View Full Version : How to accurately time long exposures?

Bernard Languillier
25-Oct-2004, 04:41
Dear all,

I have started to play with my new Ebony and am now running into a problem I had not anticipated:

- how can I accurately time long exposures?

The reason why I would like to do this is that I intend to stitch 2 images taken with my 6*9 back left/right shifted and to assemble them in PS after scanning.

This works much better if the exposure of the 2 frames is identical. To ensure identical exposures on 2 successive frames, I need to be able to reproductively apply the same lenght of exposure for the 2 shots.

Thank you in advance for any help on this.

Best regards,

Louie Powell
25-Oct-2004, 05:08
Bernard -

Great precision is not required. The longer the exposure, the greater the tolerance for small errors in timing. Remeber that to get a one stop increase in exposure, you have to increase the exposure time by 1.4. So if the exposure time is 10 seconds, a one stop increase translates to a 14 second exposure. Therefore, a 1 second error is only 1/4 stop. And if the exposure is longer, the impact of the kind of timing errors you are likely to get between sheets of film becomes even less significant.

There are three approaches you might consider. First, if the exposure is a matter of a few seconds, consider using the 1 second setting on your shutter, and use multiple exposures. So, for example, if the exposure is 5 seconds, use five separate 1 second exposures.

Second, I often will just use my wrist watch to measure exposures. Again, with longer exposures, an error of 1 or 2 seconds won't be perceptible in the final negatives.

Third, if you have a PDA, there is a piece of freeware that you can download that you can use to time exposures (or development times, for that matter). It's called FotoTimer and can be downloaded from http://www.jon-exner.de/software/ (http://www.jon-exner.de/software/).


Henry Friedman
25-Oct-2004, 05:10
Have you tried just using your wristwatch or a stopwatch, and actually experienced a problem? One can actually be quite accurate.

Also, I've been surprised at how little effect is seen from a fairly dramatic difference in exposure time, once my exposures are into reciprocity failure range.

David A. Goldfarb
25-Oct-2004, 05:28
Try using a metronome. Set it at 120 and subdivide the beat so you're counting 4 beats per second. Musicians do this all the time and are able to do things not much different from pressing a cable release with a precision of a few thousandths of a second. With older shutters, I find this method is usually more accurate than the shutter is even at one-half second.

Tadge Dryja
25-Oct-2004, 06:55
"Remeber that to get a one stop increase in exposure, you have to increase the exposure time by 1.4."

You mean by 2, right? One stop is... twice as much light? Or am I really missing something...?

Michael Kadillak
25-Oct-2004, 07:08
I would recommend getting an index card laminated with the reciprocity data of the film that you are using. Secondly, Radio Shack sells a bright yellow stop watch with a laynard that has a button for a nightime "illuminator" that is easy to use and bulletproof. Should work like a charm for what you are desiring to accomplish in your post.


Rich Long
25-Oct-2004, 07:14
Quoted from above:

""Remeber that to get a one stop increase in exposure, you have to increase the exposure time by 1.4."

You mean by 2, right? One stop is... twice as much light? Or am I really missing something...?"


Yes, I believe the 1.4 factor has to do with the increase in aperture size. Shutter speed doubles or halves (factor of 2) to achieve a one stop difference.

Emmanuel BIGLER
25-Oct-2004, 08:57
The actual maximum allowable tolerance on shutter times for a technician repairing a mechanical shutter is as high as +- 20%. Of course most of the "times" ;-) the shutter is more accurate than that, and good technicians working on good shutters will do much better than +-20%. And modern electronic shutters will simply make you forget about any inaccuracy in timing. But let us take a +-10% tolerance.

Timing 30 seconds with +-10% tolerance means timing 30 seconds +- 3 seconds. just looking at your wristwatch is accurate enough, and as mentioned do not forget to correct for reciprocity.

The range of exposures that could be the most difficult to properly determine might be the 3-10 seconds range.

For a few seconds, if you use a self-cocking shutter like the copal press or the prontor professionnal you'll simply add as many exposures of one second as needed.

Using a classical or electronic metronome sounds perfect in any cases.

Pete Watkins
25-Oct-2004, 10:09
Before you spend out on a stop watch you might not be aware that a lot of mobile phones have a stop watch facility.

Paul Butzi
25-Oct-2004, 10:16
I just look at my wrist watch, count in time with the advancing seconds (so that I get close to one count per second) and then count "zero" (as I open shutter), "one", "two", etc. until it's time to close the shutter.

For exposures longer than, say, 15 seconds, I tend to set the timer on my watch, open the shutter as I start the timer and step away from the camera, then watch the timer, step up to the camera and close the shutter as the timer runs out.

There's no need for hysterical accuracy. Letting it run for 11 seconds instead of 10 means you've overexposed by 1/7th of a stop, not a big deal.

Mark Sawyer
25-Oct-2004, 12:37
Doubling the time is an increase in exposure corresponding to one f/stop, not allowing for reciprocity failure. "Typical" b/w reciprocity is add one f/stop at one second, two f/stops at ten seconds, or three f/stops at 100 seconds. Varies by film, of course.

F/stop numbers double every other f/stop, as f/stop are measured by *diameter*, (actually the focal length divided by the diameter), but admit light as a function of *area*. Double the diameter and you increase the area 4x, or two f/stops. If that makes sense.

I find it simplifies things to do the calculations in base eight using chinese algebra...

Armin Seeholzer
25-Oct-2004, 15:19
Many good suggestions!

I do it with my watch most of the time but now I have a lens with compur electronic shutter wich goes down to 32 seconds wich is really a nice help and of course if 32 seconds are not long enough I still can make a second pop in most cases!
I really like my compur electronic no. 1

Frank Petronio
25-Oct-2004, 15:42
If you want to ensure consistent exposures, you'll need consistent lighting. Only photograph on overcast days from now on...

George Ward
25-Oct-2004, 18:13

Paul Butzi's post describes exactly the the most efficient way in the field to time exposures.
I used a stop watch initially, but it's an additional device that adds weight and time and is not any advantage
over the simple simple second hand on your wrist watch. Keep it simple.

Steve McKinney
25-Oct-2004, 20:06
I usually wear a cheap Timex Triathalon digital watch when I shoot and use exactly the same method Paul Butzi does. There's significant margin for error for shots with exposure times of a couple of seconds or more.



Bernard Languillier
26-Oct-2004, 03:29
Dear all,

Thanks a lot for your kind answers. I guess that I'll just have to buy a wrist watch... :-)

Best regards,

Graeme Hird
26-Oct-2004, 21:40

Have you tried using this shutter speed/exposure method on your Jackalopes? Maybe others got in before you and that's the reason you can't find them in the wild any more .....

Andrew O'Neill
26-Oct-2004, 23:32
I partially disagree with the 1.4 factor. By multiplying your exposure time by a factor of 1.4 will work okay up to about 10 seconds. If I have an indicated exposure of 16 seconds and apply a factor of 1.4, that's about 22 seconds. In fact with my film I should apply a factor of 2x (32 seconds). That's a significant difference. So, you cannot give a blanket factor for all exposures.

Lars Åke Vinberg
1-Nov-2004, 22:30

I have been making stitched 6x9 exposures for three years now. I frequently run into the following issues:

- Light is shifting dues to clouds moving past the sun.

- The subject matter (vegetation, water, clouds) is moving due to wind.

- Light condition changes due to earth's rotation (dawn/dusk exposures).

- Camera is moved between exposures.

Consequently, my best results are with overcast or clear skies on calm days. No surprise here. I have been able to make a few good stitches around dawn/dusk, but it takes some photoshop work to get the transition right.

At dawn/dusk I find that if I keep exposures mess than 30 seconds apart then the results are usually good. This is not always possible, as my exposure times often are minutes. Consequently I have now purchased a 6x17 back to try out over the next few months.

Kind regards,