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| Losing the low pass filter | Rate Topic |
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| Posted by Eric: Mon Feb 25th, 2013 13:04 | 1st Post |
| The D800E has lost it...the D7100 has lost it. So is it replaced with a plain glass filter? If not...How do they protect the sensor? How do they CLEAN the sensor? Has this any impact on lens hotspotting? :baffled:
____________________ Eric |
| Posted by Robert: Mon Feb 25th, 2013 13:34 | 2nd Post |
| On all DSLR cameras I have examined and investigated the sensor already has a thin glass outer shield which seals the actual sensors from any possibility of contact. I don't know if the LPF has been replaced by a replaceable piece of protective glass. I suspect it has because it will be needed to maintain optical consistency for the digitally corrected optics of modern 'digital corrected' lenses. There is a little confusion about exactly what the LPF is. On all the cameras I am aware of the LPF performs several functions. It is very slightly etched to very slightly blur the image which falls on the sensor, this cures the moir© effect but also reduces sharpness slightly, except on the D200 where it reduces sharpness a lot. Well that''s my excuse for soft images!!! The LPF also has strong UV and IR Filtration because unlike film, sensors are very sensitive to IR and UV. If it weren't there the image would include the data of the IR and UV spectrums, which apart from confusing the image, would cause blurring because of the focus shift at those wavelengths. The LPF also has the effect of interfering with the light path and bending the light path at the corners and edges of the image sensor, the further from the axis, the greater the bend. It is the corrections for this effect which justify the 'corrected for digital' label. I doubt it will have any great effect on the lens hotspot issue, because I believe there will always be a glass screen of some sort over the sensor. But who knows until you try it?
____________________ Robert. |
| Posted by jk: Mon Feb 25th, 2013 14:00 | 3rd Post |
| As Robert says the LPF does a number of things but I guess that the IR and UV filtering is now being performed by another filter in the stack. All they have done is remove the anti-more effect by removing the interference filter component that reduced the image sharpness. There is also the Bayer filter that sits directly above the sensor. See here for the current design. http://dpbestflow.org/camera/sensor This diagram shows it very well. Attachment: camera-microlenses.jpg (Downloaded 31 times)
____________________ Still learning after all these years! https://nikondslr.uk/gallery_view.php?user=2&folderid=none |
| Posted by Eric: Mon Feb 25th, 2013 15:10 | 4th Post |
Robert wrote:On all DSLR cameras I have examined and investigated the sensor already has a thin glass outer shield which seals the actual sensors from any possibility of contact. Hot spotting is a function of the proximity and geometry of the glass surfaces in front of the sensor. The prospect of removing a surface conceivably might have alleviated the issue with some lenses. Mere conjecture I accpet, and in light of JKs drawing, I suspect a vain hope I now concede. 
____________________ Eric |
| Posted by jk: Mon Feb 25th, 2013 15:31 | 5th Post |
| Yes removal of the LPF shouldnt improve hot spotting AFAIK !
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| Posted by Eric: Mon Feb 25th, 2013 16:43 | 6th Post |
jk wrote:Yes removal of the LPF shouldnt improve hot spotting AFAIK ! We draw some, we lose some. 
____________________ Eric |
| Posted by blackfox: Mon Feb 25th, 2013 17:39 | 7th Post |
| hmmm roberts statement re: digital corrected lenses gives me food for thought ,i wonder if it makes a difference to performance of older lenses ,i.e a 35mm film slr never had a filter in front of the film did it ,the shutter opened and the transmitted light fell on the film ,job done . this is throwing so many variables into the equations ,just had a hour long conflab with my lad who has recently moved back from canon to nikon ,more by chance than choice as he got a 500mm vr f4 at a good price ,and sold all his canon gear to fund it ,but my gut feeling is he still hankers after canon ,whereas i don't i,m fully committed at this point in time. i feel he his upset because they haven't brought out a d400  i digress it will be interesting to see the results and conclusions from the d7100 when it actually hits the shops and users ,either nikon have done something stupid or they have played a blinder ,time will tell 
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| Posted by Robert: Mon Feb 25th, 2013 17:53 | 8th Post |
| It would be interesting to try my D1X which has the LPF removed in a comparison with my D1 which has a standard IR pass filter (can't recall the wavelength right now) with a range of lenses known to hotspot, like the 50mm f1.4 and the Sigma 10-20 and possibly the 20mm f2.8? I am in recovery mode now the boys have gone back to their mothers after half term, so it would be a nice relaxing way of passing a couple of hours... Good weather forecast for tomorrow... Will charge up the Li-Ion batteries! Edit: No need, they are both fully charged since last year when I went to Bodnant, was that last April Jeff? I won't charge them just to see how they perform having stood best part of a year.
____________________ Robert. |
| Posted by Robert: Mon Feb 25th, 2013 18:21 | 9th Post |
blackfox wrote:hmmm roberts statement re: digital corrected lenses gives me food for thought ,i wonder if it makes a difference to performance of older lenses ,i.e a 35mm film slr never had a filter in front of the film did it ,the shutter opened and the transmitted light fell on the film ,job done . The effect is very easy to demonstrate, If you wind the window of your car down a bit, and put a pencil or a matchstick, anything slim, on the other side of the glass and visible above the glass, view it at 90º to the glass then move your head gradually sideways, the object will appear to split, the light goes through the glass at 90º no matter what angle you view it. The thicker the glass the more pronounced the effect. This is how/why lenses work. The LPF is very thin but we are talking very fine optics here, even the 0.6 mm LPF has an effect which is also more pronounced with lenses which have a deep intrusion into the mirror chamber. Some lenses particularly the longer lenses even have recessed rear elements which effect less distortion. It's nowhere near as simplistic as I describe but it does have an effect and it also affects the edge distortion and edge drop off of light, giving a vignetting effect because of the stretching of the light waves at the edge of the frame. This is also the reason why lenses with a built in filter holder must always have a filter fitted, even if it's clear glass because it's part of the optics of the lens. The old 35mm film lenses will still work and produce great images but for the most part without correction they suffer from slight distortion and apparent vignetting to a greater extent than their modern digitally corrected brethren.
____________________ Robert. |
| Posted by richw: Mon Feb 25th, 2013 20:49 | 10th Post |
| I think sometimes we get too caught up in some of this, for most of us it will make no difference to the overall impact and quality of our images.
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| Posted by Robert: Tue Feb 26th, 2013 03:10 | 11th Post |
richw wrote:I think sometimes we get too caught up in some of this, for most of us it will make no difference to the overall impact and quality of our images. Of course Rich, Nikon employ the appropriate configuration for their particular needs, but it does no harm to understand what is going on inside. Particularly for those of us who are pushing the boundaries using old lenses and making Infra Red images. Those using up to date equipment and making 'normal' images are not affected.
____________________ Robert. |
| Posted by Eric: Tue Feb 26th, 2013 04:30 | 12th Post |
Robert wrote:richw wrote: Actually that's not true! If you put a current 50mm lens (1.8or1.4) on any body and photograph dark footwear against a white background in studio...you get a very distinct hot spot. I had to routinely digitise this out ...for several years!! Whilst even this may not be classed as 'normal' photography it demonstrates that the hotspot from even some modern lenses, without any camera adaptations, is still there. It's only the confusion of colours and detail in general photography that disguises it. Try taking any high contrast subjects centred in the middle of the frame and you run the risk of seeing a hotspot.
____________________ Eric |
| Posted by Robert: Tue Feb 26th, 2013 04:40 | 13th Post |
| I stand corrected! I am easily fooled. What would your recommendation be to make a side by side test for hotspot Eric. Last time I used a sunlit pebble dashed wall, I don't have any black shoes! Perhaps a concocted still life scene with a dark object across the centre?
____________________ Robert. |
| Posted by Eric: Tue Feb 26th, 2013 04:48 | 14th Post |
Robert wrote:I stand corrected! I am easily fooled. When I get a moment I will see if I can go back through my archives and find an example of a raw imageI never edited to show the effect. It was the bane off my existance at the time and caused me to use longer, slower lenses just to avoid the extra work.
____________________ Eric |
| Posted by jk: Tue Feb 26th, 2013 05:45 | 15th Post |
| It would be very interesting to see that image Eric. I havent used my 50mm f1.8 for serious shooting for a while. Another thing to check when I get back from my travels. BTW: What was your Photoshop method of removing the hotspot?
____________________ Still learning after all these years! https://nikondslr.uk/gallery_view.php?user=2&folderid=none |
| Posted by Eric: Tue Feb 26th, 2013 06:00 | 16th Post |
| That took some finding! 2003! D1X and 50mm lens. The hotspot is a magenta spot arrowed. On white boots it didnt show...on coloured boots the discolouration disappeared but on black boots! I bought 3 different 50mm f1.8 lenses ( kept returning them to Jessops - no wonder they went bust!) Bought 2 50mm f1.4 lenses from another source - still the same! Tried all combinations of lighting to see if it was an errant reflection...even with a time exposure with modelling lights it showed this hotspot. In the end I shot the images with the 17-35 at 35mm .....and a little closer. The 17-35 is now my standard IR lens ...as it has no hotspot at all! Edit - This was an untouched nef file resampled for viewing but it doesnt show up as much when posting on the forum???? So to make the hotpot more clear on all screens I have just run a saturation brush over the area to make it a tad more obvious. Attachment: hotspot2.jpg (Downloaded 19 times)
____________________ Eric |
| Posted by Robert: Tue Feb 26th, 2013 06:46 | 17th Post |
| OK have taken some test images with the D1x normal body and the body with the LPF removed. I have managed to reproduce the hot spot effect you describe but only at f16 it is just visible at f8 and gone altogether with wider apertures. The hotspot is present on both bodies but not as distinct on the one with the LPF removed. I will take a set with the IR camera then put them up here after lunch.
____________________ Robert. |
| Posted by Eric: Tue Feb 26th, 2013 07:10 | 18th Post |
Robert wrote: OK have taken some test images with the D1x normal body and the body with the LPF removed. Yes the small apertures (I think these boots were f16) do focus and intensify the effect. In fact I invariably keep the aperture I use for IR at between 5.6 an 8...which may be a subliminal recognition of your observation? This step alone probably increases the range of lenses that are 'usable'. Ironically and bizarrely, one of the current flagship lenses (24-70) has a horrid IR hotspot even at f5.6! Which contradicts previous thinking... as its not evident when shooting in colour even at small apertures? So there is more to this than meets the eye. :baffled::baffled:
____________________ Eric |
| Posted by Eric: Tue Feb 26th, 2013 07:14 | 19th Post |
| PS...I have now got to go and shoot some large black batteries against a white background. Guess which lens I won't be using?
____________________ Eric |
| Posted by Eric: Tue Feb 26th, 2013 08:31 | 20th Post |
| Okay - I supposed after what I said I shouldnt be surprised. As I was doing the lighting set up for these batteries I thought I would try the 24-70 on this black on white scene. Below the result (left) and the repositioned shot with the 80-200 (right). This does show that the acknowledged IR big hotspot (Bjorn agrees by the way!) on this lens IS still there in colour....albeit at f16 and black on white. Although this is a very specific lighting set up and extreme settings...its not uncommon for catalogue work and frankly I cant help feeling its a big failing in this supposedly top quality lens. I now have to go and clean these batteries before I shoot them.  Attachment: _DSC7526.jpg (Downloaded 17 times)
____________________ Eric |
| Posted by Robert: Tue Feb 26th, 2013 08:45 | 21st Post |
| The hotspot I have at f16 even in the normal, visible spectrum is more pronounced than that Eric. This crop is from the D1x Normal spectrum body, the hotspot is actually in the centre of the frame. Attachment: D1x Normal f16.jpg (Downloaded 20 times)
____________________ Robert. |
| Posted by Robert: Tue Feb 26th, 2013 08:48 | 22nd Post |
| This id from the D1x which does not have a low pass filter LPF Again a crop off centre. Attachment: D1x no LPF f16.jpg (Downloaded 20 times)
____________________ Robert. |
| Posted by Robert: Tue Feb 26th, 2013 08:51 | 23rd Post |
| Thirdly from the D1 IR converted body. This id a 1:1 crop screen grab from Lightroom. Attachment: D1 IR f16.jpg (Downloaded 20 times)
____________________ Robert. |
| Posted by Robert: Tue Feb 26th, 2013 09:04 | 24th Post |
| The hotspots are barely visible at f8 and undetectable below. Minor adjustments to levels and WB to equalise the exposures but otherwise un molested.
____________________ Robert. |
| Posted by blackfox: Tue Feb 26th, 2013 09:21 | 25th Post |
| correct if i,m wrong here robert ,but has the hot spot moved left in the bottom pic
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| Posted by Robert: Tue Feb 26th, 2013 10:49 | 26th Post |
blackfox wrote:correct if i,m wrong here robert ,but has the hot spot moved left in the bottom pic No Jeff the bottom pic is from the D1 and it only has half the pixels that the D1x has so I cropped it at 1:1. The tripod probably moved while I was having my lunch too.
____________________ Robert. |
| Posted by Eric: Tue Feb 26th, 2013 12:59 | 27th Post |
Robert wrote: The hotspot I have at f16 even in the normal, visible spectrum is more pronounced than that Eric.I appreciate that... but you used the 50mm lens....which, no disrespect, is a £200? lens. The 24-70 is a £1200 lens...and frankly its not good enough to have a weakness like this. Sure we rarely use f16, but that's not the point. IMHO there is an optical defect, that compromises the full range of its potential. Interestingly my oooooold 28-70 AFS which I (fortunately) kept.... doesnt have a hotspot. Similarly the 80-200AFS I used in my test is clear. So not a step forward then?
____________________ Eric |
| Posted by Robert: Tue Feb 26th, 2013 13:35 | 28th Post |
| The 50 1.4 and 1.8 were the standard 35mm film lenses, and considered at the time to be pretty good, they were not designed for digital, which wasn't even on the horizon at the time. Even so I don't think cost is really a factor, more the reflective glass in front of the sensor. Film wasn't reflective, a factor which also messed with TTL flash. Where is Kirk? 
____________________ Robert. |
| Posted by Eric: Tue Feb 26th, 2013 15:21 | 29th Post |
Robert wrote:The 50 1.4 and 1.8 were the standard 35mm film lenses, and considered at the time to be pretty good, they were not designed for digital, which wasn't even on the horizon at the time. I totally agree its the amount (maybe type) of glass. But the point I was making was...if they charge £1200 for a lens they need to have overcome these issues. My 28-70 and 80-200 predate digital, have plenty of glass in line and yet don't have hotspots.
____________________ Eric |
| Posted by jk: Tue Feb 26th, 2013 16:53 | 30th Post |
blackfox wrote: correct if i,m wrong here robert ,but has the hot spot moved left in the bottom picThat is what I thought as well. However if Robert says No then he should know as he took the pictures.
____________________ Still learning after all these years! https://nikondslr.uk/gallery_view.php?user=2&folderid=none |
| Posted by Robert: Tue Feb 26th, 2013 17:16 | 31st Post |
| I guess I should have included the full frame! BTW It isn't a giant fury caterpillar, it's a black fury draught excluder from the kitchen! LOL Attachment: Screen Shot 2013-02-26 at 22.15.12.jpg (Downloaded 13 times)
____________________ Robert. |
| Posted by blackfox: Tue Feb 26th, 2013 19:20 | 32nd Post |
| furry draught excluders oh how the wealthy live .
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| Posted by Robert: Wed Feb 27th, 2013 04:01 | 33rd Post |
| It keeps the blast of freezing air out of the bathroom from my ankles! Essential in winter. One of my next jobs it to try to seal the source of the draughts in the bathroom. With the price of gas and the inefficiency of my boiler I only use the heating when I have visitors. The pilot light alone uses £2 worth of gas a week.
____________________ Robert. |
| Posted by jk: Wed Feb 27th, 2013 09:38 | 34th Post |
 Just looked again.It is the shadow of the leaf (used as a reference marker) that is causing the hot spot point to look as though it has moved.
____________________ Still learning after all these years! https://nikondslr.uk/gallery_view.php?user=2&folderid=none |
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