Speedbooster - extender thoughts reg. MF backs

[chrismuc]

The Metabones Speedbooster has the following function on a lens:
- decrease of focal length (of lens + speed booster) by factor 1.4
- increase of open aperture (f-stop) by factor 1.4
- decrease of image circle by factor 1.4 (which is sufficient for 1.5x crop sensor cameras)

So if one assumes that an extender has the contrary optical function of the Speedbooster, that would mean for a 1.4x extender:
- increase of focal length by factor 1.4
- decrease of open aperture (f-stop) by factor 1.4
- increase of image circle by factor 1.4

The first two aspects are obvious, the third is interesting: Could it be that a 135 lens plus 1.4x extender achieves an image circle sufficient (or quite sufficient) for a 54x40mm sensor?

It would be cool if Hcam/FPS owners/producers could test e.g. Canon EF 70-200 + 1.4x or Leica R 180f2.8 Apo + 2x on the EF mount of these cams to see which image circles would be illuminated and give a sharp image.

[gerald.d]

Unfortunately, no it doesn't, because there are typically other factors that influence the image circle - for example, the barrel of the lens can cause vignetting.

The Canon 200-400mm lens has an inbuilt 1.4x converter, and I've tested this lens on a full frame MF back with the Hartblei HCam.

Whilst the image circle does increase somewhat, it's certainly not by a factor of 1.4x.

Here's the lens at 400mm without the converter -



And here it is with the converter dropped in -

[gerald.d]

Additionally, see this post -

http://www.getdpi.com/forum/478692-post11.html

There, you can see that using the 1.4x converter on the Canon 24TS-E actually reduces the size of the image circle.

[chrismuc]

200-400 with 1.4x extender: Yes, the image circle does not cover the full 40x54mm sensor but seams to cover quite the 1.4 times 24x36mm area = 34x50mm
24TSE + 1.4x: You are right. That's the focal length where you would need the Contax 645 35f3.5 + Mirex Shift adapter to get a 35mm lens for the FPS/Hcam with slight shift capability (no tilt).

[gerald.d]

200-400 with 1.4x extender: Yes, the image circle does not cover the full 40x54mm sensor but seams to cover quite the 1.4 times 24x36mm area = 34x50mm
24TSE + 1.4x: You are right. That's the focal length where you would need the Contax 645 35f3.5 + Mirex Shift adapter to get a 35mm lens for the FPS/Hcam with slight shift capability (no tilt).

I think the thing you're not taking into account is that even without the converter, the lens is covering larger than the 24x36 sensor.

Dropping the 1.4x in would appear to increase the image circle by about 1.2x based on those images.

[EsbenHR]

So if one assumes that an extender has the contrary optical function of the Speedbooster, that would mean for a 1.4x extender:
- increase of focal length by factor 1.4
- decrease of open aperture (f-stop) by factor 1.4
- increase of image circle by factor 1.4

The first two aspects are obvious, the third is interesting: Could it be that a 135 lens plus 1.4x extender achieves an image circle sufficient (or quite sufficient) for a 54x40mm sensor?

That is a pretty tough demand for the extender, but possible. The Hasselblad HTS works more or less like that, if I remember correctly.

Of course, they only made it an extender because they needed the extra distance for the mechanics, but it could be used the way you describe.