Infrared Film iso speed

[Lolarat]

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I've started experimenting with Infrared film, unfortunately I am a little confused leading to the first film I developed being overexposed. I am using Rollei infrared 400iso when I looked it up it advised to set the iso at 25. I can't use the light meter due to using a filter on the lens. So did the same with my digital but the lowest iso was 100, so I thought I had to increase the shutter speed but I think I should have decreased it. How do I figure this out? 

For example if on my digital (again using infrared filter) the iso is 100 and shutter speed is 30secs, then how long would the shutter speed be for the film if set at iso25? 

I hope this makes sense! 

Thanks 

[gbealnz]

The quick read I had of your thread didn't make a lot of sense, but I'm sure someone will put you right.

Try, if you want to continue with film I/R, the Rollei Superpan 200, it was my choice, even though the 400 is touted as being THE I/R film.

In my case I used "about ISO6" as my base-line, omn a bright sunny day with puffy clouds etc. And with the appropriate I/R filter (I'd have to dig it out, but 720Nm rings a bell).

Tripod and using 1 second at F22, or derivatives thereof.

V/F camera works best for me, particularly my SWC.

No substitute for just trial and error.

Gary

 

[Rennrocky]

4 stops down

[jaapv]

Why don't you just use an light meter (or smart Phone app) and set it to ISO 25?  You'll have to bracket as the "sensitivity"varies with the IR  content of light.

[Pyrogallol]

I have used Rollei 400 infrared with a few different density red filters, 25A, B&W 092, Leica IR, Rd, Rh. 

Whatever the density of the filter I found that giving about 5 stops more exposure than required for a normal 400 ISO film mostly gives a negative of about the right density. I use a handheld meter and rangefinder camera so am not metering or viewing through the lens/filter.

in bright UK sunshine 30th at f5.6 seems to be about right, try not to overexpose the film as that spreads the light out in the negative.

Edited February 25, 2019 by Pyrogallol

[farnz]

The rated film speed doesn't take account of the infra-red pass (visible light blocking) filter you're using so the advice of shooting at ISO 25 is trying to compensate for that.  The problem is that different infra-red pass filters will block more light than others depending on the lower wavelength they block.  For example a 092 (=R72 'deep red' filter) will block wavelengths below 720 nm and will let 'some' visible light through whereas a 093 ('black' filter) will block all wavelengths below 850 nm and includes all visible light.  You need to work out how many stops your filter is rated at but a rough guide from my experience is that a 093 loses about 4 stops of light and a 092/R72 loses 2 to 3 stops depending on the brand (some brands let more visible light through than others).

So, using your example of ISO 100 at 1/30, then 4 stops would be ISO 100 at 1/2 sec.  Similarly, ISO 400 at 1/250 would be ISO 400 and 1/30.

BUT ... you say that you can't use your meter because of using the filter on the lens so it sounds like your camera has through the lens metering (my apologies if I've misunderstood).  If so, simply adjust the camera's ISO to the appropriate value and meter through the lens as normal and you should be fine.  For example, the Rollei IR film is rated at ISO 400 so if you used a 093 filter then adjust the camera's ISO 4 stops slower to ... ISO 25 (so that the meter can can work correctly), adjust the aperture and shutter speed to what the meter indicates and fire away.  (For an 092 filter adjust to ISO 50.)

Since you're starting out with infra-red it's probably worth mentioning that focussing can be awkward because longer 820+ nm infra-red wavelengths focus at a different plane to those in the 400 to 750 nm visible light band of wavelengths so what would be in focus in normal light is likely to be out of focus in infra-red.  Some older lenses have a red "R" on the focus scale to help; focus as you would in normal light and manually transpose the distance on the focus ring to the R.

Pete.

 

[Martin B]

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My experience with shooting IR film and 720 nm filter is to overexpose just one stop. So instead of exposing at ISO 25, you should have better exposed at ISO 200. If your digital camera in your example is ISO 100 and 30 sec (which I consider too long for even digital IR since some regular light will sneak into your IR photo with such long exposure), then you need to expose your film at 15 sec with ISO 200. 

[farnz]

Assuming you were replying to my post, Martin, I often shoot digital IR with my M8 at ISO 160 (lowest ISO so ISO 100's not available) and 1/30th with a 093 filter and I haven't experienced unwanted light sneaking into my pictures.

Pete.

[Martin B]

2 minutes ago, farnz said:

Assuming you were replying to my post, Martin, I often shoot digital IR with my M8 at ISO 160 (lowest ISO so ISO 100's not available) and 1/30th with a 093 filter and I haven't experienced unwanted light sneaking into my pictures.

Pete.

Okay, maybe we misunderstand something here: I read the OP that the exposure time with filter is 30 seconds, not 1/30 sec. Therefore my remark with regular light sneaking in. When you have a modified digital camera, you get faster shutter speeds as in your example, but 10 seconds with the filter method are not uncommon. In your modified camera the IR-blocking layer on the sensor glass cover is replaced which reduces the exposure time drastically. When using the filter method - meaning a filter is placed in front of the lens but the sensor remains the same unmodified - you have a lot longer exposure time since most of the incoming IR light is blocked in the ICF layer. 

[farnz]

1 hour ago, Martin B said:

Okay, maybe we misunderstand something here: I read the OP that the exposure time with filter is 30 seconds, not 1/30 sec

My apologies, Martin, I misread 30 sec as 1/30 so my mistake.

Pete.

[Tmx]

6 ASA for SFX 200 with Heliopan 715 or 25 ASA (spot) metering for zone III shadows