Long Exposure & ND Filters

The reason for this particular post began last week when I was “cruising” a forum on a PoD site I’m a member of, and I came across a thread started by someone about heavy ND filters and very long exposures.

Then, a couple of days later a Facebook conversation cropped up where someone I know rather well seemed to be losing the plot over things totally by purchasing a 16 stop ND.

The poor bugger got a right mauling from “yours truly” for the simple reason that he doesn’t understand the SCIENCE behind the art of photography.  This is what pisses me off about digital photography – it readily provides “instant gratification” to folk who know bugger all about what they are doing with their equipment.  They then spend money on “pushing the envelope” only to find their ivory tower comes tumbling down around them because they THOUGHT they knew what they were doing………..stop ranting Andy before you have a coronary!

OK, I’ll stop “ranting”, but seriously folks, it doesn’t matter if you are on a 5DMkIII or a D800E, a D4 or a 1Dx – you have to realise that your camera works within a certain set of fixed parameters; and if you wander outside these boundaries for reasons of either stupidity or ignorance, then you’ll soon be up to your ass in Alligators!

Avid readers of this blog of mine (seemingly there are a few) will know that I’ve gone to great lengths in the past to explain how sensors are limited in different ways by things such as diffraction and that certain lens/sensor combinations are said to be “diffraction limited; well here’s something new to run up your flag pole – sensors can be thought of as being “photon limited” too!

I’ll explain what I mean in a minute…..

SENSOR TYPE

Most folk who own a camera of modern design by Nikon or Canon FAIL at the first hurdle by not understanding their sensor type.

Sensors generally fall into two basic types – CCD and CMOS.

Most of us use cameras fitted with CMOS sensors, because we demand accurate fast phase detection AF AND we demand high levels of ADC/BUFFER speed.  In VERY simplistic terms, CCD sensors cannot operate at the levels of speed and efficiency demanded by the general camera-buying public.

So, it’s CMOS to the rescue.  But CMOS sensors are generally noisier than CCDs.

When I say “noise” I’m NOT referring to the normal under exposure luminance noise that a some of you might be thinking of. I’m talking about the “background noise” of the sensor itself – see post HERE .

Now I’m going to over simplify things for you here – I need to because there are a lot of variables to take into account.

• A Sensor is an ARRAY of PHOTOSITES or PHOTODIODES
• A photodiode exists to do one thing – react to being struck by PHOTONS of light by producing electrons.
• To produce electrons PROPORTIONAL to the number of photons that strike it.

Now in theory, a photodiode that sees ZERO photons during the exposure should release NO ELECTRONS.

At the end of the exposure the ADC comes along and counts the electrons for each photodiode – an ANALOGUE VALUE – and converts it to a DIGITAL VALUE and stores that digital value as a point of information in the RAW file.

A RAW converter such as Lightroom then reads all these individual points of information and using its own in-built algorithms it normalises and demosaics them into an RGB image that we can see on our monitor.

Sounds simple doesn’t it, and theoretically it is.  But in practice there’s a lot of places in the process where things can go sideways rapidly……..!

We make a lot of assumptions about our pride and joy – our newly purchased DSLR – and most of these assumptions are just plain wrong.  One that most folk get wrong is presuming ALL the photodiodes on their shiny new sensor BEHAVE IN THE SAME WAY and are 100% identical in response.  WRONG – even though, in theory, it should be true.

Some sensors are built to a budget, some to a standard of quality and bugger the budget.

Think of the above statement as a scale running left to right with crap sensors like a 7D or D5000 on the left, and the staggering Phase IQ260 on the right.  There isn’t, despite what sales bumph says, any 35mm format sensor that can come even close to residing on the right hand end of the scale, but perhaps a D800E might sit somewhere between 65 and 70%.

The thing I’m trying to get at here is that “quality control” and “budget” are opposites in the manufacturing process, and that linearity and uniformity of photodiode performance costs MONEY – and lots of it.

All our 35mm format sensors suffer from a lack of that expensive quality control in some form or other, but what manufacturers try to do is place the resulting poor performance “outside the envelope of normal expected operation” as a Nikon technician once told me.

In other words, during normal exposures and camera usage (is there such a thing?) the errors don’t show themselves – so you are oblivious to them. But move outside of that “envelope of normal expected operation” and as I said before, the Alligators are soon chomping on your butt cheeks.

REALITY

Long exposures in low light levels – those longer than 30 to 90 seconds – present us with one of those “outside the envelope” situations that can highlight some major discrepancies in individual photodiode performance and sensor uniformity.

Earlier, I said that a photodiode, in a perfect world, would always react proportionally to the number of photons striking it, and that if it had no photon strikes during the exposure then it would have ZERO output in terms of electrons produced.

Think of the “perfect” photodiode/photosite as being a child brought up by nuns, well mannered and perfectly behaved.

Then think of a child brought up in the Gallagher household a la “Shameless” – zero patience, no sense of right or wrong, rebellious and down right misbehaved.  We can compare this kid with some of the photodiodes on our sensor.

These odd photodiodes usually show a random distribution across the sensor surface, but you only ever see evidence of their existence when you shoot in the dark, or when executing very long exposures from behind a heavy ND filter.

These “naughty” photodiodes behave badly in numerous ways:

• They can release a larger number of electrons than is proportional to their photon count.
• They can go to the extreme of releasing electrons when the have a ZERO photon count.
• They can mimic the output of their nearest neighbors.
• They can be clustered together and produce random spurious specks of colour.

And the list goes on!

It’s a Question of Time

These errant little buggers basically misbehave because the combination of low photon count and overly long exposure time allow them to, if you like, run out of patience and start misbehaving.

It is quite common for a single photodiode or cluster of them to behave in a perfect manner for any shutter speed up to between 30 seconds and 2 minutes. But if we expose that same photodiode or cluster for 3 minutes it can show abnormal behavior in its electron output.  Expose it for 5 minutes and its output could be the same, or amplified, or even totally different.

IMPORTANT – do not confuse these with so-called “hot pixels” which show up in all exposures irrespective of shutter duration.

Putting an ND filter in front of your lens is the same as shooting under less light.  Its effect is even-handed across all exposure values in the scenes brightness range, and therein lies the problem.  Cutting 10 stops worth of photons from the highlights in the scene will still leave plenty to make the sensor work effectively in those areas of the image.

But cutting 10 stops worth of photons from the shadow areas – where there was perhaps 12 stops less to begin with – might well leave an insufficient number of photons in the very darkest areas to make those particular photodiodes function correctly.

Exposure is basically a function of Intensity and Time, back in my college days we used to say that Ex = I x T !

Our ND filter CUTS intensity across the board, so Time has to increase to avoid under exposure in general.  But because we are working with far fewer photons as a whole, we have to curb the length of the Time component BECAUSE OF the level of intensity reduction – we become caught in a “Catch 22” situation, trying to avoid the “time triggered” malfunction of those errant diodes.

Below is an 4 minute exposure from behind a Lee Big Stopper on a 1Dx – click on both images to open at full resolution in a new window.

The beastly Nikon D800E fairs a lot better under similar exposure parameters, but there are still a lot of repairs to be done:

Most people use heavy ND filters for the same reason I do – smoothing out water.

Then we change the camera orientation and get a commercial shot:

In this next shot all I’m interested in is the jetty, neither water surface texture or horizon land add anything – the land is easy to dump in PShop but the water would be impossible:

The mistake folk make is this, 30 seconds is usually enough time to get the effect on the water you want, and 90 to 120 seconds is truly the maximum you should ever really need.  Any longer and you’ll get at best no more effect, and at worst the effect will not look as visually appealing – that’s my opinion anyway.

This time requirement dovetails nicely with the “operating inside the design envelope” physics of the average 35mm format sensor.

So, as I said before, we could go out on a bit of a limb and say that our sensors are all “photon limited”; all diodes on the sensor must be struck by x number of photons.

And we can regard them as being exposure length limited; all diodes on the sensor must be struck by x photons in y seconds in order to avoid the pitfalls mentioned.

So next time you have the idea of obtaining something really daft, such as the 16 stop ND filter my friend ordered, try engaging your brain.  An unfiltered exposure that meters out at 1/30th sec will be 30 seconds behind a 10 stop ND filter, and a whopping 32 minutes behind a 16 stop ND filter.  Now at that sort of exposure time the sensor noise in the image will be astonishing in both presence and variety!

As I posted on my Book of Face page the other day, just for kicks I shot this last Wednesday night:

The image truly gives the wrong impression of reality – the wind was cold and gusting to 30mph, and the sea looked very lumpy and just plain ugly.

I spent at least 45 minutes just taking the bloody speckled colour read noise out of the 4 minute foreground exposure – I have to wonder if the image was truly worth the effort in processing.

When you take into account everything I’ve mentioned so far plus the following:

• Long exposures are prone to ground vibration and the effects of wind on the tripod etc
• Hanging around in places like the last shot above is plain dangerous, especially when it’s dark.

you must now see that keeping the exposures as short as possible is the sensible course of action, and that for doing this sort of work a 6 stop ND filter is a more sensible addition to your armoury than a 16 stop ND filter!

Just keep away from exposures above 2 minutes.

And before anyone asks, NO – you don’t shoot star trails in one frame over 4 hours unless you’re a complete numpty!  And for anyone who thinks you can cancel noise by shooting a black frame think on this – the black frame has to be shot immediately after the image, and has to be the same exposure duration as the main image.  That means a 4 hour single frame star trail plus black frame to go with it will take at least 8 hours – will your camera battery last that long?  If it dies before the black frame is finished then you lose BOTH frames……………

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