January 27, 2015, 09:36:43 AM

Show Posts

This section allows you to view all posts made by this member. Note that you can only see posts made in areas you currently have access to.


Messages - epsiloneri

Pages: 1 ... 18 19 [20] 21 22 ... 26
286
Lenses / Re: UV filter for 70-200mm f/2.8 IS II?
« on: April 08, 2011, 04:49:32 PM »
Wow, pretty flares! Thanks for checking.

287
Lenses / Re: Canon Extender EF 2x III review - at the bronx zoo
« on: April 04, 2011, 01:56:34 AM »
I have a question. Since I have the 7D and soon the 70-200mm f/2.8L IS USM II, would buying a 2x Extender actually give me an effective focal length of 640mm?

Yes, you are correct. 7D with a 2x teleconverter and 200mm lens gives a field of view equivalent to a 640mm lens on a full frame camera (e.g. 5D). This combination would likely outresolve the lens, however, resulting in a somewhat softer image at the scale of pixels.

288
Lenses / Re: Focal length limits
« on: April 03, 2011, 03:50:26 PM »
Depends on the application, of course. Amateur astronomers regularly use 500mm-2000mm lenses (telescopes) at typically f/4-f/11. No AF, rarely IS, and never hand held - a rigid mount, sometimes motorised, is required.

There is an upper useful focal length limit, but it depends on the pixel pitch of the detector. The limit is set by the resolution deterioration from the atmosphere and depends greatly on local conditions. For astronomical applications at good sites (e.g. nothing extreme like Antarctica or remote locations in the Andes) you are usually limited to a resolution of ~ 1 arcsecond (1/3600 degree), which means that there is little point in having smaller pixels than 0.5 arcsec/pixel. For terrestrial daytime applications this limit is generally much higher, as the air close to the ground is denser and more affected by turbulence than if you look straight up.

For a 7D with 232 pix/mm, a resolution limit of 0.5 arcsec/pix corresponds to a focal length of 1780 mm, while for a 5D2 you have 156 pix/mm and thus can increase the focal length to 2640 mm before getting resolution-limited by the atmosphere. The opening aperture also needs to be larger than ~10 cm to not limit the image resolution by diffraction.

The record-braking 120 MP APS-C sensor Canon showed off a while ago has 455 pix/mm, which would result in a maximal useful focal length of 900 mm. It would put very severe constraints on tolerances and the quality of the optics, however. In principle one could go even further with even higher MP detectors, but tolerances and dynamic range per pixel would likely be limiting factors. With "perfect" detectors the resolution would be completely determined by the optics. Still, I don't think it would be practically possible to get 1 arcsec resolution on anything shorter than 200mm, because already that would require an f/2 lens (to not be diffraction limited).

Looking at distant objects on the ground limits useful focal lengths to much shorter than the above estimates.


289
EOS Bodies / Re: Odds and Ends
« on: March 07, 2011, 05:44:39 PM »
Note that the portion of the picture on the left has 16-35mm on the lens

Just to clarify... the article is about recommended lenses for portraits, and goes all the way from the wide EF 16-35mm f/2.8L II USM to EF 135mm f/2L USM. The magazine layout is two columns, and the part of the 16-35mm lens seen in the image to the left belongs to the other column describing that lens. A picture of the EF 24-70mm f/2.8L USM lens (sans "II") is shown below the text displayed in the image.

290
EOS Bodies / Re: Odds and Ends
« on: March 06, 2011, 12:56:56 PM »
The same English issue of the CPN magazine doesn't have the "II" typo.

291
Lenses / Re: What ones best?
« on: March 06, 2011, 02:58:49 AM »
Apart from focal length the 24mm's biggest advantage over the 17mm is the ability to accept filters e.g. polarizers for landscape

Isn't 24mm a bit wide for a polarizer? Just asking. ND filters can be useful for flowing water etc, other than that (and polarizer/protector) I don't know what filters are used for.

Regarding the 17mm/4.0L, it is very wide on FF. Also, the depth of focus is not very narrow, making the "tilt"-part difficult to fully exploit. The "shift" parts works as advertised, but you can do almost as well in postprocessing (at the price of some resolution), so I'm not terribly excited over that feature. Mostly good for indoors architecture, where space is limited.

In all, I think the TS-E 24mm/3.5L II is more interesting (less wide and faster -> more evident "tilt" effects), and wide enough for landscapes. The "II" ability to freely rotate the relative angle between tilt and shift directions is a nice feature over "I" (apart from the increase in resolution). For macro, I would recommend the TS-E 90/2.8 instead (+extension tube).

If you have an APS-C camera, however, 24mm might be too long for landscapes. Thus, it depends a bit on your camera choice in addition to your shooting preferences (like scaleusa said).

292
Lenses / Re: Your lenses wishlist for 2011 - RESULTS
« on: February 21, 2011, 02:14:42 AM »
However, I think it's worth bearing in mind that both these stats largely reflect the readership of this forum, particularly those who would respond to posts and have the knowledge and experience to come up with a wishlist. In fact, I wonder what percentage of DSLR owners regularly read up on forums?

Let's make a poll and check: How many of you DSLR owners are reading this forum?

293
Lenses / Re: EF-S L Lenses
« on: February 12, 2011, 03:26:41 PM »
Fair question since we now have the weather-sealed 7D. But, nobody knows.

294
Lenses / Re: Canon 5200mm f14
« on: February 06, 2011, 09:34:10 PM »
Quote
Canon 5200mm F14 Prime Lens is the only ultra-telephoto lens in the world capable of taking photographs of objects 18 to 32 miles away (30km to 52kms away).

Pffft... almost any lens can easily be used to take photographs of objects up to 1e19 miles away... I've done it with my 15mm fisheye (the Andromeda galaxy). The biggest lens I've attached to a Canon dslr is a 11000mm/f11 telescope (but it wasn't autofocusing and not manufactured by Canon).

295
EOS Bodies / Re: Different camera numbers internationally - why?
« on: January 17, 2011, 04:48:39 AM »
I can see that a good name in one market might not go down well in another, but numbers? Why bother?

I have no answer to you except that I prefer the number scheme (500D, 550D etc) and find the "naming" scheme (Kiss X3/X4, T1i/T2i etc) more confusing, in particular since the number scheme fits better with the model numbers of the higher end models, which Canon uses everywhere (5D MkII is 5D MkII even in Japan).

296
Lenses / Re: Delivery of the Canon 1.4X III HAS STARTED
« on: January 16, 2011, 03:43:26 AM »
When you have some time, give THIS THREAD a read.

Thanks for the link. I read the thread and was happy to see a healthy technical discussion with actual test shots to verify theory. Now I better understand your statement that cropped-camera IQ is better than cropped FF camera IQ - you were referring to the specific examples of 5D2 and 7D.

297
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.

298
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.

299
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.

300
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.

Pages: 1 ... 18 19 [20] 21 22 ... 26