Changing aperature and shutter speed

[Chad Edgar]

I think we covered this once before, but I am just lost.

We came across a really nice scene at work the other day.  This is the dummy setting on the Lumix FZ5, f/4 for 1/200th of a sec
 


I wanted to set-up a scene with a slightly longer exposure time to catch the movement of the water.

For every stop up in speed I went one stop down in aperature and no matter how I tried it I overexposed.  I think I have to re-read Peterson.

[luv2bowhunt]

This is going to be a difficult scene to get a great exposure on at that time of day. With the sun so high and bright you are not going to be able to capture all of the details. You are either going to blow out some of the highlights or lose alot of shadow detail. Film and digital cameras can only record so much data (I believe that it is about 3 stops of light on most film and about 5 stops of light on digital sensors.)

There are a couple of things that you could do here though, one would be to put a Neutral Density Filter on the lens. These reduce the amount of light reaching the film and allow you to use a longer film speed. I also see that you shot this at f/4, you can stop that down quite a bit and get a longer shutter speed as well and it will give you a great Depth of Field.

As for overexposing, if your camera has Exposure Compensation you can try setting that to underexpose by a bit. When the camera sees this scene it will give this exposure because that is what it thinks is right. The camera is trying to keep as much detail as it can, from the highlights to the shadows.

My best suggestion though would be to return to this spot much later in the day when the sun has started to set. Then the lighting would be much more even and with less light you can use that longer shutter speed to get that silky smooth looking water    

Hope this helps.

Kevin.

[jcsnapshot]

f11 at 1/60 of a second should have given you pretty close to the same average exposure while showing movement in the water.
  You want to slow the shutter, and make the aperature smaller. Remember the f stop is a fraction, not a whole number, the bigger the f# the smaller the aperature.

Hope that helps????????????
Jim

[Fritz Brown]

Chad,

The reciprocity law says that for every stop you close down, the shutter speed should be twice as long.  In your case,  your 200 @ f/4 is equal to 100 @ f/5.6, 60 @ f/8, 30 @ f/11, 15 @ f/16, 8 @ f/22, and 4 @ f/32.  These relationships hold for normal exposure times.  As exposures get very long or very short you encounter a phenominon called reciprocity failure and you have to compensate for that in your exposure.

That said,  to get the water to blur you need your exposure time to be on the order of half a second or longer.  The longer the exposure the more blurred the water.  Your problem with this image, however, is more a problem of contrast.  As luv2bowhunt says,  film has a limited ability to register contrast.  Your "average" film will register about 5 stops between white and black.  What you see with your eyes, though is usually about 7 stops between white and black.  Your average scene has about 7 to 8 stops of light difference between fully illuminated and fully in shadow.  So you can see the problem:  film cannot replicate what you see or what is potentially in the scene unless you start messing with development times etc. (ie. the Zone System).

So the solution to your problem in this image is two fold:  decrease the contrast of the scene to the point that your film can record it, and lengthen your exposure time to blur the water.  

As was stated above, coming back in the early morning or late evening so the light is not as directly illuminating the scene, or shooting it on a cloudy day when the light is flatter will solve the contrast problems.  Coincidentally, there will also be less light on the scene so you can get longer exposures with the aperture choices you have on your lens.  If the light is still too much to get a longer exposure, throw a polarizer or a neutral density filter on.

One aditional problem with that image that hasn't been raised is that the people are back lit and thus have underexposed faces.  You need to get some light on their faces either with a flash with a fresnel on it, with a reflector bouncng sunlight on it, or by positioning them so their faces are into the light.  Otherwise they just dissappear in the image.

Hope this helped.
-Fritz

[Fritz Brown]

One more thing:  use a tripod.  You have no chance of hand holding a camera for half a second, no matter how short the lens.

-Fritz

[Chad Edgar]

Fritz, you just killed all incoming data from my hair covered computer for the rest of the weekend!    


Thanks for all the input folks, I think I need to do some studying on the reciprocity law.  I thought for every stop up or down you went the other way in equal amounts in ap speed.  Wether you were looking for long exposure times or very narrow depth of field???  Sounds like I have to go two, no!

[Fritz Brown]

Chad,  Nope, you had it right to begin with.  One stop longer exposure time with one stop smaller aperature gives the same exposure.  Keep in mind what we mean by the concept of a stop.  A one stop change is equal to a factor of two in light.  For instance if you close a lens down by one stop, you are cutting the light transmission by half.  A one stop difference in shutter speed is again a factor of two.  Going from a shutter speed of 1/125 to 1/60 is approximately twice the time during which the film is being exposed to light.  So if you stop the aperature down one stop and go one stop longer for the shutter,  you are exposing the film to half the light for twice as long resulting in the same amount of light hitting the film in the end resulting in the same exposure.

-Fritz