Smart Photography 201707

Sigma USB Dock

W

hen you buy a new lens,

you take it for granted

that it will autofocus

with 100% accuracy. And why not?

Unfortunately, that does not always

happen. Even the most expensive

lenses, sometimes, do not autofocus

with the accuracy that you may

hope for. This is true across all

manufacturers.

At times, the critical focus is a

wee-bit behind the autofocussed

point (Back Focus); sometimes in

front of the autofocussed point

(Front Focus). The normal way

out is to send the lens back to the

manufacturer for AF adjustment.

This can be inconvenient, involves

cost and you would be without the

lens for quite a few days.

Some manufacturers incorporate a

feature within their camera bodies

that allow the user to fine-tune

the autofocus. This is useful but a

quite complicated and cumbersome

procedure for most photographers.

But the question is, why is it that

some lenses do not autofocus as

accurately as we would like them to?

Anything and everything that is

man-made has what is known as

manufacturing tolerances. This is

the permitted allowance or limit

to the manufacturing accuracy of a

product. As long as the product lies

within the permitted tolerance for

accuracy, size, functioning etc, it is

considered as ‘passed’ by the quality

control department.

When it comes to cameras, we have

to deal with two such tolerances

• one for the camera body and the

other for the lens. Let us assume

that both – the camera body as well

as the lens – has a tolerance limit

of +/- 7 units (whatever be the unit

of the tolerance). This means that

if the body is, say, +5 units, it will

be considered ‘passed’. Similarly, if

the lens has a tolerance of, say, -7

units, that too will be considered

as ‘passed’. The +5 and the -7 will

cancel out each other (more or less).

But what happens if both the body

as well as the lens, are, say, +6? (or

any other number for that matter).

Now we have +12, which is beyond

the permissible limit – the autofocus

will never be accurate.

Enter Sigma USB Dock

(Available as an optional purchase

for Sigma, Canon and Nikon

mounts). To use this device, you first

need to download the free Sigma

Optimization Pro software.

The Sigma USB Dock is a device

that easily allows the user to fine-

tune autofocus issues on almost 25

Sigma lenses (Please check the Sigma

website for the list of USB Dock

Compatible Lenses). The Dock also

allows you to update firmware as

and when it is announced. Depending

on the lens, the Sigma Dock can

also adjust the Optical Stabiliser,

autofocus speed and adjust the focus

limit on your compatible Sigma

lenses.

I checked for autofocus accuracy on

my newly acquired Sigma 135mm

f/1.8 Art lens mounted on my

Nikon D810. Though I was happy

with the overall image sharpness,

Rohinton Mehta

I decided to put the lens through

a further test for AF accuracy. The

Sigma Dock allows you to fine-tune

AF performance of this lens at

4 different distances (You would

know the distances when you go

through the fine-tuning procedure

using the Dock). As required by the

Optimization Pro software, I

checked the AF accuracy at the

following four distances: 2.8’, 4.2’,

7.5’ and at infinity. While I will not

elaborate on the method used to

check the accuracy of the AF, let me

say that I felt that at 4.2’, the lens

was back-focussing (very slightly)

and at 7.5’, it was front-focussing

very slightly.

Note: The corrections that you apply

via the software, is a bit of trial and

error. There is no way to foretell

the amount of correction (+ or -).

That is why, after the procedure is

complete, you have to re-test for AF

accuracy and if necessary, repeat

the procedure. Shown below is the

procedure I followed to fine-tune the

autofocus:

Procedure

1) Download the free Sigma

Optimization Pro software from

Sigma’s Global Vision website.

(https://www.sigma-global.com/

en/download/)

2) Start the software, attach the

lens to the corresponding Dock and

plug the other end into the USB port

of your computer. The following

screengrabs show the procedure to

use the Sigma Dock to upgrade the

firmware and fine-tune the AF.

http://www.smartphotography.in 123

Smart PhotographyJuly 2017

PRODUCT FEATURE