Godfrey
somewhat colored
I did physics and optics a long long time ago too. 1:1 macro was always the gold standard for data acquisition of small things (... so that you could make precise measurements easily...), but there was always a macro range ... I recall my professor saying that he considered capturing anything with a magnification from 1:4 to 2:1 he considered photomacography and anything with higher magnification than that was slipping into micro-photography (yeah, he used those terms exactly).
1:10 isn't very magnified, but I think these are all just conventions adopted by various people at various times rather than formal definitions.
The discussion of what distance constitutes infinity focus for a given focal length lens is interesting. It's inspired me to do a little research on the subject. The simple statement it: Infinity for an optical system is defined as that distance from the camera whereby the light rays from the subject arrive as parallel rays, that is, that it falls within the size of the circle of confusion and thus cannot be distinguished from it. Calculating that gets a little complicated, and is dependent upon both the CoC convention using to calculate DoF, the working diameter of the lens (focal length divided by the physical size of the aperture), the focal length of the lens, and a magnification constant that seems to be typically 4000.
Example: Given a CoC of 0.05mm and a 50mm lens set to f/2, the working diameter of the lens is 25mm. Using 4000 as the magnification constant, infinity is 4000 x 25 = 100,000mm = 100m = 3,937 inches = 328 feet.
To show that this works, consider the same CoC, the same lens but set to f/8, and the same magnification constant, 4000. The working diameter is now 6.25mm, 6.25mm x 4000 = 25,000mm = 25m = 984 inches = 82 feet.
Fun stuff!
G
1:10 isn't very magnified, but I think these are all just conventions adopted by various people at various times rather than formal definitions.
The discussion of what distance constitutes infinity focus for a given focal length lens is interesting. It's inspired me to do a little research on the subject. The simple statement it: Infinity for an optical system is defined as that distance from the camera whereby the light rays from the subject arrive as parallel rays, that is, that it falls within the size of the circle of confusion and thus cannot be distinguished from it. Calculating that gets a little complicated, and is dependent upon both the CoC convention using to calculate DoF, the working diameter of the lens (focal length divided by the physical size of the aperture), the focal length of the lens, and a magnification constant that seems to be typically 4000.
Example: Given a CoC of 0.05mm and a 50mm lens set to f/2, the working diameter of the lens is 25mm. Using 4000 as the magnification constant, infinity is 4000 x 25 = 100,000mm = 100m = 3,937 inches = 328 feet.
To show that this works, consider the same CoC, the same lens but set to f/8, and the same magnification constant, 4000. The working diameter is now 6.25mm, 6.25mm x 4000 = 25,000mm = 25m = 984 inches = 82 feet.
Fun stuff!
G
seany65
Well-known
Thanks again for all the new replies and help.
Hmm, it does seem that we should be using the lens's maximum aperture to find the furthest "infinity" distance.
Rob_F: I'm not sure if I was specifically thinking about the placement of the marking on the lens and the actual distance of infinity, I was more wondering about the "guessed" distances between the last marked distance and infinity and how many of these "guessed" distances there should be. eg:
(I'm still not sure how to post pics on this forum. Each time i try it seems to be a different way of doing it than last time, lol). Anyway, if the pic comes up big enough for you to see, you'll notice the 2 marked distances "G" 10ft, "S" 16ft. Confusingly, this seems physically half-way between the 10ft and infinity which should really be 20ft, as far as I can tell) and the "Guessed" distances of 32ft. and 64ft. I presume as we don't know exactly, that the "infinity" mark should also be a "guessed" distance, and for somplicity I've chosen 128ft.
I was wondering if (in this case) 32ft and 64ft should be enough, or should 128ft be included and so we consider 256ft to be infinity for this lens (28mm on half-frame so about 40mm equivalent on full 35mm film).
So to put it as shortly as possible:
How many "Guessed steps" should there be between the last marked distance and the infinity mark, if a lens is used at it's widest aperture?
I presume there will be fewer "guessed steps" the shorter the focal length of the lens.
Other questions could include: Is the infinity mark at the same distance for the same focal length and the same widest aperture, for different lens/camera makers?
What about using a single accessory rangefinder for cameras that have fixed lenses of different focal lengths, either of the same format, say 35mm, or different formats, say 35mm half-frame and 6x9?
Hmm, it does seem that we should be using the lens's maximum aperture to find the furthest "infinity" distance.
Rob_F: I'm not sure if I was specifically thinking about the placement of the marking on the lens and the actual distance of infinity, I was more wondering about the "guessed" distances between the last marked distance and infinity and how many of these "guessed" distances there should be. eg:
(I'm still not sure how to post pics on this forum. Each time i try it seems to be a different way of doing it than last time, lol). Anyway, if the pic comes up big enough for you to see, you'll notice the 2 marked distances "G" 10ft, "S" 16ft. Confusingly, this seems physically half-way between the 10ft and infinity which should really be 20ft, as far as I can tell) and the "Guessed" distances of 32ft. and 64ft. I presume as we don't know exactly, that the "infinity" mark should also be a "guessed" distance, and for somplicity I've chosen 128ft.
I was wondering if (in this case) 32ft and 64ft should be enough, or should 128ft be included and so we consider 256ft to be infinity for this lens (28mm on half-frame so about 40mm equivalent on full 35mm film).
So to put it as shortly as possible:
How many "Guessed steps" should there be between the last marked distance and the infinity mark, if a lens is used at it's widest aperture?
I presume there will be fewer "guessed steps" the shorter the focal length of the lens.
Other questions could include: Is the infinity mark at the same distance for the same focal length and the same widest aperture, for different lens/camera makers?
What about using a single accessory rangefinder for cameras that have fixed lenses of different focal lengths, either of the same format, say 35mm, or different formats, say 35mm half-frame and 6x9?
Rob-F
Likes Leicas
I did physics and optics a long long time ago too. 1:1 macro was always the gold standard for data acquisition of small things (... so that you could make precise measurements easily...), but there was always a macro range ... I recall my professor saying that he considered capturing anything with a magnification from 1:4 to 2:1 he considered photomacography and anything with higher magnification than that was slipping into micro-photography (yeah, he used those terms exactly).
1:10 isn't very magnified, but I think these are all just conventions adopted by various people at various times rather than formal definitions.
The discussion of what distance constitutes infinity focus for a given focal length lens is interesting. It's inspired me to do a little research on the subject. The simple statement it: Infinity for an optical system is defined as that distance from the camera whereby the light rays from the subject arrive as parallel rays, that is, that it falls within the size of the circle of confusion and thus cannot be distinguished from it. Calculating that gets a little complicated, and is dependent upon both the CoC convention using to calculate DoF, the working diameter of the lens (focal length divided by the physical size of the aperture), the focal length of the lens, and a magnification constant that seems to be typically 4000.
Example: Given a CoC of 0.05mm and a 50mm lens set to f/2, the working diameter of the lens is 25mm. Using 4000 as the magnification constant, infinity is 4000 x 25 = 100,000mm = 100m = 3,937 inches = 328 feet.
To show that this works, consider the same CoC, the same lens but set to f/8, and the same magnification constant, 4000. The working diameter is now 6.25mm, 6.25mm x 4000 = 25,000mm = 25m = 984 inches = 82 feet.
Fun stuff!
G
This looks like what I was struggling toward, only taking into account the lens's aperture, which I didn't see at first was necessary. Good job, Godfrey! Yes, it is fun.
Beemermark
Mentor
Wow! I always considered anything past 25 feet as infinity for focusing a 35mm camera. Never gave it as much thought and always happy with the results.
David Hughes
David Hughes
I did physics and optics a long long time ago too. 1:1 macro was always the gold standard for data acquisition of small things (... so that you could make precise measurements easily...), but there was always a macro range ... I recall my professor saying that he considered capturing anything with a magnification from 1:4 to 2:1 he considered photomacography and anything with higher magnification than that was slipping into micro-photography (yeah, he used those terms exactly).
1:10 isn't very magnified, but I think these are all just conventions adopted by various people at various times rather than formal definitions.
The discussion of what distance constitutes infinity focus for a given focal length lens is interesting. It's inspired me to do a little research on the subject. The simple statement it: Infinity for an optical system is defined as that distance from the camera whereby the light rays from the subject arrive as parallel rays, that is, that it falls within the size of the circle of confusion and thus cannot be distinguished from it. Calculating that gets a little complicated, and is dependent upon both the CoC convention using to calculate DoF, the working diameter of the lens (focal length divided by the physical size of the aperture), the focal length of the lens, and a magnification constant that seems to be typically 4000.
Example: Given a CoC of 0.05mm and a 50mm lens set to f/2, the working diameter of the lens is 25mm. Using 4000 as the magnification constant, infinity is 4000 x 25 = 100,000mm = 100m = 3,937 inches = 328 feet.
To show that this works, consider the same CoC, the same lens but set to f/8, and the same magnification constant, 4000. The working diameter is now 6.25mm, 6.25mm x 4000 = 25,000mm = 25m = 984 inches = 82 feet.
Fun stuff!
G
Interesting, many thanks. Trying to find/remember how I did it the hard, back of an envelope way I came across so many rules of thumb that I now think it's not so much science as one of the black arts.
I won't bore you with the details but my version involved working out the hyperfocal distance first and then going from there.
And a useful rule of thumb was that there's twice as much behind the subject as in front. I still use it in that I tend to focus on somewhere about a third of the way along.
Regards, David
PS This thread seems to be becoming a circle of confusion too.