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Old Mar 29, 2008, 10:49 PM   #1
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I shoot with the XTI and I recently upgraded to a sigma 18-50 2.8 from the original kit lens. I love the results of the new glass, but in the past week, I have encountered a few very strange problems. When I am shooting outdoors/indoors in av mode, the camera severly overexposes anything 6.3 and higher. When it is around 2.8 and 4, it functions properly. Now the problem goes away when I fine tune the lens in Manual Mode, but sometimes I'd like the ease of dialing it to AV and firing at ease. Also, I have played with the exposure compensation (with varying results), but I should be able to find proper exposure without having to bracket each shot.

Does anybody have an idea as to why the camera might be "metering" an incorrect amount of light when I am at a small aperture value (eg f9)?

Thanks
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Old Mar 30, 2008, 1:48 AM   #2
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did u try taking the same shot twice at different settings.

one reason is that mostly metering is jsut an approximate estimate of the dynamic range roughly around the centre point in ur focus screen.

wonder why most of the high profile professional photographers in studio set up use a light meter. For more accurate metering.

Try this, half press ur shutter on different dynamic range of ur picture or different areas of ur picture. In ur eye piece u can actually see the shutter speed altering from one value to another.

Hence it may be a case where the camera spotted or metered on a darker area in the sscreen while other times it was brighter.

Best way to test it and confirm the same is to a picture indoor, on a stationary object thats lit with a single light source

Even spot metering in cameras are not as accurate as it shud be. In fact the spot metering regions are too big even in the super high priced high end cameras.

If u can create a spot meter as small as say a dot or that red light that illuminate when u lock a subject u can get more accurate metering.

AV is even moretricky during night time. thats why i have always resolved to manual metering where by i know what aperture i am going to shoot and adjust just the shutter speed for my needs.

I would def like uto test it in a more controlled environment to just elimate the n number of variables.
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Old Mar 30, 2008, 7:21 AM   #3
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DCMountaineer wrote:
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I shoot with the XTI and I recently upgraded to a sigma 18-50 2.8 from the original kit lens. I love the results of the new glass, but in the past week, I have encountered a few very strange problems. When I am shooting outdoors/indoors in av mode, the camera severly overexposes anything 6.3 and higher. When it is around 2.8 and 4, it functions properly.
That sounds like the aperture blades may be sticking open.

IOW, it's probably not the metering. The camera is probably selecting the correct aperture and shutter speed needed. But, the lens is just not responding when taking the shot (aperture blades are remaining wide open at f/2.8, when the metering tells the camera to use a slower shutter speed, and thinks the camera is closing the aperture down to the desired setting).

See if the overexposure gets worse and worse as you close the aperture using Aperture Priority mode. For example, f/2.8 is properly exposed, f/5.6 is around two stops overexposed, f/8 is three stops overexposed, etc. If you look at the EXIF, my guess is that shutter speeds are faster with the aperture open more, and the EXIF reflects the settings the camera thinks it's using. But, the lens is just not working. Remember, the camera will always autofocus and meter with the aperture wide open, only closing down to the setting used when the shot is taken.

The aperture staying open is the most likely issue when you see these kinds of symptoms, and is usually caused by oil that's leaked on the aperture blades causing them to become sticky (a problem usually found only with relatively old lenses). It could be another defect in the lens causing the aperture to stay open, too.

I'd try remounting it to make sure it's not a bad connection.

But, if it's doing what I think it's probably doing (and see if the overexposure gets worse and worse as you stop down the aperture as I mentioned above), it's probably a defective lens.

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Old Mar 30, 2008, 10:05 AM   #4
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IOW, the camera is metering the scene with the aperture wide open at f/2.8. That way, the viewfinder is brighter and the Autofocus Sensors get more light.

Then, with each one stop smaller aperture (f/2.8, f/4, f/5.6, f/8, f/11, f/16, etc.), the camera's metering is going to use shutter speeds twice as long to insure proper exposure. It assumes the lens is going to follow the instructions to close the aperture down to the desired setting when the shot is taken.

So, if the aperture blades are stuck wide open, you'll get an overexposed image (getting more and more overexposed as you use smaller apertures, represented by higher f/stop numbers).

Your EXIF should reflect what the metering thinks as needed, and you can check it for proper behavior. For example, you may see something like this as you change apertures in one stop increments with the same lighting and ISO speed (shutter speeds twice as long at each one stop change)

f/2.8, 1/2000 second
f/4, 1/1000 second
f5.6, 1/500 second
f/8, 1/250 second
f/11, 1/125 second
f/16, 1/60 second

But, if the lens doesn't respond when the shot is taken, you'll just get brighter and brighter images because the aperture iris is stuck wide open.

See if it's behaving that way (shutter speeds twice as long at each one stop aperture change from f/2.8, but images getting brighter and brighter as you change the aperture setting in Av mode). If so, the lens is probably defective (aperture blades not closing down to the desired setting when the shot is taken).

If a camera has a DOF (Depth of Field) Preview feature, you can check for stuck aperture blades real quick that way, too (the aperture should change to the desired setting, and you can usually hear the blades closing, and observe the viewfinder getting darker with greater depth of field).

But, I don't think the XTi has a DOF preview feature (unless I missed it with a quick skim of the review here).

If you unmount the lens and look through it (look at the aperture iris blades fom the back and front), you may be able to see if anything is obviously wrong with blades, too (they should be dry with no oil on them).

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Old Mar 30, 2008, 11:08 AM   #5
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JimC wrote:
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But, I don't think the XTi has a DOF preview feature (unless I missed it with a quick skim of the review here).
I missed it. ;-) You'll see a small button on the side of the camera's lens mount that's a Depth of Field preview button (it's a little lower than the lens release button).

Try setting your aperture to f/16 or so, focusing on something relatively close. Then, use the Depth of Field preview button (press and hold it down) and see if the viewfinder gets darker with Depth of Field increasing (what you can see in focus in front of and behind your subject). If not, you've probably got a bad lens. You should be able to hear the aperture closing, too.

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Old Mar 30, 2008, 11:41 AM   #6
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Jim..

Thanks for the replies. Unfortunately, I think you nailed it. I used the depth of field preview with the sigma, and got no response. I threw on the canon kit lens that I had lying around and sure enough, the screen darkened and I heard the closing sound.

To give you a little background info...I bought used, and after a few test shots, the lens worked fine. I was recently in Europe, where it was very cold; and that is when I started to encounter the aperture issues. May the cold weather have something to do with this? The lens still works, if I dial the settings in using "M" mode, but that is a bit of a pain when I'm on the go.

What do you think my options going forward are? Do you think this problem could be repaired? And at what cost do you think that would be?

Thanks!!!
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Old Mar 30, 2008, 11:54 AM   #7
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DCMountaineer wrote:
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The lens still works, if I dial the settings in using "M" mode, but that is a bit of a pain when I'm on the go.
It's not going to work that way either, unless you're metering at f/2.8.

The aperture is stuck open. ;-)

So, if you use a faster shutter speed than the meter says is needed for f/2.8, you're going to get an underexposed image.

If you use a slower shutter speed than the meter thinks is needed for 2.8, you're going to get an overexposed image.

So, you'd have exactly the same symptoms using manual exposure. If you set your aperture to something like f/8, then change your shutter speed until the exposure needle is centered in the viewfinder, you'll get an overexposed image. That's because the meter assumes that the aperture in the lens is going to close down when you take the shot, and tells you to use a slower shutter speed (resulting in overexposure).

In addition, Depth of field will always be at f/2.8 if the aperture is not closing. You'll typically have less sharpness and contrast shooting with the aperture wide open using most lenses, too.

So, keep in mind that you're always shooting at f/2.8 using it. If the aperture isn't closing, you're just wasting your time setting the camera to anything other than f/2.8. Consider it a fixed f/2.8 aperture lens for now (or just send it to Sigma and let them fix it). ;-)

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Old Mar 30, 2008, 12:04 PM   #8
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OK. So what you're saying is that when I dial the lens to either "2.8" or "18", it will produce the same exposure at a shutter speed of say, "1/6" because the lens is fixed at 2.8?

I'd rather not operate full time at a fixed aperture, so what do you think a repair would cost me? Thanks!
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Old Mar 30, 2008, 12:10 PM   #9
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Right. If you were using manual exposure, and your shutter speed, lighting and ISO speed are the same, exposure is going to be the same regardless of your aperture setting. That's because it's stuck open (not actually changing when the shot is taken).

So, just leave the camera set to f/2.8 for now, using either manual exposure or Av Mode for the meter to work right.

I don't know what Sigma would charge to fix one.

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Old Mar 30, 2008, 12:17 PM   #10
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Thanks. I just read something on the net about older sigma lenses and newer canon camera bodies. This lens is about 3 yrs old and the xti is less than a year old. The post on the other site (http://photography-on-the.net/forum/...p/t-31322.html) mentioned an err99 message, which I also got. The solution they said was to send it to sigma for a firmware upgrade. Have you ever heard of this?
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