Equipment & Techniques > Cameras, Lenses and Shooting gear
DOF and Micro Four-Thirds Format
BJL:
--- Quote from: Ray ---As I showed in post #51, when the 100-400 is focussed at a fairly long distance, a 1 stop difference is sufficient to equalise DoF between the 40D and 5D (comparing 400mm at F11 (5D) with 250mm at F8 (40D). Other shots of the same scene taken on the same occasion show that a difference of 1 1/3 stops is equally satisfactory.
--- End quote ---
So far so good!: consistent with the traditional reckoning within the experimental margin of error (and the intended precision limits of the formulas).
--- Quote from: Ray ---However, when focussing at close distances with a wider lens, such DoF differences seem more noticeable. I've found that a 2 stop difference is required to equalise DoF between the 50D and 5D, comparing 24mm at F4 with 40mm at F8. Do you want to see the images?
--- End quote ---
The examples might be interesting. How much closer? The standard basic formulas break down as the magnification become significant, with texts typically limiting their utility to m<1/10 or even m<1/20, meaning subject distance should be at least about ten or twenty times the focal length. At closer range, lens extension means that both the effective focal length and effective aperture diameter shift, and more precise formulas are needed.
Another part of the approximation is that the formulas break down when the aperture is small so that the DOF is very large: large enough to be substantially greater behind the focal plane than in front of it. (The rule of thumb about 1/3 of DOF in front, 2/3 behind has no quantitative scientific basis: DOF is close to equally distributed fore and aft when the DOF is shallow enough, but grows more behind than in front as aperture size is reduced, eventually reaching infinity behind in the hyperfocal case.) The formulas are more oriented to choosing an aperture that ensures enough DOF in situations where DOF is potentially quite limited.
All this "fine print" is why one needs to understand the physics, mathematics, and applicability of the formulas, or otherwise use them cautiously, in the range of conditions for which they were intended. More detailed formulas are available in more advanced optics texts for the harder cases!
Ray:
--- Quote from: BJL ---The examples might be interesting. How much closer? The standard basic formulas break down as the magnification become significant, with texts typically limiting their utility to m<1/10 or even m<1/20, meaning subject distance should be at least about ten or twenty times the focal length. At closer range, lens extension means that both the effective focal length and effective aperture diameter shift, and more precise formulas are needed.
Another part of the approximation is that the formulas break down when the aperture is small so that the DOF is very large: large enough to be substantially greater behind the focal plane than in front of it. (The rule of thumb about 1/3 of DOF in front, 2/3 behind has no quantitative scientific basis: DOF is close to equally distributed fore and aft when the DOF is shallow enough, but grows more behind than in front as aperture size is reduced, eventually reaching infinity behind in the hyperfocal case.) The formulas are more oriented to choosing an aperture that ensures enough DOF in situations where DOF is potentially quite limited.
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BJL,
In the comparisons below, the focussing distances, although close, are significantly greater than 20x the longest focal length used, which was 40mm. The distance to the focus point would have been about 2 metres, or 50x the longest focal length. In order to be pedantic, I've downsampled the 50D image to the 5D size. However, whether the 50D image is downsampled or the 5D image is upsampled makes no difference to the conclusion. In these samples, the 50D still retains a slightly greater DoF edge, even with a 2 stop difference, although I'm prepared to accept that such miniscule differences fall within the margin of experimental error.
As I mentioned before, my purpose in doing these tests, shortly after receiving my brand new 50D, was to check out the noise, comparing it with my 5D at equal shutter speeds and equal DoF, which involved comparing 50D noise at ISO 100 with 5D noise at ISO 320, and ISO 200 with ISO 500 etc. It was as a consequence of such tests that I discovered that the 1 1/3rd stop difference for equal DoF did not seem to apply in those circumstances and that nothing less than a 2 stop difference would produce the desired results.
Here's the over all scene: [attachment=13510:The_scene.jpg]
The general area of focus at 100%: [attachment=13511:General_...us_point.jpg]
The specific area of focus at 200% (the pale mauve, artificial flower): [attachment=13512:precise_..._at_200_.jpg]
The closest points in the foreground: [attachment=13513:Nearest_points.jpg]
The background, centre right: [attachment=13514:Centre_right.jpg]
The background centre left: [attachment=13515:Centre_left.jpg]
You'll notice that I should have used ISO 400 with the 5D instead of ISO 640. However this apparent advantage to the 5D (regarding noise) is at least partly offset by the slightly faster shutter speed (1/15th as opposed to 1/13th for the 50D) and is not as great as it might at first seem. I was simply trying to get a good ETTR. Checking out DXOmark figures later, I discover that at ISO 100, the sensitivies of both cameras is equal at ISO 93. However, at the nominal ISO of 400 the 5D is actually ISO 357, according to DXOmark, but it should be 4x93=372, so it does appear to be slightly understated. Nevertheless, according to EV compensation adjustments in ACR, the 5D has about a 1/3rd stop ISO advantage in these comparisons, that is, it requires -1 EV adjustment whereas the 50D image requires -0.67 EV adjustment.
The noise comparison images below were converted with zero black, zero contrast, linear tone curve, no sharpening, and no noise reduction of either luminance or color. Both images have been lightened to the same degree in 'levels'. The 5D comes out quite well. I was surprised that at ISO 640 the 5D would be on a par with the 50D at ISO 100. One should also bear in mind that the slight ISO advantage I've given the 5D does not translate to more photons per unit area of sensor, but a slightly greater analog amplification of the signal.
Noise comparison: [attachment=13516:Shadows_lightened.jpg]
--- Quote ---All this "fine print" is why one needs to understand the physics, mathematics, and applicability of the formulas, or otherwise use them cautiously, in the range of conditions for which they were intended. More detailed formulas are available in more advanced optics texts for the harder cases!
--- End quote ---
The problem here is that, in the field where capturing the moment is often of the essense, we don't have time to engage in complex calculations. Outside of studio conditions, it's also very difficult to get precise measurements of distance to subject, so DoF calculations, however precise theoretically, can be no more precise in practice than the precision with which can one measure distances in the field.
BJL:
Ray,
thanks; I will look at the samples more carefully when I have the time. For now, on high precisision DOF formulas:
--- Quote from: Ray ---The problem here is that, in the field where capturing the moment is often of the essense, we don't have time to engage in complex calculations. Outside of studio conditions, it's also very difficult to get precise measurements of distance to subject, so DoF calculations, however precise theoretically, can be no more precise in practice than the precision with which can one measure distances in the field.
--- End quote ---
Agreed. My point about understanding the science behind the formulas is only to avoid misusing them, or expecting more precision of them that is justified. The main use for DOF formulas as far as I can tell is to ensure enough DOF in situations when one is facing trade-offs between DOF, shutter speed, diffraction and such. So probably all that is needed in the field is a suitably good approximation, used with sufficient caution. Very careful macro photographers (including the extreme macro case of microscopy) needs more care and have use for more precise formulas.
Then again, with a sufficiently large, sharp, accurate, and bright VF image, one could stop down and verify DOF directly. But no SLR optical VF is up to that task. Live View with zoom might be the coming solution.
But the original question of this thread is much easier, and does not need details of subject distance beyond avoiding the extremes of macro range and hyperfocal distance. Because that question was only about the DOF shift between different choices of format/focal length, under conditions of equal (if unknown!) subject distance and equal display size. I have not yet seen evidence that "equal DOF at equal effective aperture" is insufficiently accurate, but I will keep looking.
P. S. Modest downsampling of Bayer CFA raw data to a RGB format like JPEG or TIFF with about half or more the pixel count might have very little effect on resolution, due to the greater information in one three color RGB pixel than in one single color Bayer CFA pixel.
I would be interested in a careful comparison of 6MP and 12MP conversions from a 12MP raw file, given that
6MP RGB has 6 millions values for each of G, R and B,
12MP Bayer CFA raw has 6 million G values, 3 million each for R and B.
So it could even be that 6MP RGB can hold more detail in some situations than a 12MP Bayer CFA raw provides.
Ray:
--- Quote from: BJL ---But the original question of this thread is much easier, and does not need details of subject distance beyond avoiding the extremes of macro range and hyperfocal distance. Because that question was only about the DOF shift between different choices of format/focal length, under conditions of equal (if unknown!) subject distance and equal display size. I have not yet seen evidence that "equal DOF at equal effective aperture" is insufficiently accurate, but I will keep looking.
--- End quote ---
BJL,
I agree completely that taking the ratio of the diagonals of the different size formats as a multiplier for determining appropriate focal length and f stop #, in order to equalise FoV and DoF, is a good starting point. However, when the formats being compared also have a different aspect ratio, as the micro 4/3rds and Canon cropped formats have, then the formula becomes slightly more 'rule-of-thumb'. When one of the formats has a significantly different pixel count, then the formula becomes even more 'rule-of-thumb'. If one of the formats has the benefit of a better lens, then that may also change the rule-of-thumbness to a greater or lesser degree in either direction.
When the subject focussed upon is fairly close to the camera (as in studio conditions rather than macro conditions), that may also affect the rule-of-thumbness to a greater extent than all the other factors mentioned above (in my view; yet to be confirmed.)
As you know, I've often battled with MFDB users on this forum about their slack approach to these matters when comparing their expensive equipment with 35mm format. I was also surprised recently when Michael compared the G10 with a P45 without also taking these matters seriously. He got the focal lengths approximately right, but was miles out with the DoF. As a consequence, when experienced photographers were invited to compare his A3+ size prints from both cameras, they were (eventually) able to identify the P45 because of its shallower DoF.
In the comparison shots of the woods scene, Michael used the G10 at F3.5 and the P45 at F11. The diagonal of the G10 sensor is 9.5mm (info available from Dpreview), and the diagonal of the P45 is 60mm. The multiplier for DoF equivalence is therefore 60/9.5=6.3. F3.5x6.3=F22. Michael should have used F22 with his P45 in this comparison. By using F11, he gave the game away.
I'm surprised that no-one seems to have pursued this issue and repeated the comparison using images with equal DoF. We are left to speculate on what the results might have been. There's no doubt that the P45 image would be less sharp at F22 (than at F11). How much 'less sharp' and at what print size is the question. Would these experienced photographers whom Michael invited to view his A3+ prints have then confused the greater sharpness of the G10 with the P45? We don't know until someone repeats the comparison.
Out of interest, I downloaded Michael's G10 and P45 images and searched for the out-of-focus areas. They were easy to find. Consider the crop comparisons below, at two different magnifications. In the second image, the G10 crop has been upsampled to the P45 size.
[attachment=13563:G10_v_P45.jpg] [attachment=13564:G10_upsa...d_to_P45.jpg]
By the way, I understand it is not true that the origins of the expression 'rule of thumb' go back to a barbaric medieval law in England, which permitted a man to discipline his wife with a stick no thicker than his thumb. No record of such a law can be found. Just thought I'd mention it .
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