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08-26-2008, 06:05 AM   #1
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One over focal length rule of thumb

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?

Earl

08-26-2008, 06:13 AM   #2
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The problem is that with dSLRs, the magnification effect means that you have to take into account the 'effective' focal length compared to full frame 35mm.

For a 200mm lens, it is equivalent to 300mm on 35mm, so you should try to stay faster than 1/300 sec to avoid camera shake.

The magnification is used to compare equivalent FoVs with 35mm, and is a rule of thumb, not an absolute law. So, if you are going to use a rule of thumb that was used for 35mm, then you need to convert the measurements to be the same as 35mm for the rule to make sense.
08-26-2008, 07:09 AM   #3
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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?

Earl
Pentax DSLRs have a smaller sensor than film. This gives more magnification of the image than what you would see with a film camera.
More magnification + more camera shake visible, which is why you need to use a higher shutter speed with longer lenses

I hate the term "crop factor" but it is useful here, as alohadave pointed out.
08-26-2008, 07:13 AM   #4
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I'm sure I am probably being dense but your explanation didn't pentetrate my skull.

It doesn't seem to me that field of view effects camera shake. It doesn't follow in my mind that a decrease in field of view equals an increase in visiblity of camera movement blur. The increase in visibility of camera movement blur would be the result of increase in magnification which is a factor of an increase in actual focal length.

I'm not saying you're not correct. I'm just saying if you are I don't understand why.

Earl

08-26-2008, 07:16 AM   #5
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The above reply was to Dave. Wheatfield maybe your explanation got me there. Is it because with the smaller fov we actually have to enlarge the image by the crop factor to get comparable sized images? And therefore that additional enlargement reveals camera blur.

Earl
08-26-2008, 07:54 AM   #6
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QuoteOriginally posted by big_ezy Quote
The above reply was to Dave. Wheatfield maybe your explanation got me there. Is it because with the smaller fov we actually have to enlarge the image by the crop factor to get comparable sized images? And therefore that additional enlargement reveals camera blur.

Earl
Exactly right.
08-26-2008, 08:35 AM   #7
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QuoteOriginally posted by Wheatfield Quote
Pentax DSLRs have a smaller sensor than film. This gives more magnification of the image than what you would see with a film camera.
.
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?
08-26-2008, 08:53 AM   #8
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Wombat,
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


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?


08-26-2008, 08:59 AM   #9
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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?
Holy crud! This sounds like quantum physics. If a tree falls in the woods and nobody is there, does it make a sound? And does it produce a breeze that eventually forms into a hurricane on the other side of the world?

Don't feel bad big ezy...I can't wrap my mind around this very easily either, and I design and test stuff for a living! I just know that when I zoom in and push the button, I get an image....and usually it's an image I have to retake or touch up!

Now I feel inferior!
08-26-2008, 10:00 AM   #10
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Didn't read fully through all these posts, but I think it also has to do with pixel size, not just sensor size. Imagine you had a 500mm lens, but your lens and image sensor was so big that it was the size of the wall. The resolution remained the same, e.g. 10MP, so your "grains" would be a lot larger. In that case, camera shake would not matter. Now, imagine you have that same 500mm lens, but a lot smaller, with a sensor size of a postage stamp, and your grains are now a thousand times smaller in each dimension. In that case, shake would make a huge difference in blurriness.

Try drawing it out on a piece of paper, that might help.
08-26-2008, 10:58 AM   #11
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It's actually quite simple: the effect of shake is dependent on field of view,. not focal length. It just so happens that focal length gave us a convenient way of doing the calculation, but if you change sensor size, you nee to throw out that calculation, or at least modify it so it really does end up produce the same answer for the same sensor size.

It is easy to demonstrate that the effects of shake are dependent on field of view, not focal length. Consider a P&S camera with a "150mm equivalent" lens (which might really be only 35mm in actual focal length), a DSLR with a 100mm lens that is basically 150mm equivalent in field of view, and a film camera with an actual 150mm lens. Same field of view on all three, but *vastly* different focal lengths.

Set all three cameras on a tripod and point them at some distant object. Since the field of view is the same, the should should the exact same scene in the viewfinder. Now, rotate all three of the tripods by the exact same amount - say, 1 degree of rotation. Is there any doubt in your mind that the object that used to be centered in the viewfinder will now be off-center by the *exact same amount* on all three cameras? Of course it will - because they are showing the same scene.

Now replace (or zoom) the lenses so they are all at 35mm equivalent. Again, they actual focal lengths will be *vastly* different (probably 8mm, 20mm, and 35mm respectively). Again, though, if you point them at the same distant subject, because they have the same field of view, they will show the exact same scene in the viewfinder. And rotating the tripods by 1 degree each will result in shifting the scene by exactly the same amount in all three cases - but of course, being a wider angle of view to start with, the shift won't be nearly as noticeable. Yet the full frame camera is at the exact same focal length the P&S camera was in the first example. That is to say, 35mm as a focal length on the P&S camera showed a *big* effect from the rotation; 35mm as a focal length on the film camera showed a much *smaller* effect - indeed, the same effect as the P&S showed at 8mm.

Consider camera shake to be the equivalent of rotating the tripod. What this shows is that the degree to which a given amount of shake affects the image is the same for the cameras with the same field of view. Change field of view, and you change the effect of shake. Focal length is *not* the determining factor - field of view is.

QuoteQuote:
I have read discussions here that have convinced me that focal length is focal length regardless of sensor size.
True enough. The problem is that is a mistake to assume that shake has anything do to with focal length. It doesn't - it is a function of field of view only. It just so happens that we can use focal length as a stand in for field of view, if we first apply the crop factor to the focal length so we're comparing apples to apples in terms of field of view.

QuoteQuote:
It seems to me that magnification is what reveals camera shake and that is a function of focal length.
Magnification is a huge red herring here, as it leads to enormous confusion when people try to sort out these issues ("but, but ... cropping n image doesn't magnify it, so in image produced on a crop sensor camera won't *really* be bigger than one on full frame..."). If you insist on thinking on those terms, the folks pointing out that the image from the smaller sensor has to be "magnified" more in order to view it at a given size than the image from the full frame sensor are correct.
08-26-2008, 11:23 AM   #12
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QuoteOriginally posted by big_ezy Quote
The above reply was to Dave. Wheatfield maybe your explanation got me there. Is it because with the smaller fov we actually have to enlarge the image by the crop factor to get comparable sized images? And therefore that additional enlargement reveals camera blur.

Earl
Amazing, I just re read it and it is total gibberish. Anyway, you seem to have a handle on it now. Also, the 1/focal length rule was invented when people made pictures from negatives, and was designed to give an acceptably sharp 8x10 from a 35mm negative.
I suspect that even 1/1.5xfocal length won't satisfy pixel peeping for sharpness. The ability to look at our pictures on a pixel by pixel basis has moved the bar a long way. Pictures that look noisier than heck, without a sharp edge anywhere at a 100% monitor view will often produce lovely 8x10 prints.
08-26-2008, 12:32 PM   #13
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Since it is a rule of thumb why not come up with another?

For sharp images hand-held and with shake reduction on, shutter speed should be at least as fast as the inverse of the focal length.

This is essentially the same as the earlier rule of thumb, but we have taken into account that our sensor size is smaller (SR takes care of that). In fact we have a bigger margin for error since SR likely takes care of more than the "crop factor" difference. But it's safer to stick with a stricter guideline. If you then want to go a stop slower that's up to you.
08-26-2008, 12:46 PM   #14
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QuoteOriginally posted by rparmar Quote
Since it is a rule of thumb why not come up with another?

For sharp images hand-held and with shake reduction on, shutter speed should be at least as fast as the inverse of the focal length.

This is essentially the same as the earlier rule of thumb, but we have taken into account that our sensor size is smaller (SR takes care of that). In fact we have a bigger margin for error since SR likely takes care of more than the "crop factor" difference.
Good point. While SR can indeed actually help more than this rule of thumb suggests, throw in the fact that as was pointed out earlier, we tend to blow up our images *much* bigger on our monitors than we ever printed in film, and we actually *need* more sharpness than we did with film in order to satisfy this sort of pixel-peeping. So the above stated version of the "rule" is probably not so conservative after all.

The important thing to realize is, these *are* just rules of thumb - guidelines to help get a very general idea of what kind of shutter speed is required. It's not like there is any guarantee that a shot taken at 200m and 1/500" shutter will be sharp enough, nor is it out of the question that you could get good enough results at 1/90". Depends on how steady your hands are, whether the lens is soft enough to hide small amounts of blur, whether igh ISO noise is masking some blur, how demanding you are about the results, etc.
08-26-2008, 12:50 PM   #15
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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
On the sensor, the image is indeed the same size. Or at least, any given object within the image is the same size - the image itself, being cropped, is physically smaller. But when was the last time you ever saw an image on the sensor? In order to see the image, you have to display it on a screen or print it. And that involves magnifying the image. And since the cropped image is physically smaller, it is going to have to be magified more to display at the same size. That is why the crop factor absolutely *does* make sense as a way of comparing what a smaller sensor does to an image taken with a particular lens.
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