what focal lenght lens for using extension tubes?
Posted 03 December 2005 - 08:57 AM
Posted 03 December 2005 - 11:49 AM
I have a set of extension tubes (m42 thread) that fit my Kras-3. Only the shortest two used singly can provide a focusable image (quite close I must say) and that is using full zoom of the lens (69mm). Ideally, I would be using a fixed focal lenth lens. What I hope to do is some basic optical printing filming a projector gate (J-K printer style). As such I need to get very large magnification. Am I correct in thinking that I need to purchase a long focal length lens for this in order to be able to use a longer combination of extension tubes? Many thanks.
the focal length of a lens is the distance from the opticle center of the lens to the film plane when focused at infinity. When the lens is at 2 times the distance from the film plane the center the object will be at 1:1. In other words if you have a 90 mm lens and set it 180 mm from the film plane then put a piece of film 180 mm from the front of the lens center it will be in focus and the image will be its original size. The problem with a zoom is that it has complex optics and the opticle center changes as you zoom and focus. Sometimes the opticle center of the lens can be outside of the lens. For working at high magnifications you should try to find a macro lens. Lenses designed for macro work are optimized for working near 1:1 magnifcation and will be sharper and have fewer cromatic aberations. I suggest you find a really good book on macro photography. Kodak used to publish a series perhaps now out of print you may be able to find at a library, Photomacrography N-12B, Cinephotomicrography N-2
Posted 03 December 2005 - 09:38 PM
The problem with a zoom is that it has complex optics and the opticle center changes as you zoom and focus. Sometimes the opticle center of the lens can be outside of the lens. For working at high magnifications you should try to find a macro lens. Lenses designed for macro work are optimized for working near 1:1 magnifcation and will be sharper and have fewer cromatic aberations.
Zooms ONLY work "right" mounted directly on the camera. even a tele-converter which works fine with a fixed lens will make a zoom go soft and smeary.
Since you have an M42 Camera, there was a "macro takumar" (for a picture see ebay Item number: 7567466153) which focuses very close, there was also a "bellows takumar" (see ebay Item number: 7564469895) which depended on using a set of Bellow to focus, and can get well past 1:1 Both may be hard to find.
A belows is somewhat better then extension subes just becuse you can pick any lens to film distance.
Another item to get is a reversing ring, using that with a "normal" (prime) SLR lens will get you in the close up range and give better sharpness than the same lens on just an extension tube. You can add extension tubes to the mix. Pentax made the reverse adaptors to fit lenses with both 49mm and 52mm filter threads. You screw the adaptor on the lens like a filter, and then mount the adaptor on the camera.
Posted 04 December 2005 - 04:37 PM