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Author Topic: Some thoughts about hyperfocal focusing  (Read 1802 times)
ErikKaffehr
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« on: February 14, 2013, 11:31:47 AM »
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Hi,

I lifted this discussion from another thread.

I have two issues with hyperfocal shooting and neither of those is absolute.

1) If you calculate hyperfocal distance the calculation will assume some CoC. What I have seen, critically sharp images would require a small CoC. In my experience I would say that setting the CoC equal to pixel pitch is needed for optimal sharpness.

2) If we use a to large CoC we get into a situation where almost nothing is really sharp. Infinity will not be in correct focus and neither will foreground be perfectly sharp.

If I assume a 6 micron sensor and use 0.006 mm CoC and f/11 the hyperfocal distance is 18.1m.

The loss of sharpness caused 0.006 mm CoC is significant, the figures below were made on a 4.7 micron pitch camera (corresponding to Nikon D800, approximately) with a 100/2.8 Minolta AF macro at 3 meter.
f/5.6, optimal focusf/11, 3 cm defocus at 3m (100 mm lens)
I would suggest that this difference by defocusing to achieve 0.006 mm CoC is probably more than between a top of the line prime and a decent zoom lens.

Similarly, stopping down to f/f11 also reduces sharpness significantly.
F/5.6, optimal focusf/11, optimal focusf/16, optimal focus


Now, I actually think that many of the differences in the above images would not be visible in say an A2 print, but it clearly shows that we loose detail even with small focusing error.

In my view, it may make little sense to invest in the very best lenses (say Zeiss 100/2 Macro Planar) or a HR Digaron-S 35/4 and than stop down f/16.

I would say that hyperfocal focusing is good if we want background and foreground be reasonably sharp, on the other hand if we have a subject that needs to be critically sharp hyperfocal focusing will not help with that, unless the subject happens to be at the hyperfocal distance. To achieve a short hyperfocal distance we need to stop down a lot, thereby limiting the resolution and contrast of the lens. If we stop down to f/11 - f/22 regularly we probably don't need a very good lens, as decent lenses usually are near diffraction limit at f/8.

Now, diffraction is benign to sharpening, so a lot of the fine detail contrast lost to diffraction can be regained with sharpening. Sharpening will increase noise, but that can be compensated by edge masking which will introduce artifacts of it's own.

Photography is always a compromise, and having a free lunch has always been an illusion.


As a side note, that HR Digaron-S 35/4 has a really mouth watering MTF curve by the way, specially considering that the lowest curve is 80 lp/mm instead of the usual 40 lp/mm.

Best regards
Erik
No I did not miss that. I have shot large format for years and know perfectly well about the use of hyperfocal focus setting.
Reportage involves a lot more than shooting acceptably sharp at infinity with the camera set to hyperfocal point.

« Last Edit: February 14, 2013, 11:49:30 AM by ErikKaffehr » Logged

scooby70
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« Reply #1 on: February 14, 2013, 08:06:01 PM »
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Intgeresting, and I'm not going to argue with or question any of that. An alternative view...

CoC gets a lot of time on the net but personally I think that for many it's a non issue or at best a red herring. For it to be crucial I suppose we'd need to start by knowing the image size and the viewing distance but these days many people never actually print the vast majority of their images and I bet rarely view at the image size and distance to make the numbers as pure and correct and vital as the internet would have us all believe that they are.

Personally I solve these issues by ignoring the whole CoC thing. To me it's an irrelevance or at least has only been an issue once in my life, when producing artwork for a stage play. When doing something like that it does matter (people sat at the back wont see small detail at the back of the stage) and the CoC could be into inches but it's never ever mattered to me in my decades of taking pictures with cameras. All I care about is that the hardware I use is capable of producing the detail that I want at an image size bigger than I'll ever print. My current gear does that. My 5Dc and lenses seem to be ok, they were designed to work together so that's no surprise, and my G1 is ok too even when ignoring the new sharp lenses designed for MFT (x2 crop, smaller CoC, sharper lenses, more image magnification etc) and going with old manual Rokkor and Zuiko lenses. I'm sure that throws the numbers out but it doesn't seem to matter, not to me anyway.

Anyway, front to back DoF. Unfortunately I think that it probably does as much damage as good. Personally I think that being selective with focus and letting it drift rather than aiming for a fudge from front to back produces a more natural looking result, but of course we do have to keep in mind some guesstimation of what the final image size and viewing circumstance will be... but I'm not going to get tied up in CoC calculations. Plus there are many who seem to strive for near to infinity "sharpness" when their scene doesn't need it. This is another thing that we see on the net IMVHO, people using setting gleamed from DoF tables that result in a DoF that exceeds their scene. Then there's the Merklinger method, easier to deal with if you don't want to either memorise or carry DoF tables.

Having got all that off my chest... I often focus on my main subject. I often shoot with shallow DoF. I often shoot with the intent that my image will look more or less natural and as the eye sees it with things in the distance being less defined than things in the foreground. I also tend to ignore diffraction, a boost to contrast and fiddling with levels and saturation post capture seems to hide the nasties well enough if and when I decide that small apertures are required and no one has ever said to me anything like "That shots ruined by diffraction" or "What's your CoC here?"

Sorry about all that Cheesy
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ErikKaffehr
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« Reply #2 on: February 15, 2013, 12:29:11 AM »
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Hi,

My take is essentially that I mostly try to focus on what is important, making sure it is sharp. I mostly shoot f/8 as it used to be a pretty optimal aperture on most lenses, but I adjust aperture to my needs. Stopping down if needed or opening up if I need shallow DoF.

What I think my examples show is that both stopping down to smallish apertures and basing focusing on some DoF tables probably will under utilize a lens or sensor. If we rutinely use smallish apertures like f/16 we can save the investment in exclusive lenses that are optimized for f/5.6 or even f/4.

On the other hand, stopping down is not an evil. Sometimes you need it.

The reason I started looking into this was in part that a reader was complaining that he had unsharp background on  an image where he focused on boat a couple 100 yards away on his Pentax 645D, probably using 150mm/f9.5. I felt that the explanation was probably that he had not the sufficient DoF, made some calculations and did some tests.

I have also noticed that on some pictures I have taken DoF was incredibly short, much less than expected. Now you can probably print a slightly unsharp image and everyone will be happy, but I feel that I should have done a better job.

The publisher of On Landscape, Tim Parkin, has written a very good article on the effects of diffraction and found that diffraction can to a great extent be compensated by sharpening.

My idea is essentially:

- Stopping down habitually to f/16 and F/22 does not make best use of lens and sensor.
- Depth of field tables and scales are calculated for smallish images
- It is good to know your gear, so you can make a well founded choice

Just as a side note, stopping down means that we either increase exposure time or reduce exposure. Reducing exposure increses noise. If we stop down hard we need to sharpen more and that sharpening also enhances noise. So we need more noise reduction.

What I try to do is to shoot several images some with optimal focus at medium aperture, perhaps one with optimal focus at f/22, I may even try to use focus bracket.

Best regards
Erik

 

Intgeresting, and I'm not going to argue with or question any of that. An alternative view...

CoC gets a lot of time on the net but personally I think that for many it's a non issue or at best a red herring. For it to be crucial I suppose we'd need to start by knowing the image size and the viewing distance but these days many people never actually print the vast majority of their images and I bet rarely view at the image size and distance to make the numbers as pure and correct and vital as the internet would have us all believe that they are.

Personally I solve these issues by ignoring the whole CoC thing. To me it's an irrelevance or at least has only been an issue once in my life, when producing artwork for a stage play. When doing something like that it does matter (people sat at the back wont see small detail at the back of the stage) and the CoC could be into inches but it's never ever mattered to me in my decades of taking pictures with cameras. All I care about is that the hardware I use is capable of producing the detail that I want at an image size bigger than I'll ever print. My current gear does that. My 5Dc and lenses seem to be ok, they were designed to work together so that's no surprise, and my G1 is ok too even when ignoring the new sharp lenses designed for MFT (x2 crop, smaller CoC, sharper lenses, more image magnification etc) and going with old manual Rokkor and Zuiko lenses. I'm sure that throws the numbers out but it doesn't seem to matter, not to me anyway.

Anyway, front to back DoF. Unfortunately I think that it probably does as much damage as good. Personally I think that being selective with focus and letting it drift rather than aiming for a fudge from front to back produces a more natural looking result, but of course we do have to keep in mind some guesstimation of what the final image size and viewing circumstance will be... but I'm not going to get tied up in CoC calculations. Plus there are many who seem to strive for near to infinity "sharpness" when their scene doesn't need it. This is another thing that we see on the net IMVHO, people using setting gleamed from DoF tables that result in a DoF that exceeds their scene. Then there's the Merklinger method, easier to deal with if you don't want to either memorise or carry DoF tables.

Having got all that off my chest... I often focus on my main subject. I often shoot with shallow DoF. I often shoot with the intent that my image will look more or less natural and as the eye sees it with things in the distance being less defined than things in the foreground. I also tend to ignore diffraction, a boost to contrast and fiddling with levels and saturation post capture seems to hide the nasties well enough if and when I decide that small apertures are required and no one has ever said to me anything like "That shots ruined by diffraction" or "What's your CoC here?"

Sorry about all that Cheesy
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scooby70
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« Reply #3 on: February 15, 2013, 07:13:54 AM »
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Erik,

"- Stopping down habitually to f/16 and F/22 does not make best use of lens and sensor.
- Depth of field tables and scales are calculated for smallish images
- It is good to know your gear, so you can make a well founded choice"


Your observations and thoughts are very interesting and I think you've hit a few nails on the head.

I don't need to go too deeply into this as I'm only an amateur and I only produce relatively small images for home and friends etc. but being slightly on the outside of all this and looking in it does seem to me that some take the CoC/DoF issue to extremes without knowing the basic things that should IMVHO be the starting point - intended image size, viewing circumstance, choice of kit - and the result you get.

One thing that impressed me in your original post was the concept of thinking for yourself what the CoC is. To me this is crucial and something that many on the net don't seem to grasp. We can all start at the beginning and construct our own DoF tables to get the result that we want but instead on many forums/threads what we see is endless links to the same on line DoF table/CoC sites and the impression that all this is carved in stone, must be adhered to and must not be deviated from.
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jrsforums
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« Reply #4 on: February 15, 2013, 02:14:20 PM »
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WHy don't DOF calculators allow you to plug in print size dimensions....and view distance, if necessary.

That way, if you were concern of printing at 20x24 or 40x60, etc. you could plug that in, and get your distances based on the new CoC.

Where CoC can be customized in a calculator, is it easy to calculate the change to CoC, then plug that value in?

John
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John
theguywitha645d
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« Reply #5 on: February 15, 2013, 11:08:05 PM »
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I have two issues with hyperfocal shooting and neither of those is absolute.

1) If you calculate hyperfocal distance the calculation will assume some CoC. What I have seen, critically sharp images would require a small CoC. In my experience I would say that setting the CoC equal to pixel pitch is needed for optimal sharpness.

2) If we use a to large CoC we get into a situation where almost nothing is really sharp. Infinity will not be in correct focus and neither will foreground be perfectly sharp.


But you are not really calculating DoF. You are trying to define the area of some kind of "perfect" sharpness.

You are also making a personal definition of sharpness, which is fine, but it only applies to you. You are also doing this under viewing condition that, for a normal viewer, would not be possible.

CoC is used to model average perception of "acceptable" sharpness in a print viewed under specific conditions. You can change the definition for your own personal reasons, but to say you have it right, but that the way it was done before is incorrect, it not very meaningful. The standard method is based on a lot more data than a personal opinion from one individual. And, no offense, but the more critical the person doing he evaluation, the more biased the result. This is not to say that it is not important to notice the fine points of your images, but it will not be perceived by the average viewer--it is possible to overestimate the significance.
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AlfSollund
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« Reply #6 on: February 16, 2013, 02:26:36 AM »
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Hi and thanks for the well grounded initial thoughts,

I must say I agree with the answers given. I have the DoF (CoC) in the back of my head, but would never use a DoF calculator or such. Instead I always focus on the main object. For some photos I try to reduce foreground or background unsharpnesses by using lens DoF scale as a starting point, but adds 1-2 stops to get the "correct" digital DoF.
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theguywitha645d
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« Reply #7 on: February 16, 2013, 12:30:26 PM »
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BTW, setting the CoC to pixel pitch simply is defining the depth of the plane of focus. That is not DoF.
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BartvanderWolf
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« Reply #8 on: February 16, 2013, 12:54:52 PM »
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BTW, setting the CoC to pixel pitch simply is defining the depth of the plane of focus. That is not DoF.

Hi,

But of course it is DoF, although personally I use 1.5x sensel pitch because when the CoC reaches that diameter I can see microcontrast starting to suffer. This is the perfect parameter to calculate Focus Stacking intervals, IOW DoF.

Cheers,
Bart
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theguywitha645d
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« Reply #9 on: February 17, 2013, 11:59:39 AM »
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Hi,

But of course it is DoF, although personally I use 1.5x sensel pitch because when the CoC reaches that diameter I can see microcontrast starting to suffer. This is the perfect parameter to calculate Focus Stacking intervals, IOW DoF.

Cheers,
Bart

Only for pixel peeping. That is not a real world viewing condition. Naturally, the problem comes up that given the same shooting conditions and optics, a given format will have different DoF just because the sensor is divided into more or less units. Naturally, that is false.

BTW, a CoC value does not change micro contrast. It actually does nothing to the image. CoC is simply modeling perception. You have a model for 100% monitor view and your personal taste.
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elf
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« Reply #10 on: February 17, 2013, 03:40:56 PM »
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Actually, it's quite useful for focus stacking, especially at the macro level. Using a COC between 1x and 1.5x pixel pitch allows you to set the focus points so there will be no focus banding in the final image. It will also allow you to calculate and shoot the minimum number of images to complete the frame. No small matter if you're combining thousands of images into one final image: http://photosynth.net/view.aspx?cid=39c28546-4202-4821-8b2d-4567cfceae6a
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ErikKaffehr
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« Reply #11 on: February 18, 2013, 12:18:28 AM »
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Hi,

CoC means circle of confusion. DoF tables are calculated based on maximal permitted CoC. That means that close and far DoF limits will be rendered with the CoC used as parameter, but somewhere in between there will be zone with much smaller CoC.

If we stop down to increase DoF we will introduce something similar to CoC because of diffraction, but that will affect the whole image uniformly.

The best way to see CoC is this (in my view):

On a very high performance system a point will be rendered as a point in the focal plane. If the image is defocused the point will not any longer be rendered as a point but as disk. The diameter of the disk is CoC.

Best regards
Erik


Only for pixel peeping. That is not a real world viewing condition. Naturally, the problem comes up that given the same shooting conditions and optics, a given format will have different DoF just because the sensor is divided into more or less units. Naturally, that is false.

BTW, a CoC value does not change micro contrast. It actually does nothing to the image. CoC is simply modeling perception. You have a model for 100% monitor view and your personal taste.
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AlfSollund
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« Reply #12 on: February 18, 2013, 02:32:32 AM »
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BTW, setting the CoC to pixel pitch simply is defining the depth of the plane of focus. That is not DoF.

There is no such thing as depth to a plane, thats why its a plane  Smiley. So except from seen from a quantum physics perspective there is no such ting as a "depth of the plane of focus", because if you move away from the plane of focus its not longer in focus.  The plane might be curved in 3D, but its still a plane.

Setting the CoC lets us decide what we perceive as DoF, that is what is sufficiently close to in focus to be called DoF. So setting the CoC is defining the DoF.
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hjulenissen
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« Reply #13 on: February 18, 2013, 03:07:42 AM »
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Is 1/width of the PSF more or less a Gaussian function of distance from the focal plane? Such that focus setting and aperture size are its two parameters?

When doing focus stacking (typically merging several shots of larger aperture, with smaller PSF in each focal plane), do you typically end up with a more rectangular function? (within some range, everything is really sharp, outside that range sharpness falls of more rapidly).

-h
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BartvanderWolf
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« Reply #14 on: February 18, 2013, 07:40:10 AM »
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CoC means circle of confusion. DoF tables are calculated based on maximal permitted CoC. That means that close and far DoF limits will be rendered with the CoC used as parameter, but somewhere in between there will be zone with much smaller CoC.

Hi Erik,

That is exactly what may be overlooked by some, the quality of focus within the DoF zone can be different between the exact plane of focus and the edges of the DoF zone, and more so when the CoC parameter is chosen to be relatively large. This effect is a bit less pronounced with Digital sensors compared to film, because the sensel size functions as an averaging filter and no finer detail than that can be recorded.

Here I've put in a chart what happens in practice, and how much can be salvaged with sharpening:



In the chart summary above, I measured the sharpness of a 100mm macro lens @ f/5.6 (on a 6.4 micron sensel pitch sensor array), focused at 3.34 metres focus distance, at 4 millimetre intervals. A focus rail was used, the camera and lens at manual focus, were moved forward with 4mm steps. According to the DOF formulae, a CoC of 0.010 mm would give a DoF range from 3.28 m - 3.40 m , or 6 mm on each side of the plane of best focus . That would indeed not require a different Capture sharpening to compensate for the contrast loss near the edges of the 12 mm narrow DoF zone.

However, choosing a different -more relaxed- CoC criterion can produce enough blur at the edges of the DoF zone to see a difference in microcontrast compared to the plane of optimal focus. For many subjects that would not create a big problem because the transition is gradual, but for critical applications such as Focus Stacking, the quality of focus must remain constant, not modulate between sharp and less sharp.

Quote
On a very high performance system a point will be rendered as a point in the focal plane. If the image is defocused the point will not any longer be rendered as a point but as disk. The diameter of the disk is CoC.

Correct, although the disc itself doesn't necessarily have a perfectly uniform resolution distribution. Residual lens aberrations/blur+diffraction combined with defocus and a sensel aperture, will affect the actual PSF shape. When the actual shape is analysed and used, then deconvolution sharpening may be able and restore some of the losses at the edge of the Dof zone, but that will over-correct the image at the focal plane which was already better focused.

So for very accurate work, it's best to minimize the CoC to something in the 1-2 sensel pitch range to achieve a uniform amount of blur/contrast loss throughout the DoF zone. It allows to optimize the quality of Capture sharpening. Hyperfocal approaches often require to use too large a CoC to achieve a reasonable DoF for landscapes, so focus stacking may also offer possibilities there (as long as the DoF zones connect well based on a small CoC).

Cheers,
Bart
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BartvanderWolf
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« Reply #15 on: February 18, 2013, 07:55:08 AM »
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Is 1/width of the PSF more or less a Gaussian function of distance from the focal plane? Such that focus setting and aperture size are its two parameters?

Hi,

It's a complex function, because of the mix of blur sources. I particularly like the following presentation which makes a distinction between defocus, and Airy disk (diffraction), and CoC (the combined response is more complex than a simple addition though):

http://www.tawbaware.com/maxlyons/calc.htm

Quote
When doing focus stacking (typically merging several shots of larger aperture, with smaller PSF in each focal plane), do you typically end up with a more rectangular function? (within some range, everything is really sharp, outside that range sharpness falls of more rapidly).

Yes, sort of. The sensel aperture will flatten the response around the plane of optimal focus to a minimum blur > 0, but the transition outside that region is gradual due to the mix of PSFs.

Cheers,
Bart
« Last Edit: February 18, 2013, 08:05:19 AM by BartvanderWolf » Logged
jgbowerman
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« Reply #16 on: February 18, 2013, 09:25:22 AM »
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I print large landscapes and generally, I'm going after "optimal" DoF. With the D800E, I'm shooting at f/8 the majority of the time, but if the foreground is getting close enough to the tripod, I'll bracket in some exposures at f/11 and decide in PP which which aperture is preferred. CoC is simply confusing to me, but I do respect the implications. My numbers with field testing proves correct what everyone says about diffraction softening and that is my advice to anyone attempting to grasp the issue of optimal DoF... do field testing. Seeing is believing. As to where one focuses and what one is after in terms of DoF, it is all a matter of style. There is no right or wrong. But I agree entirely with the premise that if one is choosing to routinely shoot smaller than f/11, it makes sense to spend less on the quality of the lens. At the same time, I don't understand why anyone would routinely shoot smaller than f/11 with the likes of a D800.

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BartvanderWolf
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« Reply #17 on: February 18, 2013, 09:53:22 AM »
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At the same time, I don't understand why anyone would routinely shoot smaller than f/11 with the likes of a D800.

Hi Greg,

It makes sense only if one doesn't print large output. That would raise a question as to why use a camera with high resolution to begin with, but that's a different subject ...

Your approach of shooting some additional images with narrower aperture makes sense, it may improve the resolution at the edges of the DoF zone more, than will be lost to diffraction (depends on distance, sensel pitch, and actual aperture). A blended/masked overlay may be possible in that case, without the need for more elaborate Focus Stacking of joining zones.

Cheers,
Bart
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