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03-17-2011, 03:36 PM   #16
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QuoteOriginally posted by Lowell Goudge Quote
But if no one prints 8x10 any more and enlarges to higher magnifications then the whole discussion is TOTALLY USELESS because the lens markings have no relevance.
Great point Lowell! In digital terms, random cropping sure makes the lens markings useless.

03-17-2011, 05:06 PM   #17
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QuoteOriginally posted by sewebster Quote
Well, it probably depends on your definition of "absolute DOF." Theoretically, the DOF is always zero. There is always only one place that is in "perfect" focus. However, based on certain factors like, how good your eyes are, what type of sensor you use, how you view the image etc., there is a region that you can't tell the difference between "perfectly" in focus and "very slightly" out of focus. This is what we call the DOF.

For your example with the ruler, definitely the picture taken with the crop sensor and then enlarged (either in print, or viewed on a computer monitor) to the same physical size will _appear_ to have less DOF. The "true" DOF in both cases is zero. Don't get too hung up on my terminology here though. DOF is basically defined as what we perceive with a certain set of assumptions.

PS I am a physicist
Physicist or not, you are incorrect

Depth of field like acceptable sharpness from camera shake is a function of "acceptably sharp" specifically what your eye can resolve from a specific distance. While infinitely sharp focus only occures at one point, acceptably sharp has a defined r ange which is a function of enlargement at a set viewing distance
03-17-2011, 05:11 PM   #18
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QuoteOriginally posted by Lowell Goudge Quote
Depth of field like acceptable sharpness from camera shake is a function of "acceptably sharp" specifically what your eye can resolve from a specific distance. While infinitely sharp focus only occures at one point, acceptably sharp has a defined r ange which is a function of enlargement at a set viewing distance
That's pretty much exactly what I said.
03-17-2011, 09:08 PM   #19
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QuoteOriginally posted by Jimfear Quote
I highlight this as this is the heart of the matter. DOF will stay absolutely the same, but it will be perceived as having changed.
Sorry but what is the difference? DOF is simply a measure of what looks acceptably sharp. It is entirely a matter of perception. DOF is not an absolute. If you enlarge a shot and crop (or use a smaller sensor with the same lens, which amounts to the same thing), 'blur' will be accentuated, hence DOF effectively reduced.

03-18-2011, 01:42 AM   #20
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QuoteOriginally posted by timo Quote
Sorry but what is the difference? DOF is simply a measure of what looks acceptably sharp. It is entirely a matter of perception. DOF is not an absolute. If you enlarge a shot and crop (or use a smaller sensor with the same lens, which amounts to the same thing), 'blur' will be accentuated, hence DOF effectively reduced.
Yes it is all about what looks acceptably sharp. My take on it is that if you increase magnification until the lens itself cannot render the difference between focal point sharp and DOF sharp you have what I called DOF. And this does not change with different formats. I guess it's all about what you think of when speaking of DOF, what you see or what is actually there. I'm for the actually there part even though that doesn't seem to be the general opinion.

Or in the words of a Savage [pun intended] "I reject your reality and substitute my own"
03-18-2011, 04:58 AM - 1 Like   #21
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QuoteOriginally posted by timo Quote
DOF is simply a measure of what looks acceptably sharp. It is entirely a matter of perception. DOF is not an absolute.
Quite correct, but incomplete. I phrase it as: DOF is a complex product of photography, presentation, and perception. We wage the DOF WARS, arguing over our attempts to simplify the complexity, leaving out crucial factors.

Photography: Frame size, focal length, aperture, subject distance, context (surroundings), even light, are factors the photographer can more-or-less control when shooting.

Presentation: Enlargement size, mounting and displaying (media, placement, light, etc), viewing distance, context, are factors the photographer might not be able to control.

Perception: Acceptable sharpness, visual acuity, viewers' attention spans and mental states and visual literacy and biases, are factors the photographer can't control at all.

We try to simplify this, by controlling certain factors and saying, ALL ELSE BEING EQUAL -- but all else AIN'T equal. We shoot, present, and view images, all in varying conditions. We may try to carefully control DOF in a shot. But if the subject is some boring crap, potential viewers won't even bother looking, at least won't pay close enough attention to see those finely-crafted details. Bother. They'll also neglect the picture if they need to piss, or to tend to other real-world stuff. Acceptable sharpness loses its meaning then. Bother.

ObTopic: The question was answered. On a crop dSLR, adjust a FF lens by just over 1 f-stop, and don't adjust DA lenses. And now, back to the DOF WARS!
03-18-2011, 06:10 AM   #22
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Sewebster and RioRico, thanks for the response. Reps to both. I didn't know I was opening a can of DOF worms. Interesting discussion but a little over my head ATM.
03-18-2011, 06:53 AM   #23
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QuoteOriginally posted by kbrede Quote
I didn't know I was opening a can of DOF worms.
DoF is a CoW just because it's murky and depends on more than just hardware and math. Read the Wikipedia entry, if you dare. The hardware and math issues alone are complex, hot enough to ignite brushfires. Throw in the human factors, and it gets to be really fun. And nobody has even mentioned diffraction yet!

03-18-2011, 07:14 AM   #24
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QuoteOriginally posted by RioRico Quote
And nobody has even mentioned diffraction yet!
That's another CoW only loosely related
03-18-2011, 07:56 AM   #25
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QuoteOriginally posted by eddie1960 Quote
That's another CoW only loosely related
Diffraction is related to DoF, sensor resolution and pixel density, and various other stuff, as contributing to (or diminishing) the bourgeois conceit that is sharpness. We can calculate very thick DoF with very small aperture openings, but then diffraction bites our butts. It's just one damn thing or another, eh?
03-18-2011, 08:03 AM   #26
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QuoteOriginally posted by RioRico Quote
Diffraction is related to DoF, sensor resolution and pixel density, and various other stuff, as contributing to (or diminishing) the bourgeois conceit that is sharpness. We can calculate very thick DoF with very small aperture openings, but then diffraction bites our butts. It's just one damn thing or another, eh?
Pretty much. mind you i have been known to shoot lenses in the diffraction zone. Meh sometimes it's just needed
the most interesting lens i've seen (though don't own....yet anyway) is the Sigma pantel 135 with an f64 setting. i imagine it's a diffraction monster (the opposite of Bokeh monster i guess )
03-18-2011, 08:11 AM   #27
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DoF simplification I hope

QuoteOriginally posted by RioRico Quote
DoF is a CoW just because it's murky and depends on more than just hardware and math. Read the Wikipedia entry, if you dare........!
Part of the reason that DoF is such a Can of Worms is that there are too any variables involved & their relative importance is hard to evaluate - I hear that people can only balance a few things at once in their minds - very few for me.

I've worked on DoF theory for a fairly long time and believe I've a reasonable understanding. It turns out that it is possible to group the important parameters into simple ratios that I think are relatively easy to remember and make sense in the context of DoF.

These ratios are:

(1) Depth of Field over Width of Field. like (one head deep) / (two heads wide) or (10' wide/30' deep)
(2) F-Stop over Display Width in Pixels. like (F:8 / 1000 display pixels wide)
(3) Scene Width over Sensor Width. like (360" scene width / 1" sensor width)

For cases where Depth of Field is small the equation is simple:



Having things expressed in ratios releases one from worrying about what dimensions to use (so long as whatever you use is the same on the top and bottom of the ratio.)

Using similar ratios also simplifies the equations for Hyperfocal conditions and Depth of Field for macros. I'll post them later to keep things simple here.

The main point is that using these ratios it is easy to envision what happens when you change one thing. Like decreasing the displayed picture width increases the apparent Depth of Field.

Last edited by newarts; 03-18-2011 at 09:34 AM.
03-18-2011, 12:03 PM   #28
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QuoteOriginally posted by sewebster Quote
That's pretty much exactly what I said.
not really, you maintain the true DOF is zero, which is different than perceived sharp.
03-18-2011, 01:53 PM   #29
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I thought that I would toss out the question on this thread - given the discussion on the one of the most current lenses of the crop. The SMC Pentax-DA 35mm F2.4 AL. How would you go about applying any sort of hyperfocal adjustment using this lens for example? No distance scale, so do you just sort of wing it?So would you flip it into manual, rotate it all the way to the right for infinity, and then back off a "smidge"? Or, would you rotate it stop to stop to see what the angular travel would be from minimum distance to infinity and then auto focus on something of interest, then back off 5 or 10% or the angular travel (less towards infinity and more towards the minimum focusing distance) and take a test shot?

03-18-2011, 02:29 PM   #30
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QuoteOriginally posted by interested_observer Quote
I thought that I would toss out the question on this thread - given the discussion on the one of the most current lenses of the crop. The SMC Pentax-DA 35mm F2.4 AL. How would you go about applying any sort of hyperfocal adjustment using this lens for example? No distance scale, so do you just sort of wing it?So would you flip it into manual, rotate it all the way to the right for infinity, and then back off a "smidge"? Or, would you rotate it stop to stop to see what the angular travel would be from minimum distance to infinity and then auto focus on something of interest, then back off 5 or 10% or the angular travel (less towards infinity and more towards the minimum focusing distance) and take a test shot?

The fstop to use to put everything behind the scene in focus is about the focal length (35 in this case) divided by the width of the scene in meters or yards.

For example a scene 5 yards (meters) wide would need
F= 35/5 = 7 so I'd use f:7 or f:8. For a scene 10 meters wide f:3.5 should be enough with your 35mm lens.

Try it.
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