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Cor
1-Jun-2010, 02:33
Hi,

I shoot quite some MACO IR 4*5 film and I use a Cokin P filter holder and an unknown brand 70 filter (50% cut off at 700 nm I believe) which serves me well.

I usually try to shoot in the morning and/or the evening because as I understood that at this times the highest amount of UV light is present, due to the angle of sunlight hitting earth..but I could be wrong on that, because I read also otherwise, but that is not were this thread is about (anyway my images show enough IR light at noon anyway).

Anyway during traveling you cannot always pick the time to shoot, and when in France 3 weeks ago I had to shoot around noon, always carefully having the sun in my back.

I had some problems with reflections of clouds behind me (my guess is that the corners of the filter, sticking out reflect light back into the lens: I realized I had this once before, also around noon, I even made a post about it on APUG: for those interested (http://www.apug.org/forums/forum37/53416-weird-reflection.html)).

The filter surely works like a mirror if you hold it in your hand.

Anyway now I even obtained the image of the diaphragm of my lens (an old 150mm Symmar mounted in a Copal Press shutter). See attached images (quick snap shot, holding the neg against a window).

I cannot really understand why, with the sun high above in my back, the diaphragm shows up.. Any thoughts?

I'll try sticking to hours when the sun is lower, and I taped a piece of black paper to the top of my square filter.

Thanks & best

Cor

Marko
1-Jun-2010, 06:46
Hi Cor,

I've had this problem with the same setup (Cokin P007). A piece of black cloth wrapped around the filter holder (so that it covers the gap between the holder and the lens) before taking the shot fixed it for me.

Hope this helps.

Marko

Cor
1-Jun-2010, 07:40
Thanks for the tip, Marko,

But did you also had the aperture of your lens reflected? Still trying to understand how that can happen, I mean I can understand that clouds etc behind me get reflected from the sticking out area of the back of my filter onto the film, but the aperture...


Hi Cor,

I've had this problem with the same setup (Cokin P007). A piece of black cloth wrapped around the filter holder (so that it covers the gap between the holder and the lens) before taking the shot fixed it for me.

Hope this helps.

Marko

Marko
1-Jun-2010, 19:10
Thanks for the tip, Marko,

But did you also had the aperture of your lens reflected? Still trying to understand how that can happen, I mean I can understand that clouds etc behind me get reflected from the sticking out area of the back of my filter onto the film, but the aperture...

No, not the aperture myself, but I did have problems with light coming in laterally between the holder and the lens and bouncing off the surfaces. I'm guessing that stray light bounced off your iris and caused the shiny surface of the filter to reflect its image back through the lens. It's a very dark filter, so the amount of light needed to cause reflections is not all that high.

Rick A
2-Jun-2010, 03:33
It it quite possible to get the double reflexion due to the distance of the filter from the front element of the lens, especially with the open gap in between. I use Cokin 'A' and 'P' series, the 'A' is slightly better , has a smaller gap, unfortunatly, has smaller everything.

al olson
2-Jun-2010, 13:27
Cokin also makes some lens hoods that fit onto the filter-holder.

There is the Modular Bellows (#350) for all series (this is a compendium type hood). E-X-P-E-N-S-I-V-E !

And there are the Modular Hoods (# A255/P255) for the A and P series. These can be coupled to the filter-holder in several ways and two or more hoods may be connected together. As I recall, the cost is around $20/hood.

I use two of them with the infrared filter except with the wide angle lens where I use only one because of the vignetting that occurs with two.

They can be positioned on the holder so that the slot nearest the lens is not obstructed which is convenient for moving the filter in or out.

vinny
2-Jun-2010, 15:27
Cor, I have two of the hoods Al mentioned of you're interested in one of them.

Marko
2-Jun-2010, 18:31
I have and use the modular hood but it won't help with this problem because it still leaves the gap between the filter and the lens. I don't know about the modular bellows, but for the price difference, a piece of black cloth works just fine for me. :)

KOG
2-Jun-2010, 20:01
I've had the lettering from the front of a lens reflected off of a Cokin P series filter. A nice semi-circle of flare showed up on film. It took me a while to figure out what was going wrong. I was using a modular lens-hood, so I knew the light was entering through the gap between the filter and the holder. Wrapping something black around the outside of the Cokin adapter should eliminate the problem.

Kevin

al olson
2-Jun-2010, 20:42
I have and use the modular hood but it won't help with this problem because it still leaves the gap between the filter and the lens. I don't know about the modular bellows, but for the price difference, a piece of black cloth works just fine for me. :)

It is possible to set the modular hood back to the farthest notch so that it surrounds the filter. This eliminates the convenience, however, of being able to remove and reset the filter without also removing the lens. It also still leaves smaller gaps behind the filter.

I have been using the Cokin system with the P007 filters to shoot EFKE, Rollei, and Ilford SFX and I have never noticed a problem with reflections from the filter. But Marko is right that relatively speaking there would be significant light reflection off the filter commensurate with the small amount of light passing through to create an image of the diaphragm.

The obvious solution would be to tape a piece of black fabric around the top and the sides that is long enough so that there is a flap that also covers part of the lens housing. To be safe in the future I intend to be doing that myself.

Cor
3-Jun-2010, 02:54
Thanks for the feedback, guys! I do have said hood which clicks in front, still leaving a gap between the filter and the holder. (I'll take a careful look again tonight at home).

I guess the best option would be to shield said gap, I already have taped a black piece of interleave paper to the top of the filter. Wrapping around the focusing cloth is also an obvious solution.

As I mentioned before I have only ran in these problems when shooting around noon, never in the morning or evening..

Best,

Cor