light and f stop

[benjiboy]

Thanks Helen

"just curious. what is the t stop system?"

Allan,

T stops are transmission numbers that correspond to the theoretical transmission of an f-stop of the same number - they take losses into account, which f-stops don't because they are simply the ratio of the focal length and the entrance pupil diameter. Most ciné lenses are marked in T-stops and f-stops, the difference being about a quarter or a third of a stop for primes and half to two-thirds of a stop for zooms. So you have a nice fast f/0.95 25 mm lens that is really only a slow old T 1.1, or an f/1.1 16 mm to 44 mm zoom that barely makes it to T 1.3, for example. Most of the losses occur at air-glass surfaces, so the lenses with a lot of elements lose out.

Strictly speaking, the f-stops are used for focus calculations and the T-stops for exposure.

Best,
Helen

Thanks Helen, you explained it much better than I could, that's exactly what I meant, I found in practice that using my zoom lenses with a separate light meter of proven accuracy resulted in enough under exposure in slide film to be significant, I found that if I used my Canon FD 70-150mm at 100mm at f8 my slides were about 1/2 stop underexposed, so I tried shooting the same thing with my prime Canon 100mm f2.8 lens at f8 and the exposure was bang on the money. I do a lot of studio portraits with studio flash, and found the same thing with the same lenses. I don't have any detailed data to support this it's just my personal experience

 

[gnashings]

Thank you Helen and Ole, I find this fascinating, and I really appreciate the time you took to explain! I love how much I learn here!

Peter.

 

[Marco Gilardetti]

Satinsnow, Dan Fromm is right, out of question. It's amazing even on how many books this topic is misconceived or badly exposed. I could cite many pages written by Ansel Adams which are... mmmh... say "borderline" right, or "oversimplified".

which in shooting the same subject at the same distance you will have less depth of field

First thing: "at the same aperture" must be added. However, this is probably where you get confused. If you do what you say, the subject photographed with the shorter focal lenght will be much smaller on the negative. Of course it seems to show more DOF: simplifying very much, it has less details and you can't really tell what is in focus and what not. Now, enlarge both images in the darkroom so that the final print shows the subject with the same dimensions and compare them. Grain and definition problems apart, they will show approximately the same DOF.

Or think it this way: take a 50mm and a 100mm. Now apply a focal duplicator to the 50mm, which does NOTHING BUT MAGNIFY THE CENTRAL PORTION OF THE IMAGE produced by the 50mm. At the same aperture, the two will show approximately the same DOF, but still one is a 50mm and the other a 100mm. Makes sense?

Do you get the trick? If you would be able to see with a wideangle all the small details that a tele reveals, you would see that what "appears" to be a well focused alpine panorama is instead formed by ultra-small misfocused tree leaves...

 

[Helen B]

Circle of Confusion is such an apt name for something to do with determining depth of field. It strikes me that whether you adopt the 'wider lenses give more DoF' or 'DoF depends only on magnification and aperture' position depends on how you consider DoF and what object distance you are talking about.

"Short lenses give more DoF than long lenses"
For constant CoC, a short lens will give a greater DoF (ie distance between the points at which the blur equals the maximum permissible CoC) than a longer one at the same magnification of objects in the plane of focus (I'll call it 'subject magnification'), and the difference will increase as subject magnification decreases (ie the farther away the subject gets). As subject magnification increases, the DoF for short lenses will approach that of long lenses so that they become effectively equal at some close point (location of which depends on a number of factors).

"DoF depends only on magnification and relative aperture"
For the same subject magnification, the diameter of the blur circle of background objects beyond the plane of focus (subject plane) is less for a short lens than for a long lens, but if the image from the short lens is magnified so that the background object is the same size as that from the longer lens, the blur circles will be identical (that's what Marco mentions above, written differently). In this sense the amount of blur is dependent only on magnification and aperture - but this is not subject magnification, and it is not the usual definition of DoF.

Format matters, as I'm sure most people on APUG appreciate. Smaller formats do give more DoF than larger formats for equivalent prints. For example, a picture taken on 4x5 with a 150 mm lens at f/22 and enlarged to 8x10 will have the same DoF as an 8x10 contact print taken from the same place with a 300 mm lens at f/45. Here the amount of blur is dependent on the absolute diameter of the aperture, not the relative diameter (150 mm at f/22 and 300 mm at f/45 both have entrance pupils 6.63 mm in diameter). This effect is the same as standing at one point with a zoom, and enlarging the centre portion of the shots taken with short focal lengths so they match the ones taken with long focal lengths. Resolution and graininess aside, the short focal length pictures have more DoF than the long focal length ones.

...but all this isn't the only way to consider acceptable focus - see Harold Merklinger's The Ins and Outs of Focus for example.

Best,
Helen

 

[Dan Fromm]

<large snip>Smaller formats do give more DoF than larger formats for equivalent prints. <snip>

Best,
Helen

This is because, other things equal, the smaller negative was taken at lower magnification. The potential loss from shooting smaller and enlarging more is that blur in the smaller image is enlarged more. There are limits, and in photomacrography they bite hard. See H. Lou Gibson's books on the subject.

Cheers,

Dan

 

[Helen B]

"This is because, other things equal, the smaller negative was taken at lower magnification. The potential loss from shooting smaller and enlarging more is that blur in the smaller image is enlarged more."

But the decrease in blur in the original more than offsets the subsequent enlargement of it, so the smaller format can produce more DoF for the same final (eg print) magnification. This is why people who shoot small format video mimic the comparatively shallow DoF of 35 mm movie film by creating an intermediate image on a ground glass (an aerial image would not work) and then relaying that to the smaller sensor.

Best,
Helen

 

[Marco Gilardetti]

Helen, it doesn't. I agree with Dan. Many comparative tests have been made over the years, and the fact that DOF is not dependent on focal lenght has always been seen clearly.

I recall at least two: one on "Popular Photography" which was called (I guess) "Depth of field 101" and the other on "Tutti Fotografi" which title I can't remember right now (but I may still have a copy at home, will check).

You can repeat the sets of experiments if you're doubtious: the one on "Tutti Fotografi" was very simple and made by photographing a metering rod placed diagonally in front of the lens, at different distances and with different focal lengths.

 

[modafoto]

I wonder how much warfare a question on shutter speed would start.

 

[Dan Fromm]

I wonder how much warfare a question on shutter speed would start.

Morten, I fear you're confusing honest disagreement with warfare. If we continue this amicable discussion much longer I think we'll come to agreement. Those whose views turn out to be mistaken -- I could be mistaken, have shared my calculations with Helen and expect her to find any errors and tell me about them so I can correct them -- will learn from the exchange of views.

Cheers,

 

[Helen B]

Quick monday morning reply:

I don't think that it even approaches 'disagreement' never mind 'warfare'; more a question of interpretation.

As my first paragraph on the subject said: "Circle of Confusion is such an apt name for something to do with determining depth of field. It strikes me that whether you adopt the 'wider lenses give more DoF' or 'DoF depends only on magnification and aperture' position depends on how you consider DoF and what object distance you are talking about." I'm just suggesting that neither of the two blanket statements are entirely correct, and neither of them are entirely wrong.

I'm still puzzled about the difference of opinion over DoF with different formats though.

Marco wrote: "Helen, it doesn't."
Marco, what are you disagreeing with? My preceding post was about the comparative difference in DoF between formats. My view on DoF for the same format is pretty much the same as Dan's - ie it is effectively independent of focal length up to a point, then steadily becomes dependent on focal length the further away you go.

I've seen a number of perfectly good tests, including my own, that show that blur is independent of focal length for the same magnification and aperture (which is backed up by theory) but that is not equivalent of saying that the appearance in the print is the same, or that the DoF is the same.

I think that Harold Merklinger's The Ins and Outs of Focus is a good alternative approach to the standard DoF methodology.

Best,
Helen

 

[Woolliscroft]

just didn't know if the longer focal length would change the amount of light even though the opening was larger

The only time you have to worry about lengthening the lens changing light transmission is if you add additional kit, such as close up tubes or a teleconverter between the lens and the camera.

David.

 

[Helen B]

Comparisons need a set of criteria and assumptions as a basis. Conclusions are then drawn. Sometimes the set of criteria and assumptions do not correspond to the practical situation we are faced with, so we should have some idea of how changing one of the criteria or assumptions changes the conclusions. Surely this is a factor that differentiates between an academic discussion and one with practical usefulness. DoF is all about the acceptability of images that are actually out of true focus, so there are plenty of assumed values.

For DoF comparisons we need to choose the limiting circle of confusion. This is not a fixed value for a given format: what is acceptable to someone using T-Max 3200P handheld at 1/15 will probably not be acceptable to someone using Tech Pan in a camera fixed to a solid tripod, for an extreme example. The choice of CoC affects the distance at which the change from DoF being effectively independent of focal length to being dependent on focal length occurs (for the same aperture and magnification). The larger the CoC, the nearer the change will occur (this could be one of the differences between my view of the usefulness of the change and Dan’s).

Perhaps of more practical use: The theoretical and practical examples of DoF being independent of focal length generally assume that the magnification required for comparison is the magnification of an object in the plane of focus. I often find that this is not the case: there is one particular item that fixes the magnification, and it isn’t conveniently in the right place in relation to the zone that I want in acceptable focus.

Having suggested that it is dangerous to make blanket statements, here’s a general observation: if the magnification is set by an object in the near part of the required zone of acceptable focus then the DoF will be greater for a shorter lens. This is the most common situation that I come across. If the determining object is in the far part of the zone, then the DoF will be greater for a longer lens.

Best,
Helen