Is f-stop universal between lenses?

Hey guys, I'm not entirely sure how to ask the question that is in my head, but I will try and shoot out some questions and see if it makes any sense.

I'm wondering if an f-stop rating from one lens will provide the same light properties as the same f-stop on another lens? For example, if I were to take a 70-200 f4 and shoot it wide open at 200mm, would it allow in the same amount of light/matching histogram of using a 35mm 1.4 at f4, provided all other camera information was identical?

My original thought is that it would be the same, but I wonder if I'm missing something? I would imagine the available light could/would change things, such as the 200mm photo may not include the sky/lights, thus altering the histogram, but would the shared elements in the photo be identical?

I would also imagine, depending on the quality of the lens, the natural vignette can have an effect, though for the sake of this discussion we can pretend that it won't

I plan to test this theory in the next day or two when I get some time, and I will post the results here, but I was wondering if anyone had any thoughts on it before I could get to it?

No. In theory one lens *should* produce the same results as another at the same aperture. Most of the time they will be close, but not entirely accurate. The only lenses that produce the same results between different lenses are cinema lenses marked with "T" stops instead of "f" stops.

The F stop is Ratio without dimensions so as I understand that the exposure requirements between different lens will be equivalent for same F stop setting - hence exposure meters do not normally require to know the lens focal length - However, I don't think this means all lens's will give identical results when set to same F stop as other factors in the Lens design and manufacturing tolerances will come into play.

As mentioned, the answer is no, not really as lenses vary in transmittance and T-stops adjust for this. But for practical purposes, it's close enough unless you get two lenses with very different characteristics.

I've had second shooters working weddings with me whose lenses produced very noticeable differences at the same f/stop-ISO-shutter speed. Even some of my own lenses will produce varying results on the same camera.

F-stops are a 'rough guide' as compared to the more precise T-stops, but it is rather unlikely that you'll really notice the difference for the most part. My Sigma 150-500 lens is noticeably dimmer for a given f-stop than any of my canon glass, but that is just a matter of remember to adjust things up by 1/3 to 2/3rd of a stop over what I would with the other lenses. Also if I meter a shot wide open, but then stop it all the way down and adjust shutter speed and ISO then I'll be off by 1/6th or so of a stop in overall brightness, but I honestly can't remember which way it was going.

It really doesn't matter much in the majority of cases for photos. Unless you're trying to photograph a perfectly even scene, such as a grey wall, and for some reason you want photos of it taken with different lenses, then odds are you'll never really notice it. However if you're shooting a movie and swapping lenses around within the same scene, or zooming in with a lens, then you are more likely to be able to actually see the effect. This is why high quality lenses for that purpose have more attention given to this detail. But there is a lot of video out there in the wild being shot with cheap lenses these days.


In short: F-stop variance is something one should be aware of so they can know and understand what is going on if they ever run into a problem with it, but it is also something that you're probably never really going to need to care about.

gjl711 wrote in post #17641554
As mentioned, the answer is no, not really as lenses vary in transmittance and T-stops adjust for this. But for practical purposes, it's close enough unless you get two lenses with very different characteristics.

Yes, I found long ago that my 100-400 is about 1/3 stop dimmer than my 50/1.4 since there's a lot more glass in there. The rest of my lenses are even less off than this.

I was recently re-introduced to this concept last week when I was shooting some flowers with my 100 f/2.8 macro and Einsteins. The Cybercommander's indicated aperture was way off. I couldn't figure it out until I remembered that at near 1:1 magnification, the macro lens eats a lot of light.

An f stop is the ratio between the focal length and the diameter of the aperture. Two lenses with the same f stop have the same aperture size relative to the focal length. F stops define geometry (and therefore things like depth of field).

Ideally, two lenses with the same f stop will give the same light intensity on your sensor. But as already pointed out, there is the issue of light transmission of the glass. Therefore the light hitting the sensor depends on both f stop and light transmission.

Also, regarding lens parameters, they are given as round numbers. And generally speaking, round numbers are always false. So if a lens is marked f/2.8, that is just a nominal aperture. The real aperture will depend on the actual engineering numbers and will be different, usually only slightly, but sometimes significantly.

But from a practical point of view, lenses with the same f stop will give the same light (ignoring little differences in brightness and color).

You're in for a wild ride on this one, almost everything is only a guide, rather than a scientifically measured value - aperture, focal length, shutter speed, and ISO.

As has been said, aperture is only a relative value and doesn't take into account light loss from glass transmissiveness, number of lens elements, and vignetting. Cinema lenses by nature have to be more truthful of their specifications, so they use the T-stop system. A lens of t/2 is guaranteed to deliver exactly the same amount of light as another t/2 lens.

Focal length is another "close enough" figure, for instance a selection of 50mm f/1.4 primes of various makes can range anywhere from being a few mm wider or longer, not even taking into account the fact that a lot of lenses change focal length as they focus (focus breathing). Normally this isn't a big problem, but it can be, on the Nikon 70-200 f2.8 VR II the breathing is so intense at 200mm that at minimum focus distance the lens is actually 180mm in FoV.

Shutter speed these days is controlled digitally, so it's pretty accurate, but don't be surprised if 1/200th isn't the same in every camera.

ISO is the biggest offender though, because it's a standardized system of measurement for sensitivity akin to F-stops, the real ISO value of your camera can vary wildly compared to the amount displayed. A displayed ISO6400 on your camera may be a real value of 4000.

"f ratio" as a concept will provide the same exposure, for the same ratio settings for all lenses. So if you use a 200mm lens at f/4 with an actual aperture diameter of 50mm, you will get the same exposure, for the same overall scene, as a 50mm lens at f/4 with an aperture diameter of 12.5mm. So if you then crop the image shot with the 50mm lens to match the field of view of the 200mm lens you would not see a difference. It is fortunate that this dimensionless ratio allows us an easy way to relate the focal length of the lens, and the area of the aperture needed to achieve the same level of exposure.

What all of the theory of the "f ratio" usually leaves out is that it requires a perfect lens, which absorbs/reflects zero light as it passes through. Add to that the fact the the aperture mechanism is mechanical and requires some level of tolerances, along with all the other manufacturing tolerances means that each lens will actually pass slightly less light than it's rated f ratio would suggest. As others have said in practice you can lose maybe 1/3rd or 2/3rd of a stop to the glass. In film making a variance of that much might be noticeable as you cut shots between multiple cameras/lenses. So to get around it they use T (Transmission Stops). If you are more interested in knowing what the Depth of Field is going to be, as is often the case with stills photographers then the f ratio is the one you want to be using, as it is the physical diameter of the aperture that is important to DoF. By the time you have produced a finished still photographic image a variance of 1/3rd or even 2/3rd of a stop is going to be pretty hard to spot, and is only applicable if using multiple lenses.

Alan

turnmybassup wrote in post #17641535
I'm wondering if an f-stop rating from one lens will provide the same light properties as the same f-stop on another lens? For example, if I were to take a 70-200 f4 and shoot it wide open at 200mm, would it allow in the same amount of light/matching histogram of using a 35mm 1.4 at f4, provided all other camera information was identical?

My original thought is that it would be the same, but I wonder if I'm missing something? I would imagine the available light could/would change things, such as the 200mm photo may not include the sky/lights, thus altering the histogram, but would the shared elements in the photo be identical?

All lenses set at f/5.6 used with the same ISO choice (either film speed or digital) and shutter speed SHOULD provide the same exposure levels. As mentioned above, sometimes they are a little different. My suspicion is that the differences are due to engineering and manufacturing tolerances and not an intended design difference.

For all practical purposes, I've never had to deal with any exposure differences between lenses. That could be because I've always used high-end lenses (the best Nikkor lenses in my chosen focal lengths for my Nikon F cameras and, similarly, "L" series lenses with my Canon cameras). I would suspect that low-priced third-party lenses may show the greatest differences.

Having a fair number of Canon 'L' lenses, I ran some tests a while back.
Assuming you're wondering about the DOF of various f/stops.

Taking pains to maintain the same "field of view", I could fine no discernible difference in depth of field
regardless of the lens used...70mm @ f/2.8 had the same DOF as a 300mm @ f/2.8.

Wow, there is a lot of information here! The main angle that I was inquiring about is testing light to see what lenses to rent for an upcoming wedding I'm shooting.

To make a long story short, I am shooting at the place that I work (and live, consequently) and I wanted to take some of my fast lenses that I have access to and see what I need to rent to supplement. ie, I can use the 24-70 2.8 to see if I need to get the 70-200 2.8 or if I could get away with the f4 to save weight. I was just wondering if it would be a comparable and reliable test. It seems like it will be based on the responses here, or again am I missing something?

It will. Any focal length lens set at f/2.8 will give you essentially the same exposure as any other lens set at f/2.8. Likewise at f/4 or any other aperture. There may be 1/3 stop difference in exposure between any two lenses, but that's negligible. Test with your 24-70 and be comfortable with the conclusions it lets you reach about a 70-200 2.8.

Jon wrote in post #17641855
It will. Any focal length lens set at f/2.8 will give you essentially the same exposure as any other lens set at f/2.8. Likewise at f/4 or any other aperture. There may be 1/3 stop difference in exposure between any two lenses, but that's negligible. Test with your 24-70 and be comfortable with the conclusions it lets you reach about a 70-200 2.8.

This is not always true. I have had second shooters who use different cameras & lenses be off by more than 1 full stop from my settings in the same lighting conditions. And this is after I tell them what f/stop/ISO/shutter speed combo to use. However it may be more of an ISO issue than lens difference.