IR help

webmonkeyuk · 22 Mar 2006 at 16:28

Just got a IR filter for my 50m and have just been out to test it but most of the images seem out of focus, anyone got some good tips for using these filters?

hoodmeister · 22 Mar 2006 at 16:44

IR is all about technique.
You're getting OOF images because you're not focus correcting.
Focus with filter off > Flip to MF > adjust focus > pop on filter > take photo.

That's just the focusing, custom WB aside.

Some lenses have markings for how much closer you need to focus. Most these days don't. If yours doesn't, just play with focusing a touch closer till you get sharp images.

webmonkeyuk · 22 Mar 2006 at 16:56

i was af focusing with the filter off to make sure the image was in focus then putting the filter back on and taking the shot but it seems alittle hit-miss, have been reading that you may need to use longer exposure times as i was only using 1/4

hoodmeister · 22 Mar 2006 at 16:58

And as I say, you need to adjust your focus, after AF.
Racking up the F numbers will help compensate, by giving you larger DoF, but it's a bad substitute for focusing properly.

webmonkeyuk · 22 Mar 2006 at 17:10

theres nothing to adjust though thats the problem the shot is in focus but when the filter is on the shot is out of focus (forgot to say that i turn the AF off when the filter is on). im starting to think its a F problem maybe i was using a low F number, will have to practice somemore

thank for the help though m8

Johnny|lucidcomposure · 22 Mar 2006 at 17:14

No the problem is that I imagine when your putting your filter onto your lens that your turning the focusing ring!. What are your camera / lens specifications please.

hoodmeister · 22 Mar 2006 at 17:40

turning the focus ring while putting the filter on is a possible problem, yes, but assuming that's not a problem, you still need to adjust your focus!
your AF without a filter will not produce a photo that is in focus with the filter. And as i'm sure you've realised, AF will flip out if you try to focus with the filter on... So you need to manually adjust your focus. A touch closer. After AF.

webmonkeyuk · 22 Mar 2006 at 18:15

so manual focus till its is absolutely in focus and then put the filter on. Im not moving the focus ring as i made sure i didnt, Camera is 300D- 50MMf1.8 lens.

Thanx guys

hoodmeister · 22 Mar 2006 at 18:19

nooooo

Sorry, i'm tired and not explaining myself well.
Autofocus
flip to manual focus
focus a teensy bit closer - not much. If your lens doesn't have Infra Red markings, then it's hit and miss. But once you get to know your lens, then you'll know how much you need to adjust it by.
Apply filter
Take photo!

I've ommited any custom WB stuff, as I assume you're already doing that.

webmonkeyuk · 22 Mar 2006 at 18:30

cheers m8 you have been a huge help, as for the wb i was just leaving it on AWB i will have to test it out in the next few days, if you wouldnt mind posting your WB thoughts that would be appreciated

hoodmeister · 22 Mar 2006 at 18:33

just take a custom WB reading off of something that should be white - grass is a good bet. Be sure to do it with the filter on. Camera shake and focus don't matter.

Simple as that.

scoop · 22 Mar 2006 at 20:25

hoodmeister said:
nooooo
Sorry, i'm tired and not explaining myself well.

Hope you don't mind, but an explaination of what's going on might help a little bit here.

When you focus without the filter, you're setting up your optics to create a sharp image on the sensor. Any filter will deflect the light slightly as it enters the lens and therefore throw the focus off a teensy bit. Now, for pretty transparent filters, this isn't an issue, as you can focus through the filter itself to compensate for this. However, with an IR filter, the filter is so dense that i) autofocus may not work and thus won't compensate for the deflection, ii) the filter bends the light rather a lot anyway and iii) IR is deflected slightly differently to the visible light your optics are designed for.

So, solutions are either i) focus, add the filter then tweak the focus to compensate, as Hoodmeister correctly suggests, or ii) try autofocussing with the filter in place. IIRC, you're on a 300D? I've no idea if you can autofocus through the filter (seems to be more of a P+S thing than a DSLR trick) but might be worth a shot.

Hope this clarifies a little.

Johnny|lucidcomposure · 22 Mar 2006 at 22:47

Your dipoter is set up correctly isnt it?

It doesnt take much to turn the focus off when screwing anything into the 50mm F1.8 .. On that note, Your not photographing stuff at F1.8 are you ? coz that would make a lot appear out of focus.

Try again tomorrow shooting with these in mind

F8
30seconds
ISO 100

webmonkeyuk · 23 Mar 2006 at 10:42

i were glasses so its set up to compensate for that, i was doing a mixture of F1.8-4 didnt really go above that, first time ive shot IR so just a bit of learning to do but once again thanx so much for the help guys

wez130 · 23 Mar 2006 at 10:50

remember, the faster the aperture (F/1.8) the lower DOF you have.

What i'd do is shoot at F/11 for 4,8 and 15 seconds, then at F/13 for the same, do it in that order so you have 6 pics, process them all and see what looks best, i have the same filter but never get round to using it lol.

Johnny|lucidcomposure · 23 Mar 2006 at 14:08

I am almost dead certian that your problem is your shooting very wide.