Macro and DOF

MainFragger · August 2, 2004

My question is this,

If you have reached the maximum F-Stop number (in this case 8) you could,
Will stacking on a ND Filter with a polarized filter gain you any more DOF?

Assuming this is the maximum F-Stop number I can get to, is there anything or anyway you could think of that I could get the relative F-Stop higher? Say even as high as 11?

MainFragger

pathfinder · August 2, 2004

MainFragger wrote:

My question is this,

If you have reached the maximum F-Stop number (in this case 8) you could,
Will stacking on a ND Filter with a polarized filter gain you any more DOF?

Assuming this is the maximum F-Stop number I can get to, is there anything or anyway you could think of that I could get the relative F-Stop higher? Say even as high as 11?

MainFragger

The f- stop is a ratio of iris aperature diameter versus focal length, thus it is limited by the iris mechanism itself. ND filters or polarizers will decrease the amount of light getting to the film plane but will not increase Depth of Field at all. Sorry. A Pin Hole filter in the center of the lens might tho......

wxwax · August 2, 2004

Pathfinder said it all. Short of a pinhole in front of your lens, which would effectively give you a smaller aperature (and a lot of vignetting

) there's not much you can do.

Just curious, why do you need more than f8? The consumer cameras I've used don't have a problem giving lots of DOF. Their weakness is creating a narrow DOF with wide open aperatures.

MainFragger · August 2, 2004

DOF too small

wxwax wrote:

Pathfinder said it all. Short of a pinhole in front of your lens, which would effectively give you a smaller aperature (and a lot of vignetting ) there's not much you can do.

Just curious, why do you need more than f8? The consumer cameras I've used don't have a problem giving lots of DOF. Their weakness is creating a narrow DOF with wide open aperatures.

The problem is that I often can't even seem to get a whole bug in focus as I get closer to it. If DOF is generally a few mm, I'd love to find a way to get it to at least be enough to get a whole bug into focus.

Now I did some checking on lens theory, and from what I can tell, changing the aperture value blocks off unwanted light, revealing detail of a depth that would otherwise be too washed out with light to get the full perspective of. So I was thinking that if I could find ways of reducing light in a way that effectively was equivalent to an f-stop. That it might further still increase the DOF.

MainFragger

zero-zero · August 2, 2004

Frag, only two things will affect DOF:

Aperture: smaller means more DOF
Magnification (how large your subject projects onto the imager, be it film or chip): smaller means again more DOF

Aperture is controlled via diaphragm (f-stop). Magnification is controlled via focal length and distance to subject. This way, and given a fixed f-stop, you get exactly the same DOF using, say, a 50 mm at 6 feet than a 100 mm at 12 feet than a 200 mm at 24 feet, and so on and on and on, because you're keeping your aperture and your magnification constant.

So when you need to increase DOF, but you are already stopped all the way down, you have only one choice: decrease magnification. How do you do that? either use a wider lens, or back off.

wxwax · August 2, 2004

MainFragger wrote:

The problem is that I often can't even seem to get a whole bug in focus as I get closer to it. If DOF is generally a few mm, I'd love to find a way to get it to at least be enough to get a whole bug into focus.

Now I did some checking on lens theory, and from what I can tell, changing the aperture value blocks off unwanted light, revealing detail of a depth that would otherwise be too washed out with light to get the full perspective of. So I was thinking that if I could find ways of reducing light in a way that effectively was equivalent to an f-stop. That it might further still increase the DOF.

MainFragger

Ah, you're trying to shoot a macro. For that, you are truly limited by your camera's capabilities.

mercphoto · August 2, 2004

Dof

MainFragger wrote:

Now I did some checking on lens theory, and from what I can tell, changing the aperture value blocks off unwanted light, revealing detail of a depth that would otherwise be too washed out with light to get the full perspective of. So I was thinking that if I could find ways of reducing light in a way that effectively was equivalent to an f-stop. That it might further still increase the DOF.

No. Using this logic you could use a cloudy day, or nighttime shooting, to increase DOF, but you can't. The reason a smaller aperture creates larger DOF is because the smaller the aperture the more you approach a pinhole camera. Its not that the aperture lets in less light (it does, but) its that it lets light hit a smaller area of the lens. And the center, most flat portion of the lens is what will have the largest DOF. Your filters let it less light, but that light still hits the entire lens surface. I don't know how well I can explain it in text. I really need a diagram.

gubbs · August 2, 2004

mercphoto wrote:

No. Using this logic you could use a cloudy day, or nighttime shooting, to increase DOF, but you can't. The reason a smaller aperture creates larger DOF is because the smaller the aperture the more you approach a pinhole camera. Its not that the aperture lets in less light (it does, but) its that it lets light hit a smaller area of the lens. And the center, most flat portion of the lens is what will have the largest DOF. Your filters let it less light, but that light still hits the entire lens surface. I don't know how well I can explain it in text. I really need a diagram.

There's a lot of confusing diagrams out there, but I thought this page explains things fairly well. The examples links don't all work though

pathfinder · August 2, 2004

MainFragger wrote:

The problem is that I often can't even seem to get a whole bug in focus as I get closer to it. If DOF is generally a few mm, I'd love to find a way to get it to at least be enough to get a whole bug into focus.

MainFragger

A scanning electron microscope will get the whole bug in focus, but may be more expensive than you had budgeted for this project.

Welcome to the world of macro photography. Most macro lenses stop down to f22 or f27 for greater depth of field. The closer the distance to the subject, the less depth of field - which is why the 180-200mm macro lenses are becoming more popular I suspect.

You can see what can be done with a 180mm Macro lens and a DSLR here http://www.dgrin.com/showthread.php?t=2188 and beginning here and on the next few pages as well http://www.dgrin.com/showthread.php?t=882&page=6&pp=10

MainFragger · August 4, 2004

Oh well...

gubbs wrote:

There's a lot of confusing diagrams out there, but I thought this page explains things fairly well. The examples links don't all work though

To an extent I understand this, but it still leaves me confused. It almost sounds to me like circles of confusion are where light from various points of the subject spread out and overlap at the film plane after passing through the aperature. As such, when you close the aperature, you get less overlap in light, which gives a truer representation of what is being viewed. The extra depth sounds like it is more of a side effect than anything. This also trashes my hope with the filters, because in essence, it would dim the light coming through the camera, which would weaken the DOF if it has any affect on it all. If I were to define depth of field, it would be- "An area of print that becomes more detailed as overlapping and repetitive (detail bluring) light is removed from the intended effective image area of the film plane."
Technically, if you slow down your film speed a hair at that point, shouldn't the extra light coming off of the intended subject intensify the remaining light, and if not taken too far, maybe a little more detail? I wonder what the best shutter speed to use is when you have found the best DOF per every foot you get from the camera.
There is a street with a tunnel of trees near me. I might go there one night and map out how well 2.8-8 can do at each shutter speed and each zoom level between 1-12. Now I just gotta figure out a way to do that without getting run over.

MainFragger

cmr164 · August 4, 2004

Wow... I do not know where to begin.

MainFragger wrote:

To an extent I understand this, but it still leaves me confused. It almost sounds to me like circles of confusion are where light from various points of the subject spread out and overlap at the film plane after passing through the aperature. As such, when you close the aperature, you get less overlap in light, which gives a truer representation of what is being viewed.

No no and no. First try to understand the a lens can only focus one precise distance at at time. The Circle of Confusion is the size of the circle blur made by taking a slice out of the cone the light rays make from a single perfect point that is in front of or behind that precise in focus distance.

The Depth of Field is a human concept that takes an arbitrary sized circle of confusion and says that anything smaller is in focus under the premise that we do not notice it. For any given lens the increase in size of the cone is effected by 3 things, the distance from the precise focus point, the diameter of the lens and the focal length. Stopping down a lens reduces the effective diameter of the cone at each point.

The extra depth sounds like it is more of a side effect than anything. This also trashes my hope with the filters, because in essence, it would dim the light coming through the camera, which would weaken the DOF if it has any affect on it all. If I were to define depth of field, it would be- "An area of print that becomes more detailed as overlapping and repetitive (detail bluring) light is removed from the intended effective image area of the film plane."

You really need to get a grasp on the concepts involved in CoC, cones of light, etc. Remember when you were a kid using a lens to start a fire, how the image of the sun changed size as you moved in front or behind the precise focus?

Technically, if you slow down your film speed a hair at that point, shouldn't the extra light coming off of the intended subject intensify the remaining light, and if not taken too far, maybe a little more detail?

I am sorry but the above is so far out that I do not know how to respond.

I wonder what the best shutter speed to use is when you have found the best DOF per every foot you get from the camera.

Only aperture, focal length, and distance from the precise focus distance have any effect on DoF. The shutter speed does not do anything at all except combine with the film speed to allow various apertures.

There is a street with a tunnel of trees near me. I might go there one night and map out how well 2.8-8 can do at each shutter speed and each zoom level between 1-12. Now I just gotta figure out a way to do that without getting run over.

MainFragger

The digicam with its 2.8 -> 4.0 -> 5.6 -> 8.0 is not going to offer a lot of change in DoF.

mercphoto · August 4, 2004

Dof

MainFragger wrote:

As such, when you close the aperature, you get less overlap in light, which gives a truer representation of what is being viewed. The extra depth sounds like it is more of a side effect than anything.

It is a side effect. A lens has only one true focus point. Everything else is out of focus. The DOF is an illusion -- it is points that are so close to being in focus that our eyes can't tell the difference.

This also trashes my hope with the filters, because in essence, it would dim the light coming through the camera, which would weaken the DOF if it has any affect on it all...
Technically, if you slow down your film speed a hair at that point, shouldn't the extra light coming off of the intended subject intensify the remaining light, and if not taken too far, maybe a little more detail? I wonder what the best shutter speed to use is when you have found the best DOF per every foot you get from the camera.

You seem fixated on this idea that letting in less light somehow changes the depth of field. Whatever material you read originally that got you onto this thinking was either misinterprted or was poorly written. Filters, shutter speeds and film speeds have zero effect on DOF. Zero. An aperture changes the DOF not because it is letting in less light per se, but because of exactly which light it is rejecting and which light it is allowing to pass -- it is selective.

DoctorIt · August 4, 2004

hey frag - go to a library right now and pick up a physics/optics book right now before you give charles a coronary.

a suggestion:
http://www.amazon.com/exec/obidos/ASIN/0486659577/qid=1091651735/sr=ka-2/ref=pd_ka_2/102-5101898-3380114

lots of equations, but if you get past those, there fundamentals are there. I use this one for research frequently.

Macro and DOF

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