How can I determine the actual f-number of an unmarked aperture stop on a manual lens?
Asked 7/29/2016
6 views
2 answers
0
I’m using a Canon FD 28mm f/2.8 on a Canon DSLR with an adapter, so I set aperture manually on the lens. The aperture ring only labels every other stop, leaving intermediate click-stops unmarked. For example, there’s one unmarked stop between f/2.8 and f/4.
Out of curiosity, is there a practical way to determine the actual f-number of that intermediate stop using just the camera, lens, and software? I’m looking for an experimental method rather than simply assuming the marked spacing.
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
10y ago
2 Answers
6
One method is to use the camera to make the measurement. This is actually surprisingly accurate, and is how most T stop measurements are done with a single reference measurement using a spectrophotometer.
Take an exposure at a known (or even "known") aperture, like f/2.8
Open the raw image in a software like RawDigger, or process it with DCRaw to get the unmanipulated image data. Find the intensity of the image in the center
Stop the lens down and repeat, the ratio of the brightness of the two informs you about the aperture.
If you get e.g. 1024 at f/? and 2048 at f/2.8, you know you have exactly halved the exposure and f/? = f/4. The new aperture will be (old aperture) + log2(ratio of exposures).
You can also do this with Jpeg files, or images processed (opening counts as processing in this case) by e.g. Adobe Camera Raw. You just have to linearize them first, when the raw image is almost always already quite linear.
Originally by user40937. Source · Licensed CC BY-SA 4.0
user40937
10y ago
0
Generated from our catalog & community — verify before relying on it.
Yes. The simplest experimental method is to compare image brightness at a known aperture and at the unknown stop.
- Photograph an evenly lit, constant scene at fixed ISO and shutter speed.
- Take one shot at a marked aperture such as f/2.8.
- Take another shot at the unmarked stop.
- Read the RAW data (or minimally processed image data) and compare the brightness in the center of the frame.
Because exposure is proportional to aperture area, the brightness ratio tells you the stop difference. If the unknown-stop image is half as bright as the f/2.8 image, that stop is f/4. If it falls between those values, it’s between them.
For reference, f-number is defined by: N = focal length / entrance pupil diameter
So on a 28mm lens:
- f/2.8 corresponds to about a 10mm entrance pupil
- f/4 corresponds to about 7mm
The usual intermediate full-stop progression would make the halfway stop about f/3.2 if it’s a 1/3-stop step, or about f/3.4 if it’s a true half-stop step. Measuring exposure is the easiest way to find which your lens is actually using.
Recommended products
UniqueBot
AI10y ago
Your Answer
Related Questions
What are the intermediate aperture settings on the Fuji GW690 III lens?
How do Canon FD lenses behave on a Micro Four Thirds camera with an FD-to-MFT adapter?
Why won't a Canon FD 28mm lens focus properly on an EOS Rebel T6/1300D?
What is Aperture Value in EXIF, and how does it relate to the f-number?
Why do 60mm and 105mm f/2.8 lenses have the same f-stop if their aperture diameters differ?