Q&A: When Is Enough, Enough?

Written by: Tim Neumann and Lorie McQuirt

Question:

One of our followers asks, “When shooting long exposure images of waterfalls, how do I know when the exposure time is just right?”

Introduction:

This question could be interpreted more than one way. Are we thinking about “exposure time as the right amount of light?”, or are we thinking about “the flow of water in the image and how it looks in the final result?” Turns out a little clarification tells us that the concern was about how the water looks in the long exposure result.

BW-SUN-0416201701-I001.jpg - A series of waterfall long exposures adjusted by ISO shift.
BW-SUN-0416201701-I001.jpg – A series of waterfall long exposures adjusted by ISO shift.

So what we really want to know here is, when have I gotten the exposure correct so that the water motion in my image will be aesthetically pleasing. Too long an exposure time and details can be lost, in an overly blurred water flow, not long enough and the feeling of movement in the frame has been lost. Below, taken from the bracketed set above, is what one person might think is the ideal amount of time for an exposure. Of course there will be some subjectivity at play here, but we can at least give you the tools to apply your own perspective to the question.

BW-SUN-0416201701-I002.jpg -
BW-SUN-0416201701-I002.jpg – “About Time” An image from the above sequence that appears to be using about the right amount of exposure time.

With that in mind, let’s dial into how long exposure waterfall images work, the elements that make up the image and finally the exposure adjustments you may want to make to change the effect of the water movement within the frame. Just to be clear, we aren’t really going to touch on composition here, other than the quality of the water’s appearance in the frame. Water location, leading lines, thirds, etc., are all still important of course, but not going to be addressed here.

Before we answer the follower’s question directly, let’s use the next two sections for a review of exposure decisions and combinations. A little review of exposure theory is up next.

Making Time – Part One

First things first, let’s have a little primer on exposure settings and how we ultimately are going to achieve these longer shutter speeds that we want to use. Our refresher is going to involve the following example:

We have our camera set to Aperture Preferred mode and our camera’s ISO set to ISO 400. Simply as a starting point, we have our aperture set to f/5.6. Using our camera’s meter we take a reading and the camera comes back with a shutter speed of 1/60 of a second. Looking at the following table (Exposure Table – ISO 400…) we’ll see that we have all of the following exposure combinations available to us…

Exposure Table – ISO 400…

ISO Aperture Shutter Speed
ISO Aperture Shutter Speed
400 f/1.4 1/1000
f/2.0 1/500
f/2.8 1/250
f/4.0 1/125
f/5.6 1/60
f/8.0 1/30
f/11 1/15
f/16 1/8
f/22 1/4

Looking at the table above, we can quickly see that shutter speeds all the way down to 1/4 of a second can be achieved, by selecting smaller and smaller apertures. Being that we are in Aperture Preferred mode, simply selecting an aperture of f/22 will cause the camera to provide us with the shutter speed of 1/4 of a second.

Now, let’s assume that that 1/4 of a second shutter speed is not going to give us the result we want. It’s simply not a long enough period of time to give, what we think is, the necessary amount of blurred water in the image. Fortunately we have a little room for adjustment here and that adjustment lies in changing the ISO. If we drop the ISO down to ISO 100, we lose two (2) stops of light in our exposure reading and correspondingly our shutter speeds will adjust to correspondingly longer times to compensate for that loss of light. Based on the premise that the amount of light, available for the exposure, needs to remain the same, the camera will adjust the shutter speeds so that they add back in two (2) stops of light that was lost due to the ISO change. Looking at our next table below (Exposure Table – ISO 100…), will show us the new exposure combinations that are now available to us…

Exposure Table – ISO 100…

ISO Aperture Shutter Speed
ISO Aperture Shutter Speed
100 f/1.4 1/250
f/2.0 1/125
f/2.8 1/60
f/4.0 1/30
f/5.6 1/15
f/8.0 1/8
f/11 1/4
f/16 1/2
f/22 1″

Looking at our new table above, you will now notice that a shutter speed of 1 second (1″) is now available, assuming that we are staying at an aperture of f/22. 1 second (1″) may indeed provide a long enough exposure to render the water the way we want it to look, but not necessarily (more on that later in the article). Let’s assume it’s not, we are now faced with a bit of a challenge. Our camera is at its lowest ISO setting, so we can’t cut out any more stops of light with that control and our aperture, is as small as it can get, so no more light can be blocked out there either. With that in mind let’s move onto the next part of our review of long exposure times.

Making Time – Part Two

So far we have explored metering the scene and using Aperture Preferred mode to provide us with an exposure time. We have seen that by making our aperture smaller, we can effectively lose stops of light and that loss leads us to longer shutter times. But our example has left us in a bit of a bind. We have no lower (less sensitive) ISO to switch to and we are tapped on aperture as well, being at the smallest setting.

In order to get to longer and longer exposure times, we need to cut out some more light in another way. Enter the Neutral Density filter (ND). This clever little piece of photo kit reduces the light entering the camera in a color neutral way, thus the name Neutral Density. Neutral, because it’s not supposed to shift the color temperature in any way. Density, because it’s supposed to provide a measurable level of density that the light must go through, in effect blocking a known portion of the light going through the filter.

ND filters come in a number of different flavors and ratings, but for now we are going to assume filters that offer a known, constant amount of light reduction. Conveniently enough that reduction is typically measured in stops of light, so you will find ND filters that are rated a -1 stop, -2 stops, -3 stops, etc. You may even combine these filters, say a -1 stop and a -2 stop and create an aggregate amount of light loss, in this case it would be -3 stops.

Though certainly not all inclusive, the following list shows the commonly available Neutral Density filters available at most camera stores:

  • -1 Stop
  • -2 Stops
  • -3 Stops
  • -4 Stops
  • -5 Stops
  • -10 Stops (not quite as common)
  • -15 Stops (even less common)

Simply applying one, or more, of these filters in front of your lens, will cut out the amount of light that the filter is rated to block and will accordingly open up new range for you in your exposure combinations. Let’s continue on with our exposure example from Making Time – Part One and we will see how helpful various ND filters can be in getting to longer exposure times:

Exposure Table – ND Filter Conversions (f/22)…

ISO Aperture Original Shutter Speed ND Filter Used Light Loss (Stop) Resulting Shutter Speed
ISO Aperture Original Shutter Speed ND Filter Used Light Loss (Stop) Resulting Shutter Speed
100 f/22 1″ ND2 -1 2″
ND4 -2 4″
ND8 -3 8″
ND16 -4 15″
ND32 -5 30″
ND64* -6* 1′
ND128* -7* 2′
ND256* -8* 4′
ND512* -9* 8′
ND1000 -10 15′

Neutral Density Filters – Naming And Equivalents…

Filter Optical Density Stop Loss
None 0 0 1/4000 1/1000 1/250 1/60 1/15 1/4 1″ 4″
ND2 0.3 1 1/2000 1/500 1/125 1/30 1/8 1/2 2″ 8″
Polarizer Up to 0.5 Up to 1 2/3 1/1250 1/320 1/80 1/20 1/5 1.3″ 3″ 13″
ND4 0.6 2 1/1000 1/250 1/60 1/15 1/4 1″ 4″ 15″
ND8 0.9 3 1/500 1/125 1/30 1/8 1/2 2″ 8″ 30″
ND16 1.2 4 1/250 1/60 1/15 1/4 1″ 4″ 15″ 1′
ND32 1.5 5 1/125 1/30 1/8 1/2 2″ 8″ 30″ 2′
ND64 1.8 6 1/60 1/15 1/4 1″ 4″ 15″ 1′ 4′
ND100 2 6 2/3 1/40 1/10 1/2 2.5″ 10″ 40″ 2’40” 11′
ND256 2.4 8 1/20 1/5 1.6″ 4″ 15″ 1′ 4′ 15′
ND400 2.6 8 2/3 1/15 1/4 1″ 6″ 25″ 1’20” 6′ 26′
ND500 2.7 9 1/8 1/2 2″ 8″ 30″ 2′ 8′ 30′
ND1000 3.0 10 1/4 1″ 4″ 15″ 1′ 4′ 15′ 60′
Filter Optical Density Stop Loss

Looking at the table above (Exposure Table – ND Filter Conversions (f/22)…), you can quickly see all of the additional exposure times you can get by applying a ND filter to your lens. Many landscape photographers will carry and assortment of ND filters with them, at different “stop” ratings, so that they can adjust their exposures to the desired time they have in mind when viewing a waterfall scene. In all reality, if you were to carry the following assortment with you:

  • -1 Stop
  • -2 Stops
  • -3 Stops
  • -4 Stops

You can achieve anywhere from a -1 stop of light all the way up to -10 stops of light, by simply combining them as desired.

Last, but not least, so far in our example we have arbitrarily selected an aperture and we haven’t really discussed the ramifications of that selection. Initially we selected it to get to longer exposure times, but now that we know that the ND filter can extend our range, we can now give more consideration to an aperture setting that we wish to use.

Aperture selection goes directly to our concern over depth of field. A larger aperture gives us very little depth of field, therefore very little of the image, outside of the focal plane, is in focus. Conversely, a much smaller aperture gives us a deeper depth of field, therefore more of the image is in focus, both in front of and behind the focal plane. Clearly depth of field is important and it really is the lynch pin of a lot of our composition decisions (which we are purposely ignoring here), but there is another consideration here as well and that is image quality.

No matter how good the lens, it is likely that it is not tack sharp all the way through its entire aperture range. A lens that goes, for example, from f/2.8 all the way to f/22, is likely not as sharp at f/2.8 as it might be at f/4.0 and again it may not be as sharp at f/22 as it is at say f/16. Most seasoned photographers avoid apertures at extreme ends of the lens’ range, to purposely avoid this softness in the resulting image. Case in point, does your waterfall image need f/16 or f/22, to completely capture all the depth in the scene that you want to be clear? Try to use the aperture that best fits the scene, without over compensating on depth of field. Below we see an adjusted table (Exposure Table – ND Filter Conversions (f/11)…), that shows the effects of the same ND filters, but at a different aperture:

Exposure Table – ND Filter Conversions (f/11)…

ISO Aperture Original Shutter Speed ND Filter Used Light Loss (Stop) Resulting Shutter Speed
ISO Aperture Original Shutter Speed ND Filter Used Light Loss (Stop) Resulting Shutter Speed
100 f/11 1/4 ND2 -1 1/2
ND4 -2 1″
ND8 -3 2″
ND16 -4 4″
ND32 -5 8″
ND64* -6* 15″
ND128* -7* 30″
ND256* -8* 1′
ND512* -9* 2′
ND1000 -10 4′

So, as we close out our review of exposure settings and combinations, keep these things in mind:

  • Adjust ISO to increase or decrease the exposure combinations that are available to you (of course the lower the ISO the better)
  • Adjust aperture to reach longer shutter speeds (but use only what you need for the composition at hand)
  • Add Neutral Density filters to extend the range to beyond what just the camera itself offers
  • As these exposure times get longer and longer, remember that a cable release not only avoid camera vibration, but it also gets you to times exposures that the camera can’t do on its own

One last thought, so far, we have looked at all of these exposure combinations, in Aperture Preferred mode. In operation, Aperture Preferred mode essentially makes us responsible for one side of the three exposure controls (ISO, shutter speed and aperture) in the exposure triangle. As you begin to extend past 30 seconds (30) of exposure time, you will likely find that you will need to begin to utilizeBulb” mode on your camera. Bulb mode in conjunction with a timer will allow you to get to much longer shutter times, if and when they are called for.

Often times, when switching to bulb mode, you’ll find that the exposure time called for will exceed the camera’s built in maximum exposure time, or the meter won’t read effectively, or the meter doesn’t offer readings at all, simply displaying bulb in the viewfinder display, rather than a recommended exposure time. No worries, the standard approach is to determine a reading without the filter on and then adjust the exposure setting, manually to compensate for the number of stops that the filter choice will effect. Most practiced photographers will use an Aperture Preferred meter reading, to set their base exposure and then after switching to Bulb mode, will adjust accordingly. Consider the following:

  • In Aperture Preferred mode, ISO 100 – f/8, the camera sets exposure time to 1/8 of a second.
  • You intend to use a 10 stop (ND1000) Neutral Density filter.
  • After switching to Bulb mode, you set your timer for 2 minutes (2′), compensating for the 10 stops of light the Neutral Density filter blocks (1/4, 1/2, 1, 2, 4, 8, 15, 30, 1′, 2′).

Keep in mind that you do not have to take all of the adjustment off of just one side of the exposure triangle, all of the following exposure combinations are the same volume of light:

  • ISO 100, 2′ @ f/8
  • ISO 200, 1′ @ f/8
  • ISO 400, 30″ @ f/8
  • ISO 1600, 8″ @ f/8
  • ISO 800, 8″ @ f/5.6

You have a tremendous amount of latitude in picking exposure combinations in any mode, just remember to adjust correctly, one or more sides of the exposure triangle, to get the exposure combination that fits the scene, the best. Once in Bulb mode, you become responsible for two, or even three, sides of the equation. It’s not difficult, just track the number of stops you are going to cut out, and adjust accordingly.

Now that we have reviewed the fundamentals of achieving longer shutter speeds, let’s get back to the question at hand, how do we know where to see our shutter speed when taking a long exposure waterfall image? Off to the next section…

The Scene

At this point, you are probably hoping for a couple of nuggets of insight that we send you on your way to photographing waterfall images with just the right amount of milky white present in the scene. Unfortunately, it’s not quite that easy and there are some additional factors that you are going to need to consider and it’s not just this one time that they need consideration. That’s right every waterfall scene that you attempt to capture is going to have characteristics that are going to make it unique and are going to require you to think through the exposure time that you wish to use.

Before we delve into some of the more obscure factors that we need to consider, there is the most obvious one of all, the amount of light present in the scene. Of course, this factor is going to effect the exposure time. More light means a shorter exposure time and less light means a longer exposure time. The difference between the two can be quite subtle as you are viewing the scene, yet dramatic when you are reviewing the resulting histogram. It is even not only possible for the light to change during the exposure, but it is likely that you will experience this shift in lighting. To a certain extent you will be chasing the changing light with adjusted exposure times while you are shooting one waterfall scene. After some experience you might even notice the light increasing, due to say the undulating cloud cover overhead and cut an exposure short in response.

BW-SUN-0416201701-I003.jpg - Possibly a little but too much exposure, given the bright light in the scene.
BW-SUN-0416201701-I003.jpg – Possibly a little but too much exposure, given the bright light in the scene.

Many photographers will plan their long exposure waterfall photography around the weather, preferring the consistent, diffused light of a mostly cloudy or overcast day. Clearly these lighting conditions will provide two benefits that need to be considered: One, the light is even, without hotspots or specular highlights appearing on the water and other shiny surfaces. Two, the light stays consistent over longer periods of time, avoiding the shifting light volumes that a slightly cloudy day might bring to the equation. Not everyone has the luxury of planning their shoots around such conditions, but if you do it is something worth considering.

BW-SUN-0416201701-I004.jpg - Exposure in this scene seems much better managed.
BW-SUN-0416201701-I004.jpg – Exposure in this scene seems much better managed.

So now that we have considered the obvious, let’s take a look at the less obvious. There are a number of factors, that the waterfalls themselves present, that we want to consider. This may all seem a little over the top, but each of these factors has an impact and consideration of all of them leads to better images. Here they are:

Flow Rate

When we are talking about flow rate, we are talking about a measurable amount of water going through a defined path. When we are talking about flow rate in waterfall photography, we are talking about more of a guess than a scientifically measured value. Long story short, the more water that’s flowing over a waterfall, in a given area, the more light that’s being reflected off the surface of the water, into the camera. So, if a waterfall is flowing fast, it is likely that you are going to need a shorter exposure time, than say a waterfall where the water is slowly meandering over the edge. We don’t really have hard, fast numbers here, but as you are working with more and more waterfalls, take note of how fast they are flowing and the adjustments you are making to get what you feel are better images. Over a period of time, you’ll begin to sense the adjustments needed based on the flow you are witnessing.

BW-SUN-0416201701-I005.jpg - Flow rate here is a tad high (actually a lot...) and the resulting image shows an exposure that is too long.
BW-SUN-0416201701-I005.jpg – Flow rate here is a tad high (actually a lot…) and the resulting image shows an exposure that is too long.
BW-SUN-0416201701-I006.jpg - Even large bodies of water have flow rate, about the appropriate time for this amount of flow.
BW-SUN-0416201701-I006.jpg – Even large bodies of water have flow rate, about the appropriate time for this amount of flow.

Waterfall Size

So you’ve probably heard, size matters, well it certainly does with lens apertures and as you are about to find out it does with waterfalls as well. The larger the waterfall, the more reflection of light we are going to get off of that increased water surface size and correspondingly the less time we will need for exposure. Conversely if the waterfall is quite small, with some relatively small water flow sections, it is possible that you may need a longer exposure time to capture more light from the waters movement. Again, as you witness more and more waterfalls and you begin to see the effect that their size has on the final image and accordingly exposure times, you’ll get a feel for the exposure time adjustments that might be appropriate.

BW-SUN-0416201701-I007.jpg - The size of this waterfall dictated how long we wanted the exposure to be.
BW-SUN-0416201701-I007.jpg – The size of this waterfall dictated how long we wanted the exposure to be.
BW-SUN-0416201701-I008.jpg - Waterfall size and distribution were a concern here and exposure was managed accordingly.
BW-SUN-0416201701-I008.jpg – Waterfall size and distribution were a concern here and exposure was managed accordingly.

Distance To Subject

As we discuss this factor, we need to keep in mind, we are not talking about how far we are from the waterfall itself, rather does the distance effect the relative size of the waterfall within the overall frame of the image. Really the consideration for this factor is the same as for waterfall size, but we wanted to call it out separately so you would keep it in mind as you are shooting waterfalls. Always consider the amount of the image that contains areas with water flow and the overall light reflectivity that many exist as a result of the water’s presence.

Perspective

One more factor to be considered is perspective and for the most part you will find that this comes down to a consideration of size again. As perspective shifts, it is likely that the overall size of the waterfall, present in the image frame, does as well. Pay attention to these shifts in both perspective and resulting waterfall size, both have the potential to effect exposure time correspondingly. One additional side note on perspective is that of angle of incidence versus angle of reflection, keep in mind that a shift in perspective can result in a corresponding shift in reflected light. Your camera’s meter, as well as your eye, should be able to note this shift. If you are seeing shifts of two or more stops of light, then a shift in exposure time is likely called for as well. Again, careful observation of your waterfall scene and changes in one or more of the factors above, so signal to you that an exposure time change is likely needed as well.

BW-SUN-0416201701-I009.jpg - Though a secondary concern here, managing perspective kept the water's footprint relatively small in this image.
BW-SUN-0416201701-I009.jpg – Though a secondary concern here, managing perspective kept the water’s footprint relatively small in this image.

Summary

As you can see there are a number of factors that can and will effect the exposure time that you select for a long exposure waterfall image. As stated earlier, the exposure time needed / desired is going to be dependent upon the effect that you wish to create and the light available in the scene you are photographing. Practice and experience will be your most effective tools in determining appropriate exposure times. When first getting started, it is likely that bracketing your exposure times will be the safest bet and will help you to come away with a worthwhile exposure from each of the differing scenes you will encounter. As times goes by and you shoot more and more long exposure waterfall images, you will begin to get a feel for what works best in different situations. Last that here before we sign off for the week…take a look at your failures and consider why they failed. Make note of what might have worked better and attempt to work that adjustment in next chance you get to shoot something similar. Along those same lines, many a photographer, having noted what didn’t work, has gone to a waterfall again and again until they have nailed the shot they want.

Keep those questions coming!

tdneumann

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