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Re: [OM] 18mm is no substitute for a 24mm shift

Subject: Re: [OM] 18mm is no substitute for a 24mm shift
From: Vaughan Bromfield <vaughan.bromfield@xxxxxxxxxx>
Date: Fri, 04 May 2001 16:31:16 +1000
Folks

At risk of sounding pedantic, comparing the 18mm and 24mm shift lenses
in this way is very misleading and not at all productive. The 24 shift
may have an angle of *coverage* as the 18mm lens, but the angle of
*view* is dependent on the format, so it's the same as any other 24mm
lens. The extra coverage of the 24 shift allows movement without
vignetting. That's all. The comparison with the 18mm is marketing hype,
and a poorchoice at that.

The 90mm Super Angulon lens has the about the same angle of view as the
21mm Zuiko, but only for 5x7 format. Slap the same SA lens onto a 35mm
film and it's got the same angle of view as any other 90mm lens.


Vaughan


> Date: Thu, 3 May 2001 13:34:03 -0500 (CDT)
> From: "Paul Farrar" <farrar@xxxxxxxxxxxx>
> Subject: [OM] 18mm is no substitute for a 24mm shift
> 
> Since the 24mm shift and the 18mm have the same angle of view, I've 
> heard people say that one can get the equivalent of a 24mm shift by 
> using an 18mm and selecting an 18*27mm subframe. I've agreed in my 
> own mind with this, without really thinking about it. But a few days 
> ago, out of the blue, I had a theophany and realized it couldn't be 
> true. Two days ago I tried it out with both lenses and found the 
> differences were quite dramatic, especially for rise and fall 
> (shifting across the narrow dimension). The 24mm shift has a 
> tremendously greater coverage.
> 
> The reason is this:
> Both lenses have the same ratio of (image circle radius)/(focal length),
> so the 24mm is like a scaled up 18mm. The 18mm fits a 24*36mm frame 
> into the image circle so that the corners just touch the image 
> circle. The 24 has a 33% large circle and moves a 24*36mm frame 
> around within it. To get a 24mm equivalent with the 18mm, you take an 
> 18*27mm subframe and shift it around. The problem is that the 
> subframe is limited to within the 24*36mm full frame.  This mean that 
> you can only get a 3mm rise or 170f the frame height. On the 24, 
> however, the 24*36mm frame is limited by the image circle, not the 
> boundaries of a slightly larger frame. This means you can get a 10mm 
> rise before your frame corners get to the image circle, or 400f the 
> frame height.  The difference is quite dramatic. The 18mm doesn't 
> come close to the ability of the 24. For lateral shifts, the 
> difference is much smaller, but detectable. This is because on the 
> sides the 18mm full frame can get much closer to the image circle.
> 
> You can get a bigger rise with the 18mm by turning it vertical and 
> shifting a 24*16mm frame for a 10mm rise, or 63%. But you can do the 
> same trick with the 24 shift and get a full 100% rise.
> 
> What lens is equivalent to the 24 shift? At full rise, the top of the 
> frame is 22mm above the axis, roughly the same as the focal length. 
> For a subframe confined to a 24*36 full frame, this means you need a 
> 12mm focal length, with a 12*18mm subframe. You will get more lateral 
> shift with the 12mm. Only Voigtlaender makes a 12mm -- but it's 
> cheaper than a 24mm shift, but by then you're getting close to Pocket 
> Instamatic format size.
> 
> The 24mm shift can do all sorts of tricks, not just prevent 
> keystoning.  For example you can control the elliptical distortion of 
> 3D objects near an edge (present even on a perfectly "nondistorting" 
> rectilinear).  You shift the object towards the center of the image 
> circle. You get a corresponding increase of distortion on the other 
> side of the frame, but if there is a person's head, or some item of 
> interest, you can choose for it to have low distortion.
> 
> Unfortunately, it looks like the 24mm shift is no longer available 
> new, at least in the US. It's not too hard to find used, but prices 
> have risen a lot in the last few years.
> 
> As a true Zuikoholic, you really neeeeeed this lens! Show the above 
> to a sceptical spouse -- but, ummmm, don't mention my name, 
> especially if you live near the Gulf Coast.
> 
> Paul Farrar
>

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