Camera/lens troubles

[GuyF]

Hi everyone,
Recently I started getting the error 99 message on my Canon 400D, and I was having problems with my images. Basically, a lot of the images appeared to be hugely over-exposed, and although I tried some methods of fixing it nothing seemed to help so I sent the camera away to be repaired. Warehouse Express were very helpful in delivering the camera up to the place I was staying on holiday in Scotland once the camera was repaired. Once I got out and started taking some shots though, I discovered I was still getting the same problems. The problem only occurs with my Sigma 105mm, I haven't noticed anything wrong with the bigma, so seeing as the camera has been repaired I'm wondering whether I also have problems with the lens.
Here is an example.
This shot was taken in strong sunlight with a shutter speed of 1/500, I'm sure that in the past I've had good images at this shutter speed in sunlight.

Here are the complete details.
Shooting mode - Shutter-Priority AE
Shutter speed - 1/500
Aperture value - 11.0
Exposure compensation - 0
ISO speed - 400
Focal length - 105.0mm
White Balance Mode - Auto

Fortunately I noticed that it was happening, otherwise I would have had hundreds of useless photos again. I tried again, this time with an shutter speed of 1/4000, which in the past I'm pretty sure has given underexposed shots in sunlight.

Here are the complete details.
Shooting mode - Shutter-Priority AE
Shutter speed - 1/4000
Aperture value - 4.0
Exposure compensation - 0
ISO speed - 400
Focal length - 105.0mm
White Balance Mode - Auto

I had similar problems when shooting on Av mode, shots with an aperture of 5.6 came out like the top image while dropping it down to 3.5 gave useable images. I've cleaned the contacts on the lens since, but I haven't tried taking anymore shots, I will have a go tomorrow.
Anyone got any ideas as to what the problem could be, should I be changing some of the settings?
Thanks for any suggestions,
Guy

[RichBrew]

GuyF:

If it's only happening with one lens, I would strongly suspect that it's not the camera that's the problem. It seems to me that the lens diaphragm is sticking and not closing down to the selected aperture at the moment of exposure, causing the gross overexposure.

RB

[GuyF]

Thanks RB,
I'll give it another try today and if I'm still getting problems I will consider sending it back to Sigma to get it sorted.
Thanks,
Guy

[WhiskyBottle]

I would sugest that the lens is not stoppong down. Switch to manual and try a range of setting. Have you got a depth of field preview - and does it work?

[GuyF]

Thanks for your suggestions WhiskyBottle. I tried doing some test shots on both shutter priority and aperture priority and I found that any shutter speed below 1/500 is over exposed and any aperture under 5.0 is over exposed (however by increasing the shutter speed I can then decrease the aperture, and vice versa). I've had a go with manual, and now if I'm careful with my settings I can get images which are properly exposed. I'm not sure whether I've got a depth of field preview, I will check later today.
One advantage of the problem is that it allows me to shoot all my shots on ISO 100 without them being too dark, but then I've noticed that I'm getting a very small depth of field in my shots.
For example this shot I took last week, I had to make sure that all of the dragonflies body was equally far away from the lens otherwise it wouldn't have been in focus, and even then the eye wasn't in focus.

I took this shot on the manual setting (at this point I hadn't worked out how to change the aperture on manual, but I now know how to do this)
Shutter Speed - 1/640
Aperture Value - 5.6
ISO Speed - 100
I will have another go changing around the settings later.
Thanks,
Guy

[WhiskyBottle]

Looks pretty good to me. Depth of field will be very limited in macro, that's the nature of things.
Try taking a shot like above, correctly exposed, in manual. Then stop the aperture down to say f/16, leaving the shutter speed the same. The resulting shot should be quite dark by comparison. If it isn't then the lens isn't stopping down correctly. If it is then you should be able to see the increased depth of field. Do the test shots on something that isn't going to fly away.

[GuyF]

Thanks again WhiskyBottle, today I tried taking a few test shots.
They're all quite dark, I should've used a slower shutter speed, but hopefully they may still help in sorting out the problem.
All of these shots were taken on the Manual setting, shutter speed 1/800 and ISO 200. I had the camera on a tiny tripod and although the shots are at slightly different angles they should all have been the same distance from the camera.
This had an Aperture Value of 6.3

This had an Aperture Value of 11.0

This had an Aperture Value of 16.0

Personally I can't see any difference between the above shots, which I think must be related to the image problems. I tried shots at every setting between an Aperture Value of 6.3 and 45.0 and they all looked like the above shots.
Thanks for your help,
Guy

[glsammy]

On your macro lens the difference in depth of field between wide open and closed isn't that great, especially when used on a subject like this. I know from my own experience when shooting at near closed focus, the difference is very small. It looks like the background to that twig is quite a distance off, so I wouldn't have expected it to be in focus at any setting.

[GuyF]

Thanks for that information Graham, tomorrow I will have some more goes to try and test out the lens a bit more. I've been seeing quite a few shots in the Gallery recently (including yours!) where I've been really impressed with the depth of field, so I think I'll have to play around with the settings a bit more to see if I can get it right.
Thanks,
Guy

[mh68]

I`m no expert on DSLRs and lenses but surely there would be quite a difference in exposure between the f6.3 and the f16 image at the same shutter speed if the lens was working properly?
Looking at the twig you focussed on there doesnt seem to be any change in DoF whatsoever (with the f16 shot i`d expect to see focus going further down the twig,as it looks to be curving away from the camera).
Just an observation Guy

Mark H