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Messages - epsiloneri

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286
Lenses / Re: Delivery of the Canon 1.4X III HAS STARTED
« on: January 14, 2011, 05:36:29 PM »
(and much better IQ than cropping your FF image).

Because of the higher pixel density? Apart from pixel density, I would expect the cropped FF image to be equivalent to the crop-camera.

As I wrote above, a crop can be considered close to the "perfect" TC. The exception is dynamic range, since the sensor well depth scales with surface area. So in some exceptionally contrasty scenes, I can see a FF+TC combo being better suited.

287
Lenses / Re: Delivery of the Canon 1.4X III HAS STARTED
« on: January 07, 2011, 06:00:51 PM »
Coming to think of it, in some cases there is a way to reproduce the image of a perfect TC, and that is to crop an image on a sensor with a pixel density that is greater than the resolution provided by the lens, that is, an image that is oversampled. Applying a TC under those conditions would be useless for that particular sensor (cropping would produce better results, except perhaps for dynamic range), but it would be a way to determine the effects of a TC for sensors with much larger (non-outresolving) pixels.

I hadn't heard of multiphoton fluorescence microscopy before, sounds pretty advanced. Unfortunately I expect the PSF to be much less well behaved for camera lenses (in particular zoom lenses), with weird shapes that change across the field. Having objects at different distances would also complicate matters. Otherwise it would be cool to try out deconvolution to squeeze out some more detail. I know the police force use it to sharpen up motion blurred license plates, but those PSFs are then simpler. In one case a few years ago they used deconvolution (sort of) to reconstruct a strongly distorted picture of a child molester (submitted by the villain himself). Very impressive work.

288
Lenses / Re: Delivery of the Canon 1.4X III HAS STARTED
« on: January 07, 2011, 05:51:44 PM »
Let me offer a real-world example from TDP's ISO 12233 crops.  Hopefully we can agree that the 2x extender will have a greater negative impact on sharpness than the 1.4x extender.

Not really :) Of course, I agree that the 1.4x sample looks sharper than the 2.0x sample from the links you provided. But these are 100% crops, and to achieve the same projected size on the sensor, the test chart has been moved 1.4x further away for the 2.0x TC (compared to the 1.4x TC). Therefore, I cannot tell from these tests how the angular resolution is affected by the TCs. It's not surprising that you see more detail if you move in closer!

A proper way would be to shoot these test charts at the same distance and then determine the angular resolution, e.g. by how much detail can be seen in the test charts. I browsed around a bit but couldn't find anything better than this experiment. Perhaps you know of similar TC tests where the angular resolution is measured - I would find that helpful.

Yes, going from 1.4x to 2.0x on the 70-200/4L (and moving the target further away) decreased the IQ more than doing the same on the 200/2L - but that's probably because the 200/2L is intrinsically sharper, and does not reflect how much of the deterioration is due to the TCs.

289
Lenses / Re: Delivery of the Canon 1.4X III HAS STARTED
« on: January 07, 2011, 06:43:00 AM »
Aha! Now I see why you think the way you do. But that doesn't seem like a proper model for how an extender works. I'm sorry I wasn't very clear. I think the following sounds much more reasonable:

Let's say "perfect" extender merely magnifies the central field given by the lens (may it be 1.4x or 2.0x), without otherwise modifying it. If we focus on resolution, then it should not change the resolution in terms of smallest resolved detail. That is, the point-spread function (PSF) would not change in angular size; on the sensor, on the other hand, the PSF would be magnified by the given magnification factor (1.4 or 2.0), so in terms of resolution per pixel, the PSF would be larger by the same factor (and thus the image appear less sharp, if the PSF is resolved by the pixel density of the sensor, of course).

Now what happens if the extender is not perfect, but adds its own fuziness? You seem to imply that the magnification factor of the PSF will change, but that doesn't seem reasonable. More realistically, the PSF of the extender will be convolved with the PSF of the lens; if the PSFs can be crudely approximated by Gaussians, the lens PSF has the full width at half maximum (FWHM) of A, and the extender a PSF FWHM B, then the combined PSF would have a FWHM C = sqrt(A^2 + B^2). (in reality the PSFs are far more complicated than simple Gaussians for non-diffraction limited optics, but this gives an approximate scaling behaviour)

Back to your example. Let's say the PSF of the 70-200/2.8L II (FWHM=2) is twice that of the 200/2L (FWHM=1), and that the extender MkII (FWHM=1) is twice that of MkIII (FWHM=0.5). Then the improvement from using the 2.0x MkIII instead of the MkII on the 200/2L would be

sqrt((2*1)^2+1^2)/sqrt((2*1)^2+0.5^2) = 1.0847, or an improvement of 8.5%.

For the 70-200/2.8L II, on the other hand, the improvement would be much smaller:

sqrt((2*2)^2+1^2)/sqrt((2*2)^2+0.5^2) = 1.0445, or an improvement of 4.5%.

Simply put, the greater the imperfections of the lens, the more they mask the small imperfections added by the extender. That is why I think it's a better to use as good lens as possible when testing for any potential differences in resolution generated by the extenders.

I don't know how the PSFs of the extenders compare to the PSFs of the lenses, but I would expect their contribution to be relatively small. That is to say, there is not much room for improvement, as the "lens flaws" themselves dominate the detoriation of resolution per pixel with magnification.

Finally, there is an easy way to measure PSFs. Just point the lens to a point source, and the resulting image will be the PSF. Unless you have a diffraction-limited telescope of aperture > 5 cm, bright stars at night make excellent point sources. Just take care to expose for short enough time or use a tracker to compensate for the Earth's rotation, otherwise stars will be motion blurred. A properly dimmed laser-pointer from a distance could also work, I guess.

290
Lenses / Re: Delivery of the Canon 1.4X III HAS STARTED
« on: January 06, 2011, 05:57:34 AM »
Extenders enlarge the flaws of any lens, but the 200mm f2.0 is THE perfect lens, so no surprise that we cant see a big difference, but i bet that on for example the new 70-200, the improvement will be more obvious.

This sounds backwards to me. Surely extenders are not able to correct lens flaws? For a "perfect" lens I would expect it to be easier to differentiate between the II and III extenders, as any flaws added by the extenders would be more visible. On a less perfect lens, however, any extender flaws might be masked by the magnified lens flaws.


291
EOS Bodies / Re: 5D Mark III [CR1]
« on: December 27, 2010, 05:10:29 AM »
I'm also looking forward to see if the dynamic range is improved. I'd trade 6-figure ISO for more DR any day of the week.

I totally agree. I find the biggest problem for the image quality from a 5D2 to be its low-iso pattern noise (banding) that severely limits the dynamic range. 50D also has this problem (it's from the same generation). The 7D is way better in this respect, and I expect a 5D3 to be as well. See http://photography-on-the.net/forum/showthread.php?t=750731 for a detailed discussion.

Unfortunately, these rumoured specs do not look very credible... at all. I'm expecting a 5D3 no earlier than by the end of 2011. Hopefully together with a 24-70/2.8L IS  :P


292
EOS Bodies / Re: Finding the optimal Micro Adjustment
« on: December 26, 2010, 06:16:37 PM »
There are several parameters that can affect the focus, and specifically the autofocus, of a lens. A few I can think of:

* Focal length of lens (for zoom lenses)
* Distance to focus target
* Aperture of lens
* Shape of focus target
* Colours (spectrum) of focus target
* Relative (3D) velocity to focus target
* Temperature of lens
* Temperature, humidity and pressure of air

As testing for MA is a bit time consuming, I have only really checked the first two parameters, and not very thoroughly. For one lens (and two camera bodies), I found that changing the distance to the target from 25x to 40x the focal length did not affect how the AF behaved as a function of MA. The set focal length on zoom lenses, however, did change the optimal MA. The contrast as a function of MA does not qualitatively look different from the prime lenses, but it changes with set focal length. I give an example in the attached figure, which shows the contrast as a function of MA for the EF 24-105/4L lens on 7D, set at 24mm (green) and at 105mm (blue).



293
EOS Bodies / Finding the optimal Micro Adjustment
« on: December 22, 2010, 05:11:48 PM »
Is focus micro adjustment a must-have feature? The absence of the feature in the recent 60D has sparked some debate. Triggered by a recent problematic lens, I've been interested in measuring the focus performance of my lenses and the possibility to micro adjust (MA) focus offsets of the auto focus (AF). The usefulness of the MA feature is dependent on the ability to accurately (and consistently) establish any potential focus offset, otherwise there is not much point in tweaking the MA. E.g., if the variance of the AF is greater than the MA, then there is very little benefit with MA.

I've experimented a bit with various methods, and the trouble I have with most of them is that they require visual inspection of images very similar to each other, which I found hard to do. Instead, I opted for the quantitative approach of measuring the contrast in an image as a proxy for its sharpness, with the assumption that more out-of-focus images will be less contrasty. That is the way "live view AF" works, which I've found to give very accurate and consistent results in good light (albeit a bit slow).

To obtain consistent results, I used a focus target properly aligned with the camera. I think any contrasty image in principle can be used as a focus target, I used one I found from Keith Cooper's MA page (below is a crop of the full image)


I found it good because it provides contrast structures on many angular scales. I displayed that image on my computer TFT screen and used the screen as a focus target. To align the the camera with the screen I taped a CD flat onto the screen, to use it as a mirror, and then checked that the center of the camera lens was seen centered in the viewfinder of the camera. This ensures that the focus target is orthogonal to the optical axis, which is again orthogonal to the image plane; that is, the focus target and the image plane are parallel. During the test shots I naturally removed the CD.

The camera was placed at a distance 25 times the focal length (i.e. at 2.5 meter for a 100mm focal length lens). I also once tried at 40 times the focal length and got consistent results, albeit with more scatter.

I took a reference image using live view contrast AF, and then obtained 4 images per MA setting using phase AF, and varied MA from -20 to +20 in steps of 5, i.e. 1+36 images in total per lens (camera, and focal length combination). I de-focused twice to infinity and twice to the near limit per MA setting, before AF.

To quantify the contrast of an image, I looked at the subimage of the focus target only (it doesn't cover the whole field) and computed the standard deviation of the intensity distribution (that is, of the grayscale image; I used MATLAB to do this, you can probably do it less conveniently in photoshop. There is also the free MATLAB clone octave if you're GNU inclined). The same procedure was repeated for both a 7D and a 5D2.

An attached figure shows an example of contrast measurements as a function of MA for a Canon EF 100/2.8 USM macro lens mounted on a 5D2. The error bars are derived from the 4 measurements per MA. A parabola is fit to the 5 best points, and in this case the optimal MA was found to be 8.9 +/- 1.2. I repeated this measurement for a number of lenses, here are the results:

Lens                                                         7D                        5D2
================================================
Canon EF 14/2.8L II                             -4.3 +/- 1.0         2.9 +/- 1.0
Canon EF 15/2.8 fisheye                     10.3 +/- 0.7           19 +/- 3
Canon EF 24/1.4L II                           -11.8 +/- 0.6        -6.7 +/- 0.8
Sigma EX 50/1.4 HSM                            3.2 +/- 0.5        13.8 +/- 0.7
Canon EF 85/1.8 USM                         -19.2 +/- 2.8        -7.2 +/- 1.2
Canon EF 100/2.0 USM                       -14.3 +/- 0.5         0.4 +/- 0.8
Canon EF 100/2.8 USM macro             -0.8 +/- 0.5          8.9 +/- 1.2

Canon EF-S 10-22/3.5-4.5 USM IS @ 10mm -11.0 +/- 0.4
Canon EF-S 10-22/3.5-4.5 USM IS @ 22mm -10.5 +/- 0.5

Canon EF-S 17-55/2.8 USM IS @ 17mm         2.9 +/- 0.5
Canon EF-S 17-55/2.8 USM IS @ 55mm      -11.9 +/- 0.5

Canon EF 24-105/4.0L USM IS @ 24mm      -1.0 +/- 0.4       6.1 +/- 0.7
Canon EF 24-105/4.0L USM IS @ 105mm    -7.4 +/- 0.5       0.5 +/- 0.9

Canon EF 70-200/2.8L USM IS @ 70mm       1.2 +/- 0.5      -2.7 +/- 1.2
Canon EF 70-200/2.8L USM IS @ 200mm    -2.6 +/- 1.8       8.2 +/- 1.5
================================================

There appear to be a fair range of optimal MA, more than I would have guessed. I will have to verify that these results hold up in the field, but that will take a while.

One question I was interested in is whether there is a constant MA offset between the 7D and 5D2. Looking at the primes only, I plot the 7D MAs versus the 5D2 MAs in the figure attached. There appear to be a clear linear correlation; I find the best fit to be that MA_5D2 = MA_7D + 9.7, with an error in the mean of 1.3 and a scatter of 3.2. The 7D seems more well centered, despite the 5D2 having been recently sent to CPS for recalibration.

The zoom lenses show a more complicated behaviour, with MA changing with focal length.

In summary, I think measuring the contrast in an image as a function of micro adjustment, with the aim to find an optimal adjustment, is an attractive alternative to visual inspection. I realise this method is not for everyone, as it involves more image analysis and data manipulation than most photographers are probably comfortable with (or used to).

294
EOS Bodies / Re: The 60D Needs AF Microadjustment
« on: December 15, 2010, 05:40:34 PM »
Thanks for your reply!

From a pragmatic standpoint, it's irrelevant - I will just continue to do an AFMA for each body+lens combo I have...

Except that it would simplify things a lot if it worked that way... you wouldn't have to measure each lens/body combination. Too bad it doesn't!

Quote
The other issue with that method (and with focusing on any image as a target) is the alignment of the target with the camera's sensor.  Ideally, you want the target to be completely flat and perfectly parallel to the sensor.  That's really a main function of the LensAlign tool - the sighting gates that allow you to align the target to the camera.  If not for that, I could just prop a ruler against a wall and have saved a few bucks. 

Actually, I have figured out a very simple way of achieving perfect alignment. Just take a flat mirror and fix it flat against the screen. Then make sure you see the reflection of the camera centered through the viewfinder, where you want it to be centered on the screen. This will ensure that the screen is perpendicular to the optical axis. I don't have a convenient flat mirror at home, so I use a CD that I tape onto the screen (it's reflective and flat enough). I haven't actually started to measure MAs yet, just wanted to poll your experience before I do.

I have found that live view manual focus works very well for me (on the 7D), I don't seem to have the problems you do. Best focus seems well defined, though I sometimes find that the focus ring could be more precise. Very seldom do I find the focus even slightly off in manually focused images. Much more common is slight motion blur (even with tripod). Contrast AF using live view usually also works as well (on well-lit objects). Both methods are always as accurate or better than AF-sensor AF (albeit much slower).

Non-integer up-scaling likely removes some contrast in the image, but the optimal focus probably still produces the sharpest image, even in the up-scaled version. Usually I find even the best resolution to be quite coarser than the pixel resolution of the sensor. This may in part be due to the anti-aliasing filter on the sensor.

Quote
If you AF on a point then switch to 10x Live View and see if it's sharp, that's static viewing; if you then rack the focus to check if the AF hit the spot, how do you know you're ending up at the same point? 

Hmmm, I don't think I understand... I imagine the procedure as follows:

1) AF focus on a target, say a properly aligned focus target. The camera registers what it thinks is the best focus.
2) Without moving the camera or the focus tagert, go to 10x live view and manually focus to what you think is the best focus, push a button or something for the camera to register what your preferred focus is.
3) The camera makes use of info from 1 and 2 to compute MA.

No need to go back to what the AF thought best... or am I misunderstanding? This procedure of course assumes that you are better than the AF at focusing (under static conditions), but in my case I've found that to always be the case. Alternatively, one could let the camera itself compare the AF between the AF sensors and the live view contrast AF, and compute MA under the assumption that live view AF is more accurate. That would be even simpler, and according to my experience, live view contrast AF is nearly always accurate (but slower). Contrast AF is not affected by front/back focus issues, since it uses the actual detected image for AF, so it would be perfect to correct for AF sensor MA. I can imagine setting up the camera on a tripod and align it to a focus target, select "calibrate AF" from a camera menu, and then let the camera automatically cycle through 10 AF measurement cycles (say), computing the best MA. Why not, Canon?

295
EOS Bodies / Re: 1Ds Mklll - and what comes next?
« on: December 15, 2010, 04:39:33 PM »
I think medium format cameras are becoming dinosaurs.

I know you asked for macfly's opinion, I'd just like to comment on this. I think medium format cameras will always find their audience, and as the sensors become better and cheaper, even more so than today! The number of pixels is not everything, cramming 50 mp into FF will not make it MF equivalent. The MF main advantage is not in their number of pixels, but in their larger sensor areas that are easier to produce excellent lenses for. I envision that in a not too distant future the sensor will be an insignificant cost of a camera, while lens prices will dominate the budget. In such a situation it is much cheaper to produce a MF camera with lens than a FF camera with lens that produces an equivalent quality picture. Today that's not the case, as MF sensors are very pricey. It's the same reason you need a really outstanding lens to produce as nice images on an APS-C as a mediocre lens is able on a FF, only amplified in the MF case.


296
EOS Bodies / Re: The 60D Needs AF Microadjustment
« on: December 15, 2010, 02:25:33 PM »
Ok, so I have a few questions for neuroanatomist:

1) You now have a 5D2 in addition to your 7D. Have you retested your lenses with you new body? Do you find a constant offset from your adjustments for 7D? In principle I think you should be able to, and that would mean it would be sufficient to only measure one lens, say a sensitive one like your 85/1.2L, and then apply the difference between the MA for that lens and 7D and MA for 5D2 to all other lenses to have the calibration to 5D2 without having to remeasure all lenses. I don't know if this works in practice, however, as it assumes the MA scale is linear which it may not be (I don't know). It would therefore be interesting to read your experience!

2) Have you tried to use an interference image on a computer screen as a focus target instead of the LenAlign Pro? I mean as detailed in this post by Keith Cooper.

3) Have you attempted to check whether there is a distance dependence to the optimal MA for your lenses? I've seen recommended distances for measurements to be 25-50 times the focal length, but I'm curious how well the adjustment works for other distances as well.

Something that would simplify MA would be to have a "calibration wizard" where you 1) autofocused on a target and 2) set the optimal focus manually (using live view). The camera should then be able to compute the corresponding MA itself. (This could even be generalised to MA as a function of distance.) This would ideally decrease the time required for a lens calibration to a few minutes instead of an hour.

Another thing I would love to see is focus bracketing. It was available on my Canon G2, but I haven't seen it since.


297
In mjardeen's defense, the OP contradicts itself.  He does put out the suggestion of the 16-35, but also says he want's something wider than his 15-85.

I don't find it contradicting to compare the IQ of two lenses of different focal length ranges. MTF, colour aberration, and vignetting, are all examples of things you can compare. You can even compare the IQ of a 14mm/2.8 lens to a 400mm/2.8, even if there of course are very few situations where you can substitute one for the other.


298
Everything I've read says the Tokina 11-16mm f2.8 is the best APS-C ultra-wide lens out there.

"Best" in some areas, but not all. Looking at the review you linked to, the Canon seems obviously sharper in the corners already from f/3.5, while the Tokina becomes similarly sharp only at f/5.6 (and is very soft at f/2.8 ). The MM of the Tokina is very low, and the chromatic aberrations not very well controlled. I agree that the light gathering power and lower price makes the Tokina a very attractive option, but not best in all areas.

299
Lenses / Re: 24-70 mm f/2.8
« on: December 12, 2010, 08:17:01 PM »
I know this is also probably not what you want to hear, but I don't think either is a very good match for your 7D. It of course depends on what you are looking for in a new lens, but if it is light gathering power, I would suggest the EF-S 17-55/2.8 IS instead. Both lighter and cheaper than the EF 24-70/2.8L, but just as good, if not better, IQ. Also a more useful focal range, IMHO. 24mm is not really wide enough on APS-C (try it on your current lens). No weather sealing though, if that's a big deal for you.

300
I don't know how the EF-S 10-22mm compares to the EF 16-35L on APS-C in the focal range where they overlap, but overall, the 10-22mm is very well regarded and I don't think you would disappointed by it. There are plenty of professional online reviews you can consult if you're uncertain.

If you're anyway planning to stay with APS-C for some time, I think it's a bad idea to insist on FF lenses and not take advantage of the EF-S format at the wide end (lighter, cheaper). My advice is to sell your EF-S lenses when/if you make the APS-C to FF switch. You will not lose that much money, and the money will be well spent. There is no current Canon FF lens on the APS-C able to compete with the EF-S 10-22mm at the wide end.

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