dckean wrote:
Really strugglin to find info on this, I have a tokina 11-20mm wide angle lens and want to check the overall sharpness by means of a spyderlenscal or similar tool, ie, what distance from the tool would you set up and at what focal length for Good average results, thanks in advance for your advice.
With a zoom you need to check both ends of the zoom range... so in your case the focal lengths you should check are 11mm and 20mm settings.
The distance you should set up the target is either 25X or 50X the lens focal length, depending upon the manufacturer. I think the Spyder instructions are 25X. With their calibration target, I know Reiken FoCal recommends 50X focal length (but I thin they say 25X is okay when calibrating really long telephoto focal lengths where that distance might be impractical).
11mm x 25 = 275mm. If you're "metrically challenged" like me, convert that to inches: 275 divided by 25.4 = 10.8". I'm sure 11 to 12 inches would be fine!
20mm x 25 = 500mm. Converted to inches: 500 / 25.4 = 19.68... call it 20 inches.
Be sure to set up the lens wide open, at it's max aperture, so that depth of field is as shallow as possible. Focus on the portion of the target that's parallel to your camera's sensor, then see where the focus falls on the angled portion of the target. The shallow depth of field from the largest possible aperture will give the clearest indication where the lens is focusing.
What to do with the info depends upon your camera. Some (most Canon with Micro Focus Adjust produced the last 8 or 10 years, for example) now have two adjustments for zooms... one for each end. If your camera only allow a single adjustment, then you may have to arrive at an average of the amount of adjustment suggested for the two different focal lengths.
I agree with a previous response... there's less need to fine tune focus with an ultrawide lens like the 11-20mm, simply because that type of lens has a lot of depth of field and can tolerate quite a bit of focus error.... especially whenever you use it stopped down to smaller apertures (which further increases depth of field).
Lens calibration is more critical with telephotos.... especially those with large apertures that are often used "wide open", or close to it, to render very shallow depth of field... such as 85mm f/1.2 and f/1.4, 135mm f/2 and f/1.8, 200mm f/2 and 300mm f/2.8 or 400mm f/2.8 lenses.
ceh1024 wrote:
Check out FoCal completely automatic but last checked works for Nikon and Canon only. I have it and it works great.
The FoCal system is good... but if you already have the Spyder LensCal or the Lens Align or other similar, those are more "manual", but will work fine, too.
It's easy to do and might be worth checking your 11-20mm, especially if you suspect a problem. But unless it's badly out of calibration, thanks to the lens deep depth of field you're unlikely to see much difference from any adjustments you make.