In our lastest video (https://www.youtube.com/watch?v=j-iD_XNpnrg&feature=youtu.be), we discussed the importance of adjusting your exposure based on the actual speed of your shutter. We've even published a handy chart that does the math for you. (See our previous blog post: https://shop.stearmanpress.com/blogs/news/about-that-shutter)
Of course, this implies that you know your shutter's actual speed. Spoiler alert: it isn't what's marked on the dial.
We've been asked if we had any suggestions for determining the actual shutter speed without buying a shutter speed test set.
After all, what good is a ZoneCalc, if you can't control the zone!
We've been measuring our shutters using a digital oscilloscope and a photo diode. It's a pretty trivial thing to do, provided you have a digital scope. If you have such a scope, you probably know how to do it and we won't bore the rest of you with the details.
Realizing that most photographers probably don't have easy access to such equipment, we started thinking about a DIY method that anyone could use.
Our original concept: photograph an object moving at a known speed and measure the "blur", we could then calculate the shutter speed. We're not the first to think of this, people tried this with turntables back when everyone owned a turntable. Nowadays, you're more likely to own a digital storage scope than a record player.
So we experimented with pendulums, flashing lights and spinning disks. All techniques that could, with a bit of effort, actually work. But they were awkward at best.
Then we realized that we were overthinking the problem: what if we could take a high speed movie of the shutter as it opened and closed? All we'd have to do is count the frames and we'd know the shutter speed!
Turns out that's actually pretty easy to do these days.
Most smartphones can record video at 240 frames per second (fps). (Even if yours doesn't, you know somebody whose phone will: your daughter, nephew, anyone who's living in the 21st century ...)
Note: Sampling theory would limit the maximum shutter speed we could capture to 1/120 of a second. However, we're capturing video images and we're just looking for a reasonable approximation. Thus, if the open shutter is visible for only one frame, we know it was faster than 1/240 of a second. How much faster is hard to tell. However, since most lenses run slow, really slow, it isn't generally a problem until you get to 1/500 second.
That said, we'd be willing to bet that 90% of large format photos are taken at speeds slower than 1/60 second. So this technique will solve the biggest part of the problem for the vast majority of photographers.
Simply setup your camera with the lens/shutter to be tested and aim it a bright light. You want the entire ground glass evenly illuminated. We used a LED panel light in our studio. It's about 10" square and we set the camera close enough that it completely fills the frame. Focus isn't important.
Set your smartphone to record at 240 fps. We're not going to tell you how; you'll have to figure that out on your own (if you're borrowing a phone, the kid that owns it knows how.)
Now mount the smartphone to record the image on the ground glass. You can rig something up using tape and rubber bands or buy a fancy tripod adapter for $5-10.
Set your shutter to 1 second, start the video and fire the shutter. Repeat for all the speeds you want to test.
Now all you need to do is count the frames. If you have the right app, you can do this on the smartphone, or transfer the video to your computer.
We find it easier to work on a full sized monitor, so we downloaded the file to our PC and used a free software video editor (https://fxhome.com/hitfilm-express) to count the frames. (There are many others, this is just one of the first free ones we found.) You'll have to figure out how to transfer the video, install the software and edit the file on your own; there are too many variables for us to try and explain it.
Here's a screen shot of our 1/4 second capture. The top image is the frame under the cursor; the sequence below it shows the shutter opening and then closing. The bottom row is the audio waveform.
Every software package does things their own way but the basic functions are the same. Here are some guidelines.
Note: the video was recorded at 240 fps but the editing software assumes it will be replayed at 30 fps. Thus the time measurements are off by a factor of eight.
Notice the numbers in the box in the lower right of the photo: "+00;00;02;13." This is a measurement of how far the cursor has moved on the timeline. (It resets to zero when released.) By positioning it at the first "opening" frame, letting it reset and then moving to the last open frame, it has counted the frames for you. (Your software may be different.)
The first three sets of numbers are hours;minutes;seconds. The last digits, "13" in this case, are how many frames are left over and not counted in the seconds. To get the total number of frames, multiply the seconds by 30 and add the remainder.
2 * 30 + 13 = 73 frames
Now to convert to time, divide by 240 frames/second:
73/240 = 0.304 seconds or 1/3.2
So on this shutter, the 1/4 second is actually 1/3 of a second. Here are the rest of our test results.
|Indicated speed||measured on
|1/2||0.56 seconds||0.55 seconds|
|1/4||0.38 seconds||0.31 seconds|
|1/8||0.169 (1/6)||0.163 (1/6)|
|1/15||0.091 (1/11)||0.088 (1/11)|
|1/30||0.045 (1/22)||0.046 (1/21)|
|1/60||0.022 (1/46)||0.025 (1/40)|
|1/125||0.013 (1/77)||0.015 (1/69)|
|1/250||0.009 (1/111)||0.010 (1/100)|
|1/400||0.0052 (1/192)||0.006 (1/167)|
So overall, pretty decent results. We didn't show the 1, 1/2 and 1/4 as equivalent fractions because you end up with a decimal in the denominator and that's just confusing. The 1 second and 1/2 second are close enough to use uncorrected. As we said above, we rounded the 1/4 second to 1/3. For the 1/250 we just round to 1/00; for 1/400 we round to 1/200.
Some other comments:
1. The preview images on the timeline aren't very accurate. You'll want to use the main image to determine when the shutter opened and closed.
2. Some interpretation is required. You can see in the sequence above that the camera captured the shutter as it opened. Do you count that as a full frame or half a frame? You get to decide.
3. You can capture the 1, 1/2 and 1/4 speeds on one video stream. They're long enough that you can keep things straight when editing. However, you'll probably find it best to record the faster speeds separately and record each three or four times and work out an average.
4. It's best to be quiet during the recording and not try to add comments like "this is 1/8 second". First, you can't understand the audio during playback because the speed is 8X. In fact, we mute the audio because it's rather annoying. Second, you can easily pick out the audio of the shutter activation visually in the audio stream below the video (see photo above). This will help you find the frames containing the faster speeds.
Someone who is familiar with the video software can probably sort this out in about ten minutes. If you don't want to mess with it, buy your nephew a pizza and let him deal with it. In fact, you can upload the file to the cloud (don't try and email it, it's a BIG file) and he can do the calculations remotely.
We know we haven't answered every question but hopefully you've got enough to get started.