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10-17-2013, 04:11 PM   #1
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Long exposure shots, ND filters and multi-exposure averaging

Hi,

it's a bit of a stupid question, but how do people manage to do landscapes with tens to hundreds of seconds of exposure time? No matter what light conditions I am in, around the dusk there is too much light to get a properly long exposure, for blurring out the sky and water in landscape shots, even with an ND filter and an F16.

So how do people go about it? An ND filter that makes the exposure 8 times longer just won't suffice. Do they simply expose several images on an intervalometer and average them in post processing? Do they use variable NDs with dual polarizers?

I would imagine multi-exposure method should bring better noise characteristics (due to sensor heating) but the interruptions in exposure could show as an interrupted motion.

10-17-2013, 04:20 PM   #2
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QuoteOriginally posted by rrstuff Quote
Hi,

it's a bit of a stupid question, but how do people manage to do landscapes with tens to hundreds of seconds of exposure time? No matter what light conditions I am in, around the dusk there is too much light to get a properly long exposure, for blurring out the sky and water in landscape shots, even with an ND filter and an F16.

So how do people go about it? An ND filter that makes the exposure 8 times longer just won't suffice. Do they simply expose several images on an intervalometer and average them in post processing? Do they use variable NDs with dual polarizers?

I would imagine multi-exposure method should bring better noise characteristics (due to sensor heating) but the interruptions in exposure could show as an interrupted motion.
Neutral density filters are generally rated by the number of stops of light they block. Each additional stop of filtering means half the light is getting through.

If your filter only lets 1/8th of the light through it is a 3-stop ND filter.

However, ND filters also come in 6-, 9-, and 10-stops (amongst others). The last one reduces amount of light passing by 2^10 = 1024 times, and would require enormously more time to reach the same exposure as a 3-stop filter. You are correct that until the very last wisps of light, a 3-stop ND filter won't lengthen exposure time to minutes, and so most people who do long exposures will use a higher stop filter.

You can also stack neutral density filters for even longer exposures, but for now I would recommend looking at B+W's ND110 10-stop neutral density filter.
10-17-2013, 04:40 PM   #3
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QuoteOriginally posted by rrstuff Quote
people manage to do landscapes with tens to hundreds of seconds of exposure time
I have no idea how they do it, my stupid question is why is that necessary? Moving water doesn't need 1 second to be blurred, clouds can be digitally blurred for a region of sky and if anything the sensor has its own inconsistencies over time that makes extremely long exposures less sharp than mildly long exposures. Something about getting too close to the sensor's noise floor at any given time. It's not like exposing glass plates or some other glacially slow medium. Electronic shutters are accurate enough that you don't get more precise exposures by using such long exposure times, either.
10-17-2013, 04:46 PM - 1 Like   #4
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QuoteOriginally posted by RGlasel Quote
I have no idea how they do it, my stupid question is why is that necessary? Moving water doesn't need 1 second to be blurred, clouds can be digitally blurred for a region of sky and if anything the sensor has its own inconsistencies over time that makes extremely long exposures less sharp than mildly long exposures. Something about getting too close to the sensor's noise floor at any given time. It's not like exposing glass plates or some other glacially slow medium. Electronic shutters are accurate enough that you don't get more precise exposures by using such long exposure times, either.
Like anything with photography, it's an artistic decision. 1 second will blur moving water, sure, but not as much as you think, and certainly not to the point of seeming completely placid or fog-like. I suppose you could blur or streak out clouds in photoshop, and some people do this to great effect, but in more complicated scenes the technical skill required becomes very high and results in a loss of natural appearance.

One final and important reason to do long exposures, especially at dawn or dusk, is it can improve the intensity of the color and the smoothness of the tonalities by giving things time to mix around, so to speak.

10-17-2013, 04:48 PM   #5
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How Dark Is Enough?

Helol rrstuff,
A couple of questions; First, are you using the lowest ISO possible?
Second, when you say '8 stops' do you mean the ND is a # 8? If so, that's the light transmittance value, not the f/stop value. A # 8 ND is a 3 f/stop value.
Here's an example. Let's say you have a bright, very sunny day, your readings are 1/500s, f/16, ISO 100. OK?
We start adding numbered ND filters. Not stacking them, individual filters;
ND 2, one f/stop reduction, shutter speed now 1/250s
ND 4, two f/stop reduction, ss now 1/125s
ND 8, 3 stop reduction, ss now 1/60s
ND 16, 4 stop reduction, ss now 1/30s
ND 32, 5 stop reduction, ss now 1/15s
ND 64, 6 stop reduction, ss now 1/8s
ND 128, 7 stop reduction, ss now 1/4s
ND 256, 8 stop reduction, ss now 1/2s.
And so on.
Now, that was under nearly optimum lighting conditions. At sunrise or sunset, the ambient light wouldn't be nearly as bright. You might be starting at 1/30s, and a ND 64 (6 stop reduction) would bring the shutter speed down to 2 seconds. The recommended 10-stop ND would result in a 32 second shutter speed (!!).
I get by very nicely with a couple of # 16's, 4 stops each. Occasionally I've added a polarizer.
Try using a couple higher-number ND's, lowest ISO and keeping experimenting.
Ron
10-17-2013, 05:07 PM   #6
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QuoteOriginally posted by v5planet Quote
Neutral density filters are generally rated by the number of stops of light they block. Each additional stop of filtering means half the light is getting through.

If your filter only lets 1/8th of the light through it is a 3-stop ND filter.

However, ND filters also come in 6-, 9-, and 10-stops (amongst others). The last one reduces amount of light passing by 2^10 = 1024 times, and would require enormously more time to reach the same exposure as a 3-stop filter. You are correct that until the very last wisps of light, a 3-stop ND filter won't lengthen exposure time to minutes, and so most people who do long exposures will use a higher stop filter.

You can also stack neutral density filters for even longer exposures, but for now I would recommend looking at B+W's ND110 10-stop neutral density filter.
Hi, thank you for the quick reply. Those filters are (a bit) pricey, although if that's the only way I can do that I can get it.
10-17-2013, 05:10 PM   #7
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QuoteOriginally posted by RGlasel Quote
I have no idea how they do it, my stupid question is why is that necessary? Moving water doesn't need 1 second to be blurred, clouds can be digitally blurred for a region of sky and if anything the sensor has its own inconsistencies over time that makes extremely long exposures less sharp than mildly long exposures. Something about getting too close to the sensor's noise floor at any given time. It's not like exposing glass plates or some other glacially slow medium. Electronic shutters are accurate enough that you don't get more precise exposures by using such long exposure times, either.
The point about blurring in post is a valuable one, but I am talking about a specific project, which will require me to do it with high levels of light, but will also require a lot of time-blur. Also, it's very helpful in busy places, where people waling in front of your camera don't disappear until a very long exposure has been applied... Another application would be light painting.
10-17-2013, 05:12 PM   #8
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rbefly, I meant ND3=8-fold reduction, that's one I got. I held off with more powerful ones due to the price. And yes, I am using an F16-22 and the lowest iso available.

10-17-2013, 05:14 PM   #9
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QuoteOriginally posted by rbefly Quote
Helol rrstuff,
A couple of questions; First, are you using the lowest ISO possible?
Second, when you say '8 stops' do you mean the ND is a # 8? If so, that's the light transmittance value, not the f/stop value. A # 8 ND is a 3 f/stop value.
Here's an example. Let's say you have a bright, very sunny day, your readings are 1/500s, f/16, ISO 100. OK?
We start adding numbered ND filters. Not stacking them, individual filters;
ND 2, one f/stop reduction, shutter speed now 1/250s
ND 4, two f/stop reduction, ss now 1/125s
ND 8, 3 stop reduction, ss now 1/60s
ND 16, 4 stop reduction, ss now 1/30s
ND 32, 5 stop reduction, ss now 1/15s
ND 64, 6 stop reduction, ss now 1/8s
ND 128, 7 stop reduction, ss now 1/4s
ND 256, 8 stop reduction, ss now 1/2s.
And so on.
Now, that was under nearly optimum lighting conditions. At sunrise or sunset, the ambient light wouldn't be nearly as bright. You might be starting at 1/30s, and a ND 64 (6 stop reduction) would bring the shutter speed down to 2 seconds. The recommended 10-stop ND would result in a 32 second shutter speed (!!).
I get by very nicely with a couple of # 16's, 4 stops each. Occasionally I've added a polarizer.
Try using a couple higher-number ND's, lowest ISO and keeping experimenting.
Ron

Hi, thank you for the detailed response. Does stacking the NDs on a wide angle lens (24mm equivalent) cause trouble? Aberrations or vignetting?
Did you try doing mulitple exposure on intervalometer too?

Last edited by rrstuff; 10-17-2013 at 05:38 PM. Reason: clarified focal length
10-17-2013, 05:47 PM   #10
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This is actually a pretty interesting topic and not as straight forward as I first thought. I'm sure someone more experienced could do a workshop on it!

I have tried stacking ND filters and yes vignetting can be an issue with wide angle lenses. I think it is worth it to invest in a filter holder system like Cokin P series. With these you can have several adaptor rings and use it with a number of lenses (and can tolerate a reasonably wide angle).

My biggest problem with multiple ND filters is colour shift (towards brown for me). Still not sure if this is a problem with my filters or something else.

Another technique for long exposure that I have just started experimenting with is photo stacking. You take a series of long exposure and combine them during Post processing. Seems to work well if shot are in close sequence. Has the advantage of shorter exposures meaning less issues with colour shift and low long exposure noise etc.
10-17-2013, 06:36 PM   #11
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QuoteOriginally posted by rrstuff Quote
where people waling in front of your camera don't disappear until a very long exposure has been applied
I assume the 'k' in walking didn't wait long enough to be exposed in your message, and I certainly don't want to get in the way of artistic expression. I do know a little bit about how our minds see images. If, for artistic reasons, you want the blurred images of moving people to remain in the final image, multiple images will work, but if the interval is too large, it won't look anything like smooth motion. Anything you can do to blur the moving objects in real time will help compensate for the slow frame rate effect, such as shortening your depth of field and focusing in the distance. Taking a sequence of second long exposures means that each frame already has a blurry object in it and if you can reduce the interval between those second long shots to a fraction of a second the denser portions of those objects should appear like a smooth motion. The entire object doesn't have to be equally blurred, as long as the densest part looks smooth, our eyes will equalize the fringes automatically.

When it comes to blurring drops of water or clouds, there is a line where too much blur removes any transparency and makes it completely opaque again, except over a larger area of the image. Transparency is how we perceive blurring in real life, our mind is stacking multiple stop action images. I just looked at a photograph of steaming mud pots in Yellowstone, and with a 10 second exposure, it is impossible to separate the vapour from the ground (and vice-versa). Which is the effect the photographer was trying to obtain, but it doesn't reflect what it is like to be there in person. To perceive placid water, you need to see sharp reflections in the water, otherwise it looks like it is out of focus or dirty.

QuoteOriginally posted by v5planet Quote
improve the intensity of the color and the smoothness of the tonalities
With film, I would agree with you, with flat plane digital sensors, I can't. The sensor only gives the camera a voltage and colour of filter for each pixel. Post processing will simulate luminosity and dynamic range by interpolating and adjusting the raw data. Basically as long as there are enough photons hitting the sensor to clearly distinguish the image from noise and not so many that the sensor is overloaded, computer algorithms will do the rest.

20 seconds is the longest exposure I've ever worked with, trying to get sharp nighttime images, not to be creative, but even then issues with wind and heat drove me nuts. From a technical standpoint, it seems to me that the kind of creative effects you describe are better done with digital manipulation than using optics to cut the amount of light reaching the sensor to the point where the sensor is not very reliable. But I certainly wouldn't describe my photography as artistic.
10-17-2013, 06:44 PM   #12
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QuoteOriginally posted by RGlasel Quote
I assume the 'k' in walking didn't wait long enough to be exposed in your message.
The 'k' was there previously but I obtained 'walking' by downsizing a much longer word with the nearest neighbour algorithm. It just lost some data.

All fair points. The reason for blurring objects in my case is in one case to draw attention to immobile objects and cleaning out the crop from high-frequency textures. I noticed that this becomes pretty good around 30-60 seconds, but that too is sometimes hard to achieve for me in the lighting I can work with.

Now regarding the PP, do you mean using a smudge/blur functions in photoshop? I am wondering if you are referring to some specific techniques I could study up on?
10-17-2013, 07:05 PM   #13
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With the craze for circular polarizing filters--you can get a baragin on a couple of used linear ones. As you rotate one w/ respect to the other you have a variable ND filter. They also work just fine (for me) for normal usage w/ the pentax dslrs. For 49 or 52mm I would think for two Tiffens $10-20.
10-17-2013, 07:07 PM   #14
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Hi dms thanks for mentioning it. Did you observe any artifacts when using stacked polarizer (uneven illumination, color shifts that are not correctable, weird color effects etc)?
10-17-2013, 07:12 PM   #15
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QuoteOriginally posted by rrstuff Quote
The 'k' was there previously but I obtained 'walking' by downsizing a much longer word with the nearest neighbour algorithm. It just lost some data.

All fair points. The reason for blurring objects in my case is in one case to draw attention to immobile objects and cleaning out the crop from high-frequency textures. I noticed that this becomes pretty good around 30-60 seconds, but that too is sometimes hard to achieve for me in the lighting I can work with.

Now regarding the PP, do you mean using a smudge/blur functions in photoshop? I am wondering if you are referring to some specific techniques I could study up on?
I assume you want to use blurry mobile objects to highlight other objects instead of unsharpening/blurring/smudging neighbouring objects that are probably not mobile. That would be interesting. I don't use Photoshop at all, and only dabbled with Corel Photo-Paint, but it seems to me there is a function to cut-out objects and spray multiple copies of them on a path, which could possibly look like the vapour trail of a person walking by if the spray object is somewhat transparent and quite blurry. I'm not sure what you mean by high-frequency textures, but you can apply irregular masks to distracting parts of your photo and un-sharpen, darken and colour adjust just those parts. If you are subtle and use more than one technique, you can definitely draw attention to parts of the picture without looking like it was deliberate. Sorry I can't be more specific, but photo-editing is really not my forte.
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