Just one more way of thinking about it:
F stops are worked out as the focal length divided by the aperture. e.g. 160mm focal length / 20mm opening = f8
If you extend the bellows to 320mm you are doubling the distance from the film to the lens (I am assuming at infinity focus the bellows extension equals the focal length). Therefore you are effectively doubling the focal length.
The new formula is: 320mm / 20mm (because the aperture stays the same) = f16
Therefore you need to give two stops more light than you would at infinity focus.
I have had success using the Quickdisc. Pretty simple to use, fast, and no calculations required.
It's here at: http://www.salzgeber.at/disc/index.html and it's FREE.
Originally Posted by Martin Aislabie
Good morning, Martin;
While welcoming Terry Bowyer to the World of Large Format is certainly a fine social gesture, for which I also commend you, I am wondering if there is a spelling error in the first line of your message.
Should that not be "Welcome to the Dark Slide?"
Have you seen the "Bellows Correction with Bacon" video yet?
Originally Posted by Terry Bowyer
To sum it up, think of it this way. In a macro situation, you are radically changing the focal plane of the lens, such that you end up firing a good deal of the light gathered by the lens into the bellows, rather than onto the focal plane.
With a view camera it can be done fairly easy with some simple tools. You just need an object of known size, then measure the size that that object apears to be on the ground glass. From those two measurments you can calculate out how many stops to open up. If you want, we could get into the specifics of the math.
I echo this, works like a charm..before that I had a metal tapeline which I used to measure the bellows extension with small colored stickers for each lens I used and the subsequent correction factors to be applied after extension in the colors matching the lens...
Originally Posted by Michael Finder
Consider a subject, and estimate some dimension of it. For example, if your subject is US quarter, then you might say the diameter is an inch or so.
Next ask yourself, how big is that subject on the ground glass (GG)?
From the actual dimension of the subject and the GG dimension, deduce an approximate magnification (m). For example if the quarter is the same size on the GG, then m=1. If it is twice as big on the GG then m=2.
Next compute the bellows factor from this simple relation:
Bellows factor= (1+m)^2
Example: if you have the quarter at life-size on your GG, then the factor is (1+1)^2=2^2=4.
Multiply your exposure by this factor. So if your hand-metered exposure for a 1:1 shot is 1/25 sec, then your exposure corrected for bellows factor will be 1 sec.
That's it. For b&w film especially, that's all I've ever needed. For slide, I sometimes reach for a ruler or set a coin or piece of paper next to my subject to get a more precise measurement. But 99.44% of the time, the magnification is what I guessed by eye.
I think it's important to remember this basic approach because I always forget rulers and meters and whatever. It's really easy. Easier than computing reciprocity factor. I mean, people often just wing it on reciprocity corrections or dev times, but when it comes to bellows factor they schlep all manner of tools and tape measures and so forth. Pack less stuff. Keep it simple ;)
The Quick Disc works for me and travels in my filter pouch. I laminated one for durability.
A friend of mine said he always at least thought about bellows compensation is the composition was less than 4x5 feet when using a 4x5 camera. In my experience, you don't need compensation at that point, but it's a good and easy-to-remember piece of advice.
I seem to recall the Kodak's Master Photo Guide had something in it on this. Again from recollection, place a two inch target at the subject and then hold a scale (provided) against the GG to determine exposure compensation. Now if I could just find mine.;)
Now that you have an adequate understanding of bellows compensation, there is one more tidbit worthy of mention. If you should decide to use a filter during the exposure, the bellows factor and the filter factor must be multiplied (not added).
In case anyone is using a monorail in the field like me (it's not that bad really), there is an extemely easy and totally free method of determining bellows ext factors using the mm scale on the rail. The only calculations are very simple and takes a few minutes. I included my chart as an example:
The inch column is the distance from the lens plane to the film plane in 1/2 inch increments for 6 inches of extension, I may change it to 1 inch increments; the mm scale is the distance from inside to inside of the front and rear standards to equal the lens and film plane distances shown--I can quickly set my focus to infinity by setting the standards apart by 126mm inside to inside anywhere along the rail. The last column is the calculated factor. I always multiply the factor by the shutter speed.
Ex: My recent image in the gallery of the leaf and bark required a bellows ext factor (BEF). I simply read the distance on the monorail from inside to inside of the standards and it was 166mm to equal about 9 3/4" from lens to film, read the chart and used a factor of 1.4x