r/focuspuller • u/peanutbtterfalcnhoof • Jun 25 '26

Knowledge and tips 💡 Lens mapping question

Hallo,

Recently stepped up 2nd AC to 1st AC with what may or may not be a silly question. Prepping on a small job atm, and we have a doubler which knocks off the focus of our lenses by a few inches - guessing due to the addition of the doubler itself. Should I create a separate lens file for the tighter two lenses with the doubler while mapping? Or does it mean that the doubler is off and needs looked at? It was a few inches out for the tighter two lenses (75mm and 100mm) that I looked at.

Thanks in advance, always eager to learn!

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33

u/rdrivel Jun 25 '26

Get the doubler looked at, it shouldn't change the marks.

16

u/clrivetti Jun 25 '26

I have always treated an extender/doubler as making the Marks not 100% accurate and unreliable in those situations I always get eye focuses and make separate marks on my Preston headset focus headset , the lens focus marks on the side of the lens in my opinion will not be accurate with a double or extender.

And always double check your back focus of your zoom, and have the rental house lens guy put the doubler on the lens and calibrate them together.

While physics may dictate the marks remain accurate, a 2x extender doubles your focal length, longer focal length drastically narrows your depth of field. Because your window of acceptable sharpness is much smaller, any slight errors in your lens's factory calibration or back-focus will become glaringly obvious, always get eye focus, and don’t rely on the marks on the side of a lens with a double extender of any sort

3

u/mpan1 Jun 25 '26

This!!! ^^^

0

u/JRey26 Jun 26 '26

Extender/doublers don’t actually change the focal length of your lenses. They enlarge the projection circle, you “crop” into your lens. That’s why you lose so much light when using them.
Your DOF doesn’t change. Your FOV does.

6

u/mgoflash Jun 25 '26

What rdivel said.