A simple question concerning f stops, ISO, and the M9

[A.G.]

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Does switching from my 35mm f2.0 lens to an f1.4 lens have the same effect as boosting the max ISO of my M9 from 2500 to 5000?

[jffielde]

No, but both are options for shooting in dimmer light. Shooting at f/1.4 will give you the option of shooting at a lower ISO with a more narrow depth of field. In that case, your images will be cleaner, but less will be in focus (at f/1.4 as compared to f/2). Shooting at twice the ISO (one stop) will render the entire image more noisy (but also brighter by a stop), but the depth of field will be unaffected.

[jdlaing]

The main penalty at that high an ISO will be noise.

[Jeff S]

Perhaps in terms of exposure differential, but certainly not in terms of ancillary effects, e.g., DOF relating to aperture, noise relating to ISO, etc.

 

Jeff

[01af]

The main difference, of course, is this: a 35 mm 1:1.4 lens exists; ISO 5000 on the M9 does not.

 

But generally spoken—yes, in terms of exposure, doubling the ISO number will have the same effect as opening up the aperture by one f-stop. Umm ... at least when both f-stops are the widest on the respective lenses, such as f/2 on a Summicron versus f/1.4 on a Summilux. Or when both are not the widest, such as, say, f/2.8 versus f/2 on a Summilux. That's because due to vignetting, the widest aperture won't give one full f-stop more exposure than the second-to-widest (or any other).

 

That doesn't mean, however, a 1:1.4 lens wasn't really 1:1.4. It is—but only geometrically. So, when opening up a Summilux lens from f/2 to f/1.4 then only the frame's center will actually receive twice the light. So in order to get the same impression of brightness across the whole frame, you must not use half the exposure time (or half the ISO) but a little more—depending on the lens, between a quarter and a half EV more.

 

Of course, basically the same is true for, say, a Summicron (or any other lens), so when comparing the widest apertures of a Summicron and a Summilux then the difference is one full f-stop again.

[LocalHero1953]

As with any digital camera, increasing the f-stop increases the amount of light captured I.e. increases the signal. Increasing the ISO setting increases both signal and noise (because increasing ISO is done by an amplification process). So, other things being equal (a big if), the f-stop route would give you a lower noise image. And noise is a big problem for te M9 at high ISO.

But the wider stop also makes it harder to nail the focus for some subjects - so it's a judgement call.