Film Tests...Basic Questiongreenspun.com : LUSENET : Large format photography : One Thread
I am currently conduting my first film tests being new to large format ...and am reading all that I can and making as many mistakes as I can...among the many things that I don't understand is the following seeming inconsistency:
- I am aware that you have to compensate for exposure for subjects that are taken at distances close to the camera...something to do with light fall-off and bellows extension.
- In many of the film test books, it mentions get a grey card/or similar, FILL the screen with the image, then focus on infinity.
I have to get fairly close to my gray card to fill my screen...Do I need to make any compensation factors due to the subject distance or not? And, if not, why not? Or, more importantly, if I use a small grey card, can I position it close to my camera (4x5, 320mm bellows, 150 or 90 mm lens) and not worry about compensations?
I have learned a tremendous amount from this forum in the past and hope that I can get some basic advice this time round...thanks in advance.
-- Malcolm Fox (email@example.com), September 29, 2000
You're asking about bellows exposure compensation for close up shooting.
There are two ways you can do this. You can measure the bellows extension and imput those measurements into a formula to derive an answer. Or you can do what I did. I spent $7.00 at Calumet and bought one of their bellows compensation rulers. This comes in two parts: a target (about 2" square) and a dedicated ruler.
You put the target in the same position as your subject. Then you focus on the target and "measure" that target with the ruler on your ground glass. The ruler is marked off in stops. If the target is small on the groundglass, it may measure out at 2/3 of a stop. If you are closer the target will be bigger and may measure out to two stops. Anyway, this method is dead easy to use and extremely accurate. If there is fly in this otherwise wonderful ointment, it's that it won't work if you are using a true "telephoto" lens. Since telephoto lenses are rare in LF photography (as opposed to "long focus" lenses") I wouln't be concerned.
-- David Grandy (firstname.lastname@example.org), September 29, 2000.
Hi Malcolm, I think for what you're doing the answer is no, you don't add any exposure time.
For your test use your 150 lens. Point it at the moon or a far away street lamp, focus it, and put a mark somewhere; you know this is infinity focus. I think it's handy to know this with any particular camera lens combo anyway. Since that is normal bellows extension, normal exposure. If you are going to use the gray card, the camera will be up close and "in focus" the bellows will be extended, but then when you have the ground glass filled with the card, before you take the shot rack the bellows back to infinity, because you don't care if it's in focus; you're only after the density on the negative. (before you go to this time and expense, it would be nice to know how your shutters are functioning by the way. If the speed is off a little it's OK, but if you now have an irratic/inconsitant shutter then you don't want to be doing what you are thinking of doing here.)To clear up a point, how small a gray card are we talking about? Mine is about the size of a standard note book page.
For calculating exposure with bellows extended, there is a handy little circular one for free; I forget the URL, but someone here will know. It's great, and it is free. And once you use it a couple of times then the mystery of exposure compensation for extended bellows will be behind you. But I don't think it is necessary for what you are doing here. I have been wrong before.
If you look in The New Zone System Manual by White/Zakia/Lorenz, I think it covers what you are about very well.
Good luck, David
-- david clark (email@example.com), September 29, 2000.
Malcom, the posters above all answered your question....what is confusing you was this....Bellows extension compensation, which is a method to determine the amount of light loss by extending the bellows wider than its spread at infinity is a necessary evil. A 150mm lens focussed at infinty will have a bellows draw of 150mm. If you focus on something closer, you may have a bellows draw of 300mm, in which case you need to add 2 stops of light into your exposure. In your case, you put a grey card in front of your lens, which is a short distance, but did not focus on it, so your bellows extension is still at the infinity distance which requires NO compensation.
I assume you are doing this to test your light metering and / or your shutter accuracy? Beware, this can be a bit tricky...assuming you are using a spot meter, you must try to spot meter the grey card at exactly the same angle the lens is seeing the grey card at...and also the same distance. If you take a grey card and hold it at arms lenght and then point your spot meter at it, and move the card around at different angles, you will be amazed at the different readings you would get. Now, if there is a bright source of light, than this problems gets much worse...so you need to beware of this, and also be sure you are using an "offical" grey card from Kodak, or any other vendor who makes them these days.... Normal $20 8x10 grey cards are not perfectly 18% grey, and also, not all spot and ambient meters are made to meter 18% grey, some are calibrated at 16%, while others are as high as 25%. So keep these variables in mind when doing your tests! In addition to this variable, you also have to be aware the lab yoy use can be 1/2 stop off in either direction...this is one good reason to try to stick with a lab you have good luck with since they usually run the same tempertures and process times.
-- Bill Glickman (firstname.lastname@example.org), September 30, 2000.
I'm fairly new to large format myself, but have been reading everything I can along the way. I agree with almost everything posted but for the fact that you need an official 18% gray card.
Your question was in regard to a film test and I'm assuming you're setting up to do something similar to the test in the "Negative" or another similar test to determine your personalized film speed, development time, and N+ and N- times. I'm also assuming that if you're going this far you're not using a lab to process your film for you.
From what I understand, as long as you meter and adjust your exposure appropriately you can shoot at any uniform and texture-free "card" you like.
Unless you always meter from an 18% gray card you're going to meter off of different colors and different values as they occur in nature, etc. anyway. I know some people meter off a gray card wherever they shoot, but it doesn't always seem practical -- it hasn't been for me.
The caveats I have encountered are that 1) some (most?) meters are not entirely "linear" and will measure reflected light differently from some colors; and 2) the very fast and very slow shutter speeds should be avoided because they are more likely to be inaccurate than the "middle" speeds. Therefore, you need a card dark or light enough so that you may use the middle speeds given your lighting.
Be sure to perform the test for each film, chem., lens, meter combo.
-- Peter Ashmore (email@example.com), September 30, 2000.
The answers above are correct as far as your basic question is concerned - you don't need to focus on the gray card to do film speed tests so you don't need to extend the bellow and, if the bellows isn't extended, there's no need to make any adjustment for the extension. However, a problem that you may be experiencing is that if you're using a small gray card it can be difficult to fill the ground glass with the card unless you extend the lens and/or get up real close to the gray card and when you get up real close it's hard to keep the shadow of the camera off the gray card. This is just a problem inherent in using a small card. I would do what someone else suggested and try to find something bigger to use for the test. Just make sure it is untextured and I wouldn't use something bright white or dark black. Theoretically it shouldn't matter but it just seems like a good idea to avoid these two extremems. You're probably going to find some other frustrations along the way, especially if you use the numbers developed in the film speed test to go on to a film development test. It isn't quite as simple to do these tests as it is when you read about them. I went through a box and a half of film trying to do these two tests because the base+fog number from the film speed test would change when I did the film development test (or something like that - it's been a couple years).
-- Brian Ellis (firstname.lastname@example.org), September 30, 2000.
If you are willing to spend about $110.00 there are two items out there which will enormously simplify your film and other zone system testing. First, get a shutter tester from Calumet. They cost about $80 and can quickly tell you how inaccurate your faster speeds are (and they are!) and, more important, whether the speeds are consistent. Knowing your shutter speeds, you can then make the appropriate adjustments after taking your meter readings.
The second item will save you mountains of frustration. Buy a Wallace Expo disk for about $30 (price depends on filter size). This is a diffusion disk (and of course you can make one yourself). No more setting up grey cards or towels or whatever. The disk diffuses the light so all detail is eliminated - only a relatively consistent light passes through the lens.
The procedure is simple. Take a meter reading for zone 5 through the disk. Attach the disk to your camera lens (easily done) and then run your tests based on the reading through the disk. In practice you can just go out and shoot the sky (assuming the light is changing dramatically).
The disk makes film testing a means to an end rather than what sometimes seems like a full-time occupation.
-- Alan Shapiro (email@example.com), October 01, 2000.
1. If you focus at infinity you do not have to compensate for the closeness of your subject. The compensation is for the focus distance, not the subject distance.
2. Get a big card from an art store. I recommend black, but you can get a medium gray one. Just don't get one with color. That way you don't have to get so close. This avoids casting a shadow on the card by the camera, which would effect the results. Still focus at infinity.
By the way, it's easiest to focus at infinity then shift the camera towards the card.
-- Charlie Strack (firstname.lastname@example.org), October 02, 2000.
Many thanks to all for posting answers, and lots of advice as usual...Malcolm
-- Malcolm Fox (email@example.com), October 02, 2000.