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TTL Metering & Depth of Field ("DOF") Control

'Through The Lens' (TTL) exposure metering allows the photographer to select a specific aperture on lens without the iris diaphragm in the lens actually closing to the selected value (opening). But the preselected aperture is one of the parameters used for correct exposure metering and control. It therefore needs to be fed into the exposure meter, and this means that lens and camera need to be equipped with the necessary method of communication either mechanically or electronically.

zuiko50illus.jpg
Okay, other than the maximum aperture is used, the effect of the depth of field is virtually similar as seen through the lens in used. However, if you are selecting an aperture value, say, f16 on a standard zoom lens of f3.5-f4.8, the scene through the lens is actually the scene corresponding to that of maximum aperture of f3.5 (depending on the focal length since the zoom is a variable aperture zoom lens - meaning that: at 35mm the maximum aperture will be at f3.5 and when you zoom to 70mm, the effective aperture at that focal length is only f4.8 - a general designing trick to keep the zoom lens "light and compact"..after all, a novice will never know the difference).

Note: Unlike most other camera manufacturers, Olympus has their Depth of Field Preview function built-in on lens instead of have it on camera body. The DOF button which has a similar look as with the lens release button but it is located on the opposite side of the lens barrel (Hidden in this Illustration). Pressing this button closes down the diaphragm to the preselected aperture for viewing depth of field.

Tips: The unusual position of the aperture ring of Zuiko lenses are aimed to distinguish the aperture ring from the shutter speed ring. The aperture ring on any Zuiko lens is designed to operate full stops with continuous adjustment in between allow fine exposure adjustments but that is not possible with intermediate shutter speed setting. Focusing is fast and precise aided by the sleeve of textured, non-slip rubber fitted to the ring. The three rings (Aperture, Focusing and shutter speed) have different texture and feel, after a few rounds of pratice, you should be able to identify them even without have to peep and confirm.

Thus, to help the photographer to find out how is the depth of field (zone of sharpness) covered in the focal length and aperture used, there are generally two methods used in providing as an alternative way of checking DOF. Older cameras and lenses were usually provide with a well illustrated and color coded depth of field scale engraved on the lens' aperture ring to give a rough estimation of the zone of sharpness covers. The other method is to make use of the depth of field preview button/Lever. Once you depress the button (other than when the maximum aperture of the lens is used) the image in the viewfinder should dim or darken (The smaller the aperture you use, the darker it will be), you have to train yourself to to identify the differences inside the dimmed image where the clarity of the image improves accordingly with the increase of depth of field with the aperture used. Check the degree of blur at fore and background, with a smaller aperture or with a larger one, it should be clearer and depth of field will increase considerably with smaller aperture used. So don't be fooled by the open aperture method and this is one of the reason sometimes why an eventual processed image can be so much different from what you envisioned an image would look like after processing.

Exposure Control Fundamental

Before continue with other sections, Again, please bear in mind there is no such thing called perfect exposure. Because this can be very subjective and such values are mainly a personal preference. Theoretically, a good exposure is defined as one (corresponding to the film in use) that visually yields the most brilliance of colors to satisfy a viewer's desire rather than appreciating the creator's vision to use over or under exposure to emphasize a subject matter. Worst still, unless you are using unforgiving and tight exposure latitude slide films, an inexperienced printer in a color lab can have a higher chance of damaging your images with inexperienced handling in print control.

The OM-1(n) uses a pair of light sensitive photocell to measure the scene with a 18% gray as standard value (correspond with most scenery such as big portion of green sceneries which in turns will interpret as around 18% gray color in black and white) within the picture frame to provide an ideal exposure comprised of a specific combination of aperture and shutter speed setting in accordance to the film speed of the film in used. Such exposure measurement reference, is what generally referred as 'metering'.

Copyright ©-Free images collection, 2000 leofoo® Malaysian Internet Resources.

Large view: 55k Jpeg)

<<<--- General scene such as this photograph with lots of background within picture frame will just be simulated as 18% gray by the metering circuit. It should be very safe for OM-1(n) meter to provide a satisfactory exposure reading. Just remember, the photocells are not human eyes and they just read light in different levels of gray and 'translate' them in electric current. In short, it 'sees' everything in black ands white.,

meterillus1.gif meteraveralgrah.jpg
The metering system employs in OM-1(n) uses a rather 'fail-safe' way of center-weighted average metering method with center portion given higher priority for exposure calculation than the rest of other area combine (One would assume where focusing is, where usually the subject of interest is). In Center-weighted average metering, light coming through the lens is refocused by an aspherical metering element and prism onto a pair of Cds light sensitive cells located near the eyepiece.

Center weighted average metering system should be safe for most photographic situations. But it may need a little extra exposure calculation when an ideal "average scene with uniform light levels" is not present. Scene such as figures is spot lit in a dark surrounding, your subject is placed in front of a white wall or with the sun or brighter scene behind the subject. We usually referred this as exposure compensation where big portion of the scene may affect the metering cells to provide a wrong exposure measurement. In such case, exposure compensation is required for subjects with unusual tone distribution, set needle off center for more or less exposure.

There are a few ways to compensate an exposure reading. Unlike automatic exposure (AE) bodies, which has a few other ways to handle the task semi-automatically; with a mechanical such as OM-1(n) camera, you have to base on experience to adjust exposure manually either by: Turning the Film Speed to a different settings to fool the camera metering circuit to provide a different reading e.g. If you are using ASA 100 film, turning it to ASA 50 will mean it will be compensate 1 stop (Scene will be brighter, or subject in front of a white wall will render grayish caused by underexposure back to a more natural white) while on the other hand, moving the film speed dial to ASA400 will mislead the camera meter to compensate exposure by minus 2 stop so as to contain a spot lit figures being overexposure in a dark surrounding.

Well, that is an old fashioned but a sure and safe way to work method especially IF you don't want the aperture shutter speed setting be changed. Naturally, nothing is more convenient that just turn your aperture a stop or two bigger or stopped down to control exposure. Naturally, if depth of field is your prime consideration and you don't intend to compromise that for exposure compensation, while you have a range of shutter speeds to spare, speeding up or slow down the shutter speeds by the shutter speed ring will yield similar effect.



The two basic elements for exposure control are: lens opening (
aperture) and shutter speed. The size of the aperture determines the amount or volume of light reaching the film from a given subject and lighting. The shutter speed determines the length of time this light acts upon the film. Apertures are expressed in f-numbers, which are larger for small openings and vice versa (e.g., f/16 represents a small opening, f/2 is referred as "large" one). Shutter speeds are expressed in seconds or fractions thereof, which are generally the reciprocals of the numbers shown on shutter-speed scales (e.g. 60 = 1/60 sec., and 2 = 1/2 sec.). At usual apertures, each f-number setting (e.g., f/8) lets in twice as much light as the next numerically larger one (f/11) and half as much as the next smaller (f/5.6). Similarly, each shutter speed (e.g., 1/60 sec.) allows light to strike the film twice as long as the next higher speed (1/125) and half as long as the next lower one (1/30). The interval between two standard f-numbers (say, f/4 and f/5.6) or shutter speeds (say, 1/15 and 1/30) is one "stop."

slr1.jpg slr2.jpg slr3.jpg slr4.jpg
Viewing, metering through the Lens (TTL)

Mirror flip-up, Lens diaphragm stopped down, light reaching the film exposure is formed

Shutter curtain closes, reflex mirror stays down, back to TTL viewing

Total exposure on the film is determined by the combination of aperture and speed. Other things being equal, using the next smaller f-number (i.e., giving one stop more exposure) will balance using the next higher shutter speed (i.e., giving one stop less exposure), and so on. A great range of combinations (e.g., f/5.6 at 1/30, f/4 at 1/60,f/2.8at 1/125, f/2 at 1/250, etc.) will thus yield the same total exposure. The specific combination you choose under given lighting conditions will depend upon the degree to which you want the greater depth of field associated with smaller apertures or greater movement-blur preventing ability of faster speeds .

apertureillus.jpg



Note
: The smaller the f-number (e.g. f1.4, f2.0, f2.8 etc.), the larger it is the aperture. Thus, allowing more light reaching the film to compensate for the duration of the shutter curtain traveling time. The larger the number (e.g. f11, f16, f22 etc.) is referred as smaller aperture, and less light will reach the film.


Exposure is determined by the combination of shutter speed and aperture. As the numbers on either the aperture ring or shutter speed dial increase by one increment, the amount of light striking the film is reduced by approximately one half. For example, the amount of light at 1/125 sec. is one half that at 1/60 sec., and the amount of light at f/16 is one half that at f/11. Brighter scenes require either faster speeds or smaller apertures or a combination of both which will give the same amount of exposure; darker scenes require the reverse. For example 1/250 sec. at f/5.6 is the same as either 1/1000 sec. at f/2.8 or 1/30 sec. at f/16.
Shutter speed (sec.)

1/1000

1/500

1/250

1/125

1/60

1/30

Aperture (f/number)

2.8

4

5.6

8

11

16

Twin Tower.jpg

These simple combinations (and other factors relates) have made photography so exciting. As apertures and shutter speeds will have respective influences in the final effect of the photograph. The camera's metering is just a guide for a theoretical good exposure, you can alter the combinations to suit the effect that youdesired.
Basically, apertures varies the
depth of field. While selection of shutter speed affects expression of movement, freeze of action or just solely avoiding camera shake.

Note: OM-1(n)'s maximum available shutter speed range is from a moderate 1 sec at it slowest speed to a fair 1/1000 sec.

<<--- Copyright ©-Free images collection, 2000 leofoo® Malaysian Internet Resources. A rare view with half of the current World's tallest Building (as at January 2000). The Petronas Twin Towers, Kuala Lumpur hidden in the clouds.

A reasonably good exposure is formed by a combination of few factors: Shutter Speed, Aperture and a subjective vision. The bare bone OM-1(n) camera featured here has taken care of the first two most important components, the third, however has to be you to execute and convert what you are seeing and your thought into a two dimensional visual art.

So, fundamentally, an OM1(n) is no difference with a top-of-the-line Canon EOS1v or a Nikon F5 when it comes to theory of formation of a basic exposure. All you will ever need is a film roll, let the camera handle the shutter timing, the lens stopping down to the aperture value you think best control the depth of field. Well, naturally, there are other differences you can find in any modern SLRs, in terms of responsiveness, the ease of handling with the levels of built-in automation, accuracy in metering and exposure control, full system integrity and possibly, durability factors it provides. Of course, every form of 'enhancement' is closely related to price. A flag ship model of a major camera manufacturer, such as a Canon EOS1v or a Nikon F5 could cost as much as 6-8 units of used OM-1n !

The true strength of OM-1(n), in this case, is not just confined to its advantage of being so compact in its size and lightweight which provide great mobility to any photographer who often on the run - but rather, in its simplicity and confidence it projects. You tends to care more about the process of image making, rather being command by electronics.

eos3om1compbk.jpg
Sometimes, I do wonder why modern SLR camera has to be made that HUGE and bulky. Isn't it electronic circuit should help to down size and dimension of cameras ?

<<<-- Comparing physical dimension between a ultra-sophisticated 1999's 45-points autofocus EOS-3 and a 1972's manual focus mechanical OM-1 with a 5 fps motor drive attached.

These may sound very boring to someone new to photography. But it is an essential route for everyone to understand the absolute basic. Since the OM-1n is being spec as such a simple SLR and thus I thought it has the need to make something useful out of nothing to talk about this little jewel from Olympus. After all, regardless how complicated or sophisticated a metering system is designed or how many types of exposure control have been programmed, all the technologies are based on the core basic fundamental elements of shutter speed + aperture value to form an exposure. So if you are happened to be a OM-1n owner, don't feel disheartened by the fact that this is the most basic camera body Olympus can offer, instead, you should maximize its full potential out of the nothing to equipped yourself . Frankly, I hope I am not biased. Sometimes, simple camera such as this fine piece of hardware could actually make you think more before you shoot. That is a valid reason for you t think about more, right ?

Back to Index Page of OM1(n) Instruction Manual
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Index Page of OM2(n) Instruction Manual

Olympus OM-1(n): Main Index Page (5 Parts) | Camera Operations (6 Parts)
Specifications:
HTML | PDF | Main Reference Map: HTML | PDF (217k)
Olympus OM-2(n): Main Index Page (6 Parts) | Camera Operations (9 Parts)
Specifications:
HTML | PDF (48k) Main Reference Map: HTML | PDF (203k)
Olympus OM-2SP: Camera Operations | Other Issues
Specifications | Main Reference Map / nomenclature

Shared Resources: Supplementary articles: TTL Metering, Depth of Field, Shutter Speed & Aperture
Motor Drive and Power Winder: Main Index Page (4 Parts)
Motor Drive 1 | Motor Drive 2 | Winder 1 | Winder 2
Flash Photography:
Main Index Page (4 Parts)
T45 | T32 | T20 | F280 | S20 | Qucik AUTO 310 | QA300, 200, 200S
Macro-Photography:
Main Index Page (3 Parts)
Macro Flash Units:
T10 Ring Flash, T28 Twin, T28 Single, T8 Ring Flash
Accessories:
Databack 1-4 | Screens | Finder Accessory | Remote | Cases

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Maintainers for OM Site & Message Board: Mr. Rick Oleson <rick_oleson@yahoo.com>; Mr. Bruce hamm <bhamm@magma,ca>; Mr. Simon Evans <sje@lrc.ruralwales.org>; Mark Dapoz <md@dementia.org>;

Credit: My old time buddy, Ahmad Ikram, Dr of Rubber Research Institute (RRI), Malaysia who shares the same passion with me and also lending his OM-1n, OM-4 and the Motor Drive 1 to me for preparing some images in this site; Mark Dapoz <md@dementia.org>for reminding some broken links; Mr Poon of Foto Poon, Ipoh, Mr Richard, Ampang Park, Mr Lim and Miss Jenny of Foto Edar for their generosity for their OM1(n), OM2n camera and some Zuiko lenses. Mr Hans van Veluwen for mistakenly using some content earlier from his OM website; J Sorensen for providing some useful images to rectify some technical "flaws"; Mr Gen Holst for helping during the early stages of development of this OM site; Mr KKLow for some of his earlier images on the OM-1appeared in this website; Miss Wati and Mirza for helping me to convert this Operation Manual into a HTML format. Mr MCLau for rectifying some mistakes made on the earlier preview sites. Site created 'unfortunately' again with a PowerMac. A personal tribute to the creator of the OM system and also a site dedicated to all the fans of Olympuses and Zuiko Optics worldwide. Some of the content and images appeared in this site were scanned from OM official marketing leaflets, brochures and instruction manual(s) for educational purposes. Olympus is a registered tradename of Olympus Optical Inc., Japan.