For Lou Jost

A forum to ask questions, post setups, and generally discuss anything having to do with photomacrography and photomicroscopy.

Moderators: rjlittlefield, ChrisR, Chris S., Pau

LVF
Posts: 66
Joined: Sun Apr 23, 2017 4:17 pm
Location: Sequim, Washington

For Lou Jost

Post by LVF »

You requested that I show the resolving power of the Sigma 180mm APO MACRO f/2.8EX lens.

I will do that next week after going to my grandsons college graduation in Spokane, Washington, this weekend.

One question. You did not say at what close-up distance you would prefer.

With the Nikon D500 camera, I get the following close-up photos versus working distance (distance from in-front of lens to the chart):

1:1 close-up at 10 inches (note - Sigma spec. say 18.5 inches for full frame camera; I measured 10 inches on my DX camera, but that would be a 1:1 photo at 19.7 inches from the D500 sensor):

Image

1:2 close-up at 16 inches:

Image

1:3 close-up at 22 inches:

Image

And,

1:4 close-up at 28 inches:

Image

If you are like me, I cannot kneel on the ground any more, so getting as close as 10 inches or 16 inches near the ground is getting difficult. So I am looking for really sharp photos at greater working distances. Then using Photoshop cs6 to get closer views of the subject.

Let me know which working distance you would like me to photograph.

By the way. I am curious how bad diffraction gets at smaller apertures, so I will be photographing the complete range of the Sigma 180mm lens apertures from f/2.8 to f/22.

Leon

Lou Jost
Posts: 5945
Joined: Fri Sep 04, 2015 7:03 am
Location: Ecuador
Contact:

Post by Lou Jost »

Leon, thanks very much for testing this lens. It is on some lists of the best current macro lenses, and Sigma has been producing some very interesting lenses lately. It could be very good.

I think a good set would be m=1, m = 1/2, m=1/5, and infinity focus. I don't think there is much use going out to f/22 but it also can't hurt.

I am confused, though, by something you said above. You stated that the magnification depended on the sensor. However, when the lens is set to 1:1 it will give 1:1 on any sized sensor. On an APS sensor the field of view would be 24mm, and on a full frame sensor the field of view would be 36mm. So the field of view changes with the sensor, but the magnification "on the sensor" does not change, for a given lens arrangement.

Are those grid lines millimeters? The 1:1 image has about 32 divisions. If those are millimeters it should only have 24 mm at 1:1.

I look forward to your tests!

LVF
Posts: 66
Joined: Sun Apr 23, 2017 4:17 pm
Location: Sequim, Washington

Lou

Post by LVF »

The small divisions are in inches. Inside the area that is the size of the sensor (0.925 X 0.618 inches) , the division is 1/32 inch, outside the sensor area, 1/16 inch.

What my confusion was, the specs state the lens gives 1:1 at 18.5 inches. I am so use to "working distance", the distance from in-front of the lens to subject, that I was thinking that the specs were, 18.5 inches from the front of the lens.

I now realize that the specs are 18.5 inches from the camera sensor. This agrees with my measurement of 19 inches with the Nikon D500 camera.

Also, I did not realize that you get 1:1 on any size sensor. I am too much a novice to understand what is taking place at this time. Although I am a scientist , I have not studied this aspect of macro photography. Lots to learn.

I hope this clears up some questions.

Leon

Post Reply Previous topicNext topic