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03-16-2014, 02:40 PM - 1 Like   #16
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I think talking about maintaining acceptably sharp focus at infinity confuses everything (although it certainly is an important detail, it's just not the most important detail). Perhaps a better way is to imagine the following thought experiment:

Imagine we need to create a photo from a scene where "everything must be in focus from 15 feet to 45 feet" -- i.e. a total of 30 ft of critically-sharp subject matter. Now the most-useful rough "guesstimate" of hyperfocal distance is that approximately 1/3 of the total critical focus will be IN FRONT of where you focus and the remaining 2/3 of what will be in focus is BEHIND where you focus. Using this 1/3 vs. 2/3 rule, in the above situation, we would want to focus precisely at 25ft, with 10 feet of important subject matter in front of the 25 foot focus point and another 20 feet of important subject matter behind where we focused our lens. So now that we have established WHERE we need to focus, the remaining issue is what aperture do we need to accomplish our task? Clearly if we shoot wide open at 25 ft we are not going to reach our goal, since only the subject matter at 25 ft is going to be in focus. Thus we need to stop down. And how much we stop down will depend on the lens you are using. A 400mm lens is not going to achieve this goal for the given situation no matter how much you stop it down simply because long focal length lenses deliver shorter total length of critical focus at every aperture compared to shorter focal length lenses. Basically, the situation described would be impossible for such a long lens. But with a wide angle lens it is a piece of cake. Without consulting any charts, I would hazard a guess that a 10mm fisheye lens would be acceptably sharp in the above situation at f/2.8!! A 20mm lens might need to stop down to f/4, a 40mm lens might need to stop down to f/5.6, and so on.

The other banana peel to walk on is who gets to define what is "critically sharp"? If we are going to enlarge our K-3 24Mp images to 24"x36" then we might decide that what is sharp for a 12"x18" print is not quite doing it at 24"x36". So the OTHER factor is how big you are going to enlarge the image. Let's say you have a 12"x18" print that has subject matter in that critically-sharp zone ranging from 15' to 45' in the above situation and that the lens was stopped down to f/8 to achieve this. Enlarging this same image to 24"x36" may reveal that the subject matter at both 15' and 45' are not as "sharp" as we first thought. In such a situation, we may need to stop down to f/11 to achieve the same relative level of focus. Likewise, if we calculate our aperture for 24"x36" prints an aperture at f/8 but we end up only making prints at 12"x18". which might have gotten away by shooting at f/5.6 instead. As you can see, the ultimate size of the image plays an important part.

While there are indeed formulas to figure all this out for your particular (many are online), most try to include infinity into the equation which in my opinion makes understanding things more difficult. In the end, it is important to get your head around the idea that as you stop down your lens, the total distance that will be in critical focus will increase. And that the distribution of that critical range of acceptable sharpness will be distributed approximately 1/3 in front of where you focus and approximately 2/3 behind where you focus. And that longer lenses deliver shorter total distances of critical focus compared to wide angle lenses which have much greater. In the end. one must match the image requirements to the lens.

Hopefully, this is clearer for some of you. For the rest, I have no doubt I just made things more complex!! :P

Michael

03-16-2014, 03:54 PM   #17
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QuoteOriginally posted by dms Quote
So using a full frame lens on a pentax dslr (1.5 cropped sensor) start by using the DOF markings for the lens 1 stop more open. e.g., taking at f/11, use the DOF markings for f/8
Other way around. For a given composition (FOV) and aperture, APS-C will have greater DOF than 35mm FF.


Steve

---------- Post added 03-16-14 at 03:58 PM ----------

QuoteOriginally posted by MJSfoto1956 Quote
In the end, it is important to get your head around the idea that as you stop down your lens, the total distance that will be in critical focus will increase. And that the distribution of that critical range of acceptable sharpness will be distributed approximately 1/3 in front of where you focus and approximately 2/3 behind where you focus. And that longer lenses deliver shorter total distances of critical focus compared to wide angle lenses which have much greater. In the end. one must match the image requirements to the lens.
Well, there ya go! The explanation in a nutshell! My experience has been that the notion of hyperfocal is only useful for situations where manual focus is difficult to determine.


Steve
03-16-2014, 05:16 PM   #18
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As far as larger vs smaller DOF--on a cropped sensor the DOF is smaller for the cropped sensor USING THE SAME LENS at the same aperture. (And I was speaking of using the DOF markings on a film/FF lens.)

But the cropped sensor "sees" a smaller FOV w/ the same lens. Using a lens on the FF that has the same FOV will give a lower DOF at the same aperture.

So yes that is the advantage of a smaller sensor/film area: that for the same FOV the DOF is greater (at the same F stop)
03-18-2014, 03:32 PM   #19
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QuoteOriginally posted by MJSfoto1956 Quote
I think talking about maintaining acceptably sharp focus at infinity confuses everything (although it certainly is an important detail, it's just not the most important detail). Perhaps a better way is to imagine the following thought experiment:

Imagine we need to create a photo from a scene where "everything must be in focus from 15 feet to 45 feet" -- i.e. a total of 30 ft of critically-sharp subject matter. Now the most-useful rough "guesstimate" of hyperfocal distance is that approximately 1/3 of the total critical focus will be IN FRONT of where you focus and the remaining 2/3 of what will be in focus is BEHIND where you focus. Using this 1/3 vs. 2/3 rule, in the above situation, we would want to focus precisely at 25ft, with 10 feet of important subject matter in front of the 25 foot focus point and another 20 feet of important subject matter behind where we focused our lens. So now that we have established WHERE we need to focus, the remaining issue is what aperture do we need to accomplish our task? Clearly if we shoot wide open at 25 ft we are not going to reach our goal, since only the subject matter at 25 ft is going to be in focus. Thus we need to stop down. And how much we stop down will depend on the lens you are using. A 400mm lens is not going to achieve this goal for the given situation no matter how much you stop it down simply because long focal length lenses deliver shorter total length of critical focus at every aperture compared to shorter focal length lenses. Basically, the situation described would be impossible for such a long lens. But with a wide angle lens it is a piece of cake. Without consulting any charts, I would hazard a guess that a 10mm fisheye lens would be acceptably sharp in the above situation at f/2.8!! A 20mm lens might need to stop down to f/4, a 40mm lens might need to stop down to f/5.6, and so on.

The other banana peel to walk on is who gets to define what is "critically sharp"? If we are going to enlarge our K-3 24Mp images to 24"x36" then we might decide that what is sharp for a 12"x18" print is not quite doing it at 24"x36". So the OTHER factor is how big you are going to enlarge the image. Let's say you have a 12"x18" print that has subject matter in that critically-sharp zone ranging from 15' to 45' in the above situation and that the lens was stopped down to f/8 to achieve this. Enlarging this same image to 24"x36" may reveal that the subject matter at both 15' and 45' are not as "sharp" as we first thought. In such a situation, we may need to stop down to f/11 to achieve the same relative level of focus. Likewise, if we calculate our aperture for 24"x36" prints an aperture at f/8 but we end up only making prints at 12"x18". which might have gotten away by shooting at f/5.6 instead. As you can see, the ultimate size of the image plays an important part.

While there are indeed formulas to figure all this out for your particular (many are online), most try to include infinity into the equation which in my opinion makes understanding things more difficult. In the end, it is important to get your head around the idea that as you stop down your lens, the total distance that will be in critical focus will increase. And that the distribution of that critical range of acceptable sharpness will be distributed approximately 1/3 in front of where you focus and approximately 2/3 behind where you focus. And that longer lenses deliver shorter total distances of critical focus compared to wide angle lenses which have much greater. In the end. one must match the image requirements to the lens.

Hopefully, this is clearer for some of you. For the rest, I have no doubt I just made things more complex!! :P

Michael
Thank you Michael, That does make sense to me

Richard

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