Ophrys Photography - Hints and tips 3 - keeping digital sensors clean

Wildlife and nature photography hints and tips

1. Be wary of using skylight or UV filters in front of certain lenses.

It is common practice to place a skylight or UV filter on to a lens to protect it from scratches. I used to do this as a matter of course until I put a Jessops 77mm filter onto my Canon 100-400 IS lens.

Take a look at the picture of the fieldfare below taken with an EOS 10D and the above filter. The bird looks fine, but look carefully at the background - it has grey diagonal lines running through it.

Be wary of using skylight or UV filters in front of certain lenses.

Here is a 100% crop of the affected area:

Be wary of using skylight or UV filters in front of certain lenses. 2

I found that once I started looking for it, I could see the effect in most shots with a brightish background.

I took up the matter with Canon EOS magazine, and they felt that the problem was due to a mismatch between the curved front element of the lens and a flat filter in front of it - possibly causing internal reflections.

Whether or not this is the reason, I have removed the filter and the problem has never recurred. Whether it is just the Jessops filter or the combination with the 100-400 lens or the EOS 10D I don't know - but my lenses now go naked - I just have to take more care!

2. Keeping digital sensors clean.

The image above is a crop from a portion of sky which clearly shows the nasty greasy looking marks that dust on the sensor of a digital camera creates.

This is a problem that film cameras do not suffer from as a new portion of film is used for each exposure. If you don't notice the marks, you can produce hundreds of images that will later on have to be repaired using the healing brush tool as described in the tutorial on this subject.

So what causes the marks? The image sensor on a digital slr camera is exposed when the mirror flips up as part of the exposure process. The sensor is reputedly charged electrostatically and so it can attract oppositely charged dust particles in the air - like your hair being attracted towards a plastic comb.

It is not actually the sensor itself which gets the dust, but the low-pass filter in front of it. Dust on the filter casts shadows onto the sensor - and that is what we see.

The dust inside the camera has come from the outside air during lens changes, but some is generated internally by the camera itself as the shutter and mirror move about.

What can you do about it?

There are new cameras such as the Canon 400D which now have a sensor which vibrates when you turn the camera on, and a sticky pad attracts the dislodged dust onto it's surface and holds it there. The technology is apparently effective and will doubtless find it's way onto other cameras as time passes - roll on that day! (Apparently it is going onto the 1D mk III in a new up rated form).

The first thing that you can do to prevent the dust problem is to turn the camera off when changing lenses. This switches the charge off from the sensor and theoretically will decrease the amount of dust attracted to it.

Always keep lens changes to a minimum and do this as quickly as possible. every second the lens is off of the camera dust can enter ! Dust tends to settle down from above (in still air) so if you can hold the camera upside down as you change lenses this will also be of benefit. Get the new lens onto the camera first and as quickly as possible and then worry about putting lens caps back on after - but do this as soon as you can to keep external dust away.
Try to not change lenses in dusty situations if at all possible - having a second camera body with a different lens on it can be very useful in extreme conditions - such as on safari. If you must change the lens, try and do it inside a plastic bin liner.

When you return home after a days shoot, clean your camera and lenses and also clean the inside of the camera, including the pentaprism and mirror. Use a combination of a soft brush and a rubber blower. This must be done in advance of sensor cleaning - otherwise the dust will be attracted onto the sensor as soon as it is exposed. Many people overlook this vital step, and this is why I'm sure a lot of people have major problems.

Clean the sensor following the manufacturer's instructions for the camera in question. This usually involves selecting the "clean sensor" option from the menu, and pressing the shutter button to lift the mirror which reveals the sensor. At this point, hold the camera over your head with the sensor pointing down towards the floor and using a rubber air blower such as the Giotto Rocket Air (obtainable from a camera shop). Give the sensor some vigorous puffs across it's surface. Do not put the tip of the blower too far into the camera as you may touch the sensor!. Most cameras need to be switched off in order for the procedure to finish. Immediately put a lens or blanking-off cap onto the camera as it is once again susceptible to attracting dust. Apparently some older camera blanking caps were made of a plastic which sheds dust as it is screwed in - so it could be worth getting a new one if you think that this is a problem for you.

When you have cleaned the sensor take a picture and check again to see if you have successfully removed the dust as described in the next section. You may have to repeat this a couple of times.

To test for dust

Periodically take a picture a piece of white card (or of the sky if you prefer) and look for dust marks by previewing the image and scrolling around at 100% zoom on the back of the camera. If you want to really be alarmed and want to see just how dirty the sensor is, take a picture at a small aperture (e.g. f22) then load the image onto your computer and view at 100% (actual pixels).

The easiest way of taking an image of a plain white card is to set the camera to manual focus (the manual/autofocus button usually located on the lens) this stops the autofocus hunting around as it struggles to focus on the low-contrast blank card. You actually don't need to focus on the card at all, in fact it is better if the image of the card is not in focus - as it will cause confusion if you are not sure if the marks that you are seeing are sensor dust or marks on the card! Also, there is no need to use a high shutter speed or use a tripod, as the dust on the sensor will move at the same time as the camera shakes or moves.

The white card will fool the camera's exposure meter into under-exposure, so I recommend dialing in about +1.5 stops of exposure compensation to keep the card looking bright to show up the grey dust against it.

It is pointless trying to clean every spec off the sensor - it is hard to do and more dust will appear soon after. Be satisfied that you have no major spots if you take your test shot at f-11. Be prepared to remove the odd remaining visible spot in post-processing.

By taking these simple precautions I have never had to resort to risky and expensive sensor cleaning methods. Most manufacturers don't recommend cleaning sensors with commercially available sticky pads. So you may need to return your camera for cleaning if you neglect to do this bit of housekeeping!

Sensor dust update - Jan 07

Despite using the precautions described, my 1d mk2 developed a number of small dust marks towards the upper edge of the frame which refused to be blown out.

I have now invested in an Arctic Butterfly sensor brush (available from Visible dust ) - which I have used just the once with very good results. The brush appears ridiculously expensive for what it is, but it does work. You have to fit batteries into the body and when you switch it on it spins the brush head. You have to spin the brush three times for a few seconds before using it on the sensor. Spinning charges up the high-tech filaments of the brush and also throws off any dust collected.

After wiping the sensor once with the brush, you have to quickly turn off the camera and refit a lens onto the body to prevent more dust getting in. Then you spin the brush again a few times to shake off the dust
collected and get it back into it's (far too tiny) cover without touching it with your greasy fingers !

I found that the brush removed every trace of dust in a single pass - so I give the Arctic Butterfly a thumbs up! After a few uses the brush has to be cleaned - so be prepared to fork out for another expensive bottle of cleaning fluid after using it a few times.

I hope to only use the Arctic Butterfly a few times a year, so I hope that I don't need to keep cleaning the brush.

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