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I've had my trusty Canon 200 mm f 2.8 USM for over 20 years and it has served me well. It's fine for terrestrial work and back in the day of 9 micron pixels seemed to work fine for CCD as well. Now using it with either a DSLR or ASI 2600 MC Pro and the smaller pixels the stars are a mess (see below 120% zoom crop of 600 s exposure - it isn't a guiding error same thing in a 1 s exposure). The image is well focused using a mask and I get the same effect regardless of F stop (I used a ring to stop down and no improvement). This lens has been round the world multiple times so I'm OK if it's time to let it go just interested to see if there is a fix. Thanks in advance. John  |
0.90
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| Looks like some spikey eccentricity and some chromatic aberration. Is this the middle, or a corner? Which of the two cameras mentioned took this pic? With the ZWO camera, how to you attach the lens to the camera? With the (budget) ZWO adapter? |
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3.31
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It's from the center this is from the ZWO camera with the ZWO adapter - things look the same when it's attached to the Canon M200 but again there is an adapter there. Here is a 300 % zoom of the same area taken from PI after WBPP gives a better view of the issue. This is 12 by 600 s frames....  Thanks |
0.90
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| What was the ambient temperature? Maybe pinching of the lens elements from the cold? |
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3.31
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I don't think so, I first saw this in the summer and it wasn't that cold last night. Thanks |
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7.90
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| Maybe an issue with the coatings, as in coating is failing. You have 2 PSF for each source one fainter than the other, apparently. The only way to get a grip on this is to study an out-of-focus image, inside and outside focus to try and figure out what the issue is. Most likely isn' t fixable and you'd need a new lens. |
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Thanks Andrea I've had my monies worth and then some so not a surprise. John |
1.20
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| How do you hold the lens to the tripod? I have seen similar effect when tightening too much the collar ring around the lens. I even see it in my 127mm refractor when the rigs are too tight. |
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3.31
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| The main attachment is via the camera body the lens only has light three point rings on it...... |
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1.20
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| The lens is too heavy to be held by the camera body, so the EOS adapter may be flexing |
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0.90
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I support the lens with a lens clamp that is made for it and the camera with the ZWO support. Of course you have to shim one somewhat accurately, since it is now over-constrained, and you don't want to induce loads tightening it down: https://www.flickr.com/photos/125134422@N06/52547747780/in/dateposted-public/ Steve |
5.61
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Have not used mine in a while but have never seen such a thing. Here is one taken with that lens and an old SBIG STT 8300. I did have a lens support as well as using an F5 aperture stop instead of the iris. I would never use it at 2.8 - too much edge distortion. You might make a simple aperture stop and try that to see what effect it has. https://www.astrobin.com/9pz60c/ |
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| Hi John. If your lens has been round the world multiple times, maybe it's due for a service by Canon. I've seen them do it. It's quite a complex procedure. They project and elaborate test chart down a corridor and adjust the various lenses from there. All the best Mark |
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3.31
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| Mark, I'd thought of that but I can't find anyone near me (Detroit) who offers the service and the Canon website is of absolutely no help!! Would be interested how you find someone who can do such a service. |
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5.61
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| One of my imaging buddies has sent lenses to Canon for repair and was quite happy. Not sure how to go about it but it is Canon itself, not a 3rd party that does the repairs. |
2.11
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John Noble: I have sent a lens to canon for repair and I went through the vendor I purchased the lens from and in my case it was Adorama. This was a while ago so things may have changed. Your canon warranty card should have information however on how to get in touch directly with canon. I've bought and sold stuff to KEH and I know they do lens repair, I had them rebuild a Canonet film camera ages ago for me. The only Canon lens that is a flat field with practically zero color distortion is the 100mm f2.8L Macro. But you should be getting better performance from your 200mm, something clearly is not right. Hope this helps. . https://www.keh.com |
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0.90
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That lens has a very flat field for a 4/3 sensor. At least my copy does. These are images taken with that lens and 294MM camera: https://www.astrobin.com/cibuek/ https://www.astrobin.com/r9f979/ https://www.astrobin.com/wz7vsn/ I am happy to share the raw data. For narrowband I stop down to f/3.7 For L-RGB I stop down to f/4.5 In both cases, I use thread-in step-down rings |
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Steven Bellavia: Oh sure I agree with you I was speaking for full frame. If you have a 4/3 or APS-C you miss/avoid a lot of it. I am not disparaging the lens really because all camera lenses behave this way particularly at different stops and the 200mm L is a particularly good lens that is mostly flat, I used to own it. People have in their minds that all lenses should be flat field and crispy sharp across all focal lengths to the very edge of the FOV and at all f-stops and that is not really the number 1 and 2 consideration of every lens design. We all know the famous quote by Henri Cartier-Bresson (1908-2004) - “Sharpness is a bourgeois concept”. There are some Leica lenses that are highly sought after because of their distortion. But I digress. Check this out and you'll see what I mean. I put the 50mmL in there for comparison. It's not that the 50 is a bad lens, it is a beautiful lens the draws your attention to the center of the frame with it's vignetting distortion so to speak. https://www.imaging-resource.com/lenses/canon/ef-200mm-f2.8l-ii-usm/review/ https://www.imaging-resource.com/lenses/canon/ef-100mm-f2.8l-macro-is-usm/review/ https://www.imaging-resource.com/lenses/canon/ef-50mm-f1.2l-usm/review/ The Zeiss Jenna Biotar has a particular type of distortion that makes it highly desirable for some photographers. It is a circular bokeh distortion. http://forum.mflenses.com/userpix/20152/big_5_carl_zeiss_jena_biotar_75mm_f1_5_exakta_nex715_1.jpg I've worn this out by now probably. |
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I have a twenty year old Nikon lens that showed similar star artifacts that on close examination of the optic under uniform white lighting revealed mold on one of the internal lens surfaces, normally the death of lenses. I'd taken it back and forth to the tropics for many years. But the affected element was near enough to the rear of the assembly that I was able to remove it and clean it with hydrogen peroxide to restore its performance. There happened to be a YouTube video about how to take apart and clean that particular lens. CS Jim |
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7.90
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| Lens for AP should work fully open with resonable un-aberrated (distortion doesn't play a part here as can be corrected during processing so we're talking chromatic aberrations, coma and astigmatism (3rd and 5th order)) field across a APS-C sensor. If they don't there is no point in using them, I believe (just get a slower lens, chepaer and lighter too). |
2.71
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These lenses don't work sharply at full aperture, but if you use a stop down ring, that removes issues about the lens not being sharp. I have the M series version of this lens and need it stopped down to f4. At the centre of the field though, lack of sharpness is not due to lens aberration. When you say that you focus using a mask, is it manual focussing, or using a stepper motor or similar? I couldn't focus mine adequately with 3.75um pixels (QHY163M) so had to make a belt/pulley based stepper system to automate focussing - the critical focus zone at f4 is 40 microns for 3.75um pixels, but nearer 160 microns for 9um pixels. If an inspection of the lens looks OK, I would suspect focus issues first of all. (Will also note that the lens adaptor for the QHY is extremely loose as well, given the weight of the camera and lens - the extra lens support is must, but I also had to shim the lens mount - I used drink cans cut open and cut into shape accordingly to do so...) |
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7.90
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Graeme Coates: A f/4 lens (let's assume a perfect one) has an Airy disk diameter of ~5.4 um. If we use 90% of encircled energy criterion then is more like 6.5 um. Real lens are hardly diffraction limited so in practice it is going to be slightly bigger (or a lot if the lens is soft) than that. This means that, at 2 pixels per sampling (3.76 um per pixel), your depth of focus (depth of focus = ~2*f/*2*3.76um) = 60 um or 0.06 mm. I'd put it at as mere minimum requirement but in a real f/4 lens it is likely that you can accomodate a 50% larger focus margin and still be on the sharp side of focus (for a real lens). On my Nikkor f/2.8 I can routinely achieve that sort of precision with the manual focus ring. |
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I had the same lens, was a very good copy and worked well for daytime photography but for astro was unuseable as made terrible star shapes and colours. Not sure why it has a good reputation for astro - maybe I used the wrong camera with it. I wanted to try the F2 L version but am told it's worse so now use a 300 2.8 II for wide field. |
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| The diffraction like abberation is probably some pinching, the axial abberation causing the blue/green to focus at different point than red is normal for that lens unless you stop down to F/4 or F/5 depending on your sensor size. |
