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I was thinking at the differences between SLR and DLSR (in Manual mode). In both cases you can change aperture and shutter speed as it suits you. But with SLR you are stuck with the ISO of the film which you happen to have in the camera at the moment, while with DSLR you can vary ISO as you wish, too.

Now maybe the question is naive, but how is this handled in practice?

I imagine that it requires more planning: you want to shoot today under this particular light and you choose a specific film roll rather than another. What if you find a nice shot but there is a shadow which changes the light: you can recover the exposure by changing the other parameters but this is not side-effects free obviously.

What are the effects of this "freedom" afforded by DSLRs? Is it making me (a complete beginner) lazy because I am not forced to think about it? Is it freeing me of a burden by removing an unnecessary constraint?

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In addition to flexible ISO, you also have a flexible white balance - you no more have worry for having correct filters for given light. –  Imre May 29 '11 at 19:20
    
@Imre: right, I had not thought about that. I thought that white balance was "simply" a matter of calibrating the digital sensor and had not a direct counterpart when shooting film. –  Francesco May 31 '11 at 8:45

4 Answers 4

up vote 12 down vote accepted

With film, you generally don't think of sensitivity as a free variable. Often your favorite emulsion is only available in one or two ISOs. So you have to reach proper exposure by adjusting aperture, shutter speed and/or lighting. Also, negative film is generally considered forgiving on some under- and overexposure.

In digital, you have more freedom to choose other variables, but you should still remember the simple truth that having more light on sensor will give you less noise in photo, so that is what you should consider first before pumping up ISO. But this does not mean that you should never use higher ISO values - noise would still be a far easier problem than excess blur or underexposure.

Actually, with film, you can change ISO by swapping film - take a note which frame you had the film on, rewind and when re-using take as many frames (plus one or two to be sure) with lens cap on. Some advanced bodies (such as Pentax MZ-S) also offer winding film to chosen frame. In medium format, many cameras employ interchangeable film backs. Large format film is handled as sheets, not roll film, so you can always change it between frames.

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whoa! i never thought about this swapping films technique when working with films! +1 –  lalli May 30 '11 at 3:55
    
Sounds useful but painful. –  Evan Krall May 30 '11 at 4:00
    
It's especially painful if your camera rewinds the film completely, without leaving end of film out of cassette. Then you'd have to rewind by hand in darkness. –  Imre May 30 '11 at 4:19
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@Imre: I didn't know that emulsions were so "selective" and only few ISO were available. I also didn't know about the swap film technique, in any case I imagine that it is more easily (relatively) done indoor than outdoor... –  Francesco May 31 '11 at 8:51
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You can always buy (or even make) a simple tool for getting the film end out of cartridge, no need to rewind in manually in darkness. –  Mart Oruaas Jun 1 '11 at 11:48

Is it making me (a complete beginner) lazy because I am not forced to think about it?

On the contrary, I think having an always available ISO selection encourages you to think about it more often. With film, ISO is basically constant, so you only have one thing you can change: the balance between shutter and aperture. With digital, you now have to consider the entire exposure tricycle, and now have two degrees of freedom for a given exposure level.

Rather than only considering the shutter-aperture balance, you now have to think whether it might be a good idea to change ISO so you can achieve a faster or slower shutter speed, more or less depth of field. Rather than being stuck with a certain amount of grain, you can (and should) make the decision before every shot of how much noise you're willing to accept.

How is this handled in practice? On a lot of cameras, particularly older DSLRs that were heavily influenced by film SLRs, ISO is only controlled via a menu or maybe a programmable function button. Even new cameras, such as the Finepix X100

Eventually, we'll start seeing program modes on the mode dials that involve ISO. Some Pentax cameras already have TAv (auto ISO, manual shutter/aperture), and most cameras should be able to select an ISO for you with auto-ISO settings, though the setting may be buried in the menus. I've set up one of my user-defined as a sort of TAv mode, and use it when I'm in a moderately dark place and everything's pretty fast-paced -- It's nice to be able to set e.g. 1/60th @ f/4 and let the ISO go as high as it needs but drop when the light is available, as opposed to using shutter or aperture priority, where I'd have to set the ISO high to begin with so I can get the shutter/aperture I need in the darker areas.

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+1 for the "TAv mode", I hadn't thought to try that –  fmark May 30 '11 at 12:33
    
thanks for the perspective, when I asked I was actually expecting that everybody would have told me that I was spoiled by the modern comforts... :-) –  Francesco May 31 '11 at 8:54

I can just say that having the ISO "locked in" once the camera is loaded is the one major pain in the backside when shooting my sweet film Leicas compared to my Canon 1Ds2 digi-beast. Everything else with the Leicas is great, this is very much not. It severely limits the flexibility; if a 100 ISO slide film is in there I can forget about night-time or indoors shooting. Conversely, if an 800 ISO colour neg is loaded, I know that daytime shots will be craptastic unless I change films. One solution is to carry two Leicas with different film... which kind of defeats the point of having such a camera instead of the big Canon in the first place.

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You could also get yourself a 3-stop neutral density filter and keep shooting with the 800 during the day. –  Evan Krall May 31 '11 at 2:20
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Err... not really. Image quality will still be craptastic, which is why I use low-ISO film whenever there is enough light. –  Staale S May 31 '11 at 7:50

With digital, you should think about what happens if you change ISO. While you can change white balance in post-production, what you set as your ISO is how your sensor captures the data. Why think about it? Because you can have more freedom to explore the creative aspect of the relationship between shutter speed and aperture if you can change ISO.

So, for example, if I know I can't get a high enough shutter speed to shoot anything other than wide open, I will often increase the ISO. If I'm looking for shallow depth of field, I will often decrease the ISO. The difference is not in whether you plan, but at what point during your shoot you plan. You can plan a sequence of shots in digital and vary the ISO rather than planning around a roll of film (or swapping a film back).

Remember, before you become too cavalier about the ISO changing, higher ISOs yield more digital noise. How much more depends on your sensor and the length of the exposure, but your lowest ISO will always give you the best image quality. From a purist's perspective.

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No, lowest ISO will not give you best image quality if using it causes underexposure or blur. Even if your tripod avoids camera shake blur, you may end up with subject blur. –  Imre May 30 '11 at 22:43
    
I agree that you should think about it, when I said that with film more planning is required I meant that you are stuck with that film roll for a good number of shots... –  Francesco May 31 '11 at 8:48
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Low ISO only yields better image quality because you inherently have to jack up the signal to noise ratio to achieve a proper exposure. High ISO means less signal multiplied, low ISO is more signal not multiplied as much. ISO results are entirely based on the signal-to-noise ratio in the exposure (and some degree of processing). If 10:1 is a normal ISO 100 exposure, a 2.5:1 exposure at ISO 400 yields the same exposure but the noise is now 28% of the result, not 9%, if you get my drift. –  Nick Bedford Jun 1 '11 at 4:45

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