Mean Time Between Failure

By Gary Gray

Estimating “mean time between failure” MTBF.

Wikipedia explains it partially as “Mean time between failures (MTBF) describes the expected time between two failures for a repairable system”

Here is the link to the Wikipedia definition.

What does this have to do with photography?

If you own a photography business, you have to make financial decisions on what equipment to buy and you will probably be maintaining some type of records for purpose of helping you make the best possible business decisions.

From a business perspective, it’s really a good idea to estimate your costs before you begin investing in hardware. Not just purchase price, but maintenance, replacement and reoccurring costs need to be calculated if you have any intention of making informed business purchasing decisions.

From a photography perspective, I like to know how durable a camera is before I swipe that credit card.

On the surface, published specs can be misleading when it comes to what you are really paying for. Those same published specs can also be used to make better educated guesses about what one is really paying for.

Take a Digital SLR for example.

I was looking at new cameras the other day and my choice came to either the Nikon D500 or the Nikon D7200.

On paper the Nikon D500 may be the best crop sensor camera on the market. It shoots fast, has a deep buffer, feels really nice and it is marketed as a Pro Level camera for wildlife photographers.

Applying the MTBF analysis to the specifications of the Nikon D500 or any other camera for that mater, I’ve boiled it down to a formula that generates a single number for making my estimates of the camera’s potential reliability.

The math: Divide the published shutter rating by the maximum fps rating of the camera and you’ll have the number of photo seconds your camera is rated for. You won’t find this value in the marketing literature. I’ve never even seen anyone pay attention to this on the internet forums. Divide the quotient by 60 to get minutes, and divide the minutes by 60 to get hours. A published shutter durability can be converted to an estimate of the number of hours the shutter will be operational before it exceeds the manufacturers rating.

Example: A camera has a published frame rate of 5 frames per second and it has a shutter rated for 100,000 actuation’s. The math says that this shutter should last, on average, 20,000 seconds of operational use. That works out to be 333.33 minutes of operation, or 5.5 hours of operation.

You can then compare apples to apples when looking at durability between camera bodies and brands.
What you’ll find is that this formula may expose a weakness in your choice of camera body.

The D500 will take photos at a rate of 10 shots per second. Very fast. The D500 shutter is rated for 200,000 actuation’s, so that gives me slightly over 5.5 hours of total shooting time before the shutter has exceeded its rating.

The D7200 will take photos at a rate of 5-6 shots per second (nothing special). The shutter is rated for 150,000 actuation’s. That’s about 7 hours of total shooting time before the shutter exceeds its rating.

Think about that. The D7200 though not built quite as tough as the D500 (according to specs) will probably last longer before requiring repair or replacement because it isn’t hammering away at the shutter like a bat out of hell. It also makes me suspect that the D500 isn’t as robust as Nikon may like you to believe. Compare it to the pro level D5, 400,000 shutter actuation rating at 12 fps, we have a camera that should give us 9.25 hours of shutter operation before it exceeds the shutter rating.

I have to question rather or not the D500 is really built to pro specifications. If it were, it would have a shutter rating at least 300,000 actuation’s which would put closer to 8.3 hours of average shutter shooting life.

Shutter rating is in essence a manufacturers published MTBF for the shutter in any particular model of camera. Big numbers don’t mean better though. Take the Nikon D500 again. A camera that racks up shots much faster with a slightly stronger shutter. Is it simply just going to fly apart quicker? MTBF makes me think it’s more likely to fail sooner than lower rated shutters on other cameras. It’s going to be hard to let off that shutter button at 10 fps and with a buffer that never fills. Are you really just paying for the joy of hearing the machine working full-tilt-boogie until it flies apart sooner?

You can determine the numbers of shutter actuation’s on Nikons as that info is embedded into the image file.

Shoot a small jpg file and upload it here, it will tell you what your shutter count is.

Canon users, there is software available that will read your camera but there is no info in the images to tell you.

One can argue this or that, but this is a simple math formula that is TRUE. Since I studied statistical process control in college and grew up in a manufacturing world that required a mathematical look at everything, I came up with this simple test. It makes my brain feel happy to know this.

I’d also bet money that some Nikon executive, “Director of Shutter Manufacturing and Repair” has a spreadsheet and could tell you the exact numbers with one phone call, if he had too.

Your mileage may vary.

December 27th, 2016