Tweet
I have been thinking that the brightness of the projected film depends not only on the size of the projected picture but also the distance (throw) of the projector from the screen. In other words for a fixed projected picture size of say 72 ins wide, you will get a much brighter picture if the projector is as close to the screen as possible, with the zoom lens zoomed out for max picture size, as opposed to projecting further back from the screen (larger throw) with the lens zoomed in to fit the same 72 ins size screen. This must be because the f number of a zoom lens (focal length/aperture) is not constant and varies with zoom setting. For example the Elmo f1.0 12.5mm to 30mm lens is only f1.0 at the 12,5mm setting and is f2.4 at the 30mm zoom setting. Do I have this right?
-
-
I think you will always get a better image without a zoom lens, but I prefer to have a zoom lens so that I can enlarge letter-boxed images in order to maintain "constant height". There is a discussion on this topic at: https://www.avsforum.com/threads/doe...ality.1226740/
If there is loss of brightness when using a zoom lens, a good light meter will give you an idea of the actual impact of zooming. You might go over to http://www.film-tech.com/vbb/ and search the subject there. I know the pros use special meters to measure screen brightness and determine how evenly the light is distributed. I believe most projected images tend to be the brightest in the center of the image.
-
Totally agree with you there Ed - you cannot beat a prime lens for best light transmission, contrast and sharpmess. Assuming its a high quality prime lens of course!
Something is wrong with my calculations though. If the Elmo f1.0 12,5mm -30mm lens is really only F2,4 at 30mm, and assuming that light transmission is inversely proportional to the square of the relative aperture (f number) then you would expect the light intensity difference from projection of the same size image at 12.5mm and 30mm would be from 100% to about 20% . I do not believe it is anywhere near that much of a drop off, so something does not add up in my assumptions and/or calculations.
Help anyone?Comment
-
Your knowledge on F stops exceed mine. The only time I thought about F stops was when I wanted to manage the depth of field and knew that lower light and a larger aperture would allow me to have sharper images in the foreground while the background was out of focus. It will be interesting to see what others have to say on the subject.Originally posted by Paul Adsett View Post
Comment
-
Ed I think I have written a load of nonsense! If you think about a zoom lens in a camera, it MUST maintain its relative aperture value when zooming from min to max, otherwise the exposure of the film would change due to darkening of the image. When you zoom the lens in a reflex camera you will see no change in image brightness when zooming while looking through the viewfinder. So they say a little knowledge is dangerous and clearly I know nothing about the optical design of zoom lenses and how they achieve constant effective aperture and constant exposure of the film or constant image brightness on projectors.Comment
-
This is an interesting topic. I'm a bit unorthodox when it comes to watching my current Super 8 home movies. My zoom lens of choice is the ever rare Eumig Suprovar 1.0 18-25 mm. This is by far my best and sharpest lens in the collection of other zooms, and primes. I generally set the Eumig Mark S 709 back about 10 feet from the screen. I use a vintage silver screen for optimal brightness. When showing home movies to the family I will choose a screen size of 3 ft by 3 ft. I know not very large, but it works well in a smaller living room area. Brightness, and sharpness at this screen size is fantastic!Comment
-
Speaking from camera lens point of view...
Yes, most camera's zoom lens will stop down when zoomed in. Looking at my Olympus 40-150mm zoom kit lens for example the min. aperture will gradually close down from f4.0 to 5.6 at telephoto end. There're some "pro" grade zoom that would retain its aperture throughout at the cost of the size, weight, and (literally) its cost.
I highly doubt if there're any "pro" grade super8 projector lens with constant aperture ever made. Large aperture lens? yes. Large & constant aperture? Hmmmm...🤔Comment
-
The other difference with camera zoom lenses is that they hold focus when zoomed, projector lenses don't. I wonder if that difference in design has any relevance here.Comment
-
Hi Paul. You’re right in your assumption that you’ll get a dimmer image with the same screen size when the projector is further away and zoomed in more.
This is fun to use;
https://www.projectorcentral.com/pro...ulator-pro.cfm
Sadly it doesn’t list super 8 projectors!
As you can see, there is not a massive difference when dealing with moves of a couple of meters and a lot of course has to do with the quality of the glass in the lens.
As you know, prime lenses are always better simply because they have less elements to get through.
You’d think with a distinction in photographic technology and a degree in film & television production I’d be able to help you work out the light loss with different f stops, but that dealt with camera lenses, not projection lenses...so I will have to look up!
It's a great question which will now have me thinking about it all day, thanks!!!
I’ll get back to you!Last edited by Rob Young; May 01, 2024, 06:51 AM.Comment
-
Don't some Pro lenses use T stops that actually show the exposure equivelent not just the focal length to aperture (in mm) of the f stop rating?Comment
-
This article is about digital projectors, but most of the information applies to all projector types
Projector Brightness and Zoom Lenses
The purpose of this article is to examine the impact of projector brightness, zoom lenses, positioning a projector, and the need for achieving a desired level of brightness and more importantly contrast and grey scale.
We are aware that there is a certain degree of variance in what projector manufacturers’ state in their marketing materials and spec sheets as the projector’s light output in “ANSI” lumens. Their published specification versus their actual output to a verifiable standard can be a difference in the way (procedure, environment, test patters, etc.) the projector’s light output is measured since there are no ANSI “police” and the ANSI “standard” is really no longer in place. In other cases, the discrepancies can be due to manufacturing variances and tolerances. The bottom line is that there can be as much as a 20% discrepancy in what a specification sheet claims and actual performance.
With this in mind, the selection of a specific projector and zoom lens combination becomes even more important because the zoom lens position in relation to where the projector is installed can further reduce light output. The claim of reduction in light output will depend on how the manufacturer measured the light output of the projector with the zoom lens. Was it measured at its brightest/closest setting at full wide angle (most likely) or in the middle range or at the least bright (unlikely) at full telephoto?
In simple terms, when a projector’s lens is in full wide angle mode which is the largest image from a given distance, more lumens exit out the lens. In full telephoto mode, less lumen light output is produced. The worst case scenario is a projector that has its light output overstated by 20% and it has a 2:1 lens and is installed at full telephoto in a given application. The implication on true system light output is not to be ignored and this affects both the screen consideration and ultimately the system performance. In the AV industry there is an “old wives tail/general rule “that there is a two to one range of brightness in most zoom lenses. This comes from the “typical” range of zoom lenses out there but this is not a standard by any means. There is in fact a significant shift in brightness that occurs with projectors that are exceptionally flexible in their placement, by virtue of wider range zoom lenses and conversely, less brightness shift or decay on shorter zoom lenses. The general rule is just that, too general.
Many projectors for the home and commercial applications offer 2:1 zoom lenses. That is a placement range from closest to furthest, of 2 to 1. Perhaps, for a 100″ diagonal 16:9 screen, that might be from 10.5 feet to 21 feet away. In general terms, if you mount it 10.5 feet from the screen, you will get almost double the lumens as mounting it 21 feet back.
There is no straight and simple universal formula, because different lens designs will have some degree of impact (some more, some less) on the actual amount of change, but, let’s say that with a typical 2:1 lens, it will be “close” to a doubling. If, on the other hand, the projector has a far more limited lens, say with a zoom ratio of 1.2:1, then, the change in brightness from one “extreme” to the other, becomes minimal, and not a serious consideration in terms of light decay from one position of the lens to another.
Today, most moderately priced DLP projectors have limited zoom lens ranges (1.1:1 to 1.3:1), so there isn’t much to concern ourselves there, but with the 3LCD, and LCoS projectors, where most of them seem to have at least 1.5:1, and many are around 2:1, where you place the projector can matter a lot. The point to consider is that depending upon the actual light output of the projector and the zoom lens in reference to where the projector is place can result in a significant light variation from the optimum suggested in the manufacturer’s specifications and the system designers original design concept.
What do lens specifications mean?
Lens specifications tell us two things:
1. The focal length of the lens is usually expressed in millimeters (example: 50?75mm). The zoom ratio of 75mm to 50mm is about 50%, or 1.5:1. This means the system designer can vary the size of the projected image by 50% without moving the projector closer or further back in the room.
2. The second number or specification of a lens is the aperture or opening of the lens glass surface that actually passes the light. This is usually expressed in f?stops (example: f2.5?3.0). F?stop specifications are guides to relative brightness capability of a given lens or optic when comparing two or more projectors with similar focal length lenses. A projector with a 50?75mm, f2.5 lens will produce brighter images over the entire zoom range than one with a 50?75mm, f3.5 lens. Note: Some manufacturers will give an f?stop specification over the entire focal length of the zoom lens, i.e. 50?75mm, f2.0 ? 2.5. This shows not only the varying f?stop in a zoom lens but also refers to that fact that there will be light loss as the f?stop number increases in the zoom range.
Focal Length and f/Stop: the Formula:
The aperture of a lens is simply the opening through which light passes and it is controlled by an adjustable diaphragm or iris. Each setting of the diaphragm is called an f/stop and is always read as a number, not as a fraction or true ratio. It is referred to as the f/stop or the f/stop of the diaphragm opening.
This value is designated by a lowercase f with a slant (/) between the f and the value. For example, f/8 means that the diameter of the opening in the diaphragm is one eighth of the lens focal length, but only “when the lens is focused on infinity.” In this example f/8 is the effective aperture. If the lens were focused at other than infinity, f/8 would then be the relative aperture. In the study of the relationship between aperture and image brightness, the term relative aperture is used frequently. The term relative aperture then refers to the ratio between the effective aperture of the lens and its focal length. This relative aperture of the lens is controlled by two factors: (1) the diameter of the beam of light passed by the lens. (2) the focal length of the lens, which governs the size of the area over which the light is spread. The formula to determine the f/stop of a lens is as follows: f=F/D where F is the focal length, D the diameter of the effective aperture and f the f/stop or the relative aperture. To find the f/stop of a lens that has a focal length of 8 inches and the diameter of the effective aperture is 2 inches, use the formula, so: f=8/2=4 Therefore, the lens has a relative aperture of f/4. When the diameter of the opening (aperture) of the lens is made smaller, less light is admitted and the image formed by the beam of light passing through the smaller opening becomes dim. As the size of the opening is reduced, the ratio between the aperture and the focal length increases. So as the f/stop becomes larger, the size of the relative aperture decreases. Since the f/stop is a ratio of focal length to the lens diameter, all lenses with the same f/stops regardless of focal length provide the same amount of light on the focal plane; that is, when all the other factors that affect image brightness remain constant.
Alan C. Brawn
Alan C. Brawn, CTS, ISF, ISF-C, DSCE
Brawn Consulting
https://www.brawnconsulting.com
2031 Jewell Ridge, Vista CA 92081
alan@brawnconsulting.com
P:760.599.6530
C:760.716.1242
F:760.231.7426Comment
-
Well, that about sums it up, Ed!
Brian, yes T stop is pretty standard on lenses used for cinematography as it is much more accurate (actual light transmission of the lens) than f stop (aperture setting of the lens).Comment
-
I am not any kind of expert on a subject like this, but it would seem that the amount of light or brightness would stay relatively the same, as the distance from the lamp does not change or, at least not by much, beyond the individual lense components within said lense. I also reiterate that I am no expert here.Comment
-
I think Osi, that Paul was asking that if move the projector back from the screen (so the distance between lamp and screen increases) and then zoom in to get the same image size as you had to start with, will it be darker? The answer is yes indeed it would. How much darker depends on how far back you move it and how much you zoom the lens. And the quality of lens, etc.Comment
-
Unfortunately it's not the case.Originally posted by Osi Osgood View Post
In most case the lens used is literally the bottleneck of the system. And by its nature the further you zoom, the smaller the bottleneck is. The illumination drop would be, as several says here, as much as a full stop - or half from the maximum brightness possible. So yes it is significant especially in commercial installation (unlike most of home cinemas like us
).
Comment
Comment