BetterPhoto Q&A
Category: New Answers

Shutter Speed for Telephoto

Nathan

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July 12, 2004

I have the Rebel 2000 which is similar, & when you shoot automatic with the flash, the shutter speed is auto set to 1/90. Were you using flash? Also, if you were shooting totally automatic (the little green square on the dial,) & if it was low lighting, the flash will pop up automatically so that may have been your case.

90 is, indeed, slow for your shutter and if your shooting anything like moving cars or something, it wouldn't be acceptible, especially telephoto. However, I am assuming the camera chose 1/90th because that is the flash sync speed. Again, I'm assuming you were using flash. If you were to use the flash and then shoot at 1/300th like that rule you mentioned says, the shutter wouldn't make it up in time for the flash to fire, so only the part of the film would be exposed.

Hope that helps...
-Steven
-http://www.vinrock.i8.com/photos/

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July 12, 2004

Welcome to "program mode" auto-exposure systems. Likely the camera doesn't know what focal length you're using when selecting the aperture/shutter combination . . . or it could be that the lens was opened up as far as possible and the only thing left to get enough light was a slower shutter speed.

In these situations when keeping a shutter speed relatively high is important, switch it to aperture priority auto exposure and move the lens aperture manually to move shutter speed around to where you want it.

-- John Lind

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July 12, 2004

When you're using flash, the shutter needs to be fully open across the entire film plane when it fires and the fastest shutter speed at which it's fully open is the X-sync speed. In addition (with flash) your exposure is controlled by much more by lens aperture than by shutter speed . . . the "effective" shutter speed being pretty much the flash duration which (with electronic flashes) is quite short . . . shorter than 1/300th.

My reference to 1/30th with an 85mm was without flash using only available light in the club.

-- John Lind

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July 12, 2004

Say your in a pitch black room...I'm talkin 0 light at all. Now say people are movin around everywhere in there...you can't see them though but you want to get a picture.

Say you set your shutter speed to 3 seconds and DON'T use a flash. Now your picture will be black, obviously (no light.)

Now say you set your shutter speed to 3 seconds and DO you flash, & also say your flash illuminates the scene for oh...1/1,000th of a second. Now you picture will be nice and brightly lit and crispy and sharp, even though your shutter speed was very very long.

This is because, really you have taken a 1/1000th exposure; the other 2 and 999/1000ths of a second didn't register at all because there was no other light when the flash was off.

So in this totally exaggerated situation, ONLY the APERTURE controls light intake. That's why you can work in slower speeds. Now, obviously you aren't always using a flash in the pitch dark. There will be ambient light to expose the scene (& register motion) during the time the shutter is open, which is why you need a relatively fast shutter.

But really, the bulk of the light from the exposure comes from that one instant the flash fires, so the shutter speed is unimportant (as long as it's slow enough to incorporate the flash [X-sync].) That's what John is saying when he mentioned "your exposure is controlled by much more by lens aperture than by shutter speed."

That's all...
-Steven

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July 12, 2004

Nathan.

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July 13, 2004