Broken Light Sensor?

[jonnojohnson]

Hi,

I have a Canon SD700IS and a few months ago it started intermitently taking very dark pictures.The view on the LCD screen is normal until I press the button to focus upon which about half the time the picture on the screen goes way too dark (as do the pictures it takes). I don't know a whole lot about the camera or photography in general so if there's something I can try to debug the issue further please let me know.

I can sometimes get a good picture by turning it off and on. It seems pretty random. I am guessing there is a light level sensor that is activated when I press the button to focus. This also happens in Manual mode. Can I manually set the light level somehow to avoid this sensor? Any explanation of what is going on would be useful. Should I bother trying to get it repaired or is the cost likely to be half the price of a new camera?

Thanks,

Mark.

[Anbesol]

Mark, welcome to the forums. It could be a number of reasons that is causing this. I think the most likely possibility is that you are inadvertently setting the exposure compensation very low by accident, something that would be reset by restarting your camera. Another possibility is that you have changed the metering type, perhaps spot, for example - and the metering data collected is incorrect for the specific shots. Another possibility is that the particular shots require a flash, and the flash is somehow not functioning the way it should, perhaps inadvertantly setting the flash compensation even. Could you tell us what mode you are using when you use the camera? And any more details to the problem you are having? The more specific and detailed you get, the easier it is for us to determine the root of the problem. If you could simulate the problem again, it would be helpful to us to do so and post up a pic of what is going on.

If it even is a problem that needs repair, repairing a camera isn't likely to be cost effective, Canon customer support usually offers significantly reduced price for upgrades if you have a damaged camera, they really take care of their customers. If it is a problem in need of repair, you're best off working with Canon for that.

I hope that helps,

Charlie.

[jonnojohnson]

Thanks for the reply Anbesol. I will try some experiments with the camera this evening.
I can tell you that this does happen in Auto mode which I use most often. If it would help I could take some pictures in specific situations. Are there some things I should look for on the display when doing this? Would it help to be in Manual mode?
It happens in bright light outdoors as well as dim light indoors.
Turning the camera off and on again doesn't always fix this but sometimes I get lucky.
I don't know anything about the metering type. How would I check this?

[Anbesol]

Not sure specifically where the metering options are available on the SD700, but there should be 3 different modes, Wide/Multi-Segment, Center Weighted Average, and Spot. Browse through your in-camera menu and see if you can find those words. If its on spot or even center weighted, it could cause some unique shots to do some strange metering. Another possibility is that you are setting the exposure compensation down, there should be an option to set whats called an "exposure value" up or down, it defaults to 0, but can go up or down 2 stops, if it was set down 2 stops, it would certainly be very dark. Both of these things can be more specifically detailed in your manual, under 'metering' and 'exposure compensation'. If its happening due to the metering type, it will be very clear to see in the shot. Try and duplicate the problem adn post the images up here, and hopefully then we can nip for certain what problem you are having. Could even be a sensor issue (though, doesn't sound like it).

[jonnojohnson]

Ok so I've tried to pin this down a little. I put the camera in Manual mode and fiddled with the ISO setting. I point the camera at a subject and gradually switch the ISO from 80-100-200-400-800 while checking at each setting. The ISO80 setting almost always goes dark when I press the button and then depending on the subject I'm looking at, somewhere at a higher ISO setting it will start operating normally

Is this the way it should work in Manual mode?

FYI the metering type is set to Evaluative (as opposed to center-weighted or spot)

Update: Note that this is not consistent even with the same subject. Ie I just tried this again with the same subject (same lighting etc) and now even at ISO80 the picture looks fine.

[jonnojohnson]

Here are 3 example photos taken in succession:
ISO 200

ISO 400

Auto


Every setting <= ISO200 is the same and every one >=ISO800 is the same (inc. auto).

However 10 minutes later I tried the same thing and AT FIRST the Auto setting AND the ISO80 setting game me the light picture. After changing the ISO a few times though it reverted to the sequence above.

[Anbesol]

In full manual mode you need to manually set the metering every time, meaning you will need to determine the shutter speed, aperture, and ISO every single shot. The higher the ISO, the brighter it will be with the same metering. For example, if a shot looks good at f2.8 - 1/60th sec - ISO 100, then at ISO 200, you would adjust the shutter speed down to 1/120th, or the aperture to f3.5, otherwise if the ISO is all you change, then the shot will become one 'exposure value' brighter.

Essentially the problem your having is that manual mode is not gathering the light data on its own, its depending on you to tell it what to do. I would stick with the automatic mode if I were you. Even most pro's stick with halfway manual modes, I primarily use Aperture Priority, meaning I tell the camera what ISO and Aperture I want, and it automatically determines the shutter speed I need for the shot. The manual mode is really for tricky shots that the metering doesn't read well. Does that help you solve your problem?

shutter speed
Aperture
ISO or Film Speed

[jonnojohnson]

I understand what you're saying Anbesol but I think I've got another effect happening on top of that ISO-ShutterSpeed-Aperture balance. There is clearly something else that is causing my pictures to be too dark in an intermittent way. I'd gladly leave the camera in Auto mode but the problem is it intermittently gives me dark shots for the same subject. I put it in manual mode only to try to nail down what is happening.

[jonnojohnson]

Ok I learned that there is metadata in the image files so I did some more experiments. There seem to be 2 situations broadly once I turn the camera on.
1. All pictures turn out normally (light)
2. Pictures in Auto mode (or Auto ISO mode) and low ISO values turn out dark and high ISO values turn out light.
In case 1. I see the following combination of ISO-aperture-speed (light/dark)
80-f/2.8-1/50 (light)
100-f/2.8-1/60 (light)
200-f/2.8-1/125 (light)
400-f/2.8-1/250 (light)
800-f/2.8-1/400 (light)
In case 2. I see:
80-f/2.8-1/250 (dark)
100-f/2.8-1/320 (dark)
200-f/2.8-1/640 (dark)
400-f/5.6-1/320 (light)
800-f/5.6-1/640 (light)

Comparing equal ISO settings it makes sense that I would get darker pictures with such fast shutter speeds right? But why would my camera choose to set the speed so fast? How would you expect a 100-f/2.8-1/320 picture to compare to 400-f/5.6-320 in terms of light level?

What is the process the camera uses to set the shutter speed and use of ND filter when I manually set the ISO?
Does it sense the light level and then choose a combination of aperture (filter on/off) and shutter speed or does it sequentially decide on the filter and then sense what shutter speed to use?

[Anbesol]

How would you expect a 100-f/2.8-1/320 picture to compare to 400-f/5.6-320 in terms of light level?

They should be the same, the ISO is two stops faster, and the aperture is two stops slower on the second one - balanced.

I don't understand what you mean by 'use of ND filter', are you really using a neutral density filter on your camera?

Does it sense the light level and then choose a combination of aperture (filter on/off) and shutter speed or does it sequentially decide on the filter and then sense what shutter speed to use?

Basically, it tries to get the middle ground, so to expose it just enough to not over or underexpose it. Sometimes this gets tricky with spot or center weight, but as you've already said you are sticking with the evaluative which would meter off of most of the frame. Yes it senses the light and then determines shutter speed and aperture, in either aperture or shutter priority, you decide one, it decides the other. The neutral density filter shouldn't 'darken' the exposure, but rather - cause a significantly longer shutter speed to be metered.

It sounds like you have a very bizarre problem with your camera, this sounds beyond a metering problem, it may be a problem with the sensor. I would contact Canon about it and see if that particular unit has a history of this particular problem. Judging by case 2 - all those shots should be equally exposed.

Comparing equal ISO settings it makes sense that I would get darker pictures with such fast shutter speeds right?

Actually, all those stats in case 2 should be nearly identical in exposure. One full ISO stop is like, 100 to 200, then 200 to 400, and 400 to 800. So, a shot at 400 at 1/80th of a second should expose the same at 1/160th of a second at ISO 800, you gain a stop and lose a stop, but the shot has the same exposure value. Make sense?

[jonnojohnson]

They should be the same, the ISO is two stops faster, and the aperture is two stops slower on the second one - balanced.

Ok, I hadn't realized that f/5/6 is 2 stops different than f/2.8. I had assumed it would be one since it is a factor of 2.

I don't understand what you mean by 'use of ND filter', are you really using a neutral density filter on your camera?

Sorry, someone else told me that my camera (SD700IS or IXUS800) does not have an adjustable iris or aperture but just has a neutral density filter. Either it is off (f/2.8) or on f/5.6).

Basically, it tries to get the middle ground, so to expose it just enough to not over or underexpose it. Sometimes this gets tricky with spot or center weight, but as you've already said you are sticking with the evaluative which would meter off of most of the frame. Yes it senses the light and then determines shutter speed and aperture, in either aperture or shutter priority, you decide one, it decides the other.

Hmm ok. I'm guessing that with my ND filter my camera would probably try to use a fast shutter speed first and if it can't go fast enough then it would switch in the filter but that's purely a guess. I haven't found anything in there to control the ND filter or shutter speed.[/QUOTE]

It sounds like you have a very bizarre problem with your camera, this sounds beyond a metering problem, it may be a problem with the sensor. I would contact Canon about it and see if that particular unit has a history of this particular problem. Judging by case 2 - all those shots should be equally exposed.

Actually, all those stats in case 2 should be nearly identical in exposure. One full ISO stop is like, 100 to 200, then 200 to 400, and 400 to 800. So, a shot at 400 at 1/80th of a second should expose the same at 1/160th of a second at ISO 800, you gain a stop and lose a stop, but the shot has the same exposure value. Make sense?

Right, so all the shots in 1. should expose about the same as each other (perhaps the last one should be a bit darker since it is not 1/500 shutter speed) and all the shots in 2. should be about the same as each other but darker than those in step 1.

So there are 2 questions:
A. What could cause my camera to sometimes under/overestimate the amount of light (intermittent sensor) and thus set the aperture/shutterspeeds to different levels?
B. Why are the pictures in 2. not all the same exposure level?

Weird.

[jonnojohnson]

I guess since I've already opened this camera up once before (I replaced a broken lcd) I'll open it up and try cleaning it a bit with compressed air to see if there is some dust or something creating the problem. Is this a bad idea or are there any tips for doing this?

[Anbesol]

The factor of 2 in stops only applies to shutter speed and ISO, so 100-200 is 1 stop, 800 to 1600 is 1, 1/30th sec to 1/60th is 1, etc. With aperture blades the division of two applies to 2 stops, just because of the way the aperture is calculated.

A. What could cause my camera to sometimes under/overestimate the amount of light (intermittent sensor) and thus set the aperture/shutterspeeds to different levels?
B. Why are the pictures in 2. not all the same exposure level?

A) It may be a problem with the aperture blades, though by the metering values you've presented that seems a bit of a stretch. If it were an aperture blade problem, the blade is between the lens elements, so taking it apart and dusting it wouldn't make a difference. And, no, it can't be taken apart and cleaned (its hard enough on full size SLR lens). Even the most stretched possibility thats created by any dust problem I dont think could apply in this situation. it may be an issue of the firmware executing incorrect ISO values, aperture values, or the sorts, in which case, dusting wont do anything.

B) They all have nearly identical exposure values (within 1/3rd stop), so again, they *should* all be the same. The fact that this is a problem that seems to revolve around the ISO, it may be issue with the firmwares execution of ISO values in the shot. I've never seen or even heard of anything like this though. But you've presented quite the unique problem here.

Could you post the case 2 shots all with the exif information attached?

[jonnojohnson]

Could you post the case 2 shots all with the exif information attached?

Do you have a suggestion where I can post them?

[Photo-John]

There's a lot of information posted here and it's hard to keep track of it all. I can't help wondering if everyone is working too hard and mossing something obvious. Like, is the exposure compensation set for -2 when the camera is in auto mode? Or maybe - and I think Anbesol may have mentioned this - the metering mode is different in manual than in auto? Remind me - is this a problem when swicthing between manual and auto, or does it appear to be completely random?

A couple of notes.

The Canon SD cameras "manual" mode isn't really manual. It's really just auto exposure with the option to change ISO, white balance, etc. But you still aren't able to manually set the shutter speed or aperture.

I believe the SD700 IS does have a real aperture. I think it may have a neutral density feature to help in really bright sunny conditions. But I would be really surprised if it didn't have an actual bladed aperture.

[jonnojohnson]

There's a lot of information posted here and it's hard to keep track of it all. I can't help wondering if everyone is working too hard and mossing something obvious. Like, is the exposure compensation set for -2 when the camera is in auto mode? Or maybe - and I think Anbesol may have mentioned this - the metering mode is different in manual than in auto? Remind me - is this a problem when swicthing between manual and auto, or does it appear to be completely random?

Although the behavior happens in Auto mode I've done all my debugging in Manual mode. I checked and even in Manual mode I have the exposure compensation set at 0 and the metering mode set at evaluative.

The Canon SD cameras "manual" mode isn't really manual. It's really just auto exposure with the option to change ISO, white balance, etc. But you still aren't able to manually set the shutter speed or aperture.

That jives with what I can tell from playing around in the menus.

I believe the SD700 IS does have a real aperture. I think it may have a neutral density feature to help in really bright sunny conditions. But I would be really surprised if it didn't have an actual bladed aperture.

Well I have never seen a photo with anything other than f/2.8 or f/5.6. Don't know where else to check this.