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Old 01-17-2005, 09:16 PM
Ray_Smith Ray_Smith is offline
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Shutter speed versus focal length

The old rule of thumb for avoiding camera shake is to use a shutter speed of 1/focal-length of the lens in use. That works on full-frame sensors but I was wondering what happens when you have a 1.3 or 1.6 sensor. For example, should I use 1/160 second when using a 100mm lens on a 20D or 1/100?

Thanks.

Ray




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Old 01-17-2005, 09:29 PM
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DonLashier DonLashier is offline
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Re: Shutter speed versus focal length

Assuming you're going to enlarge to the same size, yes - effective (35mm equivalent) focal length is the relevant factor. But in any case it's only a rule of thumb so I would pay more attention to actual results based on your own holding abilities.

- DL

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Old 01-18-2005, 05:48 PM
Ray_Smith Ray_Smith is offline
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Re: Shutter speed versus focal length

[ QUOTE ]
Assuming you're going to enlarge to the same size, yes - effective (35mm equivalent) focal length is the relevant factor. But in any case it's only a rule of thumb so I would pay more attention to actual results based on your own holding abilities.

- DL

[/ QUOTE ]

I understand your response to mean that I should use the equivalent focal-length with any sensor that is not full sized, e.g., 1/160 on a 20D using a 100mm physical focal length lens in order to minimize camera shake effects on the image recorded on the sensor. Yes?

Thanks,

Ray

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Old 01-18-2005, 06:01 PM
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DonLashier DonLashier is offline
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Re: Shutter speed versus focal length

> to mean that I should use the equivalent focal-length with any sensor that is not full sized

Ray, yes, but again bear in mind that actual "handholdability" can vary widely depending on conditions and operator. Any time I'm down in the range (or below) of the "rule" I usually shoot two or three shots as one will usually be significantly better than the others even at the same shutter speed.

- DL

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Old 01-18-2005, 08:27 PM
Ray_Smith Ray_Smith is offline
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Re: Shutter speed versus focal length

Thanks Don. I'll utilize your technique.

Ray

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Old 01-18-2005, 10:52 PM
DarinButler DarinButler is offline
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Re: Shutter speed versus focal length

I agree with Don's answer. I recently saw a posting on another forum in which this question was being passionately debated without a resolution. To me it's easiest to think of it as follows.

Let's assume we have two cameras (a 1Ds and a 20D) each shooting the same scene with a 100mm lens. The 1Ds has a 24x36 mm sensor, and the 20D has a 15x22.5 mm sensor. Let's further assume (using round numbers) that the shutter speed in both cases is 1/100. Finally, assume that while the shutter is open, the photographer's hands twitch to the extent that the camera moves horizontally by an amount we'll call x.

It's easy to see that this movement x has a larger relative effect on the smaller sensor than the bigger one, because x/22.5 is bigger than x/36. In other words, while the shutter is open the field of view of the smaller sensor changes more (in percentage terms) than it does for the bigger sensor. This, of course, means you get a more pronounced blur and loss of sharpness.

What if we set the shutter for the 20D at 1/160? Assuming that the "twitch" moves the camera at the same speed (x per 1/100 second) as in the first example, it will now move a shorter distance due to the faster shutter speed - instead of moving x, it will move x/1.6. Thus, its relative movement now becomes x/(1.6)(22.5), which is the same as x/36 as with the 1Ds. This is why it is appropriate to use the "effective" focal length in the shutter speed rule of thumb.

Of course, this says nothing about whether the heavier 1Ds is more or less prone to movement via "twitching." On the one hand, it takes a more violent twitch to move a heavier camera a given distance than a light camera. On the other hand, the increased fatigue from using a heavier camera all day may produce more violent twitches! I think I'll let somebody else do a detailed experiment on this effect... [img]/ubbthreads/images/graemlins/smirk.gif[/img]

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Old 01-19-2005, 03:48 AM
Ray_Smith Ray_Smith is offline
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Re: Shutter speed versus focal length

[ QUOTE ]
I recently saw a posting on another forum in which this question was being passionately debated without a resolution.

[/ QUOTE ]

Yes, I saw the same one which is why I decided to ask the question here. My interpretation was that the *effective* focal length was the one to use in the 1/focal-length rule but the discussion on the other forum seemed to frequently go off on tangents and no one actually answered the OP's question.

Thanks,

Ray

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