Estimate T-stop

Hi,
I would like to know if there is a way to estimate the t-stop of a lens. I have both ef lenses and old m42. I am wondering if I have one lens with known t-stop and then extract the relative t-stop by comparing histogram from the known t-stop to the unknown t-stop.

I realize that two 50mm f/1.8 (canon vs pentacon) do let let the same amount of light through.

Best regards,

You are talking about the difference in light absorption between one lens and the other.... I would suggest: take a shot of exactly the same (preferably simple) scene with one lens and with the other lens at the same camera settings and lighting situation.

If one image shows a different exposure from the other you see the difference in absorption. Yes, I think when you compare the histograms you might be able to measure the relative absorption.

Just wondering why you are interested in that? For me IQ would be the deciding factor to use a lens or not and not the T-stop.

Hi,
Thanks for the tip. I am interested in night photography, northern light to be more precise. When I go out, it takes literally one minute per shot (30 sec exposure + 30 sec. for noise reduction), so I don't have much time for experimentation. On top of that at night in the north, I am freezing my finger just taking the pictures, I cannot easily change lenses to experiment.

Best regards,

Point the known lens at a blank wall and take an exposure that results in a middle tone on the histogram. Load the image file to the computer and check the RGB values at the exact middle of the image.

Without moving the camera on the tripod, change to the lens you wish to test. Now take a series of exposures with the shutter speed varied +/- increments from the original control lens. Load these images to the computer and again check the RGB values at the exact middle of the image.

When the RGB values are the same, you have found the same amount of light transmission. Check the shutter speeds of the control shot and the matching test shot and you will see the difference as expressed by stops.

I'd be surprised if the difference was more than 1/6th of a stop. I checked some Canon lenses once for this same test protocol and the range in light transmission from the very brightest and the darkest was less than 1/3 stop total.