Sigma 180mm lens, what distance to get f/2.8

[LVF]

I made several photographs of the Sigma APO MACRO 180mm f/2.8 EX lens attached to the Nikon D500 camera. I made photographs at 1:1 at approximately 10 inches working distance, 1:2 at 14 inches, and 1:5 at 35 inches. Note - by working distance, I mean from in-front of the lens to the target being photographed.

The subject photographed was the resolution chart I made with Photoshop and described in the June 9th post under my user name "LVF".

When I took these photographs, I could not get the lens to open up to an aperture of f/2.8 as the lens is specified??? At 1:1 the largest aperture opening was only f/5, at 1:2 it was f/4.2, and at 1:5 it was f/3.5. What is going on???

After googling this strange result, I found out that the Nikon cameras record the "effective aperture" at close-up distances, whereas, Canon and Sony cameras record the lens actual physical aperture, no matter how close you are to the subject. Because of light loss, the aperture opening becomes effectively smaller as you approach 1:1 magnification.

The effective aperture can be approximately determined by the formula:

Effective Aperture = (lens aperture setting) x (1 + magnification)

So at 1:1 the effective aperture is approximately f/5.6 (I recorded f/5), at 1:2 it is approximately f/4.2 (I recorded f/4.2), and at 1:5 it is f/3.4 (I recorded f/3.5).

So at what distance does the "effective aperture" become f/2.8 on a Nikon camera?

I set out to determine this distance. I set up my resolution chart on an easel board and moved the chart at further and further distances until the Nikon camera recorded f/2.8.

Here are my results. All distances are working distances.

At 7 feet, the camera recorded f/3:



At 10 feet, the camera recorded f/3:



Well, that's all the room I had in my computer room. So I took the chart and easel board out into my hallway, and got an extension cord for my light source (Ott Lamp). I continue to move the camera away from the chart until it recorded f/2.8.

To record f/2.8 on a Nikon camera, you have to be at 12 feet or greater, away from your subject being photographed.

Here is a photo at 12 feet:



Canon and Sony cameras would have recorded f/2.8 at any of these distances.

Now that I have answered this puzzling question, I will, in coming days, post my photos I took with the Sigma 180mm lens at the 3 magnification values, 1:1, 1:2, and 1:5. In these studies, I took photographs at varying apertures from the widest aperture to f/22.

I went as small as f/22 because I was curious as to when diffraction sets in.

I will take time off and be back later.

Leon

[rjlittlefield]

So at what distance does the "effective aperture" become f/2.8 on a Nikon camera?

The exact answer would be only at infinity focus. But under reasonable assumptions, the effective aperture at 12 feet focus would be about f/2.94. I have no idea why the camera reports this as f/2.8 instead of f/3.

--Rik

[Lou Jost]

Rik, I think many cameras only give results to 1/3 of a stop, rounded to two significant figures. The next one-third-stop division above 2.8 would be 3.2. The exact 2.94 is closer to 2.8 than to 3.2. Next division above 3.2 would be 3.5 if going by one-third-stop increments, and that is the next number that Leon noticed as he got closer.

[rjlittlefield]

Lou, that would be a good explanation, except that Leon reported "At 10 feet, the camera recorded f/3". Likewise at 7 feet. Without those reports, I would not have been surprised.

--Rik

[Lou Jost]

Oops, I hadn't noticed that.