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I understand using an A-mount lens intended for APS-C sensors on a full-frame camera results in cropping of the image. According to a tech at Sony, you lose 1/3 of pixels (APS-C is a 2/3 sensor so it makes sense).

If you move the lens out by a 1/3 of focus distance using extensions, shouldn't you gain the full frame image? With possible loss of infinity focus?

The selection for E-mount full-frame lenses is somewhat limited, so I'm trying to see if there is a compromise path to re-using older lenses.

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    Were you planning on focusing this lens on anything beyond macro lengths? – inkista May 22 '15 at 17:57
  • Well I was hoping to get portrait distances. Looks like I'm only shooting nose hairs then. :) – MandoMando May 23 '15 at 13:53
  • I was reading just recently that full-frame E mount is problematic so a good selection might not appear. See some good diagrams onnthe cited page, relevant to your question. – JDługosz May 25 '15 at 5:17
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To increase the 28.8mm image circle cast by the APS-C lens to the 43.2mm image circle needed by the FF sensor, you would need to increase the focal length by a factor of 0.5, or one-half again. A 24mm lens would require a 12mm extension tube. A 50mm lens would require a 25mm extension tube. A 100mm lens would require a 50mm extension tube, and so on. That is because your modified focal length must be 3/2 of the original to offset the image circle that is only 2/3 of what you need (3/2 x 2/3 = 1). At those kind of tube lengths, you will probably not be able to focus very far beyond Macro distances.

In terms of surface area, an APS-C sensor is only roughly 44% the size of a full frame sensor. The linear measurements are two-thirds the linear measurements of a FF sensor.

A 36mm x 24mm FF sensor has an area of 864mm². A 24mm x 16mm APS-C sensor has an area of 384mm² 384 ÷ 864 = 0.4444

In terms of the light circle cast by lenses, they must be at least 43.2mm in diameter for a FF sensor and 28.8mm in diameter for an APS-C sensor.

So when you say you only lose 1/3 of the pixels, that is not exactly true. Even if you no longer require your image to be rectangular, but rather accept an image that is the shape of the intersection of a 28.8mm circle imposed over a 36mm x 24mm rectangle. If you want to stay rectangular, you lose roughly one-half the pixels.

I can mount a Tamron AF 17-50mm f/2.8 XR Di-II LD SP Aspherical (IF) Zoom Lens to my Canon 5D mk II. The image circle does not cover the entire sensor. It does have a large enough image circle to reach from bottom to top in the center of the frame. You could crop the image down to the pixels contained in a slightly larger than APS-C sized area of the center of the sensor. On the 5616X3744 (21MP) EOS 5DII at f/3.2 this yields slight corner darkening (blur from the OOF edge of the circle) at about 3900X2600 (10MP) if you want to maintain the 3:2 ratio. You can also crop to 2792X3744 (10.4MP) in either 3:4 or 4:3 orientation. Or 3200X3200 (10.2MP).

17mm APS-C lens mounted on a FF Canon 5D Mark II. The white rectangle is the area of an APS-C sensor. As you can see, the image circle is smaller than the FF sensor.

enter image description here

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Yes... kind of. You'll be projecting the image to a larger size.

If you move the lens out by a 1/3 of focus distance using extensions, shouldn't you gain the full frame image? With possible loss of infinity focus?

Putting 35mm of extension on a 100mm lens or 10mm of extension on a 30mm lens isn't just a "you can't focus at infinity" but takes you well into the range of "you can't focus on anything from one foot to infinity" range.

You will also lose some light. fe = fa * (1 + m) gets us to effective aperture is equal to the actual aperture (lets say f/2.8) * one plus the magnification ratio. The magnification ratio, as described will be 1/3. This gives us fe = 2.8 * 4/3 = 3.7. So, you're going to lose about a stop of light in this too. If your lens is an f/5.6 rather than an f/2.8, many autofocus systems will fail to work at when additional light is lost.

This may be an acceptableish solution for if you wanted to do macrophotography without buying new glass, but this setup won't be useful for much of anything else.

Furthermore, it should be pointed out that the edge of the image is where the most aberrations are. Using extension tubes or similar on a full frame lens on a full frame system helps reduce the aberrations in the glass by utilizing the best part of the lens. Its not perfect, but it helps.

Using an APS-C lens for macro would enlarge the aberrations and still have the edge of the image circle in the frame. This may result in unacceptable edge image quality.

  • It is worse than that. To offset an image circle that is 2/3 the size needed, you must increase the focal length to 3/2 the lens' original focal length. – Michael C May 22 '15 at 23:11
  • So, is there a hard drop-off for inf focus then? Assuming a 50mm lens. – MandoMando May 23 '15 at 13:49
  • @MandoMando A lens focused at infinity is at its closest to the focus plane. To focus on things closer than infinity, the lens is moved further away from the focus plane. 1/objDist + 1/imgDist = 1/focalLength. Something 10 meters away (1000 cm) would give you 1/1000 + 1/? = 1/5 (all numbers in cm) and you solve for the ? for where you have the lens at (1000/199 = 5.025cm). If you add 30mm of extension, you are describing a situation where it is 1/? + 1/8 = 1/5. Solving for ? gives you 40/3 cm as the closest you can focus at. It is a very hard drop off. – user13451 May 25 '15 at 13:28
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An extension tube won't work because the amount of extension needed would limit you to only Macro photos. You would only be able to focus a few inches in front of the lens.

A 1.4x or 1.5x teleconverter will work. Depending on the lens, you may need a 2x teleconverter to completely eliminate the vignetting at all focal lengths.

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