Angiographics:Mobile Ophthalmic Photography, Laser, HRT, and Fluorescein Processing Experts!

Exposure and Underexposure


Exposure means getting the right amount of light on the film (or digital pickup). The major cause of poor angiography is UNDER-exposure. This means not enough light reaches the film.


How To Tell If Your Angiograms Are Underexposed:


Look at your negatives - You cannot tell for sure from the prints! If the angiographic image on the negatives is only a light gray, but the numbers printed (by the manufacturer) at the top and bottom of the film are good and black, then your angiograms are probably underexposed.
(If the edge numbers are not a deep black with Kodak films, you may need to increase your development time or temperature, or you may need to switch to a more active developer. Call us for suggestions. The edge numbers with Agfa and Ilford film may be only a dark gray when acceptably developed).


How To Increase Your Exposure:


Turn up the flash power. On units rated in Watt-Seconds (like Topcon) go to the next higher number (i.e., from 150 to 200). On "automatic units" there are usually settings that increase the flash (saying "small pupil" or "+"), or you can switch to manual and use the maximums - check the manual or call us for suggested settings). On cameras where you set the film's speed (ISO, EI, ASA, etc.), use a LOWER number to fool the camera into raising the flash (it thinks you are using less sensitive film and will increase the exposure to compensate).

Advanced photographers will shoot clear, well-dilated eyes at one setting; they'll shoot eyes with moderate brunescent cataracts or small pupils at a higher setting; and theyll shoot eyes with small pupils and dense cataracts at maximum.

Remember, you should always err on the side of MORE flash. An overexposed negative is always salvageable;an underexposed one seldom is.


Do I Need New Filters?


Probably not. Most angiogram filters are interference type, not absorption type. When filters fail they usually allow more light to pass through, not less. The usual indication of failed filters is an image appearing in the "control" shots taken before the dye is injected.


Do I Need To Replace The Flash?


If most of your angiograms are underexposed and you are using maximum flash, yes. By the way, flash tubes are very expensive. You should always mark the base of the tube with the date you put it into service.
Occasionally a defective tube will fail within months of installation; if so, return it to the manufacturer.
If you replace your flash tube, keep the old one in the packaging the new one came in, with a reminder of what flash power you needed to get decent results for fundus photos and for angiography. It will make a usable backup!


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Read on for more detailed information for film angiograms:

A properly exposed negative will have highlights (the areas receiving the most light, usually NV leakage or major vessels) that are almost as dark as the numbers printed on the edge of the film by the manufacturer. A properly exposed negative will have shadow areas (areas receiving the least light, usually the macula) with at least some discernible tonal information. And a properly exposed negative will have a broad range of tones between these two extremes.

How to tell if your angiograms are underexposed:

1. The negatives have very little information on them. You can't see much except the vessels, and they are not nearly as dark as the printed edge numbers.

2. The background in the prints is a muddy, streaky gray, instead of a rich black! (You would have to give the prints LESS light than they should normally receive to keep the dye information in the negative from becoming too dark to read; thus the prints seem light).

Common causes of underexposure:

FLASH TRANSMISSION. As your camera's flash tube gets older, it usually develops deposits on the inside of the glass surface. These gray and black deposits obviously block the light coming out of the flash. So a flash set at a certain level, say 150ws, may really be delivering only 100ws or 75ws after it develops these deposits. By the way, these deposits are universal to all makes of fundus camera, and begin after as little as 100 angiograms. In addition, some manufacturers underpowered their fundus cameras; these cameras simply do not provide enough flash for patients with media opacities and/or small pupils.

PATIENT ANATOMY. It is obvious that if the patient has a small pupil the full illumination cannot get inside the eye (or back out to the film). A cataract will also block the transmission of light getting to retina, especially the blue needed to excite the fluorescein dye. Even non-cataractous lenses of older people develop some yellowing which will reduce the transmission for angiography but may not affect the fundus photos.

MANUFACTURER RECOMMENDED SETTINGS. Most manufacturers base their recommendations (or automatic settings) on "normal" patients with 9 to 10 mm pupils and no media opacities (as discussed above). We all know that the average angiography patient does not meet any of these criteria! Manufacturers' settings are also based upon their tests with new equipment; as discussed in "flash transmission" above, few cameras rarely deliver the flash power they are set on, even after moderate use. Certain camera makes and models are notorious for underexposure; call our office to discuss how best to deal with these.

Solutions for underexposure:

1. Increase your baseline flash settings. Topcon recommends 150ws. We recommend 200ws if your camera is not brand new and/or the patient older, poorly dilated, or has some media opacities. We also recommend using 300ws if the patient has an extremely small pupil and/or significant cataract. Likewise, if your negatives are underexposed you must get more light out of your camera. By the way, the increase flash almost never affects patient comfort. Patient comfort is almost always affected primarily by the viewing illumination level, not the flash intensity. Also, don't be afraid of OVERexposing the film. A negative receiving too much light can always be printed satisfactorily; a negative receiving too little light is usually unsalvageable.

2. Override your camera's "Automatic" settings. Most auto cameras do not measure the proper exposure for each picture; they just have preset flash levels. You can increase the flash by using either "Small Pupil", "+", or "Brown Eye" settings, or by REDUCING the ISO (EI, ASA) setting for cameras which you tell what film speed you use (like Nikon). When you tell the camera you are using a lower film speed, you are telling it your film needs MORE exposure to get an acceptable image. Don't fall into the trap of setting it for a high EI, like 1200; the camera then thinks you are using special supersensitive film and reduces the flash!

3. REPLACE THE FLASH TUBE. If your flash tube cannot be tweaked to provide sufficient exposure, it is time to replace it. Changing flash tubes is easy; you usually don't need to call in a repair person. We can talk you through it over the phone if you like. In any event, you should check it for deposits every 6 months, or whenever your angiograms seem to be getting progressively underexposed.

Flash tubes are absurdly expensive. I had one client tell me they were advised to use a very low flash power to extend the tube life, and to try to "push" the films in processing. This resulted in simply terrible angiograms, and in almost no image at all on patients with poor media. In our opinion, this is the poorest economy of all. It's as foolish as taking angiograms without fluorescein dye to reduce the risk of allergic reactions, or taking a medication once a day instead of four times a day to make it last longer.


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