How A Typical DSLR Shutter Works

How A Typical DSLR Shutter Works

I find cameras are often a lot like cars to people using them: they know when it works, they know what it does and can use it. But they are often clueless when it comes to the mechanics of HOW it works. To some of you, this may be a boring topic and I suggest you instead check out some great travel perspectives from Pam Mandel at Nerds Eye View.

First, a diagram with a bunny:

This diagram shows where the shutter sits in your DLSR. It is behind the mirror (and the light meter on some camera) and typically hidden from view. Let me show you what it looks like with the mirror out of the way.

That’s my thumb in the corner, in case you are wondering. What you are looking at is the first curtain of the shutter mechanism. In the picture you can see the individual leafs of the curtain. Your camera may have more or less than this Canon 7D, which has four and is what I diagram below. No matter the number, the mechanics are the same.

When activated by pressing the shutter release on your camera (or remote) the aperture on your lens is stopped down to the appropriate setting, the mirror flips up and out of the way and then the shutter magic happens. The diagram below shows a longer exposure in order to exaggerate the movements.

Looking at this diagram you may wonder why there needs to be two curtains. That is because, as shutter times get faster and faster, the mechanics of a single curtain fail to keep up with both dropping out of the way and springing back up. Also, it would allow a disproportionate amount of light to hit either the top or bottom of the sensor as it traveled. This is why two curtains are employed.

If you set your camera on a shutter time of one second, you can hear all of this going on inside. The initial slap of the mirror moving out of the way and the first curtain click are often very close. Then after the one second you will hear the second curtain activate followed by the mirror dropping and the whole mechanism resetting.

When things get faster, it looks like this:

With faster shutter speeds the speed of the shutters is increased and necessitates a narrow gap between them to help only allow so much light in.

Shutter and camera technology is constantly changing and improving. Some cameras now have a pass-through mirror and no actual shutter. Most P&S cameras don’t use a shutter curtain at all (scanning the pixels the way an old TV used to project the image by scanning). But for now, for those with DLSRs with traditional shutter curtains, this system still holds true.

Lastly, this may also help explain to some of you what is meant by “second shutter sync” when setting your flash. Basically the flash on your camera (or external flash) can set to fire in sync with the first curtain activating or the second curtain. With very fast shutter speeds, this matters little, but as speeds get slower, say 1/20 second, the decision to of when to fire the flash will make an impact on whether a moving object comes before or after the blur caused by movement.

If you want blur behind a moving object, use a second curtain sync (so the flash fires just before the exposure is ended). If you want blur in front of a moving object, set your camera to first curtain sync so the flash fires with the first curtain and then allows the object to continue moving before closing the second shutter, ending the exposure.

I hope this has been informative to those wanting to know a bit more about what goes on inside their camera.

Read more from our Cameras & Equipment category

Peter West Carey leads photo tours and workshops in Nepal, Bhutan, Seattle, Portland, Los Angeles and beyond. He is also the creator of Photography Basics - A 43 Day Adventure & 40 Photography Experiments, web-based tutorials taking curious photographers on a fun ride through the basics of learning photography.

Some Older Comments

  • Marilyn Armstrong August 31, 2011 03:14 am

    Just one extra question: I just bought (but have not yet received) a mirror-less Olympus Pen PL-1. How does that work compared to a DSLR? I think I can guess but I'd sure appreciate a diagram!

  • Marilyn Armstrong August 31, 2011 03:12 am

    Are you a technical writer (yes, I am) too? Because that was a really excellent presentation and I thank you. I did know that, but my granddaughter, who just learning. Clear diagrams help more than anything else because she is much more visual than word focused. She won't probably READ the article... but she will look at and understand the pictures. Different strokes for different folks :-)

  • Nathan August 24, 2011 04:35 am

    @ Arvind:
    Think about your window shade or blinds. Start with them closed: If you raise them and then lower them, certain parts of the window will be open for a longer time period (the bottom of the window will be open much longer than the top). In order to get correct exposure across the entire sensor, each part of the sensor needs to receive light for the same amount of time. At fast shutter times, this means you need two shutters.

  • Pinoy Photoblog August 8, 2011 09:30 pm

    Very well said. Such a great article!

  • Arvind August 1, 2011 11:54 pm

    Thanks Peter. Quite an informative article - and useful of course.

    I have a question, please see if you could spare a minute - I am not quite clear with the explanation given in these two lines, though the second explanation gives a more appropriate feeling :
    "That is because, as shutter times get faster and faster, the mechanics of a single curtain fail to keep up with both dropping out of the way and springing back up."

    - The question itself was as to why there are two of them - single curtain failing to keep up with both? Not really sure about what I've misunderstood from this line.

  • Dr. Bob July 31, 2011 07:17 pm

    @zvi harduf, israel: Letting the shutter move vertically, has the added benefit of a faster flash-sync shutter speed. Electronic flashguns produce the light needed for correct exposure in a very short time. As a consequence, of the sensor is partly covered by a curtain, as is the case at higher shutter speeds, you'll get a shadow of your curtain. The flash sync-speed is where the gap between the curtains is so big, that the entire sensor is exposed. Since the distance to travel for the curtains is shorter moving vertically then horizontally, you'll need a larger gap between the two to get the same exposure. (assuming the curtains will move at the same speed regardless of their orientation).

  • Johnny Suparman July 30, 2011 03:46 pm

    Thanks for posting and sharing this valuable knowledge. I am get more understanding about photoghraphic.
    I just have 1 question, since almost new produced cctv cameras prefer using CCD sensors, but why hi-class digicams preferring to CMOS sensors? which one the best sensor?

  • Chris Bloom July 30, 2011 01:19 am

    Great job explaining this in such an easy to understand matter. I think I'll bookmark this and share whenever I'm trying to explain this to others. Great Job!

  • Zvi Harduf, Israel July 29, 2011 06:24 am

    I think the idea was Leica's, whose device contained 2 curtains moving horizontally in front of the film, at constant speed, thus the width of the interval between the curtains determined the exposure time, Fast horizontally moving objects were shortened or elongated in accord with direction of their movement.
    Zeiss' Contax partly overcame the problem by changing curtains' direction to vertical, as one rather rarely shoots fast objects moving vertically.
    Please correct my data if mistaken

  • suresh July 29, 2011 04:34 am

    Good article! now only I get the idea of Rear curtain sync etc. I appreciate your earlier articles too. thank you.

  • Varun Raj Minocha July 28, 2011 08:08 pm

    Thanks for the explanation Peter.
    Your post made me explore the workings of the shutter further, and I came across this wonderful animation, which might help others. (under the heading Fast Shutter Speeds)

  • Johann July 28, 2011 05:07 pm

    Thanks for the nice article, and thanks for not just plopping down a youtube video, which seems to be the habit of most sites these days.

    Anyway, the reason for this comment is a question: Why does the sensor get dust on it if it is covered by the shutter most of the time?

  • timsdd July 28, 2011 05:55 am

    good stuff! 2nd shutter sync especially, I always get confused!
    now I need to know how to replace mine! canon quality....40D dead shutter @ 11K

  • SMM July 28, 2011 04:35 am

    Thank you for posting this. I also believe we can become better photographers by knowing more about our cameras, but I don't have the attention span to sit down and read my manuals. I love this quick bit of information. Please keep them coming!

  • SJCT July 28, 2011 12:39 am

    Great tutorial. Thanks for this! I've always wondered why a DSLR needed two shutters, this explained it.

  • Fuzzypiggy July 27, 2011 10:27 pm

    Wow! Now that was interesting and condensed into a very easily remembered format. Thank you very much indeed. Something to think about next time you simply click the button and a picture magically appears on the back display!

  • Ajith July 27, 2011 06:21 pm

    Thanks Peter, it is a good article. Explains the actual happenings behind the tricks etc; we play. I am an amateur in photography and would like to know whats happening inside which helps to understand the use of the equipment . Great one woud appreciate further more like these about DSLR's

  • GradyPhilpott July 27, 2011 12:49 pm

    Thanks for a very informative article. It really explains a lot that I found quite mysterious before.

  • Dr. Bob July 27, 2011 04:36 am

    I think it's good to point out that in dSLRs the shutter normally resides in 'step 1'. This is where the shutter is 'cocked' and in dSLRs the recocking happens right after exposure. Unless you're in 'quiet' mode, then the shutter is recocked when you stop pressing the shutter button. In old fashioned SLRs, the shutter is cocked when you wind the film.

  • Fred Lassmann July 27, 2011 02:01 am

    With a background in film and wet darkroom and technical (aviation) I'm convinced that the more that I know about "how it works" the better photographer I can be. Knowing how it works helps me set limits on the mechanical processes involved. Thanks for the article. Fred