Basic Lowlight Landscape - Reciprocity Question

[canoli]

Hi Folks - Can I get some advice on shooting a landscapes in very low light?


My question comes down to this - given 2 "equal" exposures, is it better to use a shorter one or a longer one?


Start with a tripod-mounted 40D at ISO 100. From there I can choose any number of Av/Tv values to get the exposure I'm looking for. But which ones are better? Is it better to leave the shutter open longer and use a small aperture, or the other way around?


Usually you make that decision based on your desired DOF and/or whether you can (or want to) freeze the motion in the scene, right?


For instance, focusing 100 feet away on a boat in a lake. There's a slight breeze that causes the boat to drift slightly ... ideally you'd freeze the motion of the boat to get a sharp image.


On the 40D using a 50mm lens @ f/2.8 gives me over 200 feet DOF. Above f/4.5 it's infinite. (0.030 CoC) So I can take DOF out of the equation right? The point of interest is the boat, there's plenty of DOF to get it all in focus, and the foreground/background will fade into OOF. (let's leave aside the aesthetic value of such an image...lol)


About the movement, the slight drift from the breeze in the scene - I'd prefer to freeze it and get a sharp image. But the cost of freezing the motion may be a high level of noise if I'm forced to use an ISO above ~400. So unless I'm going for a high-key image, none of the higher ISOs (above 400) work very well.


So - DOF isn't part of the equation; freezing the motion isn't part of the equation. I'm left with (let's say) f/2.8 at 1 sec or f/11 at 4 secs. Is one better than the other? How do I choose?


Thanks you guys, for any thoughts you'd care to share. I know it was a lengthy setup, hopefully you were able to follow it alright...

[neuroanatomist]

First off, digital sensors don't suffer from 'reciprocity failure' like film, so from that standpoint either exposure is fine. What I'd recommend is choosing an exposure that minimizes the flaws in your system. For example, the diffraction limited aperture for the 40D's sensor is f/9.3 - so, stay larger than f/9. Other than that, keep your ISO as low as possible.

[Stephen Probert]

I would go as wide as you can without running into vignetting or corner softness. If your lens is sharp in the corners wide, then vignetting can be fixed in post anyway. You want to get as much light on the sensor as you can to reduce noise. Noise is not a function of ISO, but of exposure. The less light that is hitting the sensor, the higher your noise will be. ISO actually reduces noise (for example if you took shots with the same shutter and aperture and at iso 400 and 800 and then in post upped the exposure of the first by 1 EV, the iso400 shot would be noisier). That is why long exposures in low light can be noisy even at ISO 100.

[canoli]

Thanks neuro - that makes a good bit of sense, I appreciate your help.


SP - Thanks for the noise lesson reminder - I know I alluded to higher ISOs = higher noise, and I should've been more thorough in my setup. The problem with adding exposure to minimize noise is that many times I want parts of an image to be in shadow - don't we all? So I've never understood how we get around that other than to shoot RAW and develop to taste in PP.


I can certainly flood the sensor with light, shoot at ISO 1600 and see no (or very little) objectionable noise, but what's the point if my lovely dawn landscape is now a high-key shot resembling noon?


The problem for me with the strategy above is I forget how a particular color looked at 5:50am, then at 6:25am and again at 8am. I can create beautiful images in PP but I often wonder how close I am to what was really on display...


Well I'll give it another try here very soon - it's 4:47am - sunrise at 7, be there an hour before plus 45min travel...makes for an early wake-up. But it's worth it every time - so nice when I get there and see the light begin to resolve the world into detail, especially on a still morning such as this...


Thanks again for your help you guys!

[Colin]

I'm not sure if it's addressing what you're talking about, but I try to set exposure to capture the information in RAW, and then post process using the tone curves (in addtion to exposure compensation) to set the subjective brightness. I've seen lots of things where somebody tried to keep the dark feel, and as a result wastedtons of their dynamic range, and as such, had no detail at all in the shadows.


As for the original issue, I don't deal with low light exposure issues any differently than high light. To me, it would seem that the relationshipos between Aperture, Exposure time, and ISO remain the same.

[Tim]

I tend to shoot at narrower apertures, like f10-f14 or so, I find that when using a narrower aperture the difference between highlights and shadows are less dramatic and as a whole the picture seems more balanced, therefore, If DOF and freezing motion are not issues, then I will almost always go with a narrower aperture and a longer exposure.