Amateur Photographer Magazine

Quote:

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If for example you are taking a hand held shot in low light with your ISO set to 100 and your aperture set to f4 you may need a shutter speed of 1/6 second to achieve a good exposure and this may be too slow to obtain a blur free image. Keeping the ISO set to 100 and increasing the aperture to f1.4 might give you an acceptable exposure at 1/13 second. If you increase the ISO to 400 you may find that you'll get a good exposure at f4 with a shutter speed of 1/6 and at f1.4 and 1/50.

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Eh?

ISO 100, 1/6 sec, f/4 = ISO 100, 1/50 sec, f/1.4 = ISO 400, 1/25 sec, f/4 = ISO 400, 1/200 sec, f/1.4

ISO X shutter speed in sec / f ratio squared = constant depending on light level

in this case
100 x 1/6 / 4x4 = 1
100 x 1/50 / 1.4x1.4 = 1
400 x 1/25 / 4x4 = 1
400 x 1/200 / 1.4x1.4 = 1

But for the image posted the correct exposure was very probably approx. 1/250 sec at f/11 at ISO 200 by the "sunny 16 rule".

Who needs a meter? Well, I do - but my "guess" is almost always within half a stop in nice easy situations like this! And it's useful to be able to estimate, as a gross discrepancy with the meter reading shows up anything going wrong with the meter.

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