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John
Rating: N/A
Votes: 0 (Vote!) | | Posted on Wednesday, February 08, 2006 - 03:21 pm: | 
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With the standard lens I now get very bad underexposure. With telephoto, all is OK.
I think therefore the fault is with the lens and not the camera. Any ideas, please. I am totally new to camera repair and don't even know how to take a lens apart to check it. Cheers to all. |
Tony Duell
Rating: N/A
Votes: 0 (Vote!) | | Posted on Thursday, February 09, 2006 - 05:23 am: |
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There are at least 3 different lens mounts/couplings used on Prakticas :
1) M42 screw, no contacts (LTL, MTL, etc)
2) M42 screw, EDC (Electric Diaphragm Coupling) contacts (LLC, PLC, VLC, EE, etc)
3) Bayonet mount, EDC contacts (B series)
What could be wrong depends on what sort of camera you're using (and for that matter, if you're using the internal meter, or an external one). |
john lees
Rating: N/A
Votes: 0 (Vote!) | | Posted on Thursday, February 09, 2006 - 01:24 pm: |
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Thanks, Tony. The camera is a Praktica electronic BC1. The lens is bayonet mounted.I'm looking at it for a friend. With no film in it, when I try it with the back open everything seems to be normal, i.e. the shutter works at different speeds according to light conditions both on manual and automatic. Yet still we have the problem of gross underexposure. The ASA dial setting is correct.I'm putting a film in it tomorrow and will try to use it myself on behalf of my friend and I will come back here with the results. Many thanks. John |
Tony Duell
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, February 10, 2006 - 04:35 am: |
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OK...
On the bayonet-mount cameras, the diaphragm works normally with the lens off the camera. Remove the lens, look through it, and see that the aperture changes when you turn the ring.
As I understand it (based on repairing the older EDC cameras -- my bayonet-mount B200 has never needed repair), these cameras meter at full aperture. There's a potentiometer in the lens, connected to 3 contacts on the mount, which tells the camera electronics the setting of the aperture ring, or more exactly, how many stops the lens will close down by when the camera is fired.
What this means is that if the diapragm is stuck at a particular setting, you will get _overexposure_ (apart from the special case of the aperture ring being left at the fully open position, in which case you'll get the correct exposure). You see the camera will be told that, say, the aperture will close by 2 stops, the shutter time will be set based on that, in fact the aperture doesn't close at all (it's stuck), so you get overexposure.
But you're getting underexposure, which means it's not a stuck or sticking diaphragm.
It could be one of 2 things, I think
1) The aperture mechanism has problems, and the lens is wide open when mounted on the camera, but always closes to minimum aperture when the thing fires. There's a little tab on the back of the lens which couples to the stop-down linkage in the camera. It's a bit difficult to describe this, looking at the back of the lens with the focus index on top (I'll call this 12 o'clock), the tab is at about 3 o'clock, and is moved clockwise to open the lens. When released, it is moved counterclockwise by a spring in the lens, until it hits a stop set by the aperture ring. If that stop is broken, the lens will stop right down, further than the meter is told it will by the potentiometer. Result : underexposure
2) There's a problem with the potentiometer, it's not telling the meter the lens is stoping down. Try cleaning the 3 contacts on the back of the lens. If this doesn't help, and you're happy doing electrical tests, I'll stick an ohmmeter on one of my lenses and tell you what you should measure. |
john lees
Rating: N/A
Votes: 0 (Vote!) | | Posted on Saturday, February 11, 2006 - 02:57 pm: |
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Thanks again, Tony. Very instructive and I'm getting a good idea of how these things work. Actually I have misled you because on second thought the camera is grossly OVEREXPOSING.This has meant that SLIDES are returned almost clear with very little on them (some, that is, while others are perfect). The diaphragm is NOT stuck and works normally. I have been doing tests today with an ordinary print film and have tried all sorts of exposures today using auto/manual settings, two different lenses and an old and a new battery to try to isolate the problem. I shall have the result of these tests probably by Tuesday when I get the film developed and I will come back to you with the results. I did clean the three terminals on the lens(es). Thanks so much for your help as this camera is a family heirloom and I want to get it working properly. Will come back to immediately I see the results on the developed film. Cheers, John. |
Tony Duell
Rating: N/A
Votes: 0 (Vote!) | | Posted on Sunday, February 12, 2006 - 05:47 am: |
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If it's overexposing, then either the diapragm is not stopping down properly (maybe it's sticking), or again there's a problem with the EDC coupling.
You indicated in your first message that this problem only occurs with the standard lens. I am assuming this is the case.
Firstly, does the lens stop down (viewfinder image get darker) when you push up the depth-of-field preview slider?
If you open the back of the camera (no film), set the shutter to 'B', and press-and-hold the release button, you can look through the lens. Does the lens stop down (diaphram close) then?
With the lens off the camera, set it to the smallest aperture setting, then move the coupling tab (see one of my earlier posts) in a clockwise direction. The diaphragm should open. Release the tab, and it should close smartly. If it's sluggish, then that could be the problem. |
John Lees
Rating: N/A
Votes: 0 (Vote!) | | Posted on Sunday, February 12, 2006 - 10:40 am: |
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Thanks again, Tony. I will come back to you here in a day or two when I have done all these tests.
Actually I am still trying to establish whether it is just the standard lens or not. Once the film is out (probably tomorrow)I might know the answer to that and I will also try all the tests you suggest and report the results to you.
Excuse my ignorance but what does "EDC coupling" mean. I am a complete beginner but won't be defeated by so-called specialists in the High Street who say it can't be repaired. IT CAN, I know.
Many thanks. Your interest and help is very much appreciated. Cheers, John. |
Tony Duell
Rating: N/A
Votes: 0 (Vote!) | | Posted on Monday, February 13, 2006 - 11:42 am: |
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I 'expanded' EDC in an earlier reply -- it's Electric Diaphragm Coupling.
Basically, the camera has to know the setting of the aperture ring, so that it knows how much longer an exposure to give -- the metering is down at full aperture, the lens then stops down for the actual exposure. If, say, it stops down by 2 stops (an f/2 lens used at f/5.6, say), then the camera will measure the light with the lens at f/2 (fully open) and will give 4 times as long an exposure as would be right for that aperture (this is then the correct exposure for f/5.6).
There are several ways of communicating this information. Most older cameras did it mechanically. Praktica did it electrically, there's a potentiometer in the lens wired to 3 contacts on the mount. Turning the aperture ring moves the wiper round the carbon track of the potentiometer.
I am not sure the bayonet-mount coupling was ever officially called 'EDC'. That term was certainly used for the Praktica screw-mount lenses used on the PLC, VLC, LLC, EE cameras. But since it's electically much the same as the older system, I tend to call it EDC too. |
John Lees
Rating: N/A
Votes: 0 (Vote!) | | Posted on Tuesday, February 14, 2006 - 01:10 pm: |
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Hi again, Tony. I have had the film developed after using two different lenses (perhaps the idea that the fault was just with the standard lens was a misconception), auto/manual settings (all sorts of manual settings) and an old and a new battery. Surprisingly the whole film came out fine with no problems (except one frame wasn't exposed at all - I can't explain that one).
Also I have done all the tests you suggested and everything appears to be working correctly. I can't now find a problem at all and everything happens just as you say it should.
I am now going to load a slide film in. The man in Jessops said that slide films were much more sensitive to exposure settings than normal print film. Is he right ?? (I have never had any problem taking slides with my own camera over the last forty years !!)
I will come back to you again when I get the results from the slides I take.
Many thanks for your interest. John. |
Jan Dvorak
Rating: N/A
Votes: 0 (Vote!) | | Posted on Tuesday, February 14, 2006 - 03:25 pm: |
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John,
"The man in Jessops said that slide films were much more sensitive to exposure settings than normal print film. Is he right ??"
What 'the man in Jessops' meant was that slide films have a very narrow exposure latitude and are much more sensitive to overexposure than negative films. If you underexpose a negative film by 2-3 stops, you might still end up with a decent print, while a slide would be quite useless.
All the best,
Jan |
John Lees
Rating: N/A
Votes: 0 (Vote!) | | Posted on Thursday, February 16, 2006 - 02:27 pm: |
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Thank you, Jan. Yes, that is what I have been told by a couple of "experts" behind the counter now. I am putting a slide film in tomorrow and will post the results here when I get it back. If there are failures I think there must be something wrong with the camera but it is clearly an intermittent fault because with the last slide film used about half came out perfectly and the other half were overexposed. It was not at random, though. There was a sequence of overexposed ones in amongst the good ones. I will keep you and Tony informed here. Cheers, John. |
John Lees
Rating: N/A
Votes: 0 (Vote!) | | Posted on Wednesday, February 22, 2006 - 06:16 am: |
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OK, I've had a 36 reel of slides come back and - guess what - they're all perfect! I can only assume that maybe there is some intermittent fault and will persevere with the camera. On this test reel no fault was found whether it was on automatic or manual (any F stop or speed setting gave good results) and putting an old/new battery in caused no problem. Will post here if the mystery gets solved. Thanks to all those who have made suggestions. John. |
Jacques
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, May 26, 2006 - 06:18 am: |
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Hi, I have recently been to a formulla 1 race and took photos on the side of the track but was disapointed to find some of the photos came out badly underexposed. I uesed a canon eos 20d with a tameron di 28-300mm lense, I used the auto sports mode and my own speed settings wich I measured of the passing cars. The photos I took of the automatic sports setting were perfectly exposed but whenever I switched to my own setting the photos would come out almost black very underexposed. I read in the eos 20d manual that one should never switch between settings and directly take photos thereafter, Do you think that what I did could be the cause. I tried fixing the photos with photoshop but with great effort. Your Advice would be greatly apreciated.
Thanx. |
Jan Dvorak
Rating: N/A
Votes: 0 (Vote!) | | Posted on Friday, May 26, 2006 - 11:23 am: |
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Jacques,
The Canon 20D is notorious for underexposing at the Nürnberg Ring, as well as at the Spanish Formula 1 race track. Take it to this weekend's race at Monte Carlo, exposures should be right on.
On a serious note, even though we all might own digital cameras, this forum is for tinkerers with classic film cameras. If it has autofocus, it is not a classic. If it does not use film, even less so.....
You will be better served if you post your question on a forum specific to Canon digital cameras.
All the best,
Jan |
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