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08-26-2008, 03:24 PM   #16
Lowell Goudge's Avatar

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QuoteOriginally posted by big_ezy Quote
I have seen discussion regarding the minimum shutter speed rule of thumb to the effect that it is necessary to apply the crop factor to the focal length. This doesn't make sense to me. I have read discussions here that have convinced me that focal length is focal length regardless of sensor size. The crop factor only applys to field of view. It doesn't seem to me that fov would be a factor in revealing camera shake. It seems to me that magnification is what reveals camera shake and that is a function of focal length.

Can someone reveal the flaw in my logic or confirm my reasoning?

The bottom line is that 1/focal length actually applies to a 35mm frame (24 x 36mm) blown up to roughtly 8 x 12 inches, and still have an acceptibly sharp (as defined by the confusing circle of confusion theory)

To enlarge a DSLR frame (16 x 24 mm) up to the same physical size requires 50% more magnification, therefore for the same "apparent sharpness" you need higher shutter speed.
When you consider the crop factor you are realling taking into account that you will enlarge the final image on the sensor more than the final image on film, all else being equal.

That is all!

If you don't beoileve this, simply take 2 photos, (without shake reduction) one at 1/F and one at 1/(2F) of the same subject. What you will see is that full sized, both look OK, but as you enlarge things, you will get to a point where the one shot at 1/(2f) still looks sharp, yet the one at 1/f begins to loose something.

08-26-2008, 05:01 PM   #17
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QuoteOriginally posted by Wombat Quote
With respect, I'm wondering if you're using the term magnification a bit loosely, Wheatfield. Correct me if I'm wrong, but the image isn't actually magnified, is it? There's simply less of it "on show" as it were, compared with a full 35mm film frame, giving the impression that it's magnified. If I've got this right, then surely big_ezy's first statement is correct and the rule of thumb shutter speed for a 200mm lens would be 1/200 rather than1/300?
Yes, Field of View would actually be the more correct term.
08-26-2008, 05:09 PM   #18
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a point to is that the rule of thumb is only general, personally I drink way to much coffee etc so for me 500mm lens + 1.5 = 750, for me to handhold I'd need to be over 1/1000. I try for 1/250 min on voigt 125mm
08-26-2008, 05:15 PM   #19
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QuoteOriginally posted by Big Dave Quote
You are exactly correct. The size of the sensor doesn't matter,the focal lenght effects the degree of shake. Why would cropping the image effect anything? The depth of field also remains the same for a given focal lenght, regardless of the sensor size.

Dave, the size of the sensor does matter, since that is the size of the original. It has nothing to do with cropping.
If we take a 35mm negative and want to make an 8x10 print, we are making (more or less) an 9X enlargement.
If we want to do the same thing with a APS-C sensor DSLR image, it is more like a 13X magnification.
The greater the magnification, the greater the possibility of camera shake.
This is why it is easier to handhold a camera with a wide angle lens attached than it is to do the same thing with a telephoto.
The pixel count affects resolution, in much the same way a fine grained film has better resolution than a coarse grained film, but the magnification is still greater for the smaller sensor image.
Because we are magnifying the image more, we have more problem with camera shake.

Technically, you are correct about depth of field being the same regardless of sensor size, but remember, depth of field is tied to image magnification (reproduction ratio). Given the same focal length lens and same aperture, you would have identical depth of field no matter what you put behind the lens, but you will be altering how much you see when you go to a smaller or larger capture medium.
Therefore, necessity will dictate that the camera position be moved to maintain a similar reproduction ratio, and as soon as you move the camera to maintain similar image magnification, you are altering depth of field, not to mention perspective.

I hope this clarifies things a bit. It can be a very confusing subject.

Last edited by Wheatfield; 08-26-2008 at 06:32 PM. Reason: clarifying a point of detail

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