wlaidlaw Posted August 11, 2014 Share #1 Posted August 11, 2014 Advertisement (gone after registration) What do folks think is the diffraction limitation in the M240 with its 6μm pixel size? My calculations put it around F11 for green light, where the Airy diameter is 14.7μm and f16 for blue or red, where the Airy diameter is 21.3μm. The diffraction limit for green is lower due to the Bayer sensor layout. However I think my calculation algorithm assumes there is an AA filter, so for the M240, these figures might be one stop lower for ultimate sharpness i.e. f8 and f11. Now the next question is are you better living with diffraction softening and using small apertures in very bright light or using a variable ND filter. On the basis that there is never a free lunch, I would guess that variable ND filters degrade the image somewhat. I don’t have any of these to check but I was recently a bit taken aback by how noticeably polarising filters degraded the image sharpness and I am talking about Leica, Heliopan and B+W ones, so not cheap eBay rubbish. I assume variable ND filters are based on polarisation. Wilson Quote Link to post Share on other sites More sharing options...
Advertisement Posted August 11, 2014 Posted August 11, 2014 Hi wlaidlaw, Take a look here Diffraction Limitation in M240 v ND filters. I'm sure you'll find what you were looking for!
jaapv Posted August 11, 2014 Share #2 Posted August 11, 2014 That is well beyond the diffraction limit of the lenses, so I do not think we have much to worry about. Quote Link to post Share on other sites More sharing options...
wlaidlaw Posted August 11, 2014 Author Share #3 Posted August 11, 2014 Jaap, It was the diffraction limit of the combined lens and sensor I was considering and that was the equation I used but I think the person had formulated it assuming an AA filter. Do you mean beyond the resolution limit of the lens? High quality modern lenses are supposed to be able to resolve to about 200 lp/mm in the centre of the image (that is the figure Zeiss quote for their ZM50 Planar) i.e. 5μm so with an Airy refraction limit disc size of say 14mm, diffraction will cut in before the resolution limit, so it is relevant. Wilson 1 Quote Link to post Share on other sites More sharing options...
kdriceman Posted August 11, 2014 Share #4 Posted August 11, 2014 I have not used a variable neutral density filter, so I don't know hoe they affect the image, but my experience with neutral density filters is that if you have a high quality filter the result is much better than stopping down to the point where diffraction is evident. 5 Quote Link to post Share on other sites More sharing options...
MarkP Posted August 11, 2014 Share #5 Posted August 11, 2014 I have not used a variable neutral density filter, so I don't know hoe they affect the image, but my experience with neutral density filters is that if you have a high quality filter the result is much better than stopping down to the point where diffraction is evident. Yes. Quote Link to post Share on other sites More sharing options...
wlaidlaw Posted August 11, 2014 Author Share #6 Posted August 11, 2014 I am sure you are right with non variable ND filters compared to diffraction softening. It was with variable ND filters, which have two layers of polarising filter. Now the polarising filters I used for my resolution testing were not particularly modern, as they were series VIII filters about 10 to 20 years old and maybe modern pola filters are better. I will have to test with my only modern one, a 52mm B+W circular polariser to see how it affects resolution. It is an expensive experiment to get a B+W variable ND 62mm to fit my Nocti and 80-200 with a 60-62 step up ring. I think they are around £200. Wilson Quote Link to post Share on other sites More sharing options...
wlaidlaw Posted August 11, 2014 Author Share #7 Posted August 11, 2014 Advertisement (gone after registration) I have bitten the bullet and ordered a B+W Variable ND filter and B+W 60-62 mm ring from Peter Huppert. I was trying to take some photos today with the Nocti, where I wanted shallow-ish DOF and even with pull ISO, I just could not do it. I suppose I could have bought say three different fixed ND filters of say -2EV, -4EV and -5EV but I tend to carry enough weight of junk around with me as it is. The other advantage of the variable ND filters is that you can use them as a variable polariser as well. Wilson 1 Quote Link to post Share on other sites More sharing options...
Photoskeptic Posted August 11, 2014 Share #8 Posted August 11, 2014 Wilson are you talking about the swing-away type of ND filter for M cameras? Otherwise it seems you are referring to mounting fixed filters and then checking results on the LCD. I suppose you could test different settings on a fixed variable ND - is this what you had in mind? Quote Link to post Share on other sites More sharing options...
wlaidlaw Posted August 11, 2014 Author Share #9 Posted August 11, 2014 Wilson are you talking about the swing-away type of ND filter for M cameras? Otherwise it seems you are referring to mounting fixed filters and then checking results on the LCD. I suppose you could test different settings on a fixed variable ND - is this what you had in mind? John, Is not the swing away for the M a polarising filter, which you swing up in front of the VF to get your optimum polarising effect and then swing back, which as it is 180º, does not alter the polarising effect? I don’t think that is available in the 60mm size for the Noctilux and 80-200 Vario Elmar anyway. I spoke to B+W and they assure me that any softening with their variable ND filter is close to undetectable, where diffraction softening on the M240 is certainly not undetectable. In any case I want to use wider apertures to get shallow DOF, so the only alternatives are fixed or variable ND. I wanted to get a polariser anyway for the Vario Elmar, so the variable ND does two jobs. I can therefore justify its cost (well at least to myself). In India earlier this year, I left polarising filters on my 40mm Summicron and 90mm Elmarit the whole time during the day, to get better contrast in hard light. I just have a midday position marked on the rotating part of the filter and estimate best polarisation by sun position. It seems good enough. The 62 mm filter with 60 to 62mm step up ring was €278. Wilson Quote Link to post Share on other sites More sharing options...
farnz Posted August 11, 2014 Share #10 Posted August 11, 2014 Wilson, Heliopan make a 60mm VarioND that are often available through TeamWork Photo, who very quickly ship anything they have in stock and are typically knowledgeable and pleasant to talk things over with. I bought a 60 mm VarioND from them a few months ago for around £130 I think because I'd had enough of the 60/62 step up ring. Pete. 3 Quote Link to post Share on other sites More sharing options...
wlaidlaw Posted August 11, 2014 Author Share #11 Posted August 11, 2014 Pete, Thanks for the suggestion but I have bought the 62mm B+W anyway now. I thought that for use with the Noctilux wide open, you were supposed to use a 60 to 62mm step up ring to prevent vignetting with a deep filters like the ND ones. Wilson Quote Link to post Share on other sites More sharing options...
farnz Posted August 11, 2014 Share #12 Posted August 11, 2014 Wilson, The 60mm is available in slim line and I haven't noticed additional vignetting with my f/1 Noctilux although it might be difficult to tell it apart from the lens's natural (and to me attractive) vignette. Pete. Quote Link to post Share on other sites More sharing options...
darylgo Posted August 12, 2014 Share #13 Posted August 12, 2014 Wilson, In tests with the Heliopan Variable ND there was a noticable degradation of image quality and that was after disassembling to clean the inner elements of built-up haze. The filter was returned and subsequently I have placed together several from circlular/linear B+W polarizers, all have degradation. Surprisingly, a Tiffen Variable ND has minimal to no degradation. I haven't tested others thinking that two filters will always degrade, the Tiffen being an exception and therefore it may vary by manufacturer. 1 Quote Link to post Share on other sites More sharing options...
wlaidlaw Posted August 12, 2014 Author Share #14 Posted August 12, 2014 Daryl, When I did the comparative tests on the series VIII polarising filters. The Tiffen was terrible, the B+W better but the Leica only had slight but still noticeable image degradation. So as you say, perhaps sample variation is high on these double polarisation variable ND filters. Wilson Quote Link to post Share on other sites More sharing options...
darylgo Posted August 12, 2014 Share #15 Posted August 12, 2014 Daryl, When I did the comparative tests on the series VIII polarising filters. The Tiffen was terrible, the B+W better but the Leica only had slight but still noticeable image degradation. So as you say, perhaps sample variation is high on these double polarisation variable ND filters. Wilson What you found would be my guess for image quality also. When testing the Tiffen I simply looked at image sharpness and nothing else such as color, density variation etc. Could mechanical alignment of the two filters also play a role in image quality? Quote Link to post Share on other sites More sharing options...
pgk Posted August 12, 2014 Share #16 Posted August 12, 2014 What you found would be my guess for image quality also. When testing the Tiffen I simply looked at image sharpness and nothing else such as color, density variation etc. Could mechanical alignment of the two filters also play a role in image quality? I've been wondering about variable ND filters but my experience with polarisers alone has put me off as I have found that there can be image degradation even with a good polariser. I too wonder about mechanical alignment and high variability would suggest this. It might just be worth trying some tests to see if there are shifts in the images resolution at different ND settings, which there may be if the two filters are not absolutely parallel? Quote Link to post Share on other sites More sharing options...
darylgo Posted August 12, 2014 Share #17 Posted August 12, 2014 I've been wondering about variable ND filters but my experience with polarisers alone has put me off as I have found that there can be image degradation even with a good polariser. I too wonder about mechanical alignment and high variability would suggest this. It might just be worth trying some tests to see if there are shifts in the images resolution at different ND settings, which there may be if the two filters are not absolutely parallel? There are certainly many issues with Vari ND, iirc their initial target audience was cinema. I simply gave up on them and carry a 3 stop (no ir issues) and 10 stop (ir issues). Quote Link to post Share on other sites More sharing options...
wlaidlaw Posted August 12, 2014 Author Share #18 Posted August 12, 2014 Once the B+W arrives, I will post some extreme crops of centre and corner, both using the ND and very stopped down to produce diffraction softening, with my 0.95 Noctilux. I will take boring pictures of the rough limestone walls of my house, as there is lots of very fine detail on the stone to assess resolution and it is repeatable. Daryl, I was not very clear about my testing of series VIII filters, these were single polarising filters not variable ND. Testing my 52mm B+W F-Pro multi coated circular polariser on one of my sharpest lenses, the 90mm f2.8 Elmarit-M, shows little to no image degradation but I might just have got a lucky one. Wilson 1 Quote Link to post Share on other sites More sharing options...
wlaidlaw Posted August 22, 2014 Author Share #19 Posted August 22, 2014 OK folks here are the tests. First the centre crops (all crops approx 600 x 600 px). As there are six images, I have split them into two posts Top one f2.8 no filter Middle f16 no filter Bottom f2.8 B+W Variable ND filter at -3EV So whereas there is a little softening with the filter, it is less than with diffraction Wilson Welcome, dear visitor! As registered member you'd see an image here… Simply register for free here – We are always happy to welcome new members! 1 Quote Link to post Share on other sites Simply register for free here – We are always happy to welcome new members! ' data-webShareUrl='https://www.l-camera-forum.com/topic/232020-diffraction-limitation-in-m240-v-nd-filters/?do=findComment&comment=2654643'>More sharing options...
wlaidlaw Posted August 22, 2014 Author Share #20 Posted August 22, 2014 (edited) Now the corner crops. Top one f2.8 no filter Middle f16 no filter Bottom f2.8 B+W Variable ND filter at -3EV A rather different picture appears. The corner at f16 is much sharper than at f2,8. I might have been better doing a comparison of f5.6 with f16. Again with the ND filter f2.8 is a little less sharp than without the filter but not by as much as I feared it might be. It seems to me that B+W have done a pretty fine job with this filter. Wilson Welcome, dear visitor! As registered member you'd see an image here… Simply register for free here – We are always happy to welcome new members! Edited August 22, 2014 by wlaidlaw Quote Link to post Share on other sites Simply register for free here – We are always happy to welcome new members! ' data-webShareUrl='https://www.l-camera-forum.com/topic/232020-diffraction-limitation-in-m240-v-nd-filters/?do=findComment&comment=2654646'>More sharing options...
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