EI settings help

[madgardener]

I remember reading about this on a forum here but I can not find it.

I'm reading another thread here on APUG with a lot of interest about near IR film and saw some very interesting samples taken with Foma 35mm film. One of the samples was shot at IE 1.5. Doing a lot of research on the internet I've been able to determine that with the lowest ISO setting on my camera (25) and the exposure compensation at +2, I should be at the IE equivalent of 6 (right?).

From there its aperture settings, so what settings get me below 6 and what would the IE be with each F/stop?

 

[John Koehrer]

Six is correct. You're just dividing a known(25) in half for each one stop(EI) of exposure.
Next steps are 3, 1.5 .75 and so on.

EI, exposure Index is a self determined(tested) film speed that gives you the end result you're happy with.

Depending on the limits of your equipment and the light available, it's just time & aperture just like following the scale on a meter, f stop on a lens or shutter speed.

If EI=6(ISO 100) and your settings are 11 @ 1/8 then a setting for EI 3 would be one additional stop.

 

[MattKing]

One caution though - some cameras won't let you use the exposure compensation function that way. The function doesn't change the reading, but rather changes the EI set, and if it won't normally go below 25, the exposure compensation function won't be able to move the setting either.

 

[Truzi]

For my first try (that I've not developed yet), I made it simple. I used a tripod and cable release.
I set the camera for the ISO of the film - 400 (well, I used 320), and took a picture without the filter, letting the camera decide on exposure, and making note of what the camera's built-in meter chose. (Auto mode (aperture priority) was fine for test snapshots for my purposes.)

Then I put the filter on, and I simply changed the shutter speed to get the desire EIs; each shutter speed change is one stop (the same is true for each aperture change). The camera's ISO setting does not affect anything in manual mode.

If I decide I like it, and decide on what EI I like, I will test again, a little more stringently, and will change aperture as well as shutter speed to achieve the desired EI.

 

[madgardener]

For my first try (that I've not developed yet), I made it simple. I used a tripod and cable release.
I set the camera for the ISO of the film - 400 (well, I used 320), and took a picture without the filter, letting the camera decide on exposure, and making note of what the camera's built-in meter chose. (Auto mode (aperture priority) was fine for test snapshots for my purposes.)

Then I put the filter on, and I simply changed the shutter speed to get the desire EIs; each shutter speed change is one stop (the same is true for each aperture change). The camera's ISO setting does not affect anything in manual mode.

If I decide I like it, and decide on what EI I like, I will test again, a little more stringently, and will change aperture as well as shutter speed to achieve the desired EI.

so a smaller F/stop equates to a lower ISO setting, correct?

 

[BradS]

so a smaller F/stop equates to a lower ISO setting, correct?

yes. smaller f stop number (larger diameter opening) --> smaller EI

 

[Truzi]

Remember, you must use a filter to block the "visible" light, otherwise the "visible light" will overpower any IR effect (and lower EI without a filter will basically over-expose your film).

Yes, smaller aperture (larger f-number on the lens) is lower/slower EI _if_ the shutter speed is not changed.

For example, consider an initial exposure of f=11 and shutter speed=1/125:
- Then f-8 and shutter=1/125, your EI is one stop lower.
- Or, f-11 and shutter= 1/60, your EI is also one stop lower.
- However, if f-8 and shutter=1/250, your EI is the same.

(Quick, somebody check my math.)

Basically, to get the desired exposure, you want a certain amount of light hitting the film. You make sure you get the correct amount by adjusting your f-stop and shutter speed.

Each change in f-number on the lens is one stop. Roughly speaking, each larger number (smaller aperture hole) lets in half as much light. Each smaller number (larger aperture hole) lets in twice the light.

Each change in shutter speed is one stop. Roughly speaking, each faster step (higher shutter speed) lets in half as much light. Each slower step setting lets in twice as much light.

Nearly any filter reduces the total amount of light, so you need to compensate when setting the EI manually. Also, from what I've read, IR wavelengths are not as, shall we say, strong (or perhaps near IR film is not as sensitive to IR as it is to "visible" light), so even more exposure may be warranted in addition to compensating for the IR filter factor.

I hope I got all of that right.

 

[madgardener]

Yes, I do have a cheap set of IR filters. It's not the all plastic, these are glass filters. So instead of $20 for a set of four, I am paying $50 for a set of 3.

 

[John Koehrer]

yes. smaller f stop number (larger diameter opening) --> smaller EI

Smaller f stop, not f number. The actual diameter of the aperture is smaller.

 

[BradS]

Smaller f stop, not f number. The actual diameter of the aperture is smaller.

:confused:

If I rate a film at an EI that is numerically smaller than the ISO rating, that means means I want/need to give more exposure...for a given shutter speed, I need a bigger aperture opening which means a smaller f-stop number....so for example, let's say I meter a scene at ISO100 and the meter says 1/125 second at f/11...but I want to rate the film at EI50 the exposure becomes 1/125 second at f/8....so, smaller Ei --> smaller f-stop number, and larger aperture diameter.