6/26/96
I was recently asked the following question about film resolution. I have seen a discussion of this somewhere but can't seem to locate it now. Can anyone point me in the right direction?
Bob Wise
***************
"What is the max resolution of the EM film you used to use? [This was Kodak EM film no. 4489, Estar thick base] In particular, how close can two lines be and still be distinguished, or how thin can a line be and still be detected. Also I want to know how many photograins it takes to generate this level of resolution. For example, on a 2" x 3" film, the smallest dot that can be detected has an area of x sq. inches and this area on the film contains on average y silver grains."
Can you give me any help in answering this? Is there some data on the size and concentration per unit area of grains in various films?
********************************
reply: The questions Bob is asking are interesting and I expect that he will receive some direct answers. But there are some related points which should be looked at in this context, particularly when analogies to digital imaging are to be drawn.
Line resolution of any film suitable for TEM is better than 200/mm. In TEM it is desirable to maximise electrons for exposure, this will assure better, less grainy, more contrasty and better resolved images. Slight over-exposure and a very slow film type are in fact desirable. This is fortuitous: The TEM's requirements make the highest resolution and most contrasty emulsions the most suitable.
In general terms, resolution of a TEM is equal at all magnifications but a low power image would require enlarging. At about 30x photographic magnification, insufficient electrons have formed the image and "noise" becomes intolerable. Also, above 15x and certainly by 20x, the photographic enlarging procedure becomes quite impractical.
There is a very good reason to use fairly high, perhaps 10x to 15x, photographic enlargements: The depth-of-field (or focus) is much greater at lower magnifications. It is much easier to take an in-focus picture at 20k than at 70k. Enlarging to 200k will give "identical images", but the 70k image is much more likely to be out of focus. Anybody who frequently requires very high magnification TEM images would appreciate that depth-of-field is a powerful argument.
The corollary is: Except for giant enlargements, only a small postage stamp size area from the centre of the negative is normally used. There is also a not very subliminal message here. The question now is: Which digital system maintains the huge advantages of low power depth-of-field and has the resolution to allow 20x enlargement, if only from a postage stamp sized area?
SEMs and light microscopes are another discussion and reasons for digital images from these are well advanced.
It should be noted that P&S; has an interest in film, digital cameras and printers. - We like them all!
Jim Darley
Probing & Structure
Microscopy Supplies & Accessories
Phone +61 77 740 370
Fax: +61 77 892 313
A great microscopy site http://www.ultra.net.au/~pns/
pns@ultra.net.au