Return to Answer

But it practice, especially with consumer grade lenses or even pro grade zooms, it rarely works out that way with any degree of precision.

But in practice, especially with consumer grade lenses or even pro grade zooms, it rarely works out that way with any degree of precision.

So in theory a lens with twice the focal length should project an image on the sensor that is twice as large linearly as another lens.

So in theory a lens with twice the focal length should project an image on the sensor that is very close to twice as large linearly as another lens.

• The first is focal length. The focal length of many, if not most, lenses is rounded up or down to the nearest 'standard' focal length. A lens with an actual focal length of 53.78mm will probably be labeled a 55mm lens. A lens with an actual focal length of 37.2mm will probably be labeled as a 35mm lens. Telephoto lenses almost always tend to round up, particularly zoom lenses. Your 70-300mm zoom lens is probably closer to actually being a 78-287mm lens or something similar. Common focal lengths that lenses are designated include 20mm, 24mm, 28mm, 35mm, 40mm, 50mm, 70mm, 85mm, etc. Lenses that fall in the gaps between these numbers are usually rounded to the nearest one (in whichever direction the manufacturer thinks will sell more lenses). Below about 18mm lenses seem to usually be rounded down to the nearest integer. A lens with an actual focal length of 12.6mm will almost always be marketed as a 12mm lens. And although there are 17mm, 16mm, 15mm, 14mm, 12mm, 11mm, 10mm, etc. lenses I'm not sure I've ever seen a lens marketed as a 13mm. In Western culture 13 is considered to be an unlucky number.
• The focal length of lenses is measured when they are focused at infinity. When most zoom lenses are focused closer, it tends to increase the angle of view which has the effect reducing the magnification of the len. Prime lenses, on the other hand usually reduce the FoV as they are focused closer which increases the lens' magnification and reduces the FoV. This is called focus breathing. It can be corrected in lens design, but doing so increases the cost of the lens. Certain telephoto zoom lenses can be notorious for this. Nikon's AF-S 70-200mm f/2.8G VR II is one. At maximum zoom (marked 200mm) and minimum focus distance the field of view projected onto a full frame sensor is equivalent to a focal length of only about 134mm! Nikon's latest AF-S 70-200mm f/2.8 FL VR does much better in this regard. Canon's EF 70-200mm f/2.8 L IS II breathes outward like a prime lens and gives about a 230mm FoV at MFD compared to right at 200mm at infinity focus. Cinema lenses tend to more or less fully correct for focus breathing, and their prices that can be around 5-10X the cost of their non-cinematic counterparts reflect that. A macro lens that can project a 1:1 image onto the sensor (the image projected onto the sensor is the same size as the object being imaged) has a field of view at 1:1 that is half its FoV when focused at infinity.
• Twice as big is a bit nebulous Do you mean twice as large in terms of the total area the subject covers on the sensor? Or do you mean twice as tall and twice as wide, which would give the subject four times the area on the sensor? If you mean linear measurements then a 200mm lens focused at infinity should make a subject at infinity look twice as tall and wide as a 100mm lens focused at infinity would make a subject at infinity look.

• The first is focal length. The focal length of many, if not most, lenses is rounded up or down to the nearest 'standard' focal length. A lens with an actual focal length of 53.78mm will probably be labeled a 55mm lens. A lens with an actual focal length of 37.2mm will probably be labeled as a 35mm lens. Telephoto lenses almost always tend to round up, particularly zoom lenses. Your 70-300mm zoom lens is probably closer to actually being a 78-287mm lens or something similar. Common focal lengths that lenses are designated include 20mm, 24mm, 28mm, 35mm, 40mm, 50mm, 70mm, 85mm, etc. Lenses that fall in the gaps between these numbers are usually rounded to the nearest one (in whichever direction the manufacturer thinks will sell more lenses). Below about 18mm lenses seem to usually be rounded down to the nearest integer. A lens with an actual focal length of 12.6mm will almost always be marketed as a 12mm lens. And although there are 17mm, 16mm, 15mm, 14mm, 12mm, 11mm, 10mm, etc. lenses I'm not sure I've ever seen a lens marketed as a 13mm. In Western culture 13 is considered to be an unlucky number.
• The focal length of lenses is measured when they are focused at infinity. When most zoom lenses are focused closer, it tends to increase the angle of view which has the effect reducing the magnification of the len. Prime lenses, on the other hand usually reduce the FoV as they are focused closer which increases the lens' magnification and reduces the FoV. This is called focus breathing. It can be corrected in lens design, but doing so increases the cost of the lens. Certain telephoto zoom lenses can be notorious for this. Nikon's AF-S 70-200mm f/2.8G VR II is one. At maximum zoom (marked 200mm) and minimum focus distance the field of view projected onto a full frame sensor is equivalent to a focal length of only about 134mm! Nikon's latest AF-S 70-200mm f/2.8 FL VR does much better in this regard. Canon's EF 70-200mm f/2.8 L IS II breathes outward like a prime lens and gives about a 230mm FoV at MFD compared to right at 200mm at infinity focus. Cinema lenses tend to more or less fully correct for focus breathing, and their prices that can be around 5-10X the cost of their non-cinematic counterparts reflect that. A macro lens that can project a 1:1 image onto the sensor (the image projected onto the sensor is the same size as the object being imaged) has a field of view at 1:1 that is half its FoV when focused at infinity.
• Twice as big is a bit nebulous Do you mean twice as large in terms of the total area the subject covers on the sensor (a lens with 1.4X focal length should theoretically do that)? Or do you mean twice as tall and twice as wide, which would give the subject four times the area on the sensor? If you mean linear measurements then a 200mm lens focused at infinity should make a subject at infinity look twice as tall and wide as a 100mm lens focused at infinity would make a subject at infinity look.