The Perfect Sunrise

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So, the rule of thumb for shutter speed is that it shouldn't be less than 1 / focal length. Well, that's straight forward on full frame cameras, but what about cropped sensor cameras? Is it going to be 1 / equivalent focal length? By equivalent focal length, I mean original focal length × crop factor.

My guess is: no, it's just 1 / focal length, cause the lens's focal length didn't physically change.

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See also Where does the ¹/shutter speed = focal length rule for hand shake come from?, which in some ways answers this, since knowing where the guideline comes from helps understand when it applies. –  mattdm Apr 28 '12 at 20:39
    
@DetlevCM answer is saying "Yes" the shutter speed = 1 / equivalent focal length but there is no reference to that –  akram Apr 28 '12 at 23:10
    
Okay.... Did you find the other answers more helpful? –  mattdm Apr 28 '12 at 23:55
    
@mattdm sure but no answer other than DetlevCM 's mentioned the rule for cropped sensor –  akram Apr 29 '12 at 0:02
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No? Thus I've often seen the recommendation to use the "35mm equivalent focal length" in the rule of thumb, instead of the actual focal length. –  coneslayer Apr 29 '12 at 0:58
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3 Answers

up vote 3 down vote accepted

According to this Wikipedia article on the secondary effects of crop sensors:

The old rule of thumb that shutter speed should be at least equal to focal length for hand-holding will work equivalently if the actual focal length is multiplied by the FLM [focal length multiplier] first before applying the rule.

So, yes, use the 35mm equivalent focal length as your reference for minimum shutter speed.

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looks like on crop sensors we should use the equivalent focal length but it doesn't make sense to me cause we didn't change the actual focal length of the lens. It's the same as we took the photo on full frame and crop it later in post processing! Anyway maybe this is one more thing that doesn't make sense to me but I'll have to live with it –  akram Apr 29 '12 at 4:34
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If you cropped in post-processing, and enlarged the resulting picture to the same size, the effect of camera shake would be magnified by the enlargement. As a result, you cannot tolerate as much camera shake in a cropped picture. –  coneslayer Apr 29 '12 at 5:29
    
I still don't buy into it. The FLM is misleading anyways because it's not just the sensor size that determines any magnification. If a crop sensor has the same size and density of sensels as a full frame then the crop sensor is a pure crop and has no multiplier in comparison. It can only multiply if it has more density and thus more information over a given region. –  John Cavan Apr 29 '12 at 23:10
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@JohnCavan I believe that this rule of thumb, much like DoF calculations, assumes a well-sampled image plane. That is, the "allowable blur" is large compared to the pixels. A typical "allowable CoC" for 35mm is 30 microns, which is several pixels on digital sensors. I don't think the 1/FL rule is expected to give pixel-sharp results. –  coneslayer Apr 30 '12 at 1:20
    
@coneslayer - What I'm trying to say is that in the absence of decent comparison between the resolution of 35mm film versus a given sensor, the 1/f rule of thumb cannot be a straight crop multiplier against the focal length, there are other factors. At any rate, it's just a rule of thumb and the photographer is the bigger variable in any event. –  John Cavan Apr 30 '12 at 1:40
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As long as your subject is at a large distance from the camera (non-macro), small translations in the position of the camera don't really have an impact on the image. Instead, the impact of your unsteadiness is in the pointing of the camera—that is, the rotation of the camera about its axes.

So the traditional 1/FL rule is saying that in 1/FL seconds, the angular pointing error of your hand-holding becomes significant. Significant compared to what? It has to be another angular quantity—namely the angular field of view of the camera. Thus, in the original 1/FL rule, FL is being used as a proxy for angular field of view. It's just convenient happenstance that it tends to work out well numerically for most people, when working on 35mm.

Since the FL is being used as a proxy for the quantity we really care about—field of view—it's necessary to adjust it to other sensor formats, and that means using the "35mm equivalent focal length" in the rule of thumb, not the true physical focal length.

Of course, there are lots of other caveats, not the least of which is image stabilization. These are discussed at length in another question here.

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This rule of thumb is a useful guideline because it's simple, but it is just a rough approximation with a pretty large margin of error - image stabilization, camera holding and shooting technique, the natural steadiness of your hands and even the weather can make a huge difference for the minimum hand-holdable shutter speed.

I do believe you have to use the 35mm equivalent focal length - but - this would make an about half a stop difference for crop sensor DSLRs and around 1 stop difference for most mirrorless interchangeable lens cameras, I believe this is still well inside the margin of error for this rule.

And for point ans shoots and cell phones (where the crop factor is significant) - you hold those very differently than you hold a DSLR and since this rule has stayed unchanged since the days of film when all cameras could only be used with a viewfinder (or blindly) I find it hard to believe it is relevant for those cameras at all.

So, it's useful to remember that this rule became popular because its simple, not because its accurate so adjusting it based on crop factor (or anything else that doesn't make a huge difference) is counterproductive.

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