Crop factor with MF/LF systems

[Zou]

I have seen all sorts of tasty medium and large format cameras equipped with digital backs. Obviously the backs aren't covering the 'full frame' much beyond 645-ish dimensions, so what effect is this having on images? Are MF/LF 'togs shooting with different focal lengths to 'correct' the field of vision or are there other techniques.

For example, a Phase One P65+ on a Horseman 612W - would you just use the shift functionality to take two or more images and stitch them, or just use a different focal length.

I've not really got my head around this one (not that I'll need to as I cannae afford a digi back even if I wanted one) - can anyone help?

[Malcolm_Stewart]

I have an MPP 5"x4" system and it's good fun having all the movements available, and I even found sensible uses for some of them. I made good use of a Polaroid back for a period, but felt cheated that I only got 4½"x3½" images, so I fully appreciate your point. My 90mm f8 Super Angulon is quite a wide angle on 5"x4", and requires a special recessed lens panel to allow its use on my field camera. So digi-backs on MF/LF may be convenient but restricted in angle of view, and I expect it will be some time before the sensors increase in size to allow the proper use of the excellent wide angle lenses available.

Centre filters are already available for LF lenses, and they may need to be reformulated if vignetting at the wider angles becomes a serious problem. (Unless software fixes become the norm, which is quite likely.)

[TimF]

Crop factors on medium format sensors are currently based on the 6x4.5 film format, which is the closest to most of them (though I should say there are a handful of square format at 36x36mm IIRC)

Basically, all current models (except those to be mentioned) appear to be approximately x1.1 crop (sensor size is approximately 36x48mm), apart from the Hassy & Phase 31mp backs which have a 33x44mm sensor and x1.3 crop.

The Phase One P65+ back is effectively full frame 645, as I believe the forthcoming Hasselblad 60mp back will be. The latter's 50mp IIRC is still 36x48mm.

Leaf and Sinar (who share a basic camera platform - though marketed under their respective names) plan to produce 6x6cm format backs at some point. How long this will take, who knows.

Companies such as Linhof & Cambo have produced specialist view cameras to fit these backs onto, and Schneider and Rodenstock produce digital lenses to cover the format. These are pretty much the state of the lens designers art I understand.

Other specialist solutions from such as Alpa and Cambo (again) are designed to allow the photographer to shift the back around the image circle of the lens and produce sets of images to stitch together.

Cambo
Alpa
Linhof

Phase One
Hasselblad
Leaf
Sinar

Hope this helps.

[Zou]

Thanks for the full yet concise answer Tim. As I understand it the main point of using these cameras with digital backs is for the movements and the benefits these bring. I expect that in due course as making digital sensors becomes more common and affordable, we will see larger and larger sensors, more for the sake of making money than actual need. The dynamic range that the latest MF digital backs can achieve certainly makes them look appealing to someone like me who has always preferred transparencies for colour work. The amount of detail is also quite stunning, given the much smaller sensor size when compared to 4x5 film. This is an area which I haven't really followed much, but I will certainly be paying it more attention in the future.