Hamish Gill
Tech Support (and Marketing)
I recently bought a Canon 30d for £50 ... Bargain!
Well, maybe, maybe not!
The thing with this canon that even at £50 makes it not the greatest buy is that it has a scratch across the sensor.
Unfortunately if you scratch your sensor it is likely to be very costly to fix. Dust on the other hand is fairly easy to remove with one of many DIY sensor cleaning kits.
So, I thought since I have a camera with a scratched sensor I thought I would write a bit on how to spot this issue and what the ramifications of it are.
If I take a photo with this Canon the result under certain circumstances can look like this
not ideal!
and yet under other circumstances this is the result
Perfect ...
This is dues to the first one being shot at f22 and the second one being shot at f1.8 ...
So why does this make a difference?
Well, the dust and scratch aren't actually on the sensor at all, they are on a filter that is sat on top of the sensor. What you are seeing is not actually the dust its self, but a shadow of it caused by the gap between the surface of the sensor and the surface of the filter.
In simple terms because f22 is a smaller hole than f1.8 the light is more of a thin beam and so causes a stronger shadow.
Here is a close up of the worst effected area at f22
f16
f11
f8
f5.6
f4
f2.8
f1.8
As you can see buy the time the lens is opened up to f5.6 the problem is all but gone ...
Your mileage may vary, but this is often the case!
As you can see if you want to tell if you have a dusty or scratched sensor the easiest way is to point the camera at the sky take a photo at the smallest aperture your lens will allow!
So how do I know that this is dust on the sensor and not in the lens?
Hold your hand next to any surface and see the shadow it causes, now move your hand away from the surface and see the shadow get more diffuse. Well the same applies here, tiny spots of dust on or in a lens often don't cause a shadow as they are too far away from the imaging surface.
In fact the only time that dust on a lens rears its ugly head is the the exact opposite circumstance that dust on a sensor does ...
When the lens is wide open (eg at 1.8) and areas of the image are out of focus, if within those areas there are points of light the dust can sometimes be seen
Here is a good demonstration of this happening
(thanks for the image Dan)
I'm not clever enough to tell you why this happens ... your just going to take my word for it
The reality is that this doesn't happen very often at all, I have lenses with a lot of dust in them that I have never had any problems with, a dusty lens very rarely causes any noticeable issues to image quality at all!
So back to the sensor dust issues, as as we have seen they are often significantly more problematic!
What's dust and what's a scratch?
Well I guess that's pretty obvious!
The scratch is the line the dust is the squiggle.
Scratches are also often much more visible on the surface of the sensor to the naked eye than dust are.
In this photo you can see the scratch across the bottom of the sensor ...
'But wait!' I hear you say, the scratch is at the top of the photo...
Well this is because the light travelling through a lens travels in a straight line ... your camera actually takes photos upside down!! its the computer inside that puts its the right way up for you... or if its film ... its you holding the photo the opposite way up to how the camera took it ...
So what can be done about these problems?
Unfortunately, dust landing on your sensor is almost an in inevitability...
It gets inside the camera when you change the lens!
I have been told that switching the camera off when changing the lens can reduce static on the surface that attracts dust, I have no idea how true that is though ...
The best you can really do is of course changing lenses quickly and not in the middle of a sand storm!!
If you want to avoid sensor dust, in an interchangeable lens camera the only real way is to buy a film camera ... that way you get a new sensor for every shot!!
So what causes scratches?
Well, here is the really unfortunate bit ... Often scratches are caused by the sensor cleaning process!
If you want to avoid causing scratches when cleaning you just have to be very careful!
Often lens cleaning kits consist of a swab and a form of liquid, probably some kind of alcohol.
Just make sure you follow the instructions, only use the swab once, and only swipe it from one side to the other once!
Hope that is helpful, if you have anything more to add to this please feel free!
Well, maybe, maybe not!
The thing with this canon that even at £50 makes it not the greatest buy is that it has a scratch across the sensor.
Unfortunately if you scratch your sensor it is likely to be very costly to fix. Dust on the other hand is fairly easy to remove with one of many DIY sensor cleaning kits.
So, I thought since I have a camera with a scratched sensor I thought I would write a bit on how to spot this issue and what the ramifications of it are.
If I take a photo with this Canon the result under certain circumstances can look like this
not ideal!
and yet under other circumstances this is the result
Perfect ...
This is dues to the first one being shot at f22 and the second one being shot at f1.8 ...
So why does this make a difference?
Well, the dust and scratch aren't actually on the sensor at all, they are on a filter that is sat on top of the sensor. What you are seeing is not actually the dust its self, but a shadow of it caused by the gap between the surface of the sensor and the surface of the filter.
In simple terms because f22 is a smaller hole than f1.8 the light is more of a thin beam and so causes a stronger shadow.
Here is a close up of the worst effected area at f22
f16
f11
f8
f5.6
f4
f2.8
f1.8
As you can see buy the time the lens is opened up to f5.6 the problem is all but gone ...
Your mileage may vary, but this is often the case!
As you can see if you want to tell if you have a dusty or scratched sensor the easiest way is to point the camera at the sky take a photo at the smallest aperture your lens will allow!
So how do I know that this is dust on the sensor and not in the lens?
Hold your hand next to any surface and see the shadow it causes, now move your hand away from the surface and see the shadow get more diffuse. Well the same applies here, tiny spots of dust on or in a lens often don't cause a shadow as they are too far away from the imaging surface.
In fact the only time that dust on a lens rears its ugly head is the the exact opposite circumstance that dust on a sensor does ...
When the lens is wide open (eg at 1.8) and areas of the image are out of focus, if within those areas there are points of light the dust can sometimes be seen
Here is a good demonstration of this happening
(thanks for the image Dan)
I'm not clever enough to tell you why this happens ... your just going to take my word for it
The reality is that this doesn't happen very often at all, I have lenses with a lot of dust in them that I have never had any problems with, a dusty lens very rarely causes any noticeable issues to image quality at all!
So back to the sensor dust issues, as as we have seen they are often significantly more problematic!
What's dust and what's a scratch?
Well I guess that's pretty obvious!
The scratch is the line the dust is the squiggle.
Scratches are also often much more visible on the surface of the sensor to the naked eye than dust are.
In this photo you can see the scratch across the bottom of the sensor ...
'But wait!' I hear you say, the scratch is at the top of the photo...
Well this is because the light travelling through a lens travels in a straight line ... your camera actually takes photos upside down!! its the computer inside that puts its the right way up for you... or if its film ... its you holding the photo the opposite way up to how the camera took it ...
So what can be done about these problems?
Unfortunately, dust landing on your sensor is almost an in inevitability...
It gets inside the camera when you change the lens!
I have been told that switching the camera off when changing the lens can reduce static on the surface that attracts dust, I have no idea how true that is though ...
The best you can really do is of course changing lenses quickly and not in the middle of a sand storm!!
If you want to avoid sensor dust, in an interchangeable lens camera the only real way is to buy a film camera ... that way you get a new sensor for every shot!!
So what causes scratches?
Well, here is the really unfortunate bit ... Often scratches are caused by the sensor cleaning process!
If you want to avoid causing scratches when cleaning you just have to be very careful!
Often lens cleaning kits consist of a swab and a form of liquid, probably some kind of alcohol.
Just make sure you follow the instructions, only use the swab once, and only swipe it from one side to the other once!
Hope that is helpful, if you have anything more to add to this please feel free!
). But I must admitt, I have NEVER swabbed my sensor.
