replacing shutter to stop down beyond f45 (240mm on 8x10)greenspun.com : LUSENET : Large format photography : One Thread
I have a question about the large format (8x10) camera lens. The lens which I have is Schnider 240mm with copal 3 shutter. The problem is the smallest appeture of the lens is only f45, and I am wondering if I could replace the shutter which has f-stop up to f64. The lens is still good condition, so I don't want to buy new 240 mm lens. Also, If that is possible, I 'd like to know where to buy those shutter which might fit the lens which I own. Thanks in advance Joe
-- joe (firstname.lastname@example.org), January 02, 2001
the shutter you have will most likely go beyond f45 even though it is not marked. you don't need a new shutter. if you need to be more exact than estimating one more stop from f45 you can get a new aperture scale, perhaps from steve grimes.
-- adam friedberg (email@example.com), January 02, 2001.
Joe, unless it is absolutely necessary to stop down past f22 you will experience the effects of two primary optical aberrations, spherical aberration and diffraction. As you stop the lens down, spherical aberration is reduced, and the effects of diffraction increased. This generally results in a "sweet spot" around f11, where the image has benefited from reduced spherical aberration, but not yet been degraded severely by diffraction. At small apertures such as f64, diffraction effects are quite large and the image will be noticeably softer. Your copal shutter on that 240 Schneider should be just fine. Regards Steve
-- Steve Gilb (firstname.lastname@example.org), January 03, 2001.
Steve: Can you explain about two primary optical aberrations and "sweet spot"you just mentioned a little bit easier in the bigenner's term?? Why the image will be softer if I stop down past f22?? Could anyone can explain me?? I have no idea what you are talking about here. Thanks joe
-- joe (email@example.com), January 03, 2001.
If you had a theoretically perfect lens, it would be sharpest wide open. Physics dictates that the larger the aperture opening, the sharper the optic... assuming no other factors. The reason is the phenomenon of diffraction that results from the wave nature of light. Smaller openings cause more diffraction which limits theoretical sharpness.
Of course you don't have a theoretically perfect lens, and there are other factors in the real world. The two competing factors in the real world are optical imperfections in real lenses, and the need for depth of field in most photographs of 3D scenes. Stopping down the lens corrects for some of the most common optical imperfections (aberrations) like spherical aberration mentioned above. Stopping down also increases depth of field. So there is a trade off between depth-of-field/aberration correction and diffraction effects.
In practice, diffraction effects are a function of negative size, since the amount of enlargement is typically a function of negative size. With 35mm, you need very high resolution so diffraction effects become significant at apertures like f/16-f/22. This is why you will rarely find an aperture smaller than f/22 on any 35mm lens. The larger the format, the smaller the aperture before diffraction becomes a problem (this assumes of course that you are enlarging LF less than smaller formats).
There are many threads of discussion on this issue here on the LF page, and there is also a discussion of optimizing depth-of-field vs. diffraction effects under "How to select the f-stop" on Tuan's LF homepage. Beware, the math gets messy.
In practice, you can adopt some simple guidelines based on the size prints you normally make. I usually make 16x20 or 20x24 inch prints. For roll-film (6x9cm) I am comfortable using f/11-f/16 and will sometimes use f/22 when I need the depth of field. I avoid smaller stops WHENEVER POSSIBLE. Sometimes, I sacrifice abosolute sharpness for depth-of-field and use a smaller stop. You have to decide what counts most in the image you are trying to make.
For 4x5, f/22 is fine, and I have yet to see any significant softening at f/32, but I avoid smaller stops unless I need the DOF.
For 8x10, you can certainly use f/32 with no diffraction worries, and also f/45. If you need the DOF, use f/64. A lot of great images have been made at f/64!
Now if you contact print 8x10, you can use even smaller stops since you aren't enlarging the image at all. On the other hand, if you are going to be making wall size murals, diffraction will begin to be evident at wider apertures... but it probably won't be a problem since you don't usually view wall size murals from 15" either.
-- Glenn Kroeger (firstname.lastname@example.org), January 03, 2001.
If you're contact printing with your 8x10 negatives, diffraction will be of no concern. If you're enlarging, I'd suggest that you obtain a copy of two articles that appeared in the March/April issue of "Photo Techniques" magazine entitled "Image Sharpness and Focusing the View Camera" and "View Camera Focusing." Back issues of this magazine are usually available. These two articles demonstrate, at least to my satisfaction, that the sharpest image isn't necessarily produced by the widest aperture, even assuming that you are in a position to use the widest aperture (i.e. that depth of field is of no concern). According to the tests run by the authors of these two articles, there is an optimum aperture with any lens in any given situation - if the aperture used is smaller than, or larger than, the optimum aperture then there will be some unnecessary degradation of the image. The article explains how to determine this optimum aperture in any given situation with any lens and it's quite easy to do. Of course I think anyone using 8x10, as I do, needs to remember that when we talk about "degradation" with an 8x10 negative we may not be talking about something that is even observable to the naked eye in the print and certainly not something that makes the print look horrible. I've made quite a few photographs at f 64 and enlarged the 4x5 negative to 11x14 without any problems that were obvious to me. OTOH, I remember Howard Bond saying in an article somewhere that he made one negative a F 64 and the resulting print looked so bad that he's never made another one. As between me and Howard Bond, you probably should listen to Howard Bond : - )
-- Brian Ellis (email@example.com), January 03, 2001.
The articles that Brian refered to are available on the LF page as references. See "How to select the f-stop ", http://www.cs.berkeley.edu/~qtluong/photography/lf/fstop.html.
-- Q.-Tuan Luong (firstname.lastname@example.org), January 03, 2001.
Any distortion of the image, whether caused by diffraction, lens aberration, problems with film flatness etc. will of course become more obvious the more that the negative is enlarged.
However, that is only part of the reason why diffraction is a greater problem for smaller format cameras when they are stopped down.
Diffraction occurs when light travels through a small opening. The smaller the opening then the greater the degree of diffraction. When diffraction occurs there is a scattering of some of the light which tends to degrade the quality of the image.
But whereas the amount of diffraction that occurs is directly linked to the diameter of the aperture, it is only indirectly linked to the f stop.
A 50 mm lens set at f16 will have an aperture diameter of 50/16 or approximately 3 mm. A 300 mm lens set at f16 will have an aperture diameter of 300/16 or approximately 19 mm. Therefore the degree of diffraction on the 50 mm lens at f16 is going to be much greater than on the 300 mm lens at the same f stop. In fact you would have to stop the 300 mm lens down to about f90 for the lenses to have the same aperture diameter and so the same degree of diffraction.
Large format cameras tend to be used with longer lenses and so diffraction is only going to be problematic when the lens is stopped down to a greater extent. But the difference in diffraction between the formats disappears if the same focal length lens is considered. And to say, for example, that on 4x5 cameras diffraction does not have an effect until the lens is stopped down to f32 or less is almost certainly true for a standard 150 mm lens but it is probably not true for a 47 mm lens.
-- Philip Y. Graham (PYG@plastsurg.com), January 03, 2001.
Just a note to add to the previous post. Strictly speaking, diffraction is a function of aperture size or the physical size of the hole and that is how it would be defined in a physics textbook. Which means that the larger area aperture in a 300mm lens at f/16 (as compared to a 50mm lens at f/16) should provide lower diffraction. However, diffraction patterns are angular patterns and as such are dependent on how far from the aperture you place the screen used to view it also. In photography, the aperture is at the optical center of the lens and the screen is (for infinity focus) one focal length away. The physical size of the diffraction blur is then the focal length divided by the apparent size of the aperture i.e., the definition of the f stop. Thus, in photography, diffraction is only a function of f stop and not a function of the focal length. In simpler terms, the larger aperture of the 300mm lens does offer lesser diffraction at the diaphragm (i.e., less bending around the diaphragm) but since the light now has a longer distance to travel (as compared to the 50mm lens), the smaller bending still results in a fair bit of blur at the viewing screen. Sorry to be pedantic but... We now return you to your regularly scheduled program. Cheers, DJ.
-- N Dhananjay (email@example.com), January 03, 2001.