Infrared on 400D problems

[oindypoind]

Not sure if this is down to the filter or the lens.

I purchased an IR filter for my cokin P series holder, and have been doing some test shots.
The shots are pretty huge exposure times in excess of 5mins due to the 400D's built in IR filter, but even so it seems to be playing up something chronic. It seems to get massive lens reflections in the picture, now i'm not sure if that's just because this lens is not compatible (Sigma 18-200 DC OS), though I tried it with the kit lens, and got the same problem.

I thought it was light getting in from around the edge of the filter, but I filled those holes with card and it still does this. Any ideas?

[OsmosisStudios]

Somebody more familiar with IR can surely answer this, or at least confirm, but im guessing dust on the inside of the filter gave you those spots. I cant be sure.

[oindypoind]

Originally Posted by OsmosisStudios

Somebody more familiar with IR can surely answer this, or at least confirm, but im guessing dust on the inside of the filter gave you those spots. I cant be sure.

Yeah I assume the dots are dust, i'm a bit surprised at hour pronounced they came out.
But its the big rings of light that really shouldn't be there.

[Murtasma]

probably reflections bouncing around between the front element of the lens and the cokin filer. Where is the sun in this particular scene? I've had trouble with reflections sometimes with cokin filters like this. What mounting method are you using? Try placing a dark cloth over the lens and the edges of the filter to block light from entering from the side.

[oindypoind]

In this photograph the sun was off the top left of the shot, and the filter was mounted using a standard P series filter holder, just slid in the slot closest to the lens, there was still a bit of a gap between the filter and the lens, so I filled that with card, but a bit of black cloth may have been better.

[Murtasma]

Originally Posted by oindypoind

In this photograph the sun was off the top left of the shot, and the filter was mounted using a standard P series filter holder, just slid in the slot closest to the lens, there was still a bit of a gap between the filter and the lens, so I filled that with card, but a bit of black cloth may have been better.

It really looks like the reflection from the back of the filter. Try using a dark cloth and perhaps a rubber band to keep it in place. I can make out the aperture and the front element of your lens in the reflection. It's defiantly reflections from the filter.

[kencaleno]

Look at the shapes of the offending spots-hexagonal-same shape as the lens diaphragm-They are lensflare created by the filter-always use a lens hood. regards, Ken

[oindypoind]

Originally Posted by kencaleno

Look at the shapes of the offending spots-hexagonal-same shape as the lens diaphragm-They are lensflare created by the filter-always use a lens hood. regards, Ken

Hmm, yeah good point, my normal hood for this lens won't fit with the P-Series holder in the way, is there a hood you can buy for a p-series holder... I wonder...?

[OsmosisStudios]

Originally Posted by kencaleno

Look at the shapes of the offending spots-hexagonal-same shape as the lens diaphragm-They are lensflare created by the filter-always use a lens hood. regards, Ken

There is NO way those are lensflare.

[inkista]

No, that's lens flare. IR light can bounce around between elements and cause "IR hotspot" and very weird flare. Most lenses are built to deal with visible light flare, not infrared, which is why there's a problem with IR hotspots with specific lenses. I'd say try shading the lens and filter, and not shooting into the sun first, and if that doesn't solve it, give up on the 18-200 for IR and find another lens.

Even with the lens shaded and the sun at my back, I found the 18-55 kit was useless for IR, because IR was bouncing about internally and creating a hotspot. You may just be out of luck with that lens.

I can affirm that an adapted Contax Zeiss 28 f/2.8 absolutely rocks for infrared.


Canon XT. Contax Zeiss Distagon T* 2.8/28 + adapter ring
iso 400. f/11(?), 2 minutes.

But you always have to make sure there's no lens flare, it's pretty susceptible to that with a Hoya R72 screwed onto the front of it.