TL,DR; You can find the chart of all tested cameras here: List of camera shutter speed deviations
Cameras are highly complex machines, with lots of tiny parts that we expect to move as precise now as when they were new – often after tens of thousands usage cycles. No other part in the camera has such a high workload under such tight tolerances than the shutter. But like any mechanical parts wear and tear will change tolerances – grease hardens, springs soften and running surface wear down.
A CLA can get your shutter back on track is it just sat on a shelf for years, but sometimes, certain shutter speeds are just „wrong“. However, when you know they are wrong and they are consistently wrong, you can adjust yourself. Like on my speed graphic, while it does offer 24 different shutter speeds, 3/4 of them are not even close to what they say. I checked the shutter, made my own chart, and those numbers now are correct.
What I use
You can check your shutter with various methods. I did try sound-based measuring systems, but while it is an easy and cheap way (there are even phone apps), the results were not satisfying. Instead, I went for the optical way with light sensors. My first tests where with a digital storage oscilloscope and while that worked great. it was a bit cumbersome.
Later, I upgraded to this magnificent little device – the Phochron XA. I got mine thru kickstart and might add a review of it later. It does the same thing as the DSO, just in a much more manageable package.
Usually, I take several readings for each shutter speed – to make sure there are no flukes – and then average the number. Five readings are usually good, and often the readings are very close together. If they are all over the place even on the same shutter speed, there is no point on checking the actual shutter speed – it won’t stay the same anyway.
Accuracy vs. Precision
This brings us to this important topic – my shutter can be accurate, but not precise, and also can it be precise, but not accurate. But what is the difference?
|Low Accuracy||High Accuracy|
|Low Precision||Shutter speed is all over the place||On average the shutter speed is what it should be, but lots of variation between different exposures|
|High Precision||While the shutter speed is not what it should be, all exposures are very close to each other||The shutter speed for all shots are as they should be|
What we want most is of course high precision and high accuracy. The second best thing is high precision with low accuracy – the shutter speed is not right, but you can compensate for it, and get reliable results.
With low precision on the other hand, it’s a gamble – you never know before the shot if it will be close to the right shutter speed, or completely wrong. Those cameras will keep annoying you, because just when you depend on them, the shutter will act up and ruin your shot. While using slide film in a low accuracy, high precision camera is no issue (if you know how much you have to compensate for the shutter), on a low precision camera you can pretty much forget it.
How much is to much?
You checked your camera, but what do you do with the numbers? First relex. Every camera shutter will have some variations, but often you can ignore it. Errors under 1/3 of a stop will be mostly invisible, and when you shoot negative film, you can pretty much ignore half a stop. On older cameras, the shortest times can be pretty far off – on some cameras almost up to a full stop – here you should be careful and maybe mark your camera that the 1/1000 is just 1/600 in reality.
If you want to know how my cameras perform, I made a big list of all tested cameras. You can find it here: List of camera shutter speed deviations