Andrew64673
Senior Member
You're not wrong there, I heard of a guy who sucked all his shutterblades out of his FM2. Take care please, this could get very sad.
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IDEA for a CCD Cleaning device.. Works well..
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Joseph S Wisniewski
Forum Pro
You're forgetting two things. First, and most importantly, is exactly what an SLR is. Remember, the optic path in an SLR is defined by the distance from the back of the lens to the sensor or film (when the mirror's up) and from the lens to mirror to focusing screen when the mirror's down. To allow the mirror to cover 100% of a 35mm frame, the mirror has to be 34mm long (24mm * sqrt(2)).
Photographers like their normal lenses (like the 50mm double Gauss and the 45mm Tessar) to be, for lack of a better terml, "normal". No extra elements to lengthen the optical path (make the lens retrofocus), because this kind of design increases flare, distortion, cost, and weight. So we're kinda stuck with a 45mm long optical path in a 35mm SLR.
Throw in a few more mm for the shutter mechanism itself, and a mm or two of clearance between the back element of the lens and the moving mirror, and you realize that you're right at the ragged edge of where the mirror has to be when it's down.
There's no room between mirror and shutter for any more protection than the AA filter we've already got (which, by the way, explains why AA filters are so thin).
Now, for any SLR with a sensor smaller than full frame film, you can use a smaller mirror, so there are a couple of things you can do.
The easy thing is to trim a few mm from the bottom of the mirror, and put the protective glass in front of the mirror. This requires, at most, some minor redesign of the film SLR body. The big problem here is that the moving mirror and shutter mechanism actually create dust and dirt, as metal parts grind together, paint flakes away (especially the flat paints used inside optical systems) and foam or rubber shock absorbing materials abrade (or outgas).
The "hard" thing is to reduce the size of the mirror, move the mirror forward (which implies moving the focusing screen upward) and moving the shutter forward, to accomodate a dust protector (which might be integrated with a thicker, more robust AA filter). This would mean an entire redesign of the camera.
And considering that neither Nikon nor Canon has done something as simple as reducing the length and width of mirror (with basically no other changes to the mechanism), a move that would make the mirror move faster (reducing blackout) while simultaneously reducing vibration, I have little hope that they would do anything involving a major tearup of the film camera designs. Especially if they see reduced size sensors as a "stopgap" on the way to full frame.
The second problem is the telephoto lens. Stopped down, these lenses are very highly columnated (all the light is parallel) so that the shadows of dust are cast very sharply on the sensor, even when the dust is held a few mm from the film plane by a "protective" glass.
Ciao!
Joe
Joseph S Wisniewski
Forum Pro
The Kodak-Olympus 4/3 system defines a fairly long lens to focal plane distance. So there is room for stuff between the shutter and sensor, or between the morror and lens. But the price you pay for this is that any "normal" lens such as a 25mm "normal" would have to be a radical retrofocus design. You can't just scale down the 50mm f1.4 double Gauss that virtually every SLR manufacture sells, you have to add a retrofocal back section to it. At this point, you might as well recompute whe whole lens, losing any of the optical advantages of a symmetrical configuration like the double Gauss.
(OK, if you want to get picky about it, the double Gauss focuses just a little too close to be a good 50mm normal on an SLR, so they usually have one extra element to lengthen the throw by about 5 mm. But there's a heck of a difference between a single element to increase throw 10%, and adding a bunch of elements to increase throw 100%)
Ciao!
Joe
Boxbrownie
Leading Member
Why can't the manufacturers just put a small surface area somewhere away from the CCD in the lens throat (possibly the back of the mirror) that has a greater charge than the CCD itself, that way any dust will be attracted to that rather than the CCD and we have no problem actually physically wiping the sacrificial plate clean without any fear of damage?
Again this is just an idea............
Best regards David
Again this is just an idea............
Best regards David
Boxbrownie
Leading Member
I understand the sigma DSLR does just that...........
Thank you for your very extensive reply. You certainly know
much more about SLR design than I do! (which is, by the way,
not hard to achieve).
However, your arguments only addressed my first point, i.e.
"Put the dust at some distance (on the glass), so that it won't
show up so easily on the images". You argued that it is impossible
to achieve this, at least using traditional SLR design.
The second point "Making the process of cleaning the glass
much easier and less delicate" still holds, as far as I can see.
There should be some way of covering the CCD in a sealed way
with some cheaper protective cover, that can be touched without
the risk of $1000 damage. For example, why didn't they make at
the very least the filter cover replaceable independent from the
CCD, and made it impossible for dust to enter between the
filter and the CCD? (Even the latter alone would be a big
improvement!)
Concerning the first point again, Your arguments gave me the
impression that a lot of design restrictions come from the long
35mm SLR tradition, and the fact that the DSLR's are still built
around classical bodies (using classical lenses). Obviously, this
is a huge advantage in terms of compatibility, but this may be
only a transient situation. There seem to be too many
compromises, so that a radical new design may become the
industry standard in the longer run. But again, I am by no means
an expert in SLR's or even in optics, so my opinion is really
worth nothing.
much more about SLR design than I do! (which is, by the way,
not hard to achieve).
However, your arguments only addressed my first point, i.e.
"Put the dust at some distance (on the glass), so that it won't
show up so easily on the images". You argued that it is impossible
to achieve this, at least using traditional SLR design.
The second point "Making the process of cleaning the glass
much easier and less delicate" still holds, as far as I can see.
There should be some way of covering the CCD in a sealed way
with some cheaper protective cover, that can be touched without
the risk of $1000 damage. For example, why didn't they make at
the very least the filter cover replaceable independent from the
CCD, and made it impossible for dust to enter between the
filter and the CCD? (Even the latter alone would be a big
improvement!)
Concerning the first point again, Your arguments gave me the
impression that a lot of design restrictions come from the long
35mm SLR tradition, and the fact that the DSLR's are still built
around classical bodies (using classical lenses). Obviously, this
is a huge advantage in terms of compatibility, but this may be
only a transient situation. There seem to be too many
compromises, so that a radical new design may become the
industry standard in the longer run. But again, I am by no means
an expert in SLR's or even in optics, so my opinion is really
worth nothing.
Boxbrownie
Leading Member
Wesley R. Elsberry
Well-known member
[...]
There is another approach. There may be some around who remember the Canon Pellix camera. That camera used a beam splitter rather than a moving mirror arrangement. The advantages are that one eliminates vibration due to mirror movement, there is no momentary blackout of the viewfinder, and (for our considered purpose) one could seal the area containing the media. Of course, there are disadvantages as well, the primary one being that the amount of light reaching the media is always reduced, and the viewfinder brightness is limited by how strong you make the splitter. This is a tradeoff: the brighter you make the viewfinder, the less light gets to the media.
Is the elimination of dust-on-imager worth taking, say, a 1-stop loss in sensitivity? I think that some would say "yes". Another possible benefit would be the elimination of a physical shutter, which besides getting rid of moving parts would allow a DSLR to provide a preview image on LCD or via remote connection.
This appears to be the approach taken in Olympus's E-10 and E-20 cameras.
Stan Disbrow
Veteran Member
Hi,
...by Kodak on their DCS 5xx, 6xx and 7xx series DSLRs. That leave the pure silicon surface (aka as glass) of the CCD being the part that has to be cleaned with the swabs.
Not that this has to be done nearly as often as with the designs that place the filter at the focal plane. The filter out front keeps most of the dusties from getting into the mirror box in the first place. Plus, it has to become really dirty before you can see anything in the images.
Sounds like a great idea, right? Not quite, though.
Firstly, the filter is really easy to break. It's sitting right inside the lens mounting ring, and it's pretty easy to hit it when changing lenses. A little care ges a long way, here, though.
The second flaw is a bit harder to deal with. Zoom lenses no longer hold focus from one end to the other of the zoom range. The old trick of zooming to the long end, focusing, and then zooming to the desired focal length results in out-of-focus shots. That's because the filter is so far away from the focal plane.
Of course, AF can correct this as you go, especially when using AFS lenses, but MF can be a bit tricky sometimes.
I think it was the issue with zoom lenses that caused Nikon to put the IR filter where it is, at the focal plane.
Stan
Amateur Photographer
Professional Electronics Development Engineer
More info and list of gear is in my Posters' Profile.
...by Kodak on their DCS 5xx, 6xx and 7xx series DSLRs. That leave the pure silicon surface (aka as glass) of the CCD being the part that has to be cleaned with the swabs.
Not that this has to be done nearly as often as with the designs that place the filter at the focal plane. The filter out front keeps most of the dusties from getting into the mirror box in the first place. Plus, it has to become really dirty before you can see anything in the images.
Sounds like a great idea, right? Not quite, though.
Firstly, the filter is really easy to break. It's sitting right inside the lens mounting ring, and it's pretty easy to hit it when changing lenses. A little care ges a long way, here, though.
The second flaw is a bit harder to deal with. Zoom lenses no longer hold focus from one end to the other of the zoom range. The old trick of zooming to the long end, focusing, and then zooming to the desired focal length results in out-of-focus shots. That's because the filter is so far away from the focal plane.
Of course, AF can correct this as you go, especially when using AFS lenses, but MF can be a bit tricky sometimes.
I think it was the issue with zoom lenses that caused Nikon to put the IR filter where it is, at the focal plane.
Stan
--
Amateur Photographer
Professional Electronics Development Engineer
More info and list of gear is in my Posters' Profile.
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