I brought up the problem once before of combining multiple images to control DOF and asked if a macro lens would work better than extension tubes. The consensus was that no matter what lens you use it changes the size of the image on the sensor as you change the focus. As an old time LF guy I have now recalled that the way you avoid that was to move the back of the camera, not the front. Of course there in no way to do that with digital, is there? I vaguely remember reading about one camera that moved the sensor to focus rather than the lens but I don't recall what it was. Does anyone here know anything about that?
A rail moves the whole camera Mike. The LF technique I refer to moves only the back of the camera to change focus. The change in the size of the image on the film/sensor is caused by moving the lens.
Although I mainly shoot landscapes and occasional wildlife my understanding is that the benefit of using extension tubes and / or a dedicated macro lens is to reduce the minimum focusing distance of a lens so that you can get closer to the subject and achieve higher magnification. I am not aware of any advantage offered by dedicated macro lenses or extension tubes related to depth of field.
With large format cameras you have the ability to change the angle of of the lens relative to the film plane which increases the apparent depth of field in the image. This is accomplished by changing the angle of the lens mount or the back of the camera or both.
Both Canon and Nikon offer Tilt/Shift (Canon) and Perspective Control (Nikon) lenses which allow you to tilt the lens relative to the film plane. This offers similar benefits to large format lens movements. However, the longest focal length offered is 90mm from Canon and 85mm from Nikon. Generally with Macro photography a longer focal length lens is preferred both to increase magnification and to narrow the angle of view to remove distracting backgrounds. Additionally, the Tilt / Shift lenses do not offer the same magnification as a macro lens - the Canon 90 mm tilt shift offers magnification of 1:3.44 while the Nikon offers 1:2 (according to the specs on B&H). By comparison, macro lenses generally offer magnification of 1:1. Depending on your subject matter this may or may not be significant. Tilt / Shift lenses are also manual focus and tend to be quite expensive.
You might get better response from posting this topic in either the flora forum or possibly the wildlife forums as there are likely to be more photographers who shoot macro in those forums and they might be able to give you more specific advice.
Hi Brian, I understand all that about macro lenses and shift lenses. What I was working with was focus stacking, multiple exposures focused on different points which are then combined to control the DOF. The problem I encountered in trying to manually combine exposures was that each shift in focus changes the image size on the sensor so they don't always match up. Photoshop or Helicon Focus both use an image warping routine to make the layers match so if you aren't trying to do it manually, you have no problem.
Thinking back to my large format days though I recalled that if the lens remains locked in place but you move the back to change focus (you need a monorail to do this, not a field camera) the image does not change size. All that changes is the plane of focus. I also recalled seeing something somewhere about a digital camera that focused by moving the sensor instead of the lens however I can't recall who made it and I wondered if anyone here knew. That was my question, not how to shoot macro.
Jim, I can't answer your question about a movable sensor, but from a fundamentals of optics point of view, I don't see moving the sensor doing what I think your asking. Consider a simple lens, when it's focused at infinity, the rays coming in from infinity are parallel and they are brought to a very distinct focal point at distance equal to the focal length of the lens (f). If the sensor is in front or behind that point, things at infinity are out of focus. If you move the sensor further away (closer won't work for a convex lens) you can bring into focus a plane that is closer to the lens, but now infinity is out of focus. Alternatively, if you started with a focal point at 2f in front of the lens then the rear focus is also at 2f and you can now change what is sharp by moving the sensor in either direction, (2f + delta is sharp when the focal plane is at 2f - delta and visa versa) but in each case you've changed the magnification and in my mind, it's the always changing magnification that creates problems in stacking (especially macros). If this explanation is fuzzy send me email and I can try to draw some sketches that may help.
I have never read about a movable sensor but that would certainly seem to solve some of the problems we encounter in focus stacking. For now I just use Helicon and turn the focusing ring to generate the images for the stack. Works great. There is the occasional image, particularly when dealing in the 1:1 range, that has some ghosting for the reasons you are describing but it is easy to clean up in PS.
It seems to me that assuming there is some benefit in keeping the lens in the same place and moving the sesor plane to change focus, then a macro bellows would be the only way to do it. I used to have a set that allowed both the front or the back section to move on it's rack and pinion. Such a setup with the lens section fixed should do the job, and prove whether the theory was correct or not. Seems logical enough to me.
With my internally focussing Canon 100 2.8 macro I just use a rail and move the whole kit and kaboodle back and forth. With the MP-E65 I suspect you could work out a way of keeping the appropriate part of the lens in the right place, but the maths is too complicated for me!!!
As I said, it is a technique used with large format cameras that have backs which can be moved independently (as with a monorail). I don't know the math either, just the practice. If you had a bellows with similar capability (body can be moved forward and back without moving the lens in relation to the subject) it should work the same. Because PS and Helicon Focus both compensate for the changes in image size it is academic anyway but my old brain sometimes ponders these things.
Discussion
8 repliesWith large format cameras you have the ability to change the angle of of the lens relative to the film plane which increases the apparent depth of field in the image. This is accomplished by changing the angle of the lens mount or the back of the camera or both.
Both Canon and Nikon offer Tilt/Shift (Canon) and Perspective Control (Nikon) lenses which allow you to tilt the lens relative to the film plane. This offers similar benefits to large format lens movements. However, the longest focal length offered is 90mm from Canon and 85mm from Nikon. Generally with Macro photography a longer focal length lens is preferred both to increase magnification and to narrow the angle of view to remove distracting backgrounds. Additionally, the Tilt / Shift lenses do not offer the same magnification as a macro lens - the Canon 90 mm tilt shift offers magnification of 1:3.44 while the Nikon offers 1:2 (according to the specs on B&H). By comparison, macro lenses generally offer magnification of 1:1. Depending on your subject matter this may or may not be significant. Tilt / Shift lenses are also manual focus and tend to be quite expensive.
You might get better response from posting this topic in either the flora forum or possibly the wildlife forums as there are likely to be more photographers who shoot macro in those forums and they might be able to give you more specific advice.
Thinking back to my large format days though I recalled that if the lens remains locked in place but you move the back to change focus (you need a monorail to do this, not a field camera) the image does not change size. All that changes is the plane of focus. I also recalled seeing something somewhere about a digital camera that focused by moving the sensor instead of the lens however I can't recall who made it and I wondered if anyone here knew. That was my question, not how to shoot macro.
With my internally focussing Canon 100 2.8 macro I just use a rail and move the whole kit and kaboodle back and forth. With the MP-E65 I suspect you could work out a way of keeping the appropriate part of the lens in the right place, but the maths is too complicated for me!!!