Plane Of Focus (Various Angle Lens)
When taking a picture with a large aperture opening, the depth of field is limited.
Is the plane of focus, flat (variable distance from the lens) or curved (constant distance from the lens) when comparing the center and edge of a pic?
I have seen it is possibly variable for different lens, if so, then it could change for various focal lengths of a zoom lens.
Is this data that is available somewhere? Is the plane of focus similar for similar focal lengths (i.e. are all pics taken at 105mm the same?)?
Please help me understand what the plane of focus is. Thanks.
#1. "RE: Plane Of Focus (Various Angle Lens)" | In response to Reply # 0WildIsle Nikonian since 06th Jan 2010Wed 27-Feb-13 03:08 AM
It depends on the lens' optical formula. A spherical lens has a concave plane of focus. It's the multiple elements that aim to correct the curvature. Wide angle lenses typically have more than telephotos as you'd expect. Each focal length wouldn't give exactly the same spherical aberration if the optical formulas are different. It would also change with aperture.
That's about all I know - enough to know it's complicated!
#2. "RE: Plane Of Focus (Various Angle Lens)" | In response to Reply # 0icslowmo Registered since 01st Jan 2012Wed 27-Feb-13 03:17 AM | edited Wed 27-Feb-13 03:20 AM by icslowmo
If I'm understanding your question correctly, I believe you are referring to what is known as field curviture. From what I've seen about some lenses, the 24-70 f/2.8 has it and the 50 f/1.4G has it. Other lenses have it also. On a DX camera, you wouldn't notice it because the curviture happens at about the DX corner and would look like soft corners. On a FX camera, it would show up as sharp center, slightly soft borders, and then ok sharp corners. I haven't seen any actual tests done to show the curviture in the focus plan though.. I can have you check out these links:
Granted it is the new Sigma 35mm f/1.4, but shows what it would look like on focus charts.
Also this link shows example with the Canon 24-70 f/2.8 Version 1 (Which I believe the Nikon version has the same optical curviture effect):
Hope this helps answer your question.
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#3. "RE: Plane Of Focus (Various Angle Lens)" | In response to Reply # 0Gene Duprey Registered since 08th Feb 2013Wed 27-Feb-13 03:25 AM
Boy what a question. You should get a book on optics , but here is the quick and dirty answer:
Yes, large aperture lenses have a very narrow depth of field.
The plane of focus will vary for every lens, but yes there is some field curvature in all lenses, but the degree is dependent of the lens design, and focal length. You will find the focus is always softer to some degree at the corners, and edges, some lenses are better than others.
The plane of focus for all lenses of a manufacturer, i.e., Nikon, are designed to focus at the film plane or sensor surface, for all focal lengths. However the different colors will not focus at the same point, unless the design is of an Apochromatic design. But this is only a problem, effectively, at the longer lengths. ED glass, and aspherical lens elements are usually used to help correct this.
Yes there are several books and articles on this and lens design in general. Erwin Putts has a website and he discusses this extensively. He has a couple of books on this, but are written around Leica lenses, but the theory and general issues apply to all lenses. Again the focal point is the same for all lenses of a particular manufacturer. I believe Zeiss also has several articles on lens design on their web site.
Hope this helps.
#4. "RE: Plane Of Focus (Various Angle Lens)" | In response to Reply # 0PeterBeckett Nikonian since 04th Jan 2010Thu 28-Feb-13 05:35 PM | edited Thu 28-Feb-13 05:35 PM by PeterBeckett
It is my understanding that lenses having a "macro" designation can be expected to have a flatter plane of focus than lenses without that designation. Lenses with a "copy" designation were specially designed for flat subject copying and had/have very flat fields AND very low distortion. However, I don't think I remember seeing any "35mm camera" lens touted as a copy lens apart from my old 80mm Olympus Zuiko that was designed specifically for slide copying.
I remember that another lens I used to possess was touted as a great "copy" lens with a superbly flat field. It was also spectacular for general W/A work. I'm referring to the 38mm Biogon of my Hasselblad SW-C. Ah! Memories...
#5. "RE: Plane Of Focus (Various Angle Lens)" | In response to Reply # 4KnightPhoto Nikonian since 18th Dec 2006Fri 01-Mar-13 12:05 AM
I've heard the old Nikon 55mm Micro lenses described as copy lenses. My assumption is the more modern 60mm Micros probably share this attribute as well.
On the OP, it's a wonder we ever get anything in focus
Best regards, SteveK
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