What do the numbers mean?

[Grumpy John]

I have a Sigma 17-70mm F2.8-4 DC Macro OS HSM and was wondering what do the numbers on the lens barrel mean?

[Isac]

Just a guide to let you see what the stop setting will be when you zoom in on your subject. Wide open (17mm) is f:2.8 and as you zoom, the f-stops increase until fully zoomed at f:5.8 (70mm). It's a variable aperture lens, as opposed to a constant aperture lens which will maintain the f:2.8 throughout the zoom range.

[Grumpy John]

[Ironwood]

That lens is variable aperture 2.8 - 4, so I would think that at 70mm the widest aperture should be f4.

[Grumpy John]

Give the man a cigar, or beer. His choice.

[Ironwood]

According to the lens review on DPR, it is the magnification ratio at the minimum focus distance at the various zoom ranges.
https://www.dpreview.com/reviews/sig...2-8-4-os-hsm/2

[Grumpy John]

Whatever that means, hence the original question.

[Ironwood]

Well, the best way I can explain it is, Most ‘true’ macro lenses are 1:1, that means at the lenses minimum focus distance ( MFD ), it will project the subject at its full size onto the sensor. Let’s say your sensor is 25mm across, your can take a photo of a ruler and you will have 25mm of the ruler in your photo, now if your lens is 1:2 , at the MFD your will have 50mm of the ruler in your photo, at 1:3 you will now be looking at 75mm in your photo, and so on. Canon make a 5:1 macro lens, with this lens at MFD, you will have 5mm of the ruler in your photo.
I tried to find an article on the net that explains this better than I can, but most of what I found was pretty vague in their interpretation of it.

[Grumpy John]

The sensor size on my camera is 22.4 x 15.0mm, and a crop factor of 1.6.

[Grumpy John]

The Sigma 17-70mm F2.8-4 DC Macro OS HSM C usefully delivers good macro performance. It has a minimum focusing distance of 22cm/8.7in and a maximum magnification ratio of 1:2.8. At 70mm, the working distance actually becomes 5.52cm.Mar 13, 2014

[Grumpy John]

22.4 x 2.8 = 66.72
Hand held at what I thought was about 55mm, manual focus.

[Ironwood]

To get 1:2.8 I think you would have to be at 70mm and MFD. Which should be 220mm from the sensor to the point of focus.

[Grumpy John]

MFD = Minimum Focusing Distance From the DPP article - At 70mm, the working distance actually becomes 5.52cm. 55.2mm.
I was at 70mm f4.0 ISO 100

[Ironwood]

I think the working distance they talk about is from the front of the lens to the subject , MFD is sensor to subject.
If you are focusing manually, adjust your focus ring all the way around and leave it there, now move your camera in and look through the viewfinder ( or liveview) until the subject is in focus. This is the best way to get to your lenses MFD.
Or if you wanted to get more technical, set your camera on a tripod, use a ruler to position the subject 220mm from your sensor ( my camera has a white marking on the top that indicates the sensor plane) now see if you can manually or AF focus on your subject.

[Grumpy John]

I think the working distance they talk about is from the front of the lens to the subject
That makes more sense.

[Ironwood]

I tried with my 2 macro lenses, both are 1:1 lenses. These are both handheld @ iso3200 1/60th, f5, I adjusted the focus ring to the hard stops on the closest focus end of the adjustment range and moved the camera in and out to get the image in focus, the working distance on the 55 was much less than the 105. Just looked at the 105 and it is roughly 100mm from the lens to the focus point, the 55 would be less than half this.
My sensor is 23.5 x 15.6mm ( 1.5x )

Nikon AF55mmD f2.8


Nikon AF-S 105mm G f2.8


I am getting a bit better magnification than 1:1, I assume this is because I have adjusted the focus onto the hard-stops, if auto focusing it may not go right to the hard-stop, I'm not sure on this.

[Grumpy John]

Thanks Brad, I'll do a better setup and report back. It may not be for a couple of days.