 |
 | Home | Contact Us | FAQ | Search & Site Map | Link to Us |
 | Sign In | Join | Other 45 Sites in Network |
| PhotoKB HomeDiscussion Groups Digital Photography Digital PhotoDSLR CamerasZLR CamerasPoint & Shoot CamerasFilm Photography 35 mmLarge FormatMedium formatDarkroomFilm and LabsOther EquipmentPhoto Technique Nature PhotographyPeople PhotographyTechnique GeneralGeneral Photo Topics General TopicsAustralian PhotographyUK PhotographyDirectoryPhoto Clubs |
Photo Forum / Digital Photography / DSLR Cameras / June 2006What stop are you at?
An open question for the optical geniuses around here: The recent discussion of optical design and the true definition of aperture ratio (along with the definitions of "entrance pupil" and everything that discusses actual lens design as opposed to the theoretical understanding that I had) reminded me of my first lessons in optics, which included lengthy discussions of the difference between f-stops and t-stops. My training (as I've mentioned before) and indeed most of my work is in cinematography. And in motion picture work, you encounter (not frequently, but often enough) lenses with both f-stops and t-stops marked. In case this is new for anyone, F-stops refer to the actual focal properties of the lens with reference to what the aperture ratio does to depth of field, and are used to determine focus. T-stops, on the other hand, refer to the transmission of light through the lens, so that one can know where to set exposure. It is important to know the difference, because you might need to be shooting at t/4.0 for light, but that might actually be f/3.5 for focus, and that can obviously make a difference in a critical focus situation, especially if there's a focus pull involved. On some fast Panavision lenses, there are even seperate focus marks that are used if the lens is wide open, because focus is falling in a different place than it would stopped down. All of this simply to ask: does any of this come into play in still photography at all? I've never run across a lens with different F-stops and T-stops marked. Does anyone know of any? And because they *can* be different, does anyone know which scale is used when only one is present? I'd imagine t-stops because I don't see too many photographers measuring focus or using depth of field tables, what with good autofocus and the relative ease with which one can eye focus. But perhaps that is naive of me. Anyhow, just curious about all of this and assuming that someone here surely knows. Thanks, Will SNIP > All of this simply to ask: does any of this come into play in > still photography at all? I've never run across a lens with > different F-stops > and T-stops marked. Most dSLR cameras have an exposure meter that reads through the lens (as do most Point and Shoot models), and thus automatically take care of the extention caused by closer focus and optical transmission characteristics. That actual light reading is often automatically transferred to the camera/lens settings, depending on preset preferences. Bart > you might need to be shooting at t/4.0 for light, > but that might actually be f/3.5 for focus, and that can obviously make > a difference in a critical focus situation, especially if there's a > focus pull involved. On some fast Panavision lenses, there are even > seperate focus marks that are used if the lens is wide open, because > focus is falling in a different place than it would stopped down. OK, explain to me something about filming movies. I always see a "focus puller" credit, but never understood what that is. Is there a separate person, apart from the cameraman, responsible for focusing the lens? I also gather, both from your comments, and from other things I've seen/read, that on movie cameras, focusing tends to be done not by looking through the lens, but by measuring the distance and setting that distance on the ring. Is that correct? And if so, why is it done that way? > > you might need to be shooting at t/4.0 for light, > > but that might actually be f/3.5 for focus, and that can obviously make [quoted text clipped - 6 lines] > puller" credit, but never understood what that is. Is there a separate > person, apart from the cameraman, responsible for focusing the lens? Often. These days, sometimes not even physically close to the camera, thanks to the wonders of remote control and video assist. > I also gather, both from your comments, and from other things I've > seen/read, that on movie cameras, focusing tends to be done not by > looking through the lens, but by measuring the distance and setting > that distance on the ring. Is that correct? And if so, why is it > done that way? Again, often it's done by measure, yes. The focus puller isn't the guy looking through the lens, so they mark the positions needed and he goes through them at the right times, is one way it's done. Also, think about what happens when you focus. Do you go right to the exact spot and stop? Or do you hunt around, overshoot, come back, and so forth? Well, if a film shot requires a change in focus during the take, they *really* don't want that kind of thing showing up visibly, so they have to figure out exactly what distances they should be focused at at which times in advance.  SignatureDavid Dyer-Bennet, <mailto:dd-b@dd-b.net>, <http://www.dd-b.net/dd-b/> RKBA: <http://www.dd-b.net/carry/> Pics: <http://dd-b.lighthunters.net/> <http://www.dd-b.net/dd-b/SnapshotAlbum/> Dragaera/Steven Brust: <http://dragaera.info/> David nailed it. Camera departments are (hopefully) very efficiently run. The 1st Assistant is the focus puller (in addition to other duties) and a good 1st AC can actually tell the difference between 9' and 9' 3" by eye from the side of the camera. The cinematographer (frequently not operating the camera) is in charge of too many things to worry about focus, especially if they are also operating. The camera operator, even, is too busy making sure composition is maintained and the moves look good, so the focus puller is making sure that everything is sharp, there are no lens flares, and tons of other things. It is easier and more efficient to have someone dedicated to this, and in fact many focus pulls would be impossible if someone wasn't dedicated to changing focus in shot. Television is where this becomes most critical. Think about 24, which shoots 2 44-minute episodes at a time over a period of 14 or 15 shooting days. That requires moving very fast, so a 1st on 24 needs to be on top of his game. A scene where Jack Bauer is moving around frantically in closeup on a 200mm lens presents quite a challenge, and there's no way that he's going to hit each mark the same every take. Typically, they only get 2 or 3 takes, so the 1st needs to make sure that he hits his marks every time. If Kiefer Sutherland leans forward 6 more inches then he did in the last take, the shot requires that the 1st see that and adjust focus appropriately. Will [t-stops vs f-stops] > that one can know where to set exposure. It is important to know the > difference, because you might need to be shooting at t/4.0 for light, > but that might actually be f/3.5 for focus, and that can obviously make > a difference in a critical focus situation, ... and is a non-problem for photography. The lens is marked in f/stops. If you are in a critical focus situation, set the f-stop manually. All cameras (except range finders, of which there are no digital ones, I believe) measure through the lens, so the scene will look a bit darker to the camera and thus the exposure time is increased a bit, if necessary. A 1/3rd or a half-stop is the finest step most cameras offer ... anything less is not a problem, anyway, not even for slide film and digital (both really _hate_ overexposure). For digital, rather underexpose by half a stop than the other way round. If you shoot RAW (i.e. not a finished JPEG), you can push the image a stop or even two while "developing" with comparatively small consequences. (You can push JPEGs, but with only 256 gamma-corrected steps to begin with you only have 128 steps left after pushing one stop --- not enough for smooth transitions in critical areas like the sky. RAW, with about 2^12 linear steps, suffers less, though much more than the number seems to indicate[1].) Noise, even magnified by pushing usually isn't a problem with DSLRs, not compared to analog film at comparable sensitivity, even when pushing, and there are algorithms to supress it even further. Any t-stop problem would be overshadowed by one camera calling ISO 80 "ISO 100" and another once calling ISO 160 "ISO 100". > especially if there's a focus pull involved. Such a lens is unusable by (D)SLRs, since they practically always meter and focus wide open. Many zooms were (and the good ones still are) parfocal: no focus shift by zooming! How else would you focus through the lens, using microprisms or split prisms or even just the matte screen in the days before autofocus? (D)SLRs still (auto)focus through the lens, wide open, and so do point and shoot cameras and bridge cameras[2]. > All of this simply to ask: does any of this come into play in still > photography at all? No, I believe not. > And because they *can* be > different, does anyone know which scale is used when only one is > present? f-stop. In first approximation, t-stop = f-stop, and everything else is measured through the lens, anyway. > I'd imagine t-stops because I don't see too many photographers > measuring focus or using depth of field tables, Some cameras have a Dep or A-Dep mode, in which the focus depth measured by the autofocus (several points or several measures) pre-sets the aperture. For film you probably have a well-defined output medium (projection) and size/distance range. For photograpy you may watch them on TV (not recommended), on the web (640x480, 800x600), on the computer screen (1-1.5 MPix), on paper, from 4x6 inch and smaller to 8x12 and more (30x40 inch and more, printed). You can even write the digital image on slide film and project it (or use a digital projector, though they seldom reach 1MPix resolution today and stay payable). What is critically sharp at 640x480 may not be when viewed on much larger formats. What looks good on a computer screen may differ on paper. > what with good > autofocus and the relative ease with which one can eye focus. On most prosumer class DSLRs you won't find focussing helps any more, just a matte screen, and that'll be harder to critically focus with (no thanks to mirrors instead of pentaprisms, APS-C format and small viewfinders). However, there are after market solutions. -Wolfgang [1] linear means that half of all the steps are in the highest stop. Half of all the rest is in the second stop. So at the 6th highest stop, there are only 2^6 (64) steps ... [2] typically a point&shoot with electronic viewfinder and full manual control (like an SLR) or a DSLR with a small sensor, no changeable lens and no mirror (like a P&S) > All of this simply to ask: does any of this come into play in still > photography at all? I've never run across a lens with different F-stops > and T-stops marked. Does anyone know of any? And because they *can* be > different, does anyone know which scale is used when only one is > present? There are a few T-stop 35mm lenses such as the delecatble Minolta 35mm f/2.8 [T4.5] smooth transfocus lens which is an exemplary portrait lens. You can inagine the razor thin DOF at f/2.8 for a 135mm ... probably too shallow for most portraits in fact, but then the "sweet spot" is right about where the fastest T-stop is. I've seen a lot of strong highlight background shots, model in shade, that were fantastic in model sharpness and background highlight smoothness. On digital (with a 1.5 crop factor) it gets very hard to use this lens, and the results will be a little too much on the flat side. Regardless, I hope to find a used version in good condition some day. Still photographers either read the DOF off of the lens (if so equipped), use a table or calculation, or use the DOF "preview" button. All of that is a little subjective as the marked or computed DOF's are for a print size of (typically) 8x10 for a 35mm sensor (frame) size. Using a full frame lens with a smaller sensor will result in slightly less DOF than anticipated or computed for the "standard" print size (as the magification ratio is larger). The DOF preview button is also a very subjective interpretation as the viewfinder is quite small. Watching some films with a lot of movement and focus pulls does amaze me. Cheers, Alan. Signature-- r.p.e.35mm user resource: http://www.aliasimages.com/rpe35mmur.htm -- r.p.d.slr-systems: http://www.aliasimages.com/rpdslrsysur.htm -- [SI] gallery & rulz: http://www.pbase.com/shootin -- e-meil: Remove FreeLunch. > Using a full frame lens with a smaller sensor will result in slightly > less DOF than anticipated or computed for the "standard" print size (as > the magification ratio is larger). More DOF I believe because it's cropped. >> Using a full frame lens with a smaller sensor will result in slightly >> less DOF than anticipated or computed for the "standard" print size >> (as the magification ratio is larger). > > More DOF I believe because it's cropped. Since the sensor is smaller, the magnification factor to get to a given print size is greater. Therefore less DOF. Cheers, Alan Signature-- r.p.e.35mm user resource: http://www.aliasimages.com/rpe35mmur.htm -- r.p.d.slr-systems: http://www.aliasimages.com/rpdslrsysur.htm -- [SI] gallery & rulz: http://www.pbase.com/shootin -- e-meil: Remove FreeLunch. >> All of this simply to ask: does any of this come into play in still >> photography at all? I've never run across a lens with different F-stops [quoted text clipped - 3 lines] > > There are a few T-stop 35mm lenses such as the delecatble Minolta 35mm Goof ^ "135mm" ^ Sorry. > f/2.8 [T4.5] smooth transfocus lens which is an exemplary portrait lens. > [quoted text clipped - 25 lines] > Cheers, > Alan. Signature-- r.p.e.35mm user resource: http://www.aliasimages.com/rpe35mmur.htm -- r.p.d.slr-systems: http://www.aliasimages.com/rpdslrsysur.htm -- [SI] gallery & rulz: http://www.pbase.com/shootin -- e-meil: Remove FreeLunch. >>> All of this simply to ask: does any of this come into play in still >>> photography at all? I've never run across a lens with different F-stops [quoted text clipped - 7 lines] > > Sorry. Accepted! Alan- How bout some trimming? You are often good with that. And I apologize for not correcting the Subject line, which should have read: At what stop are you? SignatureYour faithfully, john mcwilliams > Alan- How bout some trimming? You are often good with that. > > And I apologize for not correcting the Subject line, which should have > read: > > At what stop are you? Whatever. And it should "yours" in your signature line. Sorry for not trimming, I'm distracted by the Sony A100. Cheers, Alan Signature-- r.p.e.35mm user resource: http://www.aliasimages.com/rpe35mmur.htm -- r.p.d.slr-systems: http://www.aliasimages.com/rpdslrsysur.htm -- [SI] gallery & rulz: http://www.pbase.com/shootin -- e-meil: Remove FreeLunch. >> Alan- How bout some trimming? You are often good with that. >> [quoted text clipped - 5 lines] > Whatever. And it should "yours" in your signature line. > Sorry for not trimming, I'm distracted by the Sony A100. 'S'cool. I erred. Should have written "Your faithfully writhing servant", or "Yours faithfully" Signaturejohn mcwilliams Oh, what a tangled web we weave, When we first practice to decieve >An open question for the optical geniuses around here: > [quoted text clipped - 4 lines] >in optics, which included lengthy discussions of the difference between >f-stops and t-stops. [SNIP] >Anyhow, just curious about all of this and assuming that someone here >surely knows. > >Thanks, > >Will Will, With still photography, unlike cinematography, one can *usually* change the shutter speed to compensate for an diff between f & t. While there are variable shutter cine cameras, they usually really allow for less light by closing down the shutter. More often, one is stuck with the frame rate and the max shutter opening for the camera. Thanks for mentioning T/Stops - I hadn't really even thought of that since grad-school - trip down memory lane. Hunt |
Reply to this Message |
 Sign In
 Join
 My Latest Posts My Monitored Threads My Blog My Photo Gallery My Profile My Homepage Start New Thread Enable EMail Alerts Rate this Thread
|
©2009 Advenet LLC Privacy Policy - Terms of Use
This website includes both content owned or controlled by Advenet as well as content owned or controlled by third parties.