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  1. #1
    Rhoufi's Avatar
    Rhoufi is offline dPS Forum Member
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    Default Focal Length versus Shutter Speed

    There is a general rule of thumb about minimum shutter speed versus lens focal length, which goes something like this:

    Min Shutter Speed (secs) = 1/Focal Length (mm).

    So for a 50mm lens it is 1/50 sec.

    My question is, does the focal length for this "rule of thumb" relate to the specified, digital, focal length, or the equivalent film focal length? For example, I have a 70mm telephoto which has a 105mm equivalent film focal length - what would be the minimum, recommended shutter speed for this: 1/70 or 1/105?

    Second question (part of the first): does this limitation relate to the physical, constructed size of the lens? In other words, if my 70mm telephoto lens is a pancake design (physically only ~ 30mm long), can I rely on a slower shutter speed than a non-pancake telephoto design that is say, 100mm long (constructed length).

    Is the limitation therefore related to camera shake (the magnitude of the angle of rotation during shaking) multiplied by the distance from the sensor to the point of entry of light into the end of the lens.

    I hope I have described this question clearly; it seems a bit convoluted I know. Hopefully someone gets what I'm on about.
    Rhoufi -- Melbourne, Aust
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    inkista's Avatar
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    You're asking for a lot of precision from a rule of thumb.

    It's mostly a matter of personal ability to handhold. Some folks throw in the crop factor, some don't. One DPS member multiplies by 3. Remember, too, that the rule of thumb also assumes you know how to hold your camera correctly.

    Personally, I don't use the crop factor with my dSLR, but I do with my P&S. My personal lower-bound for shutter speed without stabilization is around 1/30s.

    And part of the reason that camera shake registers more with longer lenses is the magnification. This is also why you need faster shutter speeds for macro work. So, my guess would be that the physical size of the lens doesn't actually matter as much as the focal length.

    Honestly, though, the only way to really know is to try it and see. It's a guideline, not a hard and fast rule.
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    Rhoufi's Avatar
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    Default Focal Length versus Shutter Speed

    Quote Originally Posted by inkista View Post
    You're asking for a lot of precision from a rule of thumb.

    It's mostly a matter of personal ability to handhold.

    Personally, I don't use the crop factor with my dSLR, but I do with my P&S. My personal lower-bound for shutter speed without stabilization is around 1/30s.

    And part of the reason that camera shake registers more with longer lenses is the magnification.

    So, my guess would be that the physical size of the lens doesn't actually matter as much as the focal length.
    Thanks, inkista.
    I know what you mean, a rule of thumb is only just that. All my experience has been with film camera with a 50mm lens and I have come to know that 1/60 was about as slow as I could go. I was using reference to the "thumb rule" to frame my real question: what are the real variables that affect the ability to hold it steady and what features of the lens make that more difficult?

    I took an image the other day that normally I should not have been able to take properly - indoor, no flash, focal length 70mm (prime lens), f2.4 and shutter speed 1/15. It was the first day I really got to use my new (and first) dSLR. Now I cannot normally hold any camera steady enough for such a slow shutter speed (nowhere near it), but the stationary subject at the focus point was sharp. This got me thinking about what really matters (hidden factors in any "thumb rule"). Hence my post - I want to know more exactly what factors really affect this area of possible error.

    You have contributed to the picture getting clearer. Thanks.

    BTW: using the term "crop factor" makes me think you work in irrigation - could that be a shrewd guess?
    Rhoufi -- Melbourne, Aust
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    Crop factor is just the term thats been coined by the industry to explain the difference between full-frame and APS-C sensor sizes.

    I shot my grandfather's 80th birthday with a 50mm f/1.8. Shooting at f/2 i managed to handhold for 1/20 and 1/15 at times. It takes practice, but it *can* be done.
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    inkista's Avatar
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    Quote Originally Posted by Rhoufi View Post
    ... what are the real variables that affect the ability to hold it steady and what features of the lens make that more difficult?
    Ah. Yeah, it's kind of a mystery and a black art that gets condensed down to that rule of thumb. Mostly, I'd say it's physical length and weight, too, make it harder to hold a lens steady. But that more than that, the knowledge/experience the shooter has at stabilizing themselves can create a wide variance. Standing/sitting/lying down makes a difference. Leaning against something, supporting or stabilizing your elbows on something makes a difference, etc. Different holds on the camera can make a difference.

    Obviously, the biggest factor in helping you out with a lens is going to be built-in stabilization (IS/VR/etc.)

    I took an image the other day that normally I should not have been able to take properly - indoor, no flash, focal length 70mm (prime lens), f2.4 and shutter speed 1/15. It was the first day I really got to use my new (and first) dSLR. Now I cannot normally hold any camera steady enough for such a slow shutter speed (nowhere near it), but the stationary subject at the focus point was sharp.
    Sounds like your technique is better than you've been assuming.

    BTW: using the term "crop factor" makes me think you work in irrigation - could that be a shrewd guess?
    No. I'm a city girl. I earn my wage as a technical writer, so I'm very picky about terminology. The effect that a smaller sensor has on the field of view of a given focal length, to me, has more to do with cropping off the edges of an image than changing the focal length of the lens, so I use "crop factor" instead of "equivalent focal length" or "focal length multiplier." A 50mm lens is still a 50mm lens, regardless of whether it's mounted on a full-frame or a crop body.
    I shoot with a Canon 5DmkII, 50D, and S90, and Pansonic G3. flickr stream and equipment list

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    Rhoufi's Avatar
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    Default Focal Length versus Shutter Speed

    Quote Originally Posted by OsmosisStudios View Post
    Crop factor is just the term thats been coined by the industry to explain the difference between full-frame and APS-C sensor sizes.

    I shot my grandfather's 80th birthday with a 50mm f/1.8. Shooting at f/2 i managed to handhold for 1/20 and 1/15 at times. It takes practice, but it *can* be done.
    Yes, my camera has an APS-C Sensor (23.4x15.6) and in-built stabilization, and I suppose I might have fluked it, but I am still curious about the physical length of the lens (and weight) versus the focal length being the primary controlling factor. I'll just have to practise more and observe the differences between longer zooms and pancake telephotos.

    Thanks for your help.

    Quote Originally Posted by inkista View Post
    Mostly, I'd say it's physical length and weight, too, make it harder to hold a lens steady.

    Obviously, the biggest factor in helping you out with a lens is going to be built-in stabilization (IS/VR/etc.)

    Sounds like your technique is better than you've been assuming.
    I didn't have time to brace myself for the shot, and this was what struck me. The built in stabilization works on steadying the sensor against the rotation of the camera. Also the shorter and lighter the lens, the less likelihood of rotation. So the shortness of the 70mm pancake lens makes the whole package short, light and easier to hold. This is where my thinking is going. But I will test this theory out with the 18-55mm zoom and compare.

    No. I'm a city girl. I use "crop factor" instead of "equivalent focal length" or "focal length multiplier."
    Well, isn't the argot of a particular passion interesting? You both have different definitions. In irrigation "crop factor" is the multiplier given to a particular 'crop' (maize, lucerne, rye grass) to apply to the Class A Pan evaporation loss for the day, to establish how much water to apply to a field. So there you go - too much knowledge can mislead you!

    Regards to you both for the discussion. Every day is a school day.
    Rhoufi -- Melbourne, Aust
    Pentax K-7; smc-DA, 18-55mm, f3.5-5.6, AL, WR zoom; smc-DA, 70mm, f2.4, LTD prime telephoto.
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    inkista's Avatar
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    Quote Originally Posted by Rhoufi View Post
    Well, isn't the argot of a particular passion interesting? You both have different definitions. In irrigation "crop factor" is the multiplier given to a particular 'crop' (maize, lucerne, rye grass) to apply to the Class A Pan evaporation loss for the day, to establish how much water to apply to a field. So there you go - too much knowledge can mislead you!...
    That's totally cool! I like knowing that different plants are differently thirsty/abled at holding onto their water. Plus y'know, I never knew alfalfa was also called lucerne.
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    OsmosisStudios's Avatar
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    Quote Originally Posted by Rhoufi View Post
    Yes, my camera has an APS-C Sensor (23.4x15.6) and in-built stabilization, and I suppose I might have fluked it, but I am still curious about the physical length of the lens (and weight) versus the focal length being the primary controlling factor. I'll just have to practise more and observe the differences between longer zooms and pancake telephotos.
    Physical length and weight have very little to do with focal length. "Focal length" refers to the distance from the front of the lens to the nodal point of the lens (where the light beams converge and flip). 24, 35 and 50mm lenses are all about the same actual size, but the focal length varies.

    longer zooms / pancake telephotos is an interesting juxtaposition of terms. "Longer" usually refers to focal length, and a zoom referring to a vari-focal lens. That's fine. A pancake telephoto however is a complete oxymoron: pancake lenses are usually less than an inch long and are, for the most part, in the 45mm range simply as a factor of their design. Telephoto just indicates a focal length above "normal", so greater than 70mm.

    Focal length doesnt actually change with crop factor. Think of it like matting a print in a big frame. If you have a 16x20 frame and an 11x14 print, for instance, the 11x14 print represents your sensor whereas the 16x20 frame represents the image circle that's projected by the lens. If you had a 16x20 print and simply added a 16x20->11x14 matte to it, you'd only see the centre section. Your camera does the same thing: you only get the centre part of an image (the camera "crops" the rest). It just so happens that this factor is 1.5 (Nikon, sony, pentax), 1.6 (Canon) or 2 (olympus) of what the original size.

    The "equivalent" focal length just indicates the difference in what is apparent to the sensor. The better term would be "apparent focal length", as it appears youre using a longer lens, but arent.
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    Naois is offline Hobby Photographer
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    Quote Originally Posted by OsmosisStudios View Post
    Physical length and weight have very little to do with focal length. "Focal length" refers to the distance from the front of the lens to the nodal point of the lens (where the light beams converge and flip). 24, 35 and 50mm lenses are all about the same actual size, but the focal length varies.
    I think the OP was not relating them to eachother, but trying to see if the size and weight of the lens has more effect on camera shake than the focal length.

    If I have a P&S camera that weighs 1lbs with a 300mm zoom that fits in the palm of my hand and I have a dSLR that weighs 9lbs with a 300mm zoom that's a foot long, would the average person be able to hold the P&S camera with less camera shake than the dSLR due to its size and weight?

    If I can hold the P&S with less camera shake, then I could "break the rule of thumb" and go to 1/100s and get a picture that is equally as "still" (not shaken) as if I took the shot with the dSLR at 1/300s.

    The correct answer to all of this is to buy a freakin' tripod!

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    Quote Originally Posted by Naois View Post
    ...If I have a P&S camera that weighs 1lbs with a 300mm zoom...
    Actually, if you look closely at what's printed on the lens, it's probably really only a 50mm zoom. The crop factor on a P&S with a 1/2.3" sensor is 6x. So, yeah, theoretically, the average person would be able to hold it with less shake, because it's a shorter lens.
    Last edited by inkista; 02-12-2010 at 10:03 PM.
    I shoot with a Canon 5DmkII, 50D, and S90, and Pansonic G3. flickr stream and equipment list

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