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02-28-2009, 03:12 AM   #1
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Km Metering Inconsistent across modes

I got this Km in December and I'm wracking my brain trying to understand why it gives me inconsistent metering between "M" and "Av" modes. I can meter in "M" mode, roll to "Av", and get a different shutter speed.

Here are two shots as example. The light came from a window on a sunny day. The camera is on a tripod and I used the 2 second timer to take each shot. The lens is an m42 screwmount 50mm 1.4 Super Takumar. The images were taken in RAW, converted, and resized. They were not otherwise modified.

The first shot was taken in "M" mode. I focused, hit the "Av" button to meter, and pressed the shutter.



f2.8 | 1/60 | ISO 200

Immediately after the shot above I switched to "Av" mode and hit the shutter.



f2.8 | 1/250 | ISO 200

So it went from 1/60 to 1/250 by just changing the mode from "M" to "Av".

I've experienced this with all my manual lenses, and I'm sure I've seen it happen with the kit lens. I just tried recreating it with the kit lens and it didn't work, the shutter speeds were the same. I've also seen the same shutter speed with the Super Taks, though very rarely.

Is there some difference between "M" and "Av" mode that could account for this?


Last edited by wasser; 02-28-2009 at 09:33 AM.
02-28-2009, 03:51 AM   #2
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I do experience this with manual (M42) lenses. If I meter in Av, it's at least 2 stops underexposed, while M with stop down metering is usually dead on.

Last edited by jerrymouse; 02-28-2009 at 04:00 AM.
02-28-2009, 04:12 AM   #3
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I had the same problem with Penta-mirror DSLRs - K100D, K100D Super.
And no problem with Penta-prism - two K10D, *ist Ds

I don't know why, but this is a fact.
02-28-2009, 08:55 AM   #4
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Very Consistent Metering and Results *Then*

NO Pentax DSLR can meter *stop-down* at any mode other than M mode and WITHOUT the press of a button to close the aperture to stop down.

In your case, it means very consistent and accurate metering at different apertures of a Pentax lens, which is usually not the case for previous Pentax DSLR bodies before the K-m.

In your case, stopped down at f/2.8 give you 1/60th second of exposure.

At Av mode, in fact the camera has not been stopped down and you metered with aperture stayed at f/1.4 and hence there are more light received when you did the metering. How much more? Its just two stop more. Hence, the light meter gave you two more stops faster Tv, i.e., 1/250th second.

However, when you finally release the shutter, the lens' aperture closed down to f/2.8 and thus the 1/250th second Tv as given in the Av mode underexposed your pic.

For more read on the technical details and why are those, please refer to my old article: K Mount Metering & Exposure Bases - A Technical Brief

QuoteOriginally posted by wasser Quote
I got this Km in December and I'm wracking my brain trying to understand why it gives me inconsistent metering between "M" and "Av" modes. I can meter in "M" mode, roll to "Av", and get a different shutter speed.

Here are two shots as example. The light came from a window on a sunny day. The camera is on a tripod and I used the 2 second timer to take each shot. The lens is a 50mm 1.4 Super Takumar. The images were taken in RAW, converted, and resized. They were not otherwise modified.

The first shot was taken in "M" mode. I focused, hit the "Av" button to meter, and pressed the shutter.

http://i91.photobucket.com/albums/k292/duzaka/posts/IMGP4196.jpg

f2.8 | 1/60 | ISO 200

Immediately after the shot above I switched to "Av" mode and hit the shutter.

http://i91.photobucket.com/albums/k292/duzaka/posts/IMGP4197.jpg

f2.8 | 1/250 | ISO 200

So it went from 1/60 to 1/250 by just changing the mode from "M" to "Av".

I've experienced this with all my manual lenses, and I'm sure I've seen it happen with the kit lens. I just tried recreating it with the kit lens and it didn't work, the shutter speeds were the same. I've also seen the same shutter speed with the Super Taks, though very rarely.

Is there some difference between "M" and "Av" mode that could account for this?


02-28-2009, 09:32 AM   #5
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Sorry! I should have mentioned the lens is an M42 screwmount (editing the OP). The aperture ring was set to manual in both shots and is therefore physically at f2.8. The metering discrepancy exists at all apertures and different light conditions. Just pointing at the wall near the computer using f1.4 I got 1/60 in "M" and 1/250 in "Av".

However, I'm wondering if you've answered the question anyway. Could the body still be doing the math for stopping down? In other words, the camera is stuck assuming it needs to stop down a specific amount.

[edit]thanks for the article link though. I'll be reading through it. The table of the evolution of the Kmount is nice.

QuoteOriginally posted by RiceHigh Quote
NO Pentax DSLR can meter *stop-down* at any mode other than M mode and WITHOUT the press of a button to close the aperture to stop down.

In your case, it means very consistent and accurate metering at different apertures of a Pentax lens, which is usually not the case for previous Pentax DSLR bodies before the K-m.

In your case, stopped down at f/2.8 give you 1/60th second of exposure.

At Av mode, in fact the camera has not been stopped down and you metered with aperture stayed at f/1.4 and hence there are more light received when you did the metering. How much more? Its just two stop more. Hence, the light meter gave you two more stops faster Tv, i.e., 1/250th second.

However, when you finally release the shutter, the lens' aperture closed down to f/2.8 and thus the 1/250th second Tv as given in the Av mode underexposed your pic.

For more read on the technical details and why are those, please refer to my old article: K Mount Metering & Exposure Bases - A Technical Brief

Last edited by wasser; 02-28-2009 at 10:15 AM.
02-28-2009, 04:33 PM   #6
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QuoteOriginally posted by wasser Quote
Sorry! I should have mentioned the lens is an M42 screwmount (editing the OP). The aperture ring was set to manual in both shots and is therefore physically at f2.8. The metering discrepancy exists at all apertures and different light conditions. Just pointing at the wall near the computer using f1.4 I got 1/60 in "M" and 1/250 in "Av".

However, I'm wondering if you've answered the question anyway. Could the body still be doing the math for stopping down? In other words, the camera is stuck assuming it needs to stop down a specific amount.
It probably has to do with electrical contact being made with appropriate pins on the mount. My K100D behaves the same way if the DATA pin is not shorted to the mount (many people do this with aluminum foil.)


Notice that if all the remaining pins are open, the camera may think the lens has an f-stop range of 2.8-16; if all the pins are shorted (all logical 1's) the camera may think the f-stop range is 1.2-22.

I don't have this entirely sorted out yet & while it seems that insuring the DATA pin is shorted gives reasonable results with an m42 lens on a K100D, the best strategy may be to shoot in manual mode & press the button to meter.

Dave
02-28-2009, 07:51 PM   #7
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You might want to take a look at this thread, which discusses identical behavior on my K200d. The conclusion for the K200d (other bodies are different) was that to get correct metering in Av, you have to either add EV compensation or modify the lenses by adding something conductive, or leave the camera in M and use the green button.
02-28-2009, 08:25 PM   #8
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QuoteOriginally posted by newarts Quote
I don't have this entirely sorted out yet & while it seems that insuring the DATA pin is shorted gives reasonable results with an m42 lens on a K100D, the best strategy may be to shoot in manual mode & press the button to meter.

Dave
Shorting the data pin seems to be not really related to the aperture range contacts nor should it make a difference what the lens thinks the first aperture step is.
This thread and photos also prove the error.
https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/45966-metering...umar-50mm.html
http://forums.dpreview.com/forums/read.asp?forum=1036&message=23619981
The data pin is a serial transfer line from the camera to the lens chip, and back of course. What shorting it does to metering makes no sense..

02-28-2009, 08:30 PM   #9
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QuoteOriginally posted by wasser Quote
were not otherwise modified.


I've experienced this with all my manual lenses, and I'm sure I've seen it happen with the kit lens. I just tried recreating it with the kit lens and it didn't work, the shutter speeds were the same. I've also seen the same shutter speed with the Super Taks, though very rarely.

Is there some difference between "M" and "Av" mode that could account for this?
Care to (if you haven't) check the consistency by using a monotone surface at various apertures?
Is it always 2 stops off??
02-28-2009, 11:53 PM   #10
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newarts and rpriedhorsky, thank you so much for the idea and info about shorting the pins. I just shorted the data pin (topmost pin) with some foil and M and Av mode both give the same shutter speed. Now I just have to figure what method I'll use to short the pin when I use my manual lenses.

QuoteOriginally posted by jeffkrol Quote
Care to (if you haven't) check the consistency by using a monotone surface at various apertures?
Is it always 2 stops off??
Without shorting the pin, I just sat here and pointed the camera at the center of a hand written paper my daughter wrote that is taped to the wall near this desk. It's night and the light source is directly above my head.

50mm f/1.4 M42 Super Tak

Av @ "M" Tv | "Av" Tv

f1.4 @ 1/180 | 1/750
f2.8 @ 1/125 | 1/500
f4.0 @ 1/90 | 1/350
f5.6 @ 1/30 | 1/180
f8.0 @ 1/15 | 1/60
f11 @ 1/8 | 1/30
f16 @ 1/4 | 1/15

And just out of curiosity I shorted the pin and checked three stops throughout the range. The shutter speed was equal between "M" and "Av" modes for all three.
03-01-2009, 01:25 AM   #11
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QuoteOriginally posted by jeffkrol Quote
Shorting the data pin seems to be not really related to the aperture range contacts nor should it make a difference what the lens thinks the first aperture step is.
This thread and photos also prove the error.
https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/45966-metering...umar-50mm.html
Re: How to get good exposure with old Takumar with K100: Pentax SLR Talk Forum: Digital Photography Review
The data pin is a serial transfer line from the camera to the lens chip, and back of course. What shorting it does to metering makes no sense..
It is difficult to understand why the state of the DATA pin (nearest the bottom of the mount) should have a dramatic effect on exposure measurements for the k100D in Av mode. In fact, it does have such an effect.

Perhaps it is a result of how the camera's software responds to measurements it doesn't understand.

I mounted a Macro-Takumar (m42) 50/4 lens. I used a combination of paper and steel foil to force the state (open, 0, or shorted, 1) of the various pins. I set the lens at f/4 and used an LCD monitor as a uniform light source, camera in Av mode, ISO200.

I set pins 1-5 to the open state, corresponding to an f2.8/16 lens (see earlier table)

With the DATA pin open, the camera uses a time of 1/750 sec & EXIF reports F0.0, 1/750
With the DATA pin shorted, the camera uses a time of 1/125 sec & EXIF reports F2.8, 1/125

This is repeatable.

Dave

Edit: I shorted the DATA pin and set the other pins as appropriate for a 2.5/16 lens. The camera in Av mode used 1/125sec and EXIF reported F2.5, 1/125sec. Similar results obtain when the pins are set apropos f4.5/32, ie. EXIF reports f4.5

Last edited by newarts; 03-01-2009 at 02:02 AM.
03-02-2009, 09:42 AM   #12
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Thanks!

Wow this really works. I couldn't believe it Shortened DATA pin and I get correct exposure (in Av on K-m) on M42 lens (At least on pentacon 135/2.8). I will play with it more I guess
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