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02-17-2009, 08:03 AM   #1
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Zoom: hyperfocal length

Last weekend it was a beautiful day in italy so I went out with my da* 16-50 to take some photos.
Meanwhile I was taking a photo, I had to use the DOF scale but obviously there is no DOF scale in the zoom.
Using a focal length of 20mm and a f16, the hyperfocal distance is 1.27 meters: so using the programme M and M focusing I tried to focus manually at 1.27 meters, but in the zoom there is a spanning (no precise) length indicator.
I have two questions:
- what did I do is right or not? How can I set exactly 1.27 mmeters?
- Whe I tried to focus at 1.27 meters, I have to focus something or I can shoot without focus anything (and I know that the depth is from hyperfocal/2 distance to infinity)?
Best regards

02-17-2009, 10:11 AM   #2
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At least with 18-55mm kit lens the distance scale is only valid for 55mm end. You can do hyperfocal focusing with other focal lengths as well but you have to do some experimenting first:

Let say that you want to do hyperfocal focusing with 20mm and F16:

1) Place your camera on even surface, like floor or table
2) Measure up to a point in front of the camera which is 1.27m away from the surface of the sensor inside the camera
3) Place some clear object at this distance you measured
4) Focus on the object and note the reading on the distance scale. This is the reading you should use when trying to do hyperfocal stuff with your zoom @ 20mm & F16.

Of course it might be +- few centimetres. You gotta experiment which works the best.
02-17-2009, 12:07 PM   #3
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QuoteOriginally posted by Maffer Quote
At least with 18-55mm kit lens the distance scale is only valid for 55mm end.
Why only for 55mm?
QuoteOriginally posted by Maffer Quote
3) Place some clear object at this distance you measured
Why?
It's not clear to me why I should follow your steps
If I set a distance equal or little upper to my 1.27 m all should be ok: am I wrong?
It's not clear to me if I have to focus something or I can shoot in any case.
02-17-2009, 12:29 PM   #4
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Pentax lens manual says that the scale is only valid for telephoto end. Thus, when you set your lens to 20mm and you use the distance scale reading 1.27m (roughly) on the lens you are not actually focusing at 1.27m distance. Now, when you have an object at 1.27m distance from the sensor and have proper focus on it, you get the reading on the lens scale which is equal to 1.27m at the set focal length. When shooting hyperfocal, just use this reading on the lens scale and you are good to go.

Few centimetres here and there does not matter. Of course you can verify the readings you got by placing an object at hyperfocal/2 distance and see if it really gets reasonably sharp. If you can include some far away object in the same frame that's even better. Now you can fine tune the focusing and find out the exact settings for your setup.

02-17-2009, 01:41 PM   #5
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This is an interesting question. I understand hyperfocal range to be the range of distances that are acceptably in focus based upon the lens aperture and the distance of the camera lens from the subject. If a photographer does not change position and the DA* 16-50 has an F2.8 constant aperture I wouldn't think the hyperfocal range would change as the lens is zoomed. I wish I could test it myself but I do not own a constant aperture zoom.
02-17-2009, 01:57 PM   #6
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QuoteOriginally posted by B Grace Quote
This is an interesting question. I understand hyperfocal range to be the range of distances that are acceptably in focus based upon the lens aperture and the distance of the camera lens from the subject.
You're forgetting focal length. (Also the magnfiication of the final image)

I am only thinking theoretically: Since you can't focus on the hyperfocal length, why not compute the point of perfect focus which obtains coincident to the desired hyperfocal distance, and focus on *that* using the focusing screen? Would work very poorly, but it might garner you what you seek.
02-17-2009, 02:15 PM   #7
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QuoteOriginally posted by B Grace Quote
I understand hyperfocal range to be the range of distances that are acceptably in focus based upon the lens aperture and the distance of the camera lens from the subject. If a photographer does not change position and the DA* 16-50 has an F2.8 constant aperture I wouldn't think the hyperfocal range would change as the lens is zoomed. I wish I could test it myself but I do not own a constant aperture zoom.
I may be reading wrong - but that does not sound right.

Definition: Hyperfocal Distance at Wikipedia

" the hyperfocal distance is the closest distance at which a lens can be focused while keeping objects at infinity acceptably sharp; that is, the focus distance with the maximum depth of field. When the lens is focused at this distance, all objects at distances from half of the hyperfocal distance out to infinity will be acceptably sharp. "

Hyperfocal distance is dependent on the focal length (zooming changes that), aperture and the acceptable circle of confusion limit.

Last edited by UnknownVT; 02-17-2009 at 02:35 PM.
02-17-2009, 04:11 PM   #8
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QuoteOriginally posted by UnknownVT Quote
I may be reading wrong - but that does not sound right.

Definition: Hyperfocal Distance at Wikipedia

" the hyperfocal distance is the closest distance at which a lens can be focused while keeping objects at infinity acceptably sharp; that is, the focus distance with the maximum depth of field. When the lens is focused at this distance, all objects at distances from half of the hyperfocal distance out to infinity will be acceptably sharp. "

Hyperfocal distance is dependent on the focal length (zooming changes that), aperture and the acceptable circle of confusion limit.
Yeah, I know it doesn't sound right. I was troubled as I typed it. I even found a photo of an older Pentax push-pull F4 zoom on KEH's website and it did have the eliptical distance curves on the lens barrel. This would be a great excuse to buy a new zoom lens to see what is what but I digress.

I guess I forgot that when a longer focal length lens is used the magnification of the image increases the relative effect on the circle of confusion and the in-focus range is shorter.

Ok, I feel better.

02-18-2009, 01:59 AM   #9
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QuoteOriginally posted by Maffer Quote
Pentax lens manual says that the scale is only valid for telephoto end.
Where do you read this information?
I read the manual in my pentax da*16-50 and I didn't find this information.
If it is true, it's a very important notice which should be written in bold times new roman 42!
02-18-2009, 05:35 AM   #10
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http://www.pentaximaging.com/files/manual/DA%20Lens%20Manual_072208.pdf

Page 23: "The focusing index on the zoom lens is set to match the focus point when the focal length is at telephoto. The focusing index may not match the focus point for other focal lengths."
02-18-2009, 06:18 AM   #11
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QuoteOriginally posted by spinmar Quote
Last weekend it was a beautiful day in italy so I went out with my da* 16-50 to take some photos.
Meanwhile I was taking a photo, I had to use the DOF scale but obviously there is no DOF scale in the zoom......
Here's an alternate way to estimate things which may help.

There is a "hyperfocal width" and "hyperfocal f-number" that correspond to "hyperfocal distance". The f-number associated with that width is about:

N*=F/W where F is the focal length in millimeters and W is scene width in meters.

Say you focus on a scene that's 7 meters wide with a 35mm lens. The hyperfocal f-number is N*=35/7 = 5. At about f:5 everything beyond the focal plane will be in focus.

Dave
02-18-2009, 10:30 AM   #12
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QuoteOriginally posted by Maffer Quote
http://www.pentaximaging.com/files/manual/DA%20Lens%20Manual_072208.pdf
Page 23: "The focusing index on the zoom lens is set to match the focus point when the focal length is at telephoto. The focusing index may not match the focus point for other focal lengths."
Strictly speaking this is true.

Zoom lenses are supposed maintain their focus - especially when focussed at the telephoto end first.

Lenses that show difference in focus points when varying the focal length should be called Vari-Focal lenses.

However the OP's question was about getting the hyperfocal distance for a set focal length - although the calculation for the hyperfocal distance can be precise (to xx number of decimal places ) - any Scale focussing is not that precise to begin with - espcially for the scale on a zoom lens.

So the practical answer is - yes, one can use the scale to preset focus at the Hyperfocal distance and it would pretty much be OK -
if one were to be fussy, then setting the distance to somewhere between Double of and the hyperfocal distance would be a safe bet.
Double the hyperfocal distance is basically using 1/2 the circle of confusion limit - ie: double the "quality" or enlargement - often not really necessary - but if one were to do critical landscapes and wanted the maximum depth of field use the double distance.....
(as long as the near DoF distance limit is still acceptable)
02-18-2009, 10:49 AM   #13
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Call me crazy but when using a zoom I use the DOF preview. If it's too hard to see through the viewfinder, use the digital DOF preview and zoom in on the LCD. I'll focus about a third of the scene in and then preview, but if I have something somewhat close in the foreground, I'll take a couple of different shots and different focusing distances and apertures.
02-18-2009, 10:53 AM   #14
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Found this thing on flickr, its in feet though, not meters.
02-26-2009, 03:27 AM   #15
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QuoteOriginally posted by zplus Quote
Found this thing on flickr, its in feet though, not meters.
CoC for k20D is different.....
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