Recently I picked up a Canon 7D which I’m really enjoying so far. While researching the camera I found various references to both 100 and 160 being the native ISO and as I do quite a bit of still photography from a tripod was keen to determine the optimum ISO setting and also the best ISO multiples to use for available light shooting. Between various forum posts there seemed to be about a 50/50 mix of people suggesting one or the other was the native ISO so I decided to do my own test. I printed an ISO 12233 test chart replica from Cornell University on an A4 piece of paper and set it up inside a lightbox in a room with no direct sunlight. I shot using the 7D with a Canon 70-200mm f/2.8L IS USM II lens at f/2.8 at around 100mm focal length, tripod mounted with a remote cable release, manually focused once at the start of the sequence with the camera set to aperture priority mode. While I’m sure there are plenty of things that could be done better I thought this was typical of the kind of lighting situation where I’d be tempted to knock up the ISO speed. I imported the RAW files in Adobe Lightroom 3 beta 2 and didn’t perform any operations other than changing the colour balance and exposure equally on all shots to lighten them up for printing.
The results? ISO 100 and 160 were very close, but I felt 160 was just a little more defined so I might settle on that for tripod shooting. I don’t have an expert eye but I couldn’t spot any nasty steps in the ISO range that other’s have reported, in my mind it was a pretty even transition with the each ISO speed. ISO 400 remained excellent so I wouldn’t hesitate to go that high for quality shots. What I’d set out to find is if automatic ISO was a useful mode and I feel that it is. From what I gather Canon don’t release specific details on the native ISO of their cameras but the results made me wonder if on a modern DSLR it’s a myth, I’d assume the ISO setting equates to a sampling period for the sensor and can’t think of any reason it couldn’t be infinitely variable over a range. Of course at the lower end of the range there could be an optimum level such as 160 below which the sensor becomes saturated and needs to be attenuated which may explain why ISO 160 appeared marginally better than ISO 100.
I received a prompt response from Jim Rowe at Silicon Chip Magazine on the beam break trigger. Interestlingly the pulse was designed to be short to work with the time delay trigger kit but I didn’t have any success using the two together, maybe some of the components supplied in the kit were out of spec:
Greetings Mr Johnson,
You are correct in that the effective trigger pulse width from the June 2009 Beam Break Trigger will be only a little over 110us, as determined by the 10uF capacitor and the total resistance of 11.1k# in the charging circuit.
The pulse was actually made this short to prevent multiple triggering of the Time Delay Trigger published in the February 2009 issue. However if you want to use the Beam Break Trigger to trigger a camera directly, I imagine that the pulse width will be too narrow – as you have pointed out.
Your remedy of increasing the capacitor value from 10nF to 1uF is fine, but if this does not give sufficient lengthening of the trigger pulse for some cameras, the resistor from the gate of Q2 to ground can also be increased in value from its present value of 10k#. It could be increased to 22k#, 47k# or even 100k# if a much longer pulse is needed.
Thank you for your feedback, and I hope you find the projects useful.
Recently I constructed the beam break trigger and photoflash trigger kits featured in Silicon Chip magazine. Both look good in general however I ran into an issue getting the beam break kit to work with the trigger because of the short duration of the output. I thought I’d share a letter I just wrote to Silicon Chip about the problem in case anyone else runs into the same problem before it’s published in print form:
After constructing the beam break trigger from the June 2009 issue I didn’t have any luck getting it to trigger a camera either directly or via the photoflash trigger kit. Unless I’m missing something doesn’t the 10nF coupling between Q1 and Q2 lead to a time constant of mere microseconds? I didn’t measure the timing before-hand but after doing a rough calculation I placed a 1uF cap over the 10nF which lead to a trigger time of somewhere in the order of 10mS and all was fine with the photoflash trigger kit. I left it at that because it’s my intention to use it with the trigger kit however readers should be aware that some cameras require a longer pulse on their external trigger to fire. For example my Canon EOS 450D seems to require a minimum duration of about 60mS in manual focus mode, presumably if the pulse is shorter than the normal shutter lag time it gets ignored. Other than that they are a pair of excellent projects and looking forward to exploring the possibilities they offer.