WARNING: Part rant and all technical. If you simply enjoy taking photos without being a techno gearhead feel free to ignore this post. I promise to return to more readable posts after getting this out of my system.
On Facebook the other day one of my non-person friends (BorrowLenses) posted a link to a video about T stops vs f stops. Huh? I've been photographing for 57 years and never heard of T stops. I watched it and did a bit of further research. It seems some folks with lots of time on their hands (time that would have been better spent taking pictures IMO) compared the actual amount of light coming through a variety of lenses and learned that (allegedly) your lenses are lying to you. According to them f/2.8, for example, may actually be f/2.9 or even f/3.something depending on the lens. They are (accurately) surmising that some light is lost inside the lens, absorbed by the lens elements, reflection for lens surfaces, etc., which isn't being accounted for with the result that they believe the f/stops are mislabeled.
The only "mis" going on is a misunderstanding of what the f/# means. To arrive at the f/# the diameter of the diaphragm opening is divided into the distance between the lens when focused at infinity AKA the Focal Length (more on that later). The resulting number is the f/#, period, end of calculation. Thus a lens rated at f/2 has a diaphragm opening the is one half the focal length of the lens, an f/4 stop is one quarter the focal length. It's a simple ratio. The diameter of the opening:focal length with the focal length always being "1". It is not and never was a direct measure of the transitivity of the glass although it is a useful approximation for calculating exposure.
So why don't lens manufacturers measure the light each lens transmits? I'm not a lens engineer but I've been a practicing photographer for a looong time so here's my educated hard knocks school answer. They don't because light loss in the lens is only one factor in how much light will actually hit the film/sensor at any given f/stop. The nominal focal length of all lenses is the distance between the lens and the film/sensor plane when focused at infinity. That is the closest the lens can be to the film plane and have anything in focus. As you focus on anything closer than infinity you are moving the lens further from the film plane. You reach a 1:1 subject size to image size when your lens is smack in the middle between the subject and the film.
Yes, I know you can bend the light rays using a +diopter effect (basically a magnifying glass) and the distances won't exactly equal at 1:1 but that's not the point. The point is that as you move the lens further from the film plane the light on the film/sensor decreases because of the law of inverse squares. On an SLR (digital or otherwise) the diaphragm opening can be linked with the focusing helical to approximate the same amount of light to a point. The barrel of the lens however limits how far the diaphragm can open. That's why the maximum aperture on most zoom lenses is marked as variable. When you zoom out, the lens barrel isn't big enough to accommodate the larger opening of the diaphragm at the longer focal length. It's the same size as at the shorter focal length but because it is a ratio of the two measurements, iris diameter:focal length, that same diameter is a smaller portion of the focal length. And yes, it does mean you get less light at any given spot on the focal plane, again because of the law of inverse squares (spread a given intensity of light over a large area and the light on any given part of the surface is less in inverse proportion to the increased distance). Although the effect is minor at subject distances closer to infinity, the light fall off increases as you focus on closer subjects and the progression of fall off is geometric.
There are other things that reduce the light at any given f/stop, notably filters. Before Photoshop allowed you to alter the relationship between values it was necessary to add a filter to the front of your lens. Obviously any filter you place in the light path will reduce the light reaching the film/sensor but that does not alter the ratio of the iris opening to the focal length of the lens. It is something the knowledgeable photographer has to compensate for to get accurate exposure. Or I should say had to compensate for.
All we are concerned about is correct exposure which is a mix of intensity of light and time. Modern cameras all have built in light meters coupled to computer chips to read the light actually falling on the sensor. The meter measures the light directly through the lens and doesn't give rat's patootie what the labeled f/stop is. If your camera has a histogram it tells you whether the light falls within the dynamic range that the sensor can interpret into a JPG based on the chosen exposure program (if you are shooting RAW the sensor will record a somewhat broader range that you can play with later). Even if the f/stop was intended to be a measure of actual light striking the sensor, which it isn't, there is no way the manufacturer could label the diaphragm ring on your lens with T numbers that accurately reflected the intensity since it is variable due to all sorts of adjustments to focus, filtration, etc. that change with each and every image you shoot. A simple ratio of diaphragm diameter to nominal focal length, that is something they can label and that is all it is. So f/4 is f/4 no matter what other circumstances are reducing the light coming through the lens and no, the manufacturers aren't cheating to make you think the lens is faster than it actually is. Today's photographers have marvelously technological cameras that virtually eliminate the complex calculations needed to get accurate exposure 50 years ago. Be grateful, stop wasting time on silliness and go take pictures. Jeesh!
Update: I've since been told that "T" stops are important to videographers because they don't have in camera metering and they can't always trust an incident meter to read the correct f/stop because f/stops vary from lens to lens, thus the need for "T" stops. I'm not into video but the few times I've done it I never had exposure problems but hey, I suppose those who do it more are more likely to encounter such things. The point of my rant was that for still photographers (that's who I'm talking to here) it isn't something to get wrapped up in. Even when I was in the army and we had no light meters at all we never concerned ourselves over comparing the relative transivity (I don't think that is even a word) of one lens vs another yet we somehow managed to get good exposure anyway. As the old saying goes "your mileage may vary".
Wednesday, November 12, 2014
Friday, November 07, 2014
Over a month and no posts. I've been wrestling with my thoughts on the state of photography, in particular, my photography. Looking at what's on the web I feel kind of 'out of step' but at the same time I have no desire to be 'in step'.
I just finished reading Disappearing Witness: Change in Twentieth-Century American Photography by Gretchen Garner. It is an art history book which is not my usual reading matter. I read enough art history in college to last me for a lifetime. Art history is mostly dry boring stuff and there is a certain amount of that in this volume, recitation of lists of photographers and how they fit into this or that genre. But this volume attracted my attention because its premise is something I had noticed myself.
One aspect of aging, of having been around for a long time, is that you gain a perspective that younger people don't have. In a sense it's like you learned to drive at night, able to see only what your headlights could illuminate and gradually the sun comes up and then you can see all this stuff that was hidden in the darkness. The stuff immediately around you, which was all you could see in the dark, takes on a whole new context once the sun comes up. I learned and began my practice of photography in the latter end of what Ms. Garner refers to as the "witness" mode of photography, the practice of looking out at the world, connecting with it on a psychic and emotional level, and making images that attempt to convey that connection to the viewer. That largely remains my mode of expression to this day.
Somewhere in the sixties or seventies the focus of art photographers took a major turn inward. It manifested in photographs that were less about the subject in front of the camera and more about the inner life of the photographer. Images became staged performances that were recorded on film and later digitally to illustrate an idea the photographer had. Such photographs had existed for some time previously of course but most were commercial illustrations for advertising or magazines, images made by photographers under the direction of art directors or editors. In the latter '70s and onward however it became the major trend of fine art photography.
I have observed this trend even among photography enthusiasts who don't aspire to "fine art" status but are moving in this direction as well. I don't know if it is the influence of Photoshop with its infinite control or a response the art world but I see ever more constructed as opposed to observed images as time goes on, images that are less about the subject matter and more about the photographer.
Perhaps I'm just being "stuck" in my past but I'm not sure that this trend is a good thing. Yes, we need to look inward, Eastern religions with their emphasis on meditation have been doing this for millennia but always as a ground for relating outward to the rest of creation or seeing ourselves in what we see. It is my belief that is what "witness" photographers were trying to do. Whatever the trend my photography will continue to look outward, to engage the larger world.
The photo is of a tree on the corner of Rt 56 and Garfield Rd outside Potsdam, NY. It caught my eye as I was driving into town one day and I stopped to photograph it with my cell phone which was the "best" camera (the one I had with me). It has been modified in Photoshop to accentuate the mood.