Lomochrome Purple ASA?

Paul Manuell said:

how would that yield different results?

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Lomo purple is likely an emulsion with poorly matched curves. I.e. it crosses over by design; its color balance changes as it receives more or less exposure. This gives you control over the color balance through under- and overexposing it. There will be a floor to it as at some point you'll start to drastically underexpose it, at which point shadow detail gets unacceptable.

I suspect that this film has a true ISO speed of maybe 200 at best, and Lomo suggests a range of minus one underexposute to plus one overexposure.

If you're feeling experimental, don't hesitate to rate it at 50 or even 25 and see what happens. You could also try 800, but this may be disappointing in terms of shadow rendition.

koraks said:

Lomo purple is likely an emulsion with poorly matched curves. I.e. it crosses over by design; its color balance changes as it receives more or less exposure. This gives you control over the color balance through under- and overexposing it. There will be a floor to it as at some point you'll start to drastically underexpose it, at which point shadow detail gets unacceptable.

I suspect that this film has a true ISO speed of maybe 200 at best, and Lomo suggests a range of minus one underexposute to plus one overexposure.

If you're feeling experimental, don't hesitate to rate it at 50 or even 25 and see what happens. You could also try 800, but this may be disappointing in terms of shadow rendition.

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But surely the reciprocal exposure compensations I mentioned would mean each frame got the same effective exposure, no matter what ISO was set for each one. So for example, a subject shot at 1/500th with the film rated at 400, then the exact same subject shot 2 stops slower at 1/125th with the film rated at 100 would have received the same effective exposures, so why is there such a marked difference in the results of Lomochrome Purple shot at ISO400 and at ISO100

If you shoot at the same aperture, 1/125 will be exposed two stops more than 1/500. You may have set the meter to ISO100 for the first shot and to 400 for the second.

In any case, if you hold the aperture constant and you shoot the same film and the same scene, but you vary the shutter speed, you're varying exposure.

The film does not change its sensitivity just because you change the iso on your meter so if you meter at 100 you’re giving 2 stops more exposure than if you meter at 400.

With any colour negative film, as you increase the exposure, each of the colour sensitive parts of the emulsion will respond, and give you dye clouds that are more dense. Most colour negative films are designed so that a particular amount of exposure will result in the different colour sensitive parts responding in a matched or complementary way, leading to reasonably accurate colour. Less exposure will skew the results (a bit) in at least two ways (colour crossover) away from natural, as well as creating more problems with grain and loss of resolution.
Normal films are more tolerant of moderate over-exposure - the colour remains closer to natural, with relatively small amounts of colour crossover, and moderate increases in grain and loss of resolution. With normal films, greater amounts of over-exposure cause greater amounts of problems with colour accuracy, grain and resolution.
It looks like the Lomo Purple makes no effort to minimize changes in colour with changes in exposure - in fact, the marketing materials appear to celebrate those changes. It also looks like none of the changes result in particularly accurate colour, no matter what exposure you use.

Thanks Matt, that's an apt explanation as far as I'm concerned!

koraks said:

If you shoot at the same aperture, 1/125 will be exposed two stops more than 1/500. You may have set the meter to ISO100 for the first shot and to 400 for the second.

In any case, if you hold the aperture constant and you shoot the same film and the same scene, but you vary the shutter speed, you're varying exposure.

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I realise that, but I was talking about reciprocal exposure adjustments, ie., making the necessary adjustments to aperture and/or shutter speed to give the shots the same effective exposure when rated at 2 different ISOs

_T_ said:

The film does not change its sensitivity just because you change the iso on your meter so if you meter at 100 you’re giving 2 stops more exposure than if you meter at 400.

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I know that, hence why I said about making reciprocal exposure adjustments so that each shot gets the same effective exposure, no matter what ISO the film is rated at. What I DON'T understand is why you get different results with Lomo Purple based on the ISO setting the photographer chooses. I thought I'd explained why I don't understand it fairly clearly, but perhaps I haven't, so...

A photographer goes out with 2 identical cameras, each loaded with Lomo Purple, and 2 tripods. He sets the ISO on one camera to 100, 400 on the other. Wandering around he comes upon a scene which suits the Purple film perfectly - a deep blue sky and plenty of green foliage - so sets up both cameras and tripods side by side.

Taking a meter reading through the viewfinder of the camera set to ISO400 and his chosen aperture (let's say f11 for example), he gets 1/500th. Now he looks through the viewfinder of the camera set to ISO100 and the same aperture of f11, and the meter reading obviously says 1/125 because the film in that camera has been rated 2 stops slower, so the shutter speed is also 2 stops slower to compensate. He takes both shots. As the film in each camera received the same EFFECTIVE exposure, ie., the 'correct' shutter speed for their respective aperture/ISO settings, then surely each photo should look identical once developed, yes?

But this isn't the case with Lomo Purple. It's well documented that exposing it at ISO400 gives totally different results than if exposing it at 100. And that, given the scenario I've just detailed, is where my confusion comes in. Right, I've got a headache now so need to go for a lie down, haha

Stop for a second, read what has been written here.

Changing the iso setting on a film camera changes exactly nothing about the film. The difference of Lomochrome Purple's rendering when shot at different ISOs is because you DO give different exposure to the film (more at ISO 100, less at ISO 400).

There is no such think as 'effective' exposure. Or better said, there is only effective exposure.

Paul Manuell said:

I realise that, but I was talking about reciprocal exposure adjustments, ie., making the necessary adjustments to aperture and/or shutter speed to give the shots the same effective exposure when rated at 2 different ISOs

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If the exposure is the same, then it's the same. Evidently, the film won't respond any differently.

Realize that the roll of film has no knowledge of what ISO you've set your camera or light meter to.

Paul Manuell said:

What I DON'T understand is why you get different results with Lomo Purple based on the ISO setting the photographer chooses.

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This is your misunderstanding. The film responds to over/underexposure. It doesn't respond to different combinations of aperture and shutter speed that result in the same exposure. The film doesn't care if it's 1/125 f/8 or 1/500 f/4 - it'll do the same thing in both cases.

I suspect the confusion may arise because of the different significance of ISO in film and digital. In a digital camera, the ISO value is really a gain; a degree of amplification applied by the camera, so it can be used reciprocally just like shutter speed and aperture (though not without consequences for noise). But as brbo says, setting the camera's ISO value doesn't control the film's response.

If I put a roll of Ektra in my camera, and set my light meter for ISO 100, I'll get nicely-exposed photos (they'll still be rubbish, 'cause it's me holding the camera, but the exposure will be good). If I set the meter to ISO 400 instead, and don't tell the lab to push the film, I'll get pale, thin negatives and maybe darkish prints (though they will be able to correct the brightness somewhat in the printing or scanning stage: colour negative film has quite a high degree of tolerance for bad exposure).

Here, Lomography haven't given an exact speed value to their film. If I expose Purple at ISO 100, I'll get relatively dark negatives, with one sort of colour response. If I expose it at ISO 400, I'll get thinner, paler negatives with a different colour balance. The printing/scanning stage will offset the difference in brightness more or less, but the colour difference will remain. So Lomography are kind of abandoning the idea of a 'correct' speed, because the variable colour response is the feature they are selling.

In fact, if I scan my Purple negatives, I suppose poor old Silverfast (scanner-driving software) will be doing its damnedest to correct the novelty colours for me too. I usually start from Silverfast's defaults for the film, and auto brightness, and then apply corrections. If I were to use any of this, I'd need to look up how to not autocorrect the effects.

Dustin McAmera said:

If I put a roll of Ektra in my camera, and set my light meter for ISO 100, I'll get nicely-exposed photos (they'll still be rubbish, 'cause it's me holding the camera, but the exposure will be good). If I set the meter to ISO 400 instead, and don't tell the lab to push the film, I'll get pale, thin negatives and maybe darkish prints (though they will be able to correct the brightness somewhat in the printing or scanning stage: colour negative film has quite a high degree of tolerance for bad exposure).

Here, Lomography haven't given an exact speed value to their film. If I expose Purple at ISO 100, I'll get relatively dark negatives, with one sort of colour response. If I expose it at ISO 400, I'll get thinner, paler negatives with a different colour balance. The printing/scanning stage will offset the difference in brightness more or less, but the colour difference will remain. So Lomography are kind of abandoning the idea of a 'correct' speed, because the variable colour response is the feature they are selling.

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Ok, I think I've got it... at last! The Lomo Purple has a true ISO, but one which Lomography don't actually state. Instead, they get creative by saying you can expose it as 100 or 400 (and presumably everything in between) for different results. The different results being because one ISO setting is it's true ISO, anything else you set being under or over (depending on what that true ISO is)

Paul Manuell said:

The different results being because one ISO setting is it's true ISO, anything else you set being under or over (depending on what that true ISO is)

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Yes, you got it!
And from all this, you might deduce that the 100-400 range is somewhat arbitrary, so nobody's holding you back in trying 25, 50 and 800 as well.