Tilting Wide Angle Lens - Focus Soft At Sides Of Image

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When I tilt a very wide angle lens to obtain near-far focus (either a 58 mm or 65 mm in 6 cm x 9cm format; roughly equivalent to 75 mm or 90 mm in 4 x 5), the image is sometimes soft in the sides of the background, but still relatively sharp in the center of the background. What is causing this? Too much or too little tilt? Curl in the roll film back? Thanks.

-- Howard Slavitt (info@naturelandscape.com), November 05, 2000


You are getting too close to the edge of the coverage field: even though you aren't getting a full vignette, you are exceeding the area of _good_ image-quality coverage. One thing you could try is to do your tilting at the rear of the camera. This puts less strain on lens coverage, but will change the geometry or "drawing" of the image.--- Carl

-- Carl Weese (cweese@earthlink.net), November 05, 2000.

Howard Just a thought.....but are you stopping the lens down far enough, I would imagine that a 65mm /58mm should adequately cover 6x9 unless you are either a) tilting too much or b) not using a small enough aperture. I use a 47mm (non XL) on 6x9 and always stop down to at least f22 and don't suffer from what you describe. As I said, just a thought!! Regards Paul

-- paul owen (paulowen_2000@yahoo.com), November 05, 2000.

Howard: Try the "focus in" method and see if that does not help. Focus on the subject and then focus nearer and let the depth of field carry the image. That takes care of the curvature of the focus on wide angle lenses. For a scenic, focus on about 10 or 12 feet and stop down to f16 or 22.

Hope this helps,

-- Doug Paramore (dougmary@alaweb.com), November 05, 2000.

Howard, I second Doug's reply, I think you are seeing field curvature which is very pronounced in some wide angle designs. Don't focus so far away...

-- Glenn Kroeger (gkroeger@trinity.edu), November 05, 2000.

I'm inclined to think that both Carl and Doug have nailed it between them. If you're tilting the front standard, and not compensating by raising or lowering the lens panel, then you're effectively using the edge of the field of coverage, making field curvature more pronounced, and asymmetrical from the centre of the image.
You should re-centre the image in the coverage circle whenever you use swing or tilt. (Offset tilt minimises this effect, but necessitates refocussing. On axis tilt does the opposite. Neither are perfect.)

-- Pete Andrews (p.l.andrews@bham.ac.uk), November 06, 2000.

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