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How To Resurrect a Lens!

A Golden Oldie Revived - Sigma 500mm F4.5 APO EX

We recently had a customer send us a combo which to be honest we had never come across before, a Sigma 500mm F4.5 APO EX and a 1DX Mk IV. This lens was first introduced in May 2005 and was considered a classic, considerably cheaper than a genuine Canon 500mm. This was one of the first series of lenses which really brought affordability to the masses and at the time was well received.
The customer had purchased the lens second hand but complained that he was unable to obtain sharp results when used on his 1DX MK IV, judging by the condition of the lens it had seen some use and was a little battered around the edges. In his own words, if we couldn’t do anything with it no one could....
So up on the calibration rig it went and we duly started the calibration process. However, as you can see from the Focal test report below, we were unable to fully complete the test as the final recommended calibration value was +23 out of range.
Sigma 500mm lens calibration
Looking at the 'Before' calibration image there was definitely a problem. This was verified by the 'Lens Profile' test chart.  The 'Lens Profile lists lens quality at various calibration values as LINES PER MM.  For this lens, at a '0' value only returned a value of 900 LPMM. For reference, typically a good quality example with a corrected adjustment (or a lens that doesn’t need any adjustment) scores a result of between 1500 – 1600 LPMM. This example was far from that and the calculated final result was +23 which was out of range. (Both Canon & Nikon models allow for -/+ 20 micro adjustments).

 What's To Blame - Lens Optics or AF Sensor?

The question was, how much of this was down to displacement of the lens optics or the AF sensor in the camera, after all the 1D MK IV had also seen better days. In order to determine which, we decided to calibrate the lens to our studio bench model. In this case a corrected Eos 7D Mk 2. This body has been corrected so that the in-camera AF sensor is perfectly aligned.
Sigma 500mm lens calibration
The result as the second calibration was +17, the difference of +6 micro adjustments was obviously due to the displacement of the AF sensor in the 1D Mk IV.


What aperture could this lens effectively be used at?

So from our tests we had determined that the recommend correction was +23 for the 1D Mk IV. However the maximum amount of adjustment was +20.  How about aperture wise? What aperture could this lens effectively be used at?
Sigma 500mm lens calibration
Generally, when a lens returns such a large amount of correction a corresponding Lens Optimisation test (please hyper link this to a description of what this does / service) normally reports that the lens has to be stopped down considerably in order to obtain sharp results. So after adding the maximum amount of correction (+20) to the body up went the combo back on to the test right. The result, the lens at + 20 returned the best results wide open at F4.5. The last thing to do was to perform a BEFORE / AFTER comparison, this was achieved by taking two shots of the test target and dropping both images into Photoshop, cropping to just the target size followed by inspection of the smallest detail (the writing at the bottom of the test target which is 1/100th the size of the whole target) at 200%. The results speak for themselves....
Sigma 500mm lens calibration
Sigma 500mm lens calibration
A true testimony to the importance of calibration and the phenomenal difference it can make. This lens was resurrected from being previously unusable to something that can now be used with confidence it will deliver sharp results.
So if you have a lens with similar problems why not give our services a try..