Question about Aperture Scale Compatibility

[jamgolf]

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!

[Dan Fromm]

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.

[jamgolf]

Thanks for explaining, Dan.

[Luis-F-S]

No. No. And no. Manufacturer has nothing to do with it.


++1!

[Kevin Crisp]

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.

[Graeme Hamilton]

I have a post (https://www.largeformatphotography.info/forum/showthread.php?147102-Recreating-a-Missing-Aperture-Scale-A-How-To&p=1453267#post1453267) 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.

[jamgolf]

Thanks all, for the information.

[J. Patric Dahlen]

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.

[reddesert]

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.

[jamgolf]

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.

[robphoto]

My only suggestion, if you want to experiment before you spend money, is to use the ratio of the focal lengths to make yourself a conversion scale.

Because the f:stop is the ratio of the focal length to the aperture diameter, if both lenses fit into the same shutter (an unlikely if), you can make a conversion scale.

For a simple example, if the scale is for a 90mm lens, and I mount a 180mm in the same shutter, the f:stops are proportional to the focal length, so:

90mm f:8 ---- for 180mm lens in same shutter, that would be f:16 (8 x 180/90)

note this is if both are mounted in the same shutter: you can't convert between different shutters

in your case, if you mounted the 400mm lens into the shutter that had held the 150:

150mm f:8 ----- 8 x 400/150 = 21.3, roughly f:22, or three stops different.

Another way to get a real rough scale would be to measure the maximum aperture opening, mark it on a tape scale on the shutter, figure out what that aperture number would be (focal length / diameter), stop down to half that diameter (two stops different) etc., and fill in the intermediate points by eye. This is very rough and ready, but would be fairly close. The advantage is you get to try out the lens before investing money in it.

(edit: I see the above poster saying that what you measure isn't the actual aperture circle, but the apparent size of the aperture looking through the front element, otherwise this should be correct)

[dahlen59]

Ok if you want to spend money. My way to do it was to unscrew the scale flip it around an write on the back. To get the right meusure you have to unscrew the front element and measure the iris. Easy in the beginning but as the f stop gets smaller it is more problematic. I calculated the diameter for ex f 11 and then adjusted the aparture to that diameter. Different tubes and pencils was handy.

[maltfalc]

Ok if you want to spend money. My way to do it was to unscrew the scale flip it around an write on the back. To get the right meusure you have to unscrew the front element and measure the iris. Easy in the beginning but as the f stop gets smaller it is more problematic. I calculated the diameter for ex f 11 and then adjusted the aparture to that diameter. Different tubes and pencils was handy.no. read the previous replies.