My old Philips 31CT computer flash unit

▼ I have an old Philips 31CT "computer flash". No, it has nothing to do with computer memory—it's a flash unit that sits on top of your camera. And although calling it a computer grossly overstates matters, but it has a small amount of automation built in that allows the flash to measure the amount of light returning from the scene that's being illuminated, and actually stop flashing mid-flash once a proper exposure has been accomplished. These units can't actually vary their brightness, only the length of the flash, which can be anything between 1/500th and 1/100000th of a second.

Hooking up an old flash unit is actually kind of risky, as these flashes can send hundreds of volts through the camera. I measured the voltage, which was 230 V. The Nikon D7100 is good for 250 V, though.

But why risk it? After all, modern cameras have a built-in flash. Simple: the built-in flash isn't very powerful, and it illuminates the subject straight on. Which is good when you need a "fill-in" flash to soften harsh shadows when photographing portraits in the sun, but it sucks as the main source of light: everything turns flat, you often get annoying reflections or red eyes, foregrounds that are overexposed and backgrounds that are underexposed.

Most external flash units on the other hand, can be aimed to the side or up at the ceiling, which makes the resulting light much more natural and pleasing. The photos below were taken with the flash pointing straight ahead or up at the ceiling, with one taken with the built-in flash for comparison.

Unfortunately, the ancient computer and/or Xenon flash bulb didn't age gracefully, and failed to provide consistent illumination. What you see here is a dial on the back of the unit that allows you to select one of three color-coded programs. You turn the dial to point to your selected ISO (well, ASA or DIN) sensitivity and then you set the camera's aperture to one of the options the colored arrows point at, and then move a slider on the front of the unit until the little eye that measures the light is surrounded by the chosen color. For best results, the shutter time should be set to the highest flash sync speed, 1/250 on the D7100. Then, turn on the flash, wait for the orange light to come on and snap some photos. But those were all underexposed, and not always to the same degree.

So I think I'll invest in a Yongnuo or similar third-party i-TTL flash unit at some point. Like Nikon's flashes, these will let the camera measure the required flash strength through the lens for optimal results. Interestingly, computer flash units seem to have died out these days, flashes are either controlled by the camera through Nikon's or Canon's proprietary systems, or they only function in manual mode, where they simply fire at a preset strength. Unfortunately Nikon's cheaper flashes don't tilt and swivel and are hampered by tiny batteries, while the bigger ones are very, very expensive.

Permalink - posted 2014-03-05

The 31CT flash revisited

▼ After yesterday's experimentation with my old "computer" flash unit, someone on the dpreviews forums suggested that maybe I was getting bad exposures because the flash was a bit too slow for the D7100's 1/250 sync speed. So I set up a little still life to take some more test shots.

The sync speed wasn't the problem. But I did get very consistent results now. I think I need to shoot under some more varied circumstances to really get a handle on the flash unit's idiosyncrasies. But with the aperture opened up an extra stop the results weren't bad at all. These shots are straight out of the camera, save for resizing/cropping. (I had to apply auto levels on yesterday's test shots).

Click on the different links to see the same scene with the flash pointed directly at it, or titled up 30, 60 or 90 degrees. Straight on provides strong illumination, but it kills all the textures. Also, the beige-ish background is completely washed out. When tilted up, the light basically overshoots the scene, so it's not illuminated very well.

At 60°, all the light bounces off of the ceiling right above the subject, so it's illuminated from above. Last but not least, by aiming the flash straight up (90°), the light is quite diffuse and comes from both above and from the front, which is what I prefer here.

Permalink - posted 2014-03-05

Ethernet energy efficiency

▼ How power-hungry are various permutations of Ethernet on modern MacBook Pros? Tests performed and written up by Jeroen van der Ham and Iljitsch van Beijnum.

For some reason, we thought it would be a good idea to make the students of the System and Network Engineering master program test whether various permutations of IPv4/IPv6, energy-efficient Ethernet (eee) and jumboframes use more or less power. While the students were busy measuring, we were bored so we decided to run the same tests ourselves using our MacBook Pros (with Retina Display®)—Jeroen the 15" mid-2012 model and Iljitsch the 13" late 2013 model. These computers are too thin to house an Ethernet port, so we used Apple's Thunderbolt - Gigabit Ethernet adapter. We looked at the number of milliamps taken from the battery to see how much power the entire system used during various tests.

We didn't realize it at first, but the most surprising result was actually the first one, looking at system power use with the adapter plugged into the computer, but without attaching a network cable. This increased system power use by 300 mA, or about 3.4 Watts. On the 15" MBP this is already significant, increasing power use from 1250 to 1550 mA. But the 13" MBP only used 370 mA without the adapter plugged in, so its power use almost doubled after plugging in the adapter!

Could it be that the secret to the long battery life of Apple's recent laptops is the fact that they come without an Ethernet port?

We continued our testing trying different possibilities with Energy Efficient Ethernet (eee) turned on or off. Students in 2012 already looked at the general impact of eee on energy usage for clusters. They found that it provides an energy saving of up to 40% in switches, depending on the amount of traffic. Energy usage goes up when traffic goes up, but it already maxes out when you are using roughly 40% of the bandwidth. We confirmed this in our testing: there is not much difference with eee when the interface is idle, or with full traffic.

More significant energy savings started to appear when we enabled jumbo-frames (MTU 9000). The amount of data that goes through the adapter remains virtually identical, but with only one-sixth as many packets per second, the impact of per-packet overhead is greatly reduced, and this shows in the energy consumption. With full traffic in one direction this saves you over 10%. These savings also add up if you have traffic in both directions, in our experiments we saw a savings 16-19% for a bidirectional fully-utilized link.

Not surprisingly, the biggest difference in energy usage is to scale down the interface speed. When we forced the interface down to 10Mbps we saw a saving of up to 49% with full utilisation. Then again, if you're sending a large file, it is going to take a lot longer, and you basically use a lot more energy, only more distributed over time.

The results of our measurements are in the table below, but note that our measurements (using the System Profiler battery current figures) weren't very precise. The numbers give a pretty rough indication. If you want to do something serious with this data, perform your own experiments.

Total system power use	MBP15	MBP13
Interface attached vs baseline	+24%	+92%
Baseline vs idle eee	+27%	+110%
Idle eee vs no eee	-6%	-1%
Idle eee vs iperf 1 Gbps TCP eee	+74%	+97%
iperf 1 Gbps TCP eee vs no eee	-3%	-1%
iperf 1 Gbps TCP jumbo eee vs no jumbo eee	-13%	-12%
iperf 0.5 Gbps UDP eee vs no eee	+1%	+1%
iperf 1 Gbps TCP bi-di eee vs unidirectional eee	-0%	-3%
iperf 1 Gbps TCP bi-di jumbo eee vs no jumbo bi-di eee	-19%	-16%
iperf 10 Mbps TCP no eee vs 1 Gbps	-38%	-49%

We did some extra tests to look at the impact of jumboframes on CPU use. Please note that our Thunderbolt-to-Gigabit Ethernet adapters don't support TCP segmentation offloading (TSO). With 1500-byte packets the CPU utilization on the MBP13 was about 16%, with 9000-byte packets about 90%. Despite being idle more than 80% of the time, the CPU still ran at its advertised 2.4 GHz clock speed:

So the difference in CPU power use was 18% (7.9 W for 1500 bytes vs 6.4 W for 9000 bytes), despite the difference in CPU utilization only being 5%.

Permalink - posted 2014-03-03

Testing cameras on the oil rig

▼ As you may know by now, there's an oil rig 12 km off the coast of The Hague. 12 kilometers is pretty far away, so it's not easy to photograph. Even with a long zoom lens it shows up pretty tiny:

Today the weather was clear and sunny, so I tried once again with both my new Nikon D7100 with the Nikon 70-300 mm lens and the not-so-new Canon SX280 point-and-shoot. I mounted them both on a tripod and tried 300, 240 and 200 mm on the Nikon and the full 90 mm (about equivalent to 300 mm on the Nikon) with and without digital zoom on the Canon. I cropped the images to show just the oil rig.

The results (click the links to load the different images):

It looks like the Canon is slightly sharper than the Nikon combo. And although the 70-300 mm lens is supposed to be a bit soft at 300 mm but shaper at below 200 or 240 mm, it still shows a tiny bit more detail at 300 mm than at 240 mm and certainly more than at 200 mm. And as expected, the results for using digital zoom or simply cropping the photo on the computer later are virtually identical.

Permalink - posted 2014-03-02