I am looking for information on the use of wide angle lenses and extension tubes for close up photography. The images will be used to capture surface contours using PhotoModeler software. This requires the use of a prime lens. I purchased a Canon 60mm macro lens, but the focal length is a bit long and the surface data contained a lot of noise as the light rays are nearly parallel. The target area containing reference marks, which should fill the image area of my 7D, is about 1.5" square and the subjects, located in the center, are generally smaller than 0.75". I'm looking at the Canon 20 mm lens (or alternatively a 24 mm lens) for use with extension tubes. Does anyone have experience with these lenses in close up photography? If so, what depth of field could I expect.
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The majority of macro lenses are short telephotos, such as the 60mm you bought. The reason for this is to give adequate working distance between the front of the lens and the subject. About the shortest focal length, modern, true macro lens I'm aware of is the Tokina 35mm, which I'm not sure is still in production. This short a lens would put you very close to your subject.
I have used a Canon EF 20/2.8 for macro work, but only rarely. The intent and reason for using it was to maintain some of the distant background detail (most tele macros will completely blur down backgrounds to near nothingness). I used it with a 12mm extension tube and was shooting California poppies on a full frame (film) camera. At somewhat less than full 1:1 magnification, petals from the flowers were touching the front element of the lens. Any more extension, and I'd have been trying to focus on things inside the lens! Way too close for most macro work.
I am not entirely certain what you are trying to do. It sounds as if you are seeking a lot of depth of field, but at a macro scale. Let me throw out a couple ideas for your consideration.
First, adequate depth of field is always a struggle with macro photography. For that reason, many macro lenses have incredibly small apertures... f32 and even f45 or smaller are available on some. You have to weigh using such small apertures against getting a steady shot, shutter speed and ISO of course, but also have to consider diffraction, which is an effect where there is increasing loss of fine detail when using smaller apertures. I suggest you read this tutorial for more info about diffraction, before resorting to really small apertures in search of more depth of field. I would do a test with my particular lens and camera combo, to see the effects of diffraction and help make choices about what aperture to use for different purposes. I can tell you that on the 18MP 7D, diffraction begins to occur at f7.1 (assuming an 8x10 print final product), but isn't very noticeable until f16, is fairly strong at f22 or smaller.
One way to increase depth of field beyond what's possible optically is with "focus stacking". This is a technique combining the sharpest portions of multiple shots in post-production, each shot using a slightly different point of focus. There are softwares to help with this, read more about the technique and see some examples at this website, of one of the companies offering a software for the purpose. It means making a series of focus-bracketed shots, which might also call for a camera hack such as Magic Lantern (I haven't done and don't know if it's possible for 7D), in order to be able to make the focus-bracketed images.
EDIT: It's been a while since I poked around the Helicon Focus website linked above. I see they now also offer "Helicon Remote" software to make focus bracketed shots. Haven't used it myself, but it sounds interesting. It does appear to require shooting tethered to a computer.
Another tool you might find useful is a Tilt-Shift lens. Canon's 45mm and 90mm TS-E lenses are close focusing on their own, though not quite as high magnification as it sounds you are seeking (fitting a 1.5 inch area onto a 7D's APS-C sensor is about 0.60X magnification, on the longer axis of the image). On crop cameras like 7D, I often use the 45mm for small product/tabletop studio photography. It can be fitted with a short extension tube to increase magnification, as well.
The tilt and shift movements of the lens open up new possibilities. Tilt allows you to align the plane of focus with the subject, to some extent. Shift can be used to dodge around problems with reflections or shadows. Both movements can be used to correct perspective distortions (i.e., "keystoning"). The current 45mm comes with the tilt and shift functions aligned 90 degrees out of phase with each other, but that can be modified by Canon service center (Note: Newer TS-E 17mm and 24mm have an additional plane of rotation built in that allows the user to change the movement alignment on-site, with a simple turn of the lens... the 45mm and 90mm TS-E lenses have not been updated with this feature yet). Best guess, you might find the tilt useful to align plane of focus with your subject, but still may need to use it in combination with focus stacking and/or smaller apertures.
Another feature of true macro lenses is that most are "flat field" designs. This helps insure better image sharpness edge to edge at close focusing distances. Many non-macro lenses are not this type and will tend to be soft in the corners when forced to focus extremely close. This might be useful at times, for some photographs. But just as easily could be a problem in other instances.
Here is one of the images I made with Canon 20mm lens and 12mm extension tube....
I don't recall the aperture used (film camera, so no EXIF to refer to), but I used the wide lens specifically to keep the poppies in the background enough in focus to remain recognizable. With any longer focal length, they became more meaningless blobs.
DOF is always tough with macro. The longer the focal length and higher the magnification, the shallower DOF gets. Here - using a 180mm macro lens - you can see the plane of focus is only a few millimeters deep (sorry, I don't recall what aperture was used, this also was shot with a film camera)...
The next image is an example of a product shot made with the 45mm TS-E lens (on a crop sensor camera). This object is about four or five inches tall, so the magnification isn't as great as what you are seeking and no extension tubes were needed. But the lens could be made to focus closer and higher magnification with extension tubes (Note: I believe Canon states the TS-E 45mm lens is limited to 12mm of extension, but I suspect that's on a full frame camera only. I imagine on a cropper like 7D you could use 20mm or possibly 25mm of extension... but haven't tried it personally).
Finally, the image below illustrates what I mentioned above about how a non-macro, non-flat field lens can go soft in the corners (and show some vignetting), when forced to focus close by adding extension tubes. In this case, I wanted the effect and so deliberately used a 20 or 25mm extension on a Canon EF 50/1.4 lens. I used a fairly large aperture as well...
In the end, I think the lens you have has a lot of potential, if you use a small aperture and possibly a technique such as focus stacking. The EF-S 60mm is an excellent macro lens in many respects. A tilt-shift lens might be helpful, too. I don't think you'll get the results you want with a wide angle lens forced to focus extremely closely with extension tubes.
I hope this helps.
I am not sure exactly what you wnat to accomplish either.
You know there is no free lunch. You give up something to get something.
You are aware that the Canon 20mm can focus to less than 10" as is? With certain extension tubes, it can focus to an object nearly touching the glass itself. Is this what you want? Remember this will lessen your DOF, there is no free lunch!
This may not be anything you have seen before. For details go to the PhotoModeler website. I'm taking this technology beyond anything they show on their web site. The images here were taken with my 7D and an EF-S 60mm macro lens. All photos are taken using B&W settings because that's what the software converts them to anyway.
The software I'm using is PhotoModeler Scanner. Here's a typical photo with the 'Smart Points' shown.
The cavity I am trying to measure is in the center of the image. Surrounding it are coded targets printed on a piece of glossy paper and taped to the surface of the tool.
I pushed the wrong button and sent this before I was ready....
The surrounding targets are printed on 5mm centers and are automatically recognized by the software. A photo is taken every 15°, resulting in a set of 24 photos. These targets allow the software to determine the camera position for each image. If you look carefully at the cavity, you will see many white dots. These are the 'Smart Points.' I do not know what the software detects on the surface, but it identifies 'unique' surface characteristics and tracks them between adjacent images.
When you tell the software to compute the locations of all those points, it gets interesting. I'm running an Intel i7 CPU running at over 3 GHz with 16 Gb of memoryon a 64 bit system. It still takes 3 to 4 minutes for it to run all the calculations. That's a LOT of computation. The output is a point cloud that when viewed from one end looks like this:
The reference targets are not shown. The problem is the noise. Looking at the horizontal top surface, all the points should be in the same plane, but there are many points that are above and below the bulk of the data. To date I have been using a non-linear regression program to fit surfaces to these points, and then stripping out any that were in excess of 6 sigma from the calculated plane and then running the regression again on the remaining points. It works, but it is not very satisfactory.
The source of the problem is the focal length of the lens. Although the lens is listed as 60mm focal length, when you get up close it increases to about 100mm. The focal length is calculated during a lens calibration procedure that determines distortion and some other characteristics in addition to focal length. As focal length increases, the light rays creating the image become more parallel. This results in a much higher potential for error, thus my interest in using a wide angle prime lens for marco photography.
The test article is placed in a light tent for uniform lighting. The camera and test article are sitting directly on a concrete slab to minimize vibration. The camera is operated directly from a PC with the mirror locked up to inimize vibration. The diaphram was set to f8, what seemed a good comprimise between depth of field and light diffraction.
In the final steps of the process, the point colud and fitted surfaces are imported into a CAD program. A solid model of the cavity is created from which I can take accurate measurements. To date I have demonstrated an accuracy of .002". This degree of accuracy is comparable to a laser scanner, but is pretty poor considering the accuracy of todays metrology instruments. I'd like to get the accuracy down below .001" for these small objects. Improved accuracy would benifit the regulatory officers in my customer's facilities when they present their case to the FDA.
I've been considering a tilt/shift lens. I've never used one, except once 50 years ago on a friends 8" x 10" plate camera. It would help a lot with the depth of field problem, but would have to be calibrated for each session. That isn't a terrible price to pay as it only takes a few minutes to shoot a set of targets at a few angles.
I looked into the tilt/shift lens in more detail. On the surface it looked really promising, but calibrating it is a problem. To calibrate a lens it must photograph a target pattern in 'normal' position and while rolled left and right. with the lens tilted the focus goes way out when tilted. Bummer.
I recently discovered the Zeiss Distagon 25mm / f2.8 lens that can focus to very short distances. It isn't very good with full frame sensors, but it's exactly the lens I need for use with my 7D. Now I'm looking for an adaptor to mount this lens (intended for a Nikon mount) on my Canon EOS. It must be mechanically as solid as possible - I don't much care about automatic focus, exposure, etc. because I set all these manually and they stay exactly the same throughout the photo session.
This one seems a good design
but there are plenty of others from $200 to $15 including0
Does anyone have raally good experience with a Nikon F mount adaptor to a Canon EOS body?
Nikon sensors have much more electronic noise than my Canon so I prefer not to purchase a Nikon body.