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Old Feb 27, 2010, 8:24 AM   #1
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Default Rule of reciprocity and crop factor

How does the rule of reciprocity work with the crop factor adjustments to the zoom on a lens?

In case I'm not using the correct terms, let me illustrate:

With a 200mm lens my shutter speed needs to be at least 1/200 sec. But what if the crop factor indicates my lens is actually a 300mm equivalent?

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Old Feb 27, 2010, 10:46 AM   #2
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What's important, when trying to deal with motion blur due to camera shake, is not focal length, but angle of view. An APS-C dSLR has a narrower angle of view than a 'Full Frame' dSLR, so motion blur due to camera shake will be more pronounced in a photo from an APS-C dSLR than one from a 'Full Frame' dSLR, if both use a lens of the same focal length.

That rule (Shutter Speed = 1 / Focal Length) just happens to work out well for 35mm film and 'Full Frame' dSLRs, but because of their narrower angle of view, that rule needs to include the 'Crop Factor' (as in, Shutter Speed = 1 / Focal Length * Crop Factor).
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Old Feb 27, 2010, 11:23 AM   #3
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BTW, that rule also doesn't work for larger image sensors. Cameras with 6x4.5cm image sensors have a 'Crop Factor' of 0.6, so while a FF dSLR with a 200mm lens should use a shutter speed of 1/200 second, a Medium Format camera can get away with 1/120 second.
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Old Feb 27, 2010, 11:58 AM   #4
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I always look on the 1/focal-length shutter speed to be a guideline rather than a rule - some people are steadier than others. And it also depends on your strength - I find I sometimes struggle to keep a really heavy camera/lens combination steady at at any shutter speed, but can get away with significantly slower shutter speeds with lighter lenses. Others who are bigger and stronger than I am could probably manage better.

The lovely thing about digital is that it costs nothing to "push the envelope" and experimenting with slower shutter speeds, there's always the delete key. After a while you'll get a good idea of just how slow you can manage with a particular lens/camera.
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Old Feb 27, 2010, 2:29 PM   #5
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Quote:
Originally Posted by TCav View Post
BTW, that rule also doesn't work for larger image sensors. Cameras with 6x4.5cm image sensors have a 'Crop Factor' of 0.6, so while a FF dSLR with a 200mm lens should use a shutter speed of 1/200 second, a Medium Format camera can get away with 1/120 second.
There is no rule. It's a guide. Rules are useful, although of course they can be broken. A "guide" on the other hand is just meant to let people know what to expect.

I regulartly shoot handheld with an 800mm lens at speeds under 1/200.

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Old Feb 27, 2010, 4:21 PM   #6
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It is called a 'rule of thumb' - something easily remembered and quick to apply. Photography is full of them. Most were developed to help get decent shots before there was auto-metering, auto-focus, and auto-everything else built into cameras.
Some cameras have a shake warning based on this rule built in and will show a hand waving (usually red) when focal length (35mm eq.) is longer than the reciprocal of shutter speed.

Hand holding 800mm lens at most any shutter speed is an accomplishment. I don't think I would care to arm-wrestle with you.

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Old Feb 27, 2010, 5:08 PM   #7
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Hand holding 800mm lens at most any shutter speed is an accomplishment. I don't think I would care to arm-wrestle with you.

brian
It's not as impressive as it sounds. Took six months of practice before I could do it, and I will Never be able to handhold the Bigma...

I've met people who can handhold the Bigma, and they could bench press me and my 120 pound dog...

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Old Mar 1, 2010, 3:20 AM   #8
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Basically - use the 35mm equivalent.

Image Stabilization helps a lot.

Some people can hold steadier than the rule of thumb. Some people less than.

Some people are finding that with the new very high resolution sensors the rule of thumb is no longer giving the results they need.

I find for example that with my 22Mp 5DMkII that I have had to adjust to 1/(2*focal length) to avoid the worst effects of camera shake. Others are reporting similar things with 65Mp digital medium format, and more recently on this forum with the 18Mp Canon 7D.

The point really is to start with the rule of thumb and adjust to your own specific circumstances.
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Old Mar 1, 2010, 4:47 AM   #9
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There is no rule. It's a guide.
I agree. It's not a 'Rule' so much as a 'Rule of Thumb'. I was just continuing to use the terminology that iowa_jim started for the sake of clarity. And this is not the first time someone here referred to the 'Rule of Reciprocity', which I think is an odd term all by itself. 'Rule of Reciprocity' sounds like a modern, civilized way of expressing the Old Testament's "... an eye for an eye and a tooth for a tooth ..." as opposed to the New Testament's "... turn the other cheek ...".
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Old Mar 1, 2010, 7:43 AM   #10
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It has sounded odd to me as well, but I didn't know how to refer to it otherwise. What would be a good way to refer to the method for estimating the minimum shutter speed based on the focal length of the lens to achieve a sharp focus image?
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