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03-17-2009, 12:28 PM   #1
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Spotmatic F - open aperture metering

A few months ago, I swapped the standard lens on my Spotmatic F. I had always used my Spottie with stop-down lenses (most often the 55/1.8 Super Tak, and a generic 135mm.) I shot a test roll, and exposure seemed pretty good, maybe a touch overexposed, but you never really know with the age of the meter and the potential effects of the batteries we have to use in PX625 cameras.

This week I've been shooting with the Spottie again. For whatever reason, I decided to switch between stop-down and open aperture metering. (On the Spotmatic F, that's the switch on the side of the lens mount where earlier Spots had the meter switch.)

Now to the crux of my question.... In daylight, I notice a SUBSTANTIAL difference in metering between open aperture and stop down! Probably 2 stops worth. In dimmer light the difference is smaller, but still there. I haven't yet tested it against my handheld meter to see which is the accurate reading, but it almost doesn't matter.

Does anyone have any idea what's going on? Is the aperture coupling out of whack? Any suggestions for troubleshooting/adjusting/fixing? I'd rather not have to send the camera out for a CLA -- it is in perfect user condition otherwise.

03-17-2009, 02:09 PM   #2
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Just checking - is that a SMC or S-M-C lens or just a Super? I believe for the open aperture metering to work you need the former, with the Super only stop down works.

Look here for the difference.
Screwmounts

Of course, if you already know this and are using a SMC, my apologies, and yeah you have some problem or another...
03-17-2009, 03:37 PM   #3
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Sorry, yeah, I should have been clearer. I retired the Super Tak, and now I'm shooting with an SMC Takumar.

Open aperture metering definitely works with the SMC Tak, it's just that the metering between open and stopped down metering don't jive...
03-17-2009, 04:15 PM   #4
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I just checked the Spotmatic meter against my handheld. The open-aperture metering is very accurate -- within 1/3 stop. The stop down metering is wonky...

Oh well, there can be worse things. At least it's not the other way around.

03-17-2009, 04:28 PM   #5
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QuoteOriginally posted by filmamigo Quote
I just checked the Spotmatic meter against my handheld. The open-aperture metering is very accurate -- within 1/3 stop. The stop down metering is wonky...
Good news. Mine behaves the same way.

Now take the SMC Tak off and replace it with a Super Tak. You should see that the stop down metering is accurate again.

Stop down is for Non-SMC lenses. I do not think that SMC Taks will meter correctly with stop down, and I am not sure why... Something about the linkage no doubt. You do use stop down with SMC for DOF preview but not for metering...

Every F I have ever owned worked this way.

Manual here. Difference treated on pages 20-21, but no word about SMC won't work right with stop down.

http://www.cameramanuals.org/pentax_pdf/pentax_spotmatic_f.pdf

If you read closely here http://www.kyphoto.com/classics/forum/messages/2/13854.html?1227772497 you will see that this apparently the case according to others. To wit: "Now I could be wrong, but I seem to recall you should not use stop-down on the Spotmatic F with an SMC lens, only open aperture. Stop-down was for Super Takumar and similar which lack the coupling for full aperture metering, at least that's what the manual says.
You would get an incorrect reading trying to use stop-down with an SMC lens."

The responses to this thread say the same thing: http://www.mail-archive.com/pdml@pdml.net/msg22288.html To wit: "You should NOT use the switch on the side of the camera with the SMC lens except as a depth-of-field preview. It will cause your meter to behave incorrectly when using open-aperture metering capable lenses."

In all simple stuff. Use the switch with SMC for DOF preview but ignore the meter. Use the switch with Super Takumars for metering (and DOF preview).

woof!

Last edited by woof; 03-17-2009 at 07:27 PM.
03-17-2009, 06:23 PM   #6
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You guys are scaring the hell out of me.

I have 3 Spotties with God knows how many lenses that I'm starting to shoot with...Taks, Super Taks and 3rd party...and now THIS comes up?

Screw it. I'm just going to use my 1968 Minolta Auto Spotmeter for everything, and finally learn Ansel's zone system once and for all.
03-17-2009, 07:10 PM   #7
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QuoteOriginally posted by Ira Quote
You guys are scaring the hell out of me.

I have 3 Spotties with God knows how many lenses that I'm starting to shoot with...Taks, Super Taks and 3rd party...and now THIS comes up?

Screw it. I'm just going to use my 1968 Minolta Auto Spotmeter for everything, and finally learn Ansel's zone system once and for all.

Blink...
03-18-2009, 02:54 AM   #8
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Okay....

But you just saw that Spot F + S-M-C or SMC Tak + open aperture metering = good exposure reading. Stop-down is for Super Taks and earlier (and S-M-C/SMC lenses on Spotmatic I and II), and it still gives a good reading.

03-18-2009, 05:09 AM   #9
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QuoteOriginally posted by Ira Quote
You guys are scaring the hell out of me.

I have 3 Spotties with God knows how many lenses that I'm starting to shoot with...Taks, Super Taks and 3rd party...and now THIS comes up?

Screw it. I'm just going to use my 1968 Minolta Auto Spotmeter for everything, and finally learn Ansel's zone system once and for all.
Unless you own a Spotmatic F, ES, or ESII this doesn't apply to you at all.
03-18-2009, 06:16 AM   #10
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Thanks to everyone for the input. It's nice to know that my Spot F is still behaving according to spec! As a side benefit, it forced me to test the meter which I am pleased to find is very accurate
03-18-2009, 06:27 PM   #11
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Open aperture metering

I think that SMC lenses work ONLY in open-aperture mode. That is because the linkage is still working in stop-down mode.

In open aperture metering mode, the lens remains wide open, so the amount of light passing through the lens to the meter is constant. The linkage tells the meter what aperture you have selected. The meter then combines the information about how much light is actually coming through the lens, with the aperture selection and the shutter speed to tell you if you're overexposed or underexposed.

If you stop an SMC lens down, the linkage still tells the meter the selected aperture, but the light actually passing through the lens decreases. So, I would expect the meter to be wrong. Since non-SMC lenses lack the linkage, the camera always thinks that it is wide open, even if it is at f/16. So, the actual light received is what matters. With an SMC lens, the camera adjusts the actual light based on the selected aperture.

IOW, it knows that while you metered with the lens at f/1.4, you have selected f/2.8, so it knows that at exposure time, the light received will be 1/4 what is seeing during the metering phase. When you stop an SMC lens down, you are letting 1/4 the light through AND telling it that you are going to stop down two stops. Does that make sense?

BTW, the meter on a Spotmatic F is always on. The switch on the side of the lens mount is just for DOF preview or stop-down metering with non-SMC lenses. For a Spotmatic F, a lens cap is imperative.

Why would you WANT to use stop-down metering, if you don't have to? I learned on a plain Spotmatic, so I had to use stop-down metering, but I don't miss it on my newer cameras.
03-19-2009, 05:50 AM   #12
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QuoteOriginally posted by noblepa Quote
Why would you WANT to use stop-down metering, if you don't have to? I learned on a plain Spotmatic, so I had to use stop-down metering, but I don't miss it on my newer cameras.

I don't really want to use stop down metering with my SMC lens. But when I checked the DOF and saw the meter go nuts, I started to doubt the accuracy of the whole thing.

By the way, I think learning on a stop-down camera is the best! When my wife wanted to get serious about photography, I handed her a Canon TL set to stop-down metering. She spent the day shooting that way, and gained a much better understanding of the effects of the aperture. At the end of the day, I knew she understood completely when she said "I wish there was a way to check the meter reading and set the aperture without having to stop the lens down and darken the viewfinder..." I showed her the stop down switch, and explained that this "open aperture metering" was the latest thing!

Seriously though, sometimes I think camera designers have abstracted us away from understanding what really happens to make a photograph.
03-19-2009, 06:47 AM   #13
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QuoteOriginally posted by filmamigo Quote
Seriously though, sometimes I think camera designers have abstracted us away from understanding what really happens to make a photograph.
Put another way... automation is a great thing, but it is still incumbent upon the photographer to learn and understand his tools. Nothing better than a manual camera as a teaching aid, especially with respect to the concepts of aperture/shutter speed/film speed and to a certain degree, the optimal ways to handle these variables.

The comment your wife made about darkening the viewfinder is usually made in response to having difficulty focusing "in the dark." This shows a fundamental flaw in the use of the camera.

This is not a criticism of her of course, but I have said it a few times in the context of "Katz Eye/Split Screen black out" issue threads... Pentax has always advocated in the manuals something that they refer to as "Bright Field Focusing."

On a Spotmatic with Stop-down metering this looks like this:

Pre-req
A. Insure lens is set to 'Auto"

1. Focus camera;
2. Stop down and meter via switch
3. Shoot.

This is the pimary flow they advocate and it is one of the reasons why the Spotmatic turns the meter off on every shot. You can see that you go right back to having a bright field and you can recompose/focus, etc...

This can be varied to be:

1. Stop down and meter via switch;
2. Turn off meter
3. Focus
4. Shoot

Some efficiencies are lost here, but not much.

It varies a little on other cameras, but all of it harkens back to the basic "workflow" used with a preset lens, and Pentax has literally documented "Bright Field Focusing" since at least 1957.

My point with respect to Katz Eye/Split Screen darkening is just this... "Who cares?" By the time you've stopped your lens down and are seeing a blacked circle you have already focused... haven't you? Granted, the process is a little different, but if you think about and adapt what I am saying about Bright Field Focusing to digital, you will see what I mean.

Ultimately, my point is this: even digital users have something to learn from the tools and techniques used in manual film cameras for well over half a century. And that feeds right back into your point about designers having removed much of the interface that required really understanding the variables and their interaction.

woof!

Last edited by woof; 03-19-2009 at 06:53 AM.
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