#### mathbias

##### TPF Noob!

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I'm trying to find the focal plane of my camera (Sony a7 iii, with FE 28-70mm F3.5-5.6 OSS). The camera has the location of the sensor plane marked, but all focus math requires the location of the focal plane.

A hypothetical single element lens would have the focal plane ahead of the sensor plane by the focal length. I thought that separation would be lower for a real lens. My test results (below) make no sense. Please explain what I'm doing wrong and/or tell me what I should do instead and/or which product stats to disbelieve.

Camera on tripod with center of view aimed EXACTLY (11.7x digital zoom assisted) at one end of 4-foot "yardstick), with sensor plane 70 inches from that centered tip of the stick. I take a photo, then zoom within the edge of that image to get a precise measure of half the subject width, which is 41.125 inches at the 28 end of the zoom and 16.25 at the 70 end.

I looked up the sensor width and found 36mm.

Using H = half the subject width and D=distance subject to focal plane and F=focal length, I'm pretty sure the right formula is:

D = (2*H)*F/36

Notice D and H are in the same units as each other, while F and 36 are in the same units as each other, so D,H don't need to be in the same units as F.

The distance from the center of the subject to the focal plane is then 63.2 inches for F=70 and 64 inches for F=28. But that puts the focal plane 6 to 6.8 inches ahead of the sensor plane. But that is further away than the front of the lens. I don't understand how the focal plane could be entirely outside the physical structure.

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Before posting, I searched this forum and elsewhere for similar subjects. I found many threads that describe the sensor plane symbol on the camera and say that it is the "sensor plane" but then treat it (in all the related instructions) as if it were the focal plane. I don't think I misunderstood. But please correct me if you think I did and/or point to a discussion that has correct focal math given a sensor plane marking and no focal plane marking.

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On a related subject, the bundle I bought included a Vivitar 55mm 2.2x Professional Telephoto Lens HD4. All the online info I could find claims it multiplies focal length by 2.2. With testing similar to what I described above, I'm pretty sure I measured that it multiplies focal length by about 1.4; I checked the product name on the product, not just on the box. Both say 2.2

Is it not really the product it is labeled as? Does it interact with my Sony lens to produce less magnification than it would with a different lens? Is there some misleading standard whereby 2.2x means it multiplies focal length by 1.4?

If I later buy some other lens for multiplying focal length, how can I translate before purchase from what it says the multiple is to what the multiple actually is?

A hypothetical single element lens would have the focal plane ahead of the sensor plane by the focal length. I thought that separation would be lower for a real lens. My test results (below) make no sense. Please explain what I'm doing wrong and/or tell me what I should do instead and/or which product stats to disbelieve.

Camera on tripod with center of view aimed EXACTLY (11.7x digital zoom assisted) at one end of 4-foot "yardstick), with sensor plane 70 inches from that centered tip of the stick. I take a photo, then zoom within the edge of that image to get a precise measure of half the subject width, which is 41.125 inches at the 28 end of the zoom and 16.25 at the 70 end.

I looked up the sensor width and found 36mm.

Using H = half the subject width and D=distance subject to focal plane and F=focal length, I'm pretty sure the right formula is:

D = (2*H)*F/36

Notice D and H are in the same units as each other, while F and 36 are in the same units as each other, so D,H don't need to be in the same units as F.

The distance from the center of the subject to the focal plane is then 63.2 inches for F=70 and 64 inches for F=28. But that puts the focal plane 6 to 6.8 inches ahead of the sensor plane. But that is further away than the front of the lens. I don't understand how the focal plane could be entirely outside the physical structure.

-------

Before posting, I searched this forum and elsewhere for similar subjects. I found many threads that describe the sensor plane symbol on the camera and say that it is the "sensor plane" but then treat it (in all the related instructions) as if it were the focal plane. I don't think I misunderstood. But please correct me if you think I did and/or point to a discussion that has correct focal math given a sensor plane marking and no focal plane marking.

------

On a related subject, the bundle I bought included a Vivitar 55mm 2.2x Professional Telephoto Lens HD4. All the online info I could find claims it multiplies focal length by 2.2. With testing similar to what I described above, I'm pretty sure I measured that it multiplies focal length by about 1.4; I checked the product name on the product, not just on the box. Both say 2.2

Is it not really the product it is labeled as? Does it interact with my Sony lens to produce less magnification than it would with a different lens? Is there some misleading standard whereby 2.2x means it multiplies focal length by 1.4?

If I later buy some other lens for multiplying focal length, how can I translate before purchase from what it says the multiple is to what the multiple actually is?

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