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Dave Passmore
Rating: N/A
Votes: 0 (Vote!) | | Posted on Tuesday, August 24, 2004 - 06:03 pm: | 
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Hello,
I have been trying to calibrate the meter in my Petri color 35. I take it in the back yard with my Fm2. I meter off the side of the house with the Fm2, then adjust the Petri to match. I also meter off a fence and go in the shade and meter off another wall. I change the aperature and shutter speed as I go , adjusting the Petri. I can get it dead on on the walls and fence but......when im done and go out on a shoot ,the Petri meter will be as far as 2 stops off taking general scene shots as compared to the Fm2. What to do? Is there any instructions or methods to calibrate meters around?I realize no two meters are exactly alike, but I would think I could get it closer than this ......Thanks , Dave |
M. Currie
Rating: N/A
Votes: 0 (Vote!) | | Posted on Tuesday, August 24, 2004 - 08:33 pm: |
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Since the Fm2 has center-weighted metering, it may not ever match an averaging meter in the field. If the Petri tracks the Fm2 reading evenly lit surfaces, I think it's as accurate as it will get. |
Jackson
Rating: N/A
Votes: 0 (Vote!) | | Posted on Tuesday, August 24, 2004 - 10:11 pm: |
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In addition to weighting differences, the CDS cell in the Petri probably has a much different angle of response than the TTL metering in the Fm2. You'll need to compensate differently for various situations compared to the Fm2. |
Jim Brokaw
Rating: N/A
Votes: 0 (Vote!) | | Posted on Tuesday, August 24, 2004 - 10:34 pm: |
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I'd check it using a '18% gray card' as this is the old standard (before "1000-sensor 3D color-matrix meters") and I'd do it from close up. The Petri Color-35 (a great little camera BTW) probably 'sees' about a 40-degree angle, and you want it to all see the gray card. The FM2 will be easy, you want it to be the full viewfinder image. Just be sure to not shade the card with the camera body or your hands, etc. A reading off the 18% gray card should be the same for both cameras if the meters are calibrated the same. If you know you get good exposures with your FM2, then set the Petri to match the gray-card readings of the FM2. Set the meter in bright direct light, open shade, and maybe inside, but don't expect it to be exactly linear in all conditions... probably your FM2 will be, but its got a more sophisticated meter circuit I'm sure. |
dave passmore
Rating: N/A
Votes: 0 (Vote!) | | Posted on Tuesday, August 24, 2004 - 11:25 pm: |
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Hello,
Im going to give the gray card a shot next,it only makes sense.I'll put a 35mm lens on the Fm2 to match as close as I can to the Petri 40mm lens.I guess the main problem is that the Petri has two trim pots to adjust the meter with, so coming up with a balance between them that will measure correctly at different speeds and aperatures is a challenge.......Dave |
Michael Linn
Rating: N/A
Votes: 0 (Vote!) | | Posted on Wednesday, August 25, 2004 - 08:11 am: |
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Hi Dave-
The trim pot on the rewind side is the low-light adjustment, meant to be set to a light box standard of LV 9, which would be ASA 100, 1/30, f4 for example. The wind side trim pot is the high light adjustment at LV 15, ASA 100, 1/250/ f 11. The low light should be set first, the rechecked after any adjustment of the other pot, and the checked at LV 12.
I would test your Petri against a known good reference on a uniform background at LV 9 and LV 15, or as close as you can come to these values. The original factory spec for this camera's metering system is +/- 1/2 stop, so I wouldn't bother with adjusting it if it's in that range or close. If you try to improve on that, you'll probably be chasing that needle around all day!
Jim is correct that this type of metering system is not going to be as linear as your FM2, particularly below LV 9. It's a great little camera, however, with a very sharp lens.
Mike |
Dave Passmore
Rating: N/A
Votes: 0 (Vote!) | | Posted on Wednesday, August 25, 2004 - 01:10 pm: |
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Michael,
Thanks for the info! I dont have access to a light box,but I will try to simulate the conditions you mentioned.The meter didnt work at all when I got the camera and I saw that some one had tried to make other repairs on the camera. Everything is functioning now ,just need to touch up the meter.......thanks ,Dave |
Conrad
Rating: N/A
Votes: 0 (Vote!) | | Posted on Saturday, September 04, 2004 - 08:47 am: |
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My experience with CdS meters using a 2-pot calibration arrangement, is that they almost always have a 1 or more stop error compared to very linear photodiode meters. The intensity response curve is just that, a curve, so it's just a question of where you want that error to show up. It's easiest to figure out what's going on if you can get access to a light box, but they're few and far between. I try to get things calibrated at the normal maximum brightness I encounter, and let the higher values go off. Same thing at the bottom end. Since I don't do much dim light photography, I calibrate the lower end at a moderate light level. That gives me excellent accuracy over the range I use, but probably puts the meter out of spec at the high and low ends. BTW, you can use a dimmer on a lamp pointed at a wall to set various light levels for comparison. |
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