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Cameras, Lenses and Accessories: Discuss Lens sharpness...Sorry if this is a bit of a newbie question but I was wondering what the easiest/most accurate way of ...
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Old 05-08-2008, 12:09   #1 (permalink)
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Lens sharpness

Sorry if this is a bit of a newbie question but I was wondering what the easiest/most accurate way of testing lens sharpness is?
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Old 05-08-2008, 12:47   #2 (permalink)
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Re: Lens sharpness

Well, I guess taking a shot of an open book at different apertures will be as good a test as any if all you're after is sharpness - using a tripod and probably mirror lock up (latter only really necessary at speeds around 1/30 but won't do any harm for all!)
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Old 05-08-2008, 13:57   #3 (permalink)
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Re: Lens sharpness

An open book is one way but the curvature of the pages (around the spine) would not be useful. Better would be a very flat piece of newspaper (iron it and tape it to a flat board). Also, you need to make sure the camera and target are parallel (sensor plane and target parallel) so that the plane of sharpness (focus) is aligned with the surface of the newspaper. This is important when checking the wide apertures like f/2.8, f/1.8 etc.

Take a series of images: start at the widest f/No (e.g. f/1.4) and a corresponding shutter speed for correct exposure then close the aperture and adjust the shutter to make a series at each aperture value on the lens (if the camera is on auto use aperture priority and you can forget about the shutter speed). It can also be useful to write the appropriate aperture on a small bit of paper attached to the target so you can see it in the final images.

Examine each image and check the corners and the centre. The first aperture which shows the least difference between the corners and centre is the first one for best sharpness (not depth of field since this is a different aspect). Since you are checking the corners with the centre, it is important to set the target and camera parallel at the start (really a tripod would be the best way).

Don't over-expose the images as too much exposure of white will spread (flare) into the darker text and make it appear less sharp.

As Markulous said "tripo and mirror lock" and a cable release or the self-timer for a 'hands off' exposure.

These steps assume you really want a reasonable test!

Cheers
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Old 05-08-2008, 15:05   #4 (permalink)
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Re: Lens sharpness

Welcome back Les You been busy then ?
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