Rule Of Thumb

[arpinum]

just some back of the envelop calculations. Since angle of view is a trig function and not a linear one, we should expect some variation of the rule with different focal lengths. From about 22mm focal length on a 35mm camera, you will need to add a correction factor to the denominator greater than 1. I assume that the original rule was for a 46.8 degree angle of view.
Taking these corrections into account, a more precise rule of thumb should be about 1/250 for a 200mm lens and 1/20 for a 25mm lens. Given that most cameras back then, and many now, don't have 1/200 as a shutter option, nor a 1/20, Using the standard rule of thumb for focal lengths greater than 22mm on a 35mm camera should be fine. Smaller than 22mm, you would have to get near fisheye territory to need adjustments.

If you find the rule works for you, no need to tweak it for different focal lengths.

[RalphLambrecht]

arpinum

It's not worth the effort. Do the math, the relationship is almost linear.

[wiltw]

After thinking about it a bit more, I'm not so sure that it really is a 35mm rule. I remember my Dad teaching me this rule at a time when we used nothing but medium-format cameras. Could it be that the rule is an older medium-format rule of thumb and was carelessly carried over to 35mm?.

It is my understanding as well, that 1/FL was originally the rule applied to medium format. And when 135 became popularized, the world neglected to factor in the smaller format via a multiplier. So any safety margin that existed for medium format rule got eaten up in the 135 format rule. Only when the world adopted the smaller format of the original APS-C did they properly add in a multiplier again.

[Sirius Glass]

arpinum

It's not worth the effort. Do the math, the relationship is almost linear.

Thank you for saving me the effort of posting the same. Trigonometry is based on ratios and the scaling in the Rule of Thumb is based on ratios. Both are linear.

Steve

[arpinum]

Thank you for saving me the effort of posting the same. Trigonometry is based on ratios and the scaling in the Rule of Thumb is based on ratios. Both are linear.

Steve

I did the math, that's why I found and reported that its not worth varying the rule based on focal length. Yes they are both ratios. But tangent is not linear. The rule works well because tangent is close to linear over most regular fields of view. Fisheyes however are on a non-linear section of the tangent curve compared to other lenses. Here the rule can break down by a stop.

[Sirius Glass]

I did the math, that's why I found and reported that its not worth varying the rule based on focal length. Yes they are both ratios. But tangent is not linear. The rule works well because tangent is close to linear over most regular fields of view. Fisheyes however are on a non-linear section of the tangent curve compared to other lenses. Here the rule can break down by a stop.

Especially if your feet are in the photograph! This is not rocket science. It is a linearization for back of the envelop calculations.

[Rick A]

Rules are meant to be broken, so watch out for your thumb, it too can be broken.

[lxdude]

Rules are meant to be broken, so watch out for your thumb, it too can be broken.

Rules of thumb are important. I consider the rule of finger even more important.
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