For the past few days, I’ve been doing tests of image sharpness with the Sony 70-200 f/4 OSS FE lens mounted on a Sony a7R and a a7II camera. I’ve been using a really sturdy tripod — the RRS TVC-43 — and head — the Arca Swiss C1 Cube — for the testing when I’ve not been handholding.
I wondered what would happen if I used a much lighter weight tripod and head. I dug up a Gitzo 6X carbon fiber travel tripod with a light RRS ball head. Here are the two tripods next to one another:
You can see that the legs of the travel tripod get pretty skinny towards the floor. You can also see that I extended the column nearly all the way. Yeah, I know that’s not a good idea, but I wanted to create a pretty flexible camera mounting device. You’ll note that the RRS monster tripod doesn’t even have a center column; RRS probably figures that, if you want a tripod this big, you’re not going to want to take any chances with a center column detracting from the stiffness that you paid all that money for.
I used my usual protocol:
- RRS L-plate on camera base. This is not the usual way to mount the 70-200, which has its own collar mount, but it produces less image-blur from shutter shock with the a8R than the conventional mounting method using the collar, so I used it for both cameras.
- Landscape orientation.
- Lens zoomed to 200mm.
- No filter.
- The lighting: a single Fotodiox LED-200WA-56 daylight balanced variable-output flood.
- ISO set to 1000, f-stop set to f/8.
- Focusing using single shot autofocus. The focus point is a Siemens star on the target.
- Drive set to single
- EFCS on, in the case of the a7II
- Manual exposure mode.
- Self-timer set to 2 seconds
- OSS set to off. On the a7II, this turns off IBIS.
- Lamp to full, shutter to 1/1000 second, make 16 exposures with new focusing for each, turn the light down a stop, turn the shutter speed down a stop, make 16 exposures… until you get to 1/60 of a second.
- OSS to on. On the a7II, this turns on IBIS.
- Repeat exposure sequence.
- Develop in Lightroom 5.7.1 with standard settings.
- Crop, export as TIFFs, analyze for horizontal edge MTF50 in Imatest.
- Export the results to Excel, crunch the stats, and graph.
Here’s what happens with the a7R on the heavy tripod:
The bold lines are the average (aka mean, aka mu) values. The lighter lines are the average value plus and minus the standard deviations (aka sigma). If the statistics are Gaussian, about two thirds of the expected results will lie between the two pair of lighter lines. Orange is OSS on, and blue is OSS off.
And here are the results on the light one:
You can see that the camera’s vibration affects the lighter tripod much more than the heavier one. Now let’s zoom in a bit:
You can see that leaving the OSS on on the heavy tripod doesn’t make much difference, but it produces materially worse results with the light tripod than leaving it off.
Here’s what the a7II — with EFCS on — looks like on the heavy tripod:
On the light tripod:
The light tripod is a little worse with OSS on at moderate shutter speeds, but the difference is not dramatic at all. At 1/60, OSS on is actually a tad better.
Our test situation was a bit special. It was indoors. There was no wind. The footing was vinyl tile over concrete. Therefore, almost all the camera motion can be expected to be the result of the camera’s internal motions. Under those circumstances, a camera with large and violent internal motions like the a7R needs a much heavier tripod than one with not much internal motions that affect the exposure, like an a7II with EFCS enabled.
I probably could have guessed that, couldn’t I? Still, it’s nice to have it quantified.