astronaut wrote in post #3214033
Did anyone explain already why the "bright blurry spot" apprears in the middle of the image when IR filter is used on a normal dSLR ? I actually don't know the explanation myself as well..
In my experience this is not a function o the camera, but the lens. For example I never get it when using my EF50mm f1.8, however with my old 300D + kit lens I always got it - but only at focal lengths >35mm.
I have had pretty good results with un-modded cameras. Firs with my old 300D and now with my 5D:
A while back i wrote a quick guide to IR because I get a lot of questions from people who look at my IR site on PBase. I have cut and pasted it below (without the embedded exampe images I am afraid):
How to take & process digital IR images.
First how to take them
You will need an IR filter and a good steady tripod. It helps if you are using a very fast (f/2.8 or faster) lens as well.
So to the basics (even though you probably already know this – so skip it if you want) Your digital camera can detect both the visible and the infrared spectrum. In normal photography this is a pain, as the focal length needed to get a sharp image with IR is a bit offset from visible light, so there is a risk of an out of focus IR image over the top of all your ‘normal’ pictures. This is why all digital cameras have some king of low pass filter which attempts to block out the IR spectrum. With my Cannon 300D this is a very strong filter (which is great for non-IR pics), so I have to use very long exposures to compensate – you may not have to.
The IR filter you fit to the front of your lens (I use a Hoya IR72) works the other way round to your camera’s internal filter. It blocks out most of the visible spectrum leaving only the IR spectrum to play upon your camera’s sensor.
You will not be able to see through the filter so you will need to compose the picture on a tripod and then put on the filter. The great thing about digital is that you can check the exposure, I usually start somewhere around 10sec at f2.8 and see how it comes out – then adjust till the exposure is spot on. I have found that a long exposure with low ISO (100) gives better results than a shorter exposure with a high ISO. This may only be the case with my camera, so you should experiment. On very bright days you can let the f stop out a bit to give better DOF at the expense of a longer exposure.
There is a lot of confusion over focus and IR. This is because in the manual focus world of film photography if you focus by eye and then put on an IR filter the shot will be out of focus because of the different wavelengths of IR light. So most lenses have an IR offset on the focusing scale so you can adjust for this. With auto focus this is not an issue – the AF system will focus on whatever light it is getting & with an IR filter fitted it is only ‘seeing’ the IR spectrum and so will focus on this.
Image settings – if your camera allows you to create RAW images I strongly suggest that you do so, as this gives you a good deal of control of the post processing without losing any image quality (which may well be worse than you are used to because of the long exposures). If you don’t have RAW use the highest quality settings your camera will allow.
Finally – this is digital, not film so take lots and lots of shots. With long exposures there is much more scope for movement issues, hot pixels, light changes etc and it’s costing you nothing.
(I am assuming Photoshop as the processing software)
There are two approaches here: The first creates a mono black and white image; and the second a pseudo colour one.
The first is much the easiest:
• Open your picture
• Crop to taste
• Open the channel mixer window (images, adjustment, channel mixer)
• Click in the mono checkbox and adjust the sliders till the image looks right. (trying to keep the sum of the values to around 100) click ok
• Go to levels and move the outside sliders to just clip the curve and move the mid tome one round till you are happy
• Use unsharp mask to sharpen it all up (settings will vary depending on the image, but 100, 1.9, 3 will be a good place to start.
• Save As (so the original is unchanged so you can try it again)
The second is a bit trickier (but not when you know how!):
With my set-up this is what IR images look like straight from the camera
You can see there is a very heavy red cast to them. The first job is to swap the red and blue channels. I do this by using the channel mixer. You will see a drop down at the top of the window showing you which channel you are working with, and 3 sliders. It opens in the red channel and all you have to do is move the red slider to 0 and the blue to 100. Then, using the drop down at the top, select the blue channel and move the blue slider to 0 and the red to 100. Then click ok.
The picture should look something like this:
The next bit is where you can play around a lot, open up the levels tool. You will see the 3 pipette buttons in the bottom right. To use these click on the black one and then click on your picture in an area you want to be black, do the same for the white. Then click the grey one and try clicking on different parts of the image until you get an effect that you like. In this case I selected one of the white buildings and ended up with this: