f/1.7 will give you extraordinarily narrow depth of field. With studio work, f/5.6 is a more typical aperture. This aperture (or thereabouts) will give you maximum sharpness.

If you want to give f/1.7 a go, I would advise some kind of focus aid - either a magnifier; such as an O-ME53 or Magnifier FB; or a focussing screen with split prism, as it is really easy to stuff critical focus in these cases!

If you are using P-TTL flashes, you can set your pop up flash to work as a controller.

Chris,

As the lens is a purely manual lens, your camera cannot tell what aperture the lens has and thus cannot meter like it normally does. You therefore have two options when shooting with such a lens:

1 - Shoot in AV mode. This assumes that you are using the lens wide open (at f/1.7), and constantly meters depending on where you point the camera - hence the changing numbers you saw, which are for your shutter speed.

2 - Shoot in M mode. This allows you to set the shutter speed, and also to use stop down metering as John described.

There are a lot of guides on how to use M lenses with your camera, but I will give you a brief overview here.

First, you need to set the custom function and the focal length of your lens. You've already done this, so good!

Secondly, fit the lens on your camera and go to "M" mode on the shooting wheel. Select your ISO as normal. Set your camera to Manual Focus.

Now you are ready to start metering and shooting. First of all, set the aperture to the value you want by using the ring on the lens where it mounts to the camera. I have a similar lens to you, and like to use f/2.0 to f/2.4 as it gives you a bit more sharpness and depth of field than shooting wide open at f/1.7.

Once you have set your aperture, look through the viewfinder and compose as normal. When you are ready to shoot, press the green button. You should hear a swish and notice that the viewfinder gets slightly darker for a moment. This is your camera stopping down the lens to the selected aperture and taking a meter reading. You should notice your camera has now calculated a shutter speed. If it is too high or low, considering changing your ISO or aperture.

You are now ready to take a test photo! Make sure to check it afterwards to see how well it came out - and feel free to manually adjust the shutter speed if you don't quite like how it came out.

You have to choose between one stabilization method and the other - using both at the same time would wreck your images.

Sigma now make one or two lenses in KAF mount with OS - you must disable SR to use them (or turn off OS). If you don't, then the SR mechanism moves the sensor enough to adjust for the shake when the lens is producing a stable image, so your image is artificially blurred.