HC 100mm diaphragm(?) problem

[nainmaslun]

Hi everyone,

Maybe some of you read my post in 'Lenses' about getting a new lens. Well, here's why Im looking for that. I have a H5D-40 with 2 lenses, an HC 35mm and an HC 100mm. The thing is that I had to send my camera a year ago to repair because it had a lot of issues and also the sensor died. When it came back it was absolutely great, working really good, no more bugs and with new sensor. The thing is I had to keep struggling with a 'no sync' malfunction of the camera, but then after making some test with both lenses I realized that it is the HC 100mm that has something wrong.

The issue. If I shoot IN STUDIO with strobes at 1/250 second, the image is black or very underexposed. If I shoot at a not very close distance, and at 1/500 or 1/800 the image will be well exposed. If I get very close to the subject and the focus is to the minimum, the exposure goes down 2 stops. I've been testing a lot and they told me maybe I had 'True Exposure' on. In fact, I had it on. So when I tried with 'True Exposure' off, it happened the same.

To make it very simple, if I shoot at 10 or 13 feet away from the subject the images will be good exposed and sometimes will show me some under exposed. If I shoot with that set up 10 images, 2 o 3 will be under exposed. If I shoot 4 feet from the subject, a very close portrait, the images will be under exposed. 8 out of 10 images will be under exposed.
After testing, I started to look the images and I realized that the DOF was different from the ones that came out 'good exposed'. So yes, it has to be the diaphragm I think.
I did the same test with the HC 35mm, I shoot from 10 feet and very close, with minimum focus, and the images were just fine.

Does anyone of you have experienced this same issue with some of your lenses???

Thank you!