How Do I Read Program Graphs?greenspun.com : LUSENET : Canon EOS FAQ forum : One Thread
My EOS5 manual (as well as articles in photo magazines) have so-called "Program Graphs." As far as I understand, they describe the decision-making process of a camera in the P mode. Yet I don't understand how I should interpret them??? In the X-direction they have shutter speeds (at the bottom of the graph) and EV-scale (at the top); in the Y-direction they have aperture values (right) and some unidentified scale, say, from -5 to +5 (on the left). The question is, how exactly do I get information from these? Thank you.
-- Peter Larin (email@example.com), July 20, 2000
Yes, they are an explaination of how the camera picks aperture and shutter speed combinations in P mode. On top of the graph is an EV (Exposure Value) scale. As the brightness of the scene increases (from left to right) the camera chooses increasingly faster shutter speeds while holding the aperture of the lens wide open. Once the brightness increases to the point that the shutter speed is equal to (or nearly equal to) the reciprical of the lens length in "mm", the camera starts to progressively shut the aperture as well as increase the shutter speed. Although not as fast as the graph shows. Once the maximum shutter speed is reached any increase in lighting will only reduce the aperture, if there is any more aperture to reduce left. I have no idea what they are trying to say with the +5 to -5 scale on the right, unless they are getting at the fact that you can change the exposure + or -, or perhaps that you can "shift" the program without changing the EV.
There does seem to be errors in the way they count EV stops and the shutter/aperture combiinations though. They show a 1 EV change when actually there is a two EV change from one has mark to the next on the right side of the graph.
My experience is that, after seriously studying these charts, there is absolutely no photographic value to be found in them. The only thing you might want to remember is that the lens stays wide open until you reach a shutter speed that is nearly equal to the reciprical of the lens lengs in "mm."
It bugs me that Canon does such a poor job of explaining some details (particularly flash related details) and then wastes space in these little booklets for worthless charts. They have been in the EOS booklets since the first (I have a 650 book in front of me now). Obviously no body reads them, because they are still wrong. Does anybody out there get anything from them?
-- Jim Strutz (firstname.lastname@example.org), July 22, 2000.
Jim: Thanks for your analysis, since I must confess that these graphs have been rather meaningless for me, and I never took the time to begin deciphering them as you have.
-- kurt heintzelman (email@example.com), July 22, 2000.
I decided to take a plunge and make my own graph of P-mode and compare it to a printed one. I used EOS5 + 28-70/2.8 at 70mm end. Here's what I got:
In low light, camera would set 2.8+shutter speeds from 30" to 90. Next steps were: 3.5+90, 3.5+125, 4.0+125, 4.0+180 etc. i.e. every time EV increases by 1/2 stop, the camera will increase either aperture or shutter speed, one after another. Thus, my graph looks like a straight line, and then "steps". When I compared it to a EOS1v+28-70 (I took it for granted that programs for 5 and 1v are identical) i noticed a striking similarity: Canon's graph looked like a "log" put on my "steps" in the sense that it was a straight line connecting my "top-left" vertices. Thus I concluded that:
1) Program graph indeed shows the possible combinations "aperture- shutter speed" 2) But it doesn't show all of them. For horizontal segments, it shows all. For sloped segments, it "approximates" the real "step- wise" graph. 3) Why there's only an approximation? I have two answers: first is that it's meant to give an idea of the program, not exactly the algorithm. Second is that the real stepwise graphs will look differently depending on what exposure increase we select on a camera, 1/2 or 1/3. 4) The "EV" scale on top should be used in the following way: if we want to know the EV of a certain point on the graph, we have to ADD the EV readings from the left and from the top. Hence, the +/- scale is a "correction" scale for the main EV scale on top.
-- Peter Larin (firstname.lastname@example.org), July 24, 2000.