The Online Photographer

Ahh, don't mind me......but this does seem to take the whole pixel peeping, technophile thing to an extreme.

Bron

OK, I guess I'm just stupid, but I never realize that DSLRs even had shutters. I always assumed that the sensor was activated only for the needed duration (1/125 of a second, or whatever). What am I missing? It seems like a computer could activate a sensor with greater accuracy than a complicated mechanical device could expose it to the light. Not that I actually know anything about electronics or anything. But, wait, why do digicams have to add sound effects to let you know that they've taken a picture? Isn't that because there's no shutter? I always thought the sound my D200 makes was just the mirror. So, here's my dumb question: Why do these cameras have shutters, and do digicams have shutters that just doesn't make any sound?

In response to David Rodrigues

I don't think this is entirely true. I remember being in a studio with a (I think) Nikon D1. I was using the studio strobes.

I was very surprised to discover the *actual* flash sync speed was 1/4000, and not 1/500 as advertised.

Of course there is no way that it could possibly open the shutter all the way, trigger the strobe and close it again in 1/4000 second. Therefore, at speeds above the *shutters* flash sync speed it must have used the cmos ccd to shorten the exposure time.

I've never really gotten to the bottom of it, but I think that some types of ccd can act as an "electronic" shutter.

Alex

Mike,

Curtain shutters, such as in SLRs, reach their fast shutter speeds by sliding both curtains across the film plane at the same time. There is then no moment that the entire frame is exposed, just a thin line that slides across it. The shorter the shutter time, the thinner that line.

This is why SLRs have a flash sync speed, usually 1/125 or /250 or so: That is the shortest exposure time where it will still expose the entire frame.

You can try this by setting the shutter speed to something shorter than the sync time and using a flash. You'll only get part of the frame exposed properly. (careful though, modern SLR flash system often have something called "high-speed sync", where the flash will actually flash multiple times in short succession. Turn that off, or use a flash that can't do that)

The studio strobes actually have a fairly long duration, compared to the electronic flashes you normally find. This became obvious as the image got darker as the shutter time got shorter.

Mike - I'll try to clarify what Alex is talking about :

The max sync speed for your camera is the fastest shutter speed for which the entire sensor is exposed at once. At speeds faster than this, the second shutter starts to drop before the first shutter has fully opened. So at any given time only part of the sensor is exposed.

If you trigger a flash at one of these higher speeds, you will only get a line across the image where the sensor was exposed, since flash durations are so short.

High speed sync allows you to use a faster shutter speed by pulsing the flash over a longer duration, so that all parts of the sensor "see" the flash.

It's a bit counter-intuitive that you need a longer flash duration to use a higher shutter speed, but it's true.

You know, I figured you must know that Mike. (Yet I still typed it out...sorry!) I wonder if Alex's original point was that he thought the CCD in the D1 did something special, when in reality the studio strobes probably just had a longer duration than his shutter speed?

But you mentioned studio strobes having a much shorter duration compared to regular strobes, which is what confused me about what you were getting at...

Flaneur has the real info, and explained it better than I could.

I just observed this in the studio once and spent an hour or so trying to decide if I was going mad before the model got bored and I continued shooting :)

Shortly after that I switched to leaf shuttered cameras, and they sync at any speed...

That fast shutter sync is the best least-known feature of the Nikon D40 and D50, IMHO. Too bad it comes with a price. These cameras show a curious artifact when shooting into the sun. Whenever I'd include the sun in a wide-angle outdoors shot, the photo had what appeared to be a jet contrail crossing the sun, though I'd seen no airplane there. Wait, was this evidence of the mythical "chemtrails" that cross our skies, marking a secret governmental science experiment intended at controlling the climate, or our minds?

No, it wasn't. After consulting various Nikon experts on the boards, I learned it was the mark of sunlight "bleeding" laterally along pixel rows during the long, 1/90 sec time the shutter was open. It happens every time, go see for yourself, if you have one.

Look at that thing shake. That is why mirror lock up is a nice feature to have.