RE: filmscanners:Focusing film flatness
At 12:49 PM 4/04/01 EDT, Rich wrote: ...they ended up relenting and giving me a much better lens with sufficient depth of field This is an interesting statement. The only things that affects depth of field in a lens is its apeture or focal length. A 'much better lens' doesn't necessarily imply either. ... But ... a lens with a somewhat concave field is actually an advantage since it would give you a sharp image on a curved surface. In the projector business, it is likely that a 'better lens' has a slightly curved field to match the assumed curvature of a slide. Yep, they told me that the 'average' projector lens is just designed for a flat plane, as you suggest. But the new one they gave me, a Leitz Colorplan I think it is, was designed for a slightly curved plane, in the direction that most slides bow. I presume they designed it for a middle ground - it certainly does cope very well with flat slides as well as the typical cardboard job, and of course those slides that 'pop' as they heat up from the light. (Was the bowing deliberately done by Kodak to avoid that, I wonder?? Back in those days, I would imagine the vast majority of slides were destined for projection, rather than the enlarger or a film-scanner, where flatness is much more of a virtue!) Increasing a projector lens' depth of field by reducing it's apeture is impractical since it would result in a much dimmer image on the screen. I didn't take notice of any aperture restriction. (It's stored at the moment, and I'm too lazy to drag it out and look!) But I wonder just how much restriction you would need to gain the required result? This is heading off-topic, so no answer required! So the question is, are the lenses in film scanners flat field, or are they slightly dished to accomodate film curvature? Or are some small apeture, high depth of field lenses working with more sensitive ccds. Tony praises a fixed focus Minolta scanner which would have to fit the later category. How about some others? I know my Acer copes well in this area, so I just decided to push it and find out. I put 2 bowed slides in, one reversed, got it to focus on the first (which I presume it does towards the centre of frame) and scanned both at the same focus plane. Sure enough, first one was sharp, inc. corners, second one was blurred in centre, although the edges were OK.. That tells me that the depth of field is just about right, maybe 0.4mm (?) as a wild guess.. Regards, Mark T. == Mark Thomas [EMAIL PROTECTED] http://www.adelaide.net.au/~markthom
RE: filmscanners:Focusing film flatness
--- You wrote: When I bought an expensive slide-projector about 10 years ago, I took it straight back when it gave out of focus edges on curved slides. After some argument, they ended up relenting and giving me a much better lens with sufficient depth of field. It copes easily with flat and curved slides, and so does my current scanner, a low end 2720 model. --- end of quote --- This is an interesting statement. The only things that affects depth of field in a lens is its apeture or focal length. A 'much better lens' doesn't necessarily imply either. What would affect edge sharpness is the flatness of the lens' focal field (not necessarily the correct technical term.) Not all camera lenses focus correctly on a flat surface and we are likely to spend extra bucks on a flat field macro lens for really accurate copying work, for example. We assume our expensive enlarging lenses are flat field. But under some practical conditions, where film planes aren't actually flat, a lens with a somewhat concave field is actually an advantage since it would give you a sharp image on a curved surface. In the projector business, it is likely that a 'better lens' has a slightly curved field to match the assumed curvature of a slide. Increasing a projector lens' depth of field by reducing it's apeture is impractical since it would result in a much dimmer image on the screen. So the question is, are the lenses in film scanners flat field, or are they slightly dished to accomodate film curvature? Or are some small apeture, high depth of field lenses working with more sensitive ccds. Tony praises a fixed focus Minolta scanner which would have to fit the later category. How about some others? Rich
RE: filmscanners:Focusing film flatness
So the question is, are the lenses in film scanners flat field, or are they slightly dished to accomodate film curvature? Or are some small apeture, high depth of field lenses working with more sensitive ccds. Kodak and others used to make projection lenses with field curvature designed to match the expected curvature of cardboard mounted slides. Worked well unless the slide was reversed or mounted in glass. It would be worthwhile to reverse the film in a scanner showing insufficient depth of field (a.k.a. the recent Nikon 4000 review). If the field curvature of the lens looks like ) but the film looks like ( then reversing the film would make both look like ).
RE: filmscanners:Focusing film flatness
And so it should! If the new (and I gather old) 4000 dpi Nikons can't handle a curved slide, there is no way I would consider them. I can just imagine ripping all of my (and my client's) Kodachromes out of their mounts before I scanned them. Oh what fun.. When I bought an expensive slide-projector about 10 years ago, I took it straight back when it gave out of focus edges on curved slides. After some argument, they ended up relenting and giving me a much better lens with sufficient depth of field. It copes easily with flat and curved slides, and so does my current scanner, a low end 2720 model. This is not rocket science.. I think it is a VERY fair criticism of the Nikon scanner. If Nikon has chosen a scanning method that doesn't work well for curved slides, I reckon they have just lost a significant portion of the market. Or can someone give me a really easy, quick and painless way of transferring a piece of film from a glued cardboard mount into a glass slide And, for that matter, a recommendation for glass slides that REALLY don't suffer from Newton's rings. My experience is that even those that are supposedly Newton-proof generally are not. Mark T At 07:01 PM 31/03/01 -0500, you wrote: Because of the light source the lens has significant depth of field. You need not be concerned. David -Original Message- From: Stan Schwartz [mailto:[EMAIL PROTECTED]] As many of my cardboard mounted transparencies are bowed to various degrees, I have been curious whether this focusing scheme tracks the curvature of the film--or does it just focus on a single plane. snip == Mark Thomas [EMAIL PROTECTED] http://www.adelaide.net.au/~markthom
Re: filmscanners:Focusing film flatness
Or can someone give me a really easy, quick and painless way of transferring a piece of film from a glued cardboard mount into a glass slide Mark T. For getting out of the cardboard pick up a Wess paper mount opener. Should be available in any good camera store. Aside from being quicker it protects against slips of knife. John M.
Re: filmscanners:Focusing film flatness
Most darkroom workers interested in quality wouldn't use a mounted slide for projection in an enlarger. If you've ever had a large custom print made by a good lab you'll see it's been taken out of the mount and replaced. Not too hard, cut the cardboard half way through with a single edge razor along the center of any of the 4 wide sides of the mount, bend one of the near corners down a bit and it'll come apart. (Don't remove the layer completely and you can re-use the mount. Replace the film and tape along the cut. Practice on outtake slides.) Then place the film in a glassless carrier, possibly using tape along the sprocketed sides to fix in place and flatten a bit if needed. I've scanned a few mounted slides in my LS-30 and don't recall any particular problem with focus at the edges. Some mounted slides are more curved than others however, it depends on type of film, processing conditions, how old the film was before processing, how it's mounted, and how it's stored. I did see a problem using the auto film strip feeder and neg strips, but the fix was easy, use the film strip holder. We've only seen one complaint about this so far? Perhaps we should wait a bit before coming to any hard and fast conclusions. And even if the DOF is on the shallow side with Nikon's new scanners, there will surely be easy fixes for careful workers. These new scanners are pretty compelling, and I doubt this will turn out to be a major problem, if any problem at all. Dave - Original Message - From: Mark T. [EMAIL PROTECTED] To: [EMAIL PROTECTED] Sent: Sunday, April 01, 2001 1:17 AM Subject: RE: filmscanners:Focusing film flatness And so it should! If the new (and I gather old) 4000 dpi Nikons can't handle a curved slide, there is no way I would consider them. I can just imagine ripping all of my (and my client's) Kodachromes out of their mounts before I scanned them. Oh what fun.. When I bought an expensive slide-projector about 10 years ago, I took it straight back when it gave out of focus edges on curved slides. After some argument, they ended up relenting and giving me a much better lens with sufficient depth of field. It copes easily with flat and curved slides, and so does my current scanner, a low end 2720 model. This is not rocket science.. I think it is a VERY fair criticism of the Nikon scanner. If Nikon has chosen a scanning method that doesn't work well for curved slides, I reckon they have just lost a significant portion of the market. Or can someone give me a really easy, quick and painless way of transferring a piece of film from a glued cardboard mount into a glass slide And, for that matter, a recommendation for glass slides that REALLY don't suffer from Newton's rings. My experience is that even those that are supposedly Newton-proof generally are not. Mark T At 07:01 PM 31/03/01 -0500, you wrote: Because of the light source the lens has significant depth of field. You need not be concerned. David -Original Message- From: Stan Schwartz [mailto:[EMAIL PROTECTED]] As many of my cardboard mounted transparencies are bowed to various degrees, I have been curious whether this focusing scheme tracks the curvature of the film--or does it just focus on a single plane. snip == Mark Thomas [EMAIL PROTECTED] http://www.adelaide.net.au/~markthom
