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Old Apr 19, 2009, 9:36 PM   #11
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TCav wrote:
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Forget focal length. For the same angle of view, a larger image sensor has a shallower depth of field.
Only because it requires a different focal length lens to accomplish the same FOV. DOF is a lens property.
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A 100mm lens on a Canon 5D (FF) at f/16 focused at 10 feet has a total depth of field of less than 3 feet. A 50mm lens on an Olympus E520 (4/3) at f/16 focused at 10 feet has the same angle of view, but a toal depth of field of over 6 feet.


There is no doubt that a 100mm lens has less DOF than a 50mm lens at equal f-ratios. In absolute terms, you can also look at same DOF by going to f8 on the 4/3 lens. In other words, not only is it a lens property, but it is an absolute aperture property of the lens once equal FOV is established.

Aside from COC, DOF is a lens property, not a format property.
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Using the focal length and the circle of confusion is going around the block to get next door.
The equation of DOF is based on those exact things. To state what you did is to replace science so you can fly by the seat of your pants. I must say it makes no sense to me.


Greg



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Old Apr 19, 2009, 11:08 PM   #12
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I really appreciate all the help you guys TCav, Bill, Greg and Brian gave. Bill, I checked out the circle of confusion and just as it stated, it confused me. lol I will not give up though and will study the formulas when I get more time. Brian, my camera goes from 2.8 to 3.7 so I will put it on a tripod (I almost always do) and use the telephoto at the max 432 mm which should default to 3.7 and see if that gets me some blury background. Thanks again to everyone and even though you don't know me, I have read so many of all of your previous posts in this site, I feel like I know all of you.

Bob


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Old Apr 20, 2009, 12:36 PM   #13
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A telephoto lenses gives the impression of a shallower DOF because it compresses the background. But the actual DOF isn't really a function of focal length.

In Bob's case he wants some foliage 3 inches behind a flower to be blurred so the flower stands out. He finds that shooting macro at 35mm (eq) doesn't sufficiently blur the background. Stuff 1 foot away will be blurred, but not the close foliage. Framing the flower the same standing back and using 432mm isn't going to make the close foliage blur any more. With the higher f stop he might have a greater rather than lesser DOF. But the stuff that is blurred a foot away will appear to be only a couple of inches from the flower because of telephoto compression, giving the impression of a shallower DOF. But the foliage 3 inches from the flower will still be as sharp because he hasn't significantly altered his DOF.

This is a quote and chart from a good article on DOF:

"Note that I did not mention focal length as influencing depth of field. Even though telephoto lenses appear to create a much shallower depth of field, this is mainly because they are often used to make the subject appear bigger when one is unable to get closer. If the subject occupies the same fraction of the viewfinder (constant magnification) for both a wide angle and a telephoto lens, the total depth of field is virtually* constant with focal length! This would of course require you to either get much closer with a wide angle lens or much further with a telephoto lens, as demonstrated in the following depth of field chart:"
http://www.cambridgeincolour.com/tut...h-of-field.htm

You will notice that even a 10mm has less than 20% more DOF than a 400mm, and the DOF is constant from 50mm to 400mm. So it is reasonable to think Bob might actually increase his DOF a little going to telephoto with a higher f stop.

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Old Apr 20, 2009, 6:35 PM   #14
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Thanks Slipe, while playing around in the Cambridge tutorials you suggested, I decided to google kodak p850 macro and got this little trick which I can't wait to try out tomorrow (it is raining here now).

http://video.google.com/videoplay?do...70879980855669

There is even a short video showing the trick of how to get closer to the subject than the auto focus wants to allow. Don't know if it would work on other cameras but apparently it does on the P850. Anyway, I will try tomorrow to see if it works. Again, thanks everyone.
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Old Apr 21, 2009, 9:00 AM   #15
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Slipe's reference makes a good point, though it is worth seeing the footnote:

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*Note: We describe depth of field as being virtually constant because there are limiting cases where this does not hold true. For focal distances resulting in high magnification, or very near the hyperfocal distance, wide angle lenses may provide a greater DoF than telephoto lenses. On the other hand, for situations of high magnification the traditional DoF calculation becomes inaccurate due to another factor: pupil magnification. This actually acts to offset the DoF advantage for most wide angle lenses, and increase it for telephoto and macro lenses. At the other limiting case, near the hyperfocal distance, the increase in DoF arises because the wide angle lens has a greater rear DoF, and can thus more easily attain critical sharpness at infinity for any given focal distance.
Also nothing in that article contradicts the fact that DoF depends on focal length, aperature, distance, and circle of confusion: not format size.
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Old Apr 21, 2009, 11:14 AM   #16
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BillDrew wrote:
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Also nothing in that article contradicts the fact that DoF depends on focal length, aperature, distance, and circle of confusion: not format size.
The Circle of Confusion varies with the format size. See http://en.wikipedia.org/wiki/Circle_...ed_on_d.2F1500
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Old Apr 21, 2009, 12:18 PM   #17
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TCav wrote:
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BillDrew wrote:
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Also nothing in that article contradicts the fact that DoF depends on focal length, aperature, distance, and circle of confusion: not format size.
The Circle of Confusion varies with the format size. See http://en.wikipedia.org/wiki/Circle_...ed_on_d.2F1500
To drive this point home:

I have both a Canon 20d (APS C sensor) and 1d (aps H sensor). If I use a 100mm lens on both cameras, same aperture, same distance to subject they produce two different depths of field. At the same DISTANCE, the smaller sensor has shallower DOF. However, for the same Field of View (i.e. adjusting distance to subject so subject is same size in the frame) the larger senser has shallower DOF.

However, this isn't a whole lot of help to the OP. The OP isn't going to change their sensor size. They have control over aperture, distance (until you get to minimum focus distance) and focal length.

My suggestion to the OP is this - start at the minimumum focus distance for the camera. Open up the aperture to it's widest value and zoom until your subject fills the frame. Take the shot. If you still have zoom left, back up a bit and zoom further and take a shot. Repeat until you're at full zoom. Get the photos on your computer and review the results. Don't worry about the math. Let your experience guide you. You'll quickly learn for your camera which produces the shallowest dof (closest distance/widest aperture or more zoom).

Reading about circle of consusion is nice theory to understand WHY. But, for most people I think practical experience / tests drive home the practical lessons. You're not going to do calculations in the field most times anyway.
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Old Apr 22, 2009, 11:22 AM   #18
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TCav wrote:
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The Circle of Confusion varies with the format size. See http://en.wikipedia.org/wiki/Circle_...ed_on_d.2F1500
True, the circle of confusion depends on the magnification used in the final image which for the same print size does depend on the format size. The circle of confusion depends on print size, the eyesight of the viewer, the viewing distance, whether the viewer is using a loupe, any limiting factor in the printing process, ... The circle of confusion is a concept that boils all of these into a single number.

The circle of confusion is a number assigned to define what is "good enough" in terms of focus. Different people will have different ideas of what is good enough so there is no single answer. To claim the CoC depends on only one factor such as format size misses the point.

John has exactly the right idea: experiment to find out what works for you with the equipment you have. I think the theory is worth figuring out to guide those experiments, and it is worth having the theory cast in the clearest possible form. Jonh's exeriment with different formats and the same lens shows the effect of magnification and distance on DoF.
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