Comment by haikuginger | Hacker News Reader

Comment by haikuginger | Hacker News Reader

Parent Full thread

Quick explanation of the groundwork for this piece:

Typical DSLR shutters are focal-plane shutters, which mean they open and close directly above the image sensor. They are mechanically primed, and when triggered, the "first curtain" opens across the sensor, followed by the second curtain after a brief delay corresponding to the relevant exposure time.

In the case of very fast shutter speeds (depending on the camera, typically 1/250s or faster), the second curtain begins its traversal of the sensor before the first curtain is fully open. This means that every portion of the sensor is exposed for the appropriate amount of time, but that not every portion of the sensor is exposed at the exact same time. This is due to mechanical limitations, and is hard to get around.

In general, this isn't an issue. However, flashes and studio strobes are of a very short duration - often 1/10000th of a second or less. In the case of a focal plane shutter, this can mean that not the entire sensor will be exposed to the flash's light, causing a stripe in the final image.

There have been various ways to compensate for this - for example, high-speed flash synchronization that fires the flash at lower power levels several times during the exposure such that each portion of the image sensor is properly exposed.

In earlier DSLRs (Nikon D50 and D70 models with the 6MP Sony CCD), a "digital shutter" was used, which allowed for native flash syncing at high speeds - the mechanical shutter only operated up to its "sync limit" of 1/250, and the digital shutter took over at speeds higher than that.

In comparison, leaf shutters always open fully, which means that there's a period where, if a flash is fired, the entirety of the sensor will be exposed evenly to that flash. Because of the very short duration of the flash, shutter speed does not affect how much of the flash's light falls on the sensor, and can be used to control for background light instead, as compared to aperture and ISO controls that would affect both background light and the impact of the flash's light in the overall image.

To get a sense of how a regular camera's focal-plane shutter moves, here's a great video from Slow Mo Guys:

https://www.youtube.com/watch?v=CmjeCchGRQo&feature=youtu.be...

This is one of the coolest youtube videos I have ever seen.

I could never figure out why we generally focal plane shutters now. I suppose there must be some advantage to them.

Edit: Thanks a lot for the explanations

It boils down to higher effective shutter speeds. Barring a global electronic shutter (such as was in the Nikon D100 and D70 CCD cameras), you can get much faster shutters (or, really, lower exposure times at the sensor) with a focal plane shutter than you can with a leaf shutter. The limiting factor is not how much stuff you can move how far in how little time, but how narrow you can make the moving slit. Not only does a leaf shutter need to fully open and fully close to create the exposure, it needs to do it quickly enough to have a negligible effect on the len's aperture (both for exposure and for depth of field control reasons).

That speed does come at a cost, though, since there is a point at which you cannot expose the entire sensor at once. (That's also true of rolling electronic shutters, by the way.) Flash needs to be worked around in various ways, and anything moving in the frame will suffer distortion.

I still have good reasons to use an old Nikon in some circumstances (lower resolution requirements, high ambient light levels and flash). While they do have focal plane shutters, they only go up to 1/250 of a second. Faster than that and you're using a global electronic shutter, which doesn't reduce effective aperture when the lens is wide-open or nearly so, but still expose the entire sensor in one go. That means I can use a higher shutter speed to control the ambient exposure while using very low flash power. That means no waiting for recycling or carrying around large amounts of lighting equipment.

Aren't there modern sensors that have electronic shutters? Why isn't this capability found on newer cameras?

It's usually called a global shutter, and it adds complexity to the sensor design, because each pixel has a tiny amount of "memory" with a gate and capacitor to store charge into. There is a signal bus that simultaneously moves all the charges from the constantly exposing pixels into the storage areas, where the charge is no longer being increased by exposure, and can be read out asynchronously using a switching matrix and a bank of ADCs. This avoids the problem where the charges of the pixels you haven't read are still increasing through light exposure.

Electronic shutters don't work well with fast moving objects. You can get distortion. I think they don't read out the whole sensor at once.

The global shutter on the Nikon 6MP CCD sensor was really global, not like the rolling electronic shutter on a CMOS sensor. You got an all-at-once still, the electronic equivalent of a leaf shutter; the current scanning versions are basically a "no moving parts" version of a traditional focal plane shutter.

* Price: one shutter for many lenses versus one shutter in each lens. * Range of shutter speeds: leaf shutters on large lenses struggle to get to 1/1000 of a second. They can do 1/2000 on digital sensors by electronically beginning the exposure and mechanically ending it. The fastest focal plane shutters went to 1/12000 of a second (but most today are either 1/4000 or 1/8000).

I'm almost certain it's because of SLRs and through the lens viewing. Leaf-shutter SLRs were more expensive to make and were more complicated. To get an idea how, read about halfway down the page here: https://simonhawketts.co.uk/2016/11/05/kodak-retina-reflex-i...

Basically, focal plane shutter didn't get in the way of the viewer's light path since they were behind the mirror. They were cheaper to make, more reliable, and operated more consistently at the rated speeds. They were almost certainly more durable. Tungsten focal plane shutters can be rated for a 100,000 activations.

They're cheaper. You need one focal plane shutter in your camera, vs a leaf shutter in each lens you own. The only place where leaf shutters dominate these days are in large format cameras where focal plane shutters would be much more expensive and unwieldy.

So many answers here already - but simply:

1. Very fast natural light shutter speeds (i.e. beyond 1/8000th second), whereas most leaf shutters might hit 1/500th or greater. He talks about some very expensive lenses that can go to 1/1600th, but this is fairly extreme.

The downside is a slower (1/200 - 1/250th) flash sync speed.

Rear curtain sync can also be useful for some shots for instance if you want your flash exposure to be "on top" of your ambient light exposure.

A Fujifilm X100F can make use of it's supplementary electronic shutter (in place of its leaf shutter) to simulate rear curtain sync.

I don't know if it's the only reason, but historically focal plane shutters could be significantly faster than leaf shutters.

I figured this out while reading the article (since I know how conventional curtain shutters work and why they have need 1/60th second to use a flash), but was puzzled that the article used so many words and didn't bother explaining the main concept.

This was a really awesome and helpful explanation! Thank you very much! I read the other article and was left thinking "Yeah but what's wrong with the other style?" which I might've just missed but this explanation was great!

Reply on news.ycombinator.com