OM-1 viewfinder marks do indicate 1/3 EV increments (1/3EV when the needle just touches the black 'tab', 2/3 EV when the needle just covers the tip of the black 'tab', 3/3 EV when the needle just touches the edge of the white window opening) The needle position for f/1.8 was NOT equivalent to 1/3 EV nor 2/3EV nor 3/3EV positions of the needle.

Click to expand...

It is hard to tell what you're trying to show there as your example changes both shutter speed and aperture, creating essentially the same three exposure with very different meter indications. And while one may be able to infer 1/3 stops on an OM 1 or 2, Olympus' intention was for these meters to indicate half stop increments.

thuggins said:

Apparently you have never heard of this rule called "Sunny 16"? The entire premise of Sunny 16 lies in the fact that every film speed has a "standard" or "natural" shutter speed, and that speed is in theory the inverse of the ISO.

Click to expand...

ASA (predecessor of ISO) predates Sunny 16. It is a happy coincidence. There is no "standard" or "natural" shutter speed for any given ASA/ISO rating.

thuggins said:

This would indicate that you don't shoot transparency film.

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Interesting how assumptions can be so wildly WRONG.

I shot Kodachrome extensively for over 50 years.

I also shot and processed Ektachorome in the early days, but gave it up when Kodak refused to correct the color bias.

- Leigh

1

faberryman said:

There is no "standard" or "natural" shutter speed for any given ASA/ISO rating.

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As I stated earlier, ASA is the reciprocal of film speed when exposing film at f/16. That's the definition of ASA speed.

- Leigh

I only used Minolta cameras and lenses for four decades mostly using MC and MD lenses and I never had a problem with them. I would recommend that you choose something else to worry about since the aperture spacing is nonlinear.