Wednesday, May 22, 2019

Soft Images With Longer Focal Length Lenses

I was recently asked a question about using the  Nikon 105mm macro lens for IR.  This person wanted to take macro and close-up shots of flowers but the lens wasn't as sharp as he expected.

The poster had a converted D90 camera (590nm Super Color conversion) and standard IR focus adjustment.  As a long time macro shooter, he was not happy with the lack of sharpness from his favorite Nikkor 105mm f/2.8 VR AF-S lens. This lens is super sharp on his normal camera. 

I shoot with the 105mm macro lens and it can be a real challenge unless you have the focus adjusted for that focal length and make the 105/D90 a dedicated lens/camera combination. That worked for me because I rarely shoot landscapes.

With the standard focus adjustment, the camera auto focus predicts the actual IR focus when using focal lengths from about 18mm to about 70mm when using reasonable apertures. After that point, the images get softer as the focal length increases. You can improve the sharpness somewhat by stopping down (increasing the DOF) but that introduces more noise as the camera struggles with the low-light conditions (After conversion, >90% of the visible light never makes it to the sensor).

What can you do?
1. Use moderate focal lengths and crop the image.
2. Use wide angle macro techniques to get good DOF and minimize focus shift problems.

If you want to use longer foal lengths :
1. Use LiveView and LiveView focusing.
2. Use LiveView and manual focusing using the LiveView zoom feature.

If you want to use the auto-focus and longer focal lengths:
1. Use smaller apertures to compensate for focus shifting and do that only when you have bright conditions. This will minimize softness and noise. You may have to use EV adjustments to make the visible light exposure calculations approximate the IR conditions.
2. Use smaller apertures but add light with a flash. Flashes kick out a lot of IR light.
3. Use smaller apertures and use longer exposure times (with EV adjustments).

If you need more light you can increase the D90's ISO to 400 or 800 as necessary.

Do you have other questions?  Just leave me a message in the comments section.


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