Will the aperture scale for a 150mm f5.6-f45 lens from manufacturer A be interchangeable with the aperture scale for a 400mm f5.6-f64 lens made by manufacturer B?
Will the markings from f5.6 through f45 match?
In other words, would setting the aperture to f22 result in the same physical opening of the iris, using either aperture scale?
Thanks in advance for your help.
Cheers!
No. No. And no. Manufacturer has nothing to do with it.
The f/ number is defined as diameter of entrance pupil/focal length. People often use the diaphragm's diameter in place of entrance pupil's diameter.
For a 150 mm lens, f/5.6 implies entrance pupil's diameter = focal length/5.6 = 26.8 mm. For a 400 mm lens, 71.4 mm.
Thanks for explaining, Dan.
++1!Originally Posted by Dan Fromm
And to complicate it a bit more, the aperture scales of some shutters are linear and some are not. That said, you can make measurements, do a little division, and come up with your own scale that is plenty accurate. I find a blank "P Touch" label usually fits on the shutter somewhere and you can write down the f stops with a fine point sharpie.
I have a post about etching a scale from hobby brass if you want to get ambitious!
It includes determining the scale based on focal length and entrance pupil.
Thanks all, for the information.
And it's not always as easy as measuring the diameter of the front element to calculate the aperture. For example, the Ross Xpres 3,8/105 has a larger front element diameter than the Zeiss Tessar 3,5/105. Maybe someone with more knowledge in lens design can explain that.
The f-number is the focal length divided by the diameter of the entrance pupil. The entrance pupil is the image of the aperture formed by the elements in front of the aperture. So if you look at the lens from the front, with a ruler laid across the lens, the diameter of the apparent aperture is what you need to calculate f-number (approximately, due to a little bit of parallax error).
Some lenses have magnifying power in the front elements and some don't, and that can change the image diameter versus the physical diameter of the aperture.
The front element size is sometimes the diameter of the entrance pupil wide open, but especially for lenses with a wide angle of coverage, the front element is often larger than the pupil, because it has to accommodate off-axis rays. Also lens manufacturers may vary in how consistent they are with stating an accurate f-number, compromising it for vignetting wide-open, etc.
I knew it was a long shot but thought I’d ask and maybe save a couple of hundred bucks on aperture scales. But I think its best I send them to Grimes and get proper scales. You almost always learn something when you ask a question instead of assuming, as I have here. Thanks again.
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