(X100) What is the maximum shutter speed at F5.6 F4 F2.8 F2?

Started Feb 20, 2011 | Discussions thread
plevyadophy Veteran Member • Posts: 4,258
Re: (X100) What is the maximum shutter speed at F5.6 F4 F2.8 F2?

prime wrote:

plevyadophy wrote:

Well, I don't think the fact that it has an in-lens shuther has much to do with it, other than the fact that Fuji may have opted for an implementation that is rather limited.

The fact that it has an in-lens shutter has everything to do with it. You need to think carefully about optics and how focal plane shutters differ from in-line (whether in lens or behind the lens) leaf shutters. Follow the example below.

Assume that you are shooting a scene at f/2 and 1/1000 second.

If you have a leaf shutter, immediately after you press the shutter button, the lens is open at f/2 , but then, say. 1/4000 second later, the lens -- through the leaf shutter mechanism which also os the aperture iris -- will have stopped down to f/2.8 ; 1/2000 second later, to f/4 ; 3/4000 second later, to f/5.6 ; and, by the end of the 1/1000 second, when the electronics turn off the photosites in the sensor, it will have stopped down to f/8 . (These are approximations, for the purposes of an example.)

The reason why an aperture ring -- any aperture ring -- works in any camera is that if you limit the light passing through the lens to the light in the middle of the lens, then, rather than getting a smaller image circle -- exposing only the middle part of the frame -- you get an image across the whole frame, but dimmer. However, as the aperture gets smaller and smaller, there is increasing likelihood of diffraction effects in the exposed image.

If, on the other hand, you have a focal plane shutter, there are two curtains in front of the sensor, one deployed across the sensor, the other rolled up. When you press the shutter button, the one that has been deployed starts to roll up, allowing a portion of the sensor to be exposed to the light from the lens. After 1/4000 second, the first curtain will have exposed, say, about 1/4 of the sensor.

Sometime close to that time, the second shutter curtain will start to unroll, following the trailing edge of the first curtain across the focal plane with its leading edge. After about 1/2000 second, the trailing edge of the first shutter curtain will have traveled halfway across the focal plane, and the leading edge of the second curtain will have followed it to about 1/4 of the way across the focal plane. You see what is happening here: the exposure is a moving slit that travels across the focal plane. The first quarter of the exposure is exposed, on average, at one edge of the image; the second quarter is exposed, on average, at one of the middle two quarters of the image; the third quarter of the exposure is exposed, on average, on the other middle quarter of the image; and the last quarter of the exposure is exposed at the "far" edge of the image. Some creative photographers have taken advantage of this effect, which makes fast-moving subjects in the image frame look as if they have been stretched like taffy. You cannot get those effects with an in-line leaf shutter. But -- the other sign of the coin -- you will never get those same distortions at all with a leaf shutter.

Now, consider that the two curtains of the focal plane shutter each travel only one direction only, until they run into a bumper at the end of their travel. In contrast, the leaves of a leaf shutter must reverse direction at the end of their travel. Each leaf must travel very fast in one direction to stop down, then slow to a stop, then reverse direction to go back to the set aperture.

Fujifilm has (apparently) determined that, rather than increase the speed of the iris leaves, need to put on bigger brakes, and need to increase the power to reverse direction, instead, it will allow the leaves of the shutter leaves to travel more slowly, and allow the proper exposure to be achieved with a neutral density filter inserted in the path of the light. And, by not stopping down from a wide f/2 all of the way to " f/infinity ," the choice that Fujifilm has made avoids diffraction effects of very small apertures at the stopped-down phase of the exposure. Is that a "limited implementation" or just plain smart engineering?

I have noted your interesting comments, in addition please note my comments below at : http://forums.dpreview.com/forums/read.asp?forum=1020&message=37809877

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