Modern optical designs allow the focal planes to be optically re-positioned within the lens assembly. This is done for extreme wide-angle to move the lens away from the mirror and for telephoto to shorten the length of the lens. The zoom ring on a lens will move one or more elements to change the focal length. This will affect the magnification, angle of view and aperture value. This is why most zoom lenses give different aperture values at minimum and maximum zoom. Within limits, some lenses can adjust the actual diameter of the aperture as the zoom range changes. This is why some zoom lenses have the same aperture at all zoom settings.
In theory, the focus ring alters the distance between the image sensor and the optical image plane. This does not alter the focal length of the lens, but it can affect the angle of view, thus the image size. Given a fixed distance between the film plane and the subject, there are three ways to move the optical image plane(s). The obvious choice is to physically move the lens assembly within the barrel. This demands heavy-duty mechanics and physically large lens barrels. Another choice is to move the focal plane(s) within the lens assembly. This is an elegant choice. But, without extreme diligence in the design, it can lead to changes in the overall focal length of the lens assembly. The radical approach is to change the focal length.
Lens specifications usually include the nearest distance you can focus and the corresponding reproduction ratio. This near focus limit is measured from the film plane to the subject. When stated as the working distance, it is measured from the front of the lens barrel without filters or hoods to the subject. If the near focus limit is given at 1:1, you can calculate a thin lens adjusted focal length. It will be the near focus limit divided by four. If the reproduction ratio is not 1:1, other formulas provided here may be used to approximate this adjusted focal length. For precise answers, you need to know the separate image and object distances relative to the center of the lens and these are not provided. Without knowing these, the math available can only be an approximation. The good news is that any focal length changes are insignificant at normal shooting distances. Read and heed the owner's manual that comes with the lens.
The point of all this is that calculations for depth of field and field of view are affected. All the readily available tools for calculating metrics of photography are based on the thin lens assumptions and formulas. At distant focusing or where the focal length change is small, the artifacts are negligible. Caveat Emptor.
As the lens (or focal point) moves significantly further from the image sensor, there is also an inherent loss of light (the inverse square law), leading to an effective aperture which is smaller than the actual opening. This effective aperture affects exposure but it does not affect depth of field. Some lens/camera combinations will adjust for this automatically, some will not. The same effect will be observed when using extension tubes.
The Nikkor D-type lenses relay subject to camera distance information to the body. It is used for 3D Multi-Sensor Balanced Fill-Flash with Speedlights. I see how this can be calculated since the image distance and the focal length are known. Unfortunately, Nikon does not seem to include this in any EXIF data that I can find. Nikon Capture does show an EXIF field for "subject distance" but it is always empty. And focal length changes from zoom are reflected in the EXIF data, but any change resulting from focus operations is not. These are features I would like to see in the next generation of digital SLRs. Understanding lens operations is already getting very complicated. As technology advances, the advanced consumers will need more and more information just to use them effectively.
There are many variables in choosing and using an SLR lens. Focal length for distance and framing, maximum aperture affecting the ability to collect light, minimum aperture affecting the DOF, MTF ratings affecting sharpness, and more. No single lens can address all the possible shooting challenges. Thatís why we chose to go the SLR route in the first place.
Higher quality lenses are going to cost more. Fact of life. But even the best of lenses will degrade at the limits of their operational design. So we should follow the proven techniques of the trade such as using medium apertures, tripods, good lighting, proper exposure, and others. We pay attention to depth of field, perception, and subject movement. Sometimes we have to make tradeoffs. Then again, the sharpest image isnít always the most artistic image and a unique perspective can be captivating.
I hope you found this article informative. It is not intended to be a definitive, final answer to all lens questions, but an introduction to the concepts and terminology. There was some math, but only the simplest forms. The intent was simply to reinforce the concepts presented. It also illustrates the fact that some metrics such as image/subject distances and magnification ratios need to be presented with a proper contextual reference. This exercise has certainly helped me to have a better understanding of the terminology and components of a modern lens system. As part of my research, I placed several of the formulas used here into a Microsoft Excel Spreadsheet. You are welcome to use them (unwarranted) for your own exercises and experiments. As I hope you can see, the laws of physics are still immutable, but with advances in optical technology there are many design options available to the manufacturers, thus many choices for the SLR photographer. Don't forget that 90% of the contribution to a good image will still be the photographer's artistic and creative ability. Happy shooting.
If you have any comments, suggestions, or insight, I would welcome your input. Especially if you find any errors in my assumptions, formulas, or conclusions. Consider this a working document that I want to update when new information is available. I do no want this just to contribute to the open sewer pipe of misinformation being brought on by the Internet. Please send me an Email.
Georgia State University:
University of Colorado at Boulder: Physics: Light and Color
Nikon Microscopy U: Microscope Optical Systems
Don Fleming: Depth of Field Articles
Luminous Landscape: Understanding MTF
NikonLinks.com: Nikon Lens Nomenclature
NikonLinks.com: MTF tests of Nikkor lenses
Rags Int., Inc.: A spreadsheet for exposure calculations
Rags Int., Inc.: A spreadsheet of various lens formulas
Optics in Photography: Rudolf Kingslake: ISBN-0819407631
Applied Photographic Optics: Sidney F. Ray: ISBN-0240515404
I would like to thank Len Shepard (UK) for his patience and thoughtful explanations as I tried to understand what I was reading.
Rags Int., Inc.
204 Trailwood Drive
Euless, TX 76039
July 25, 2003