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Flash + Ambient, OCFTDWPP p. 50

edited February 2011 in flash & lighting
I am reading Neil's "On-Camera Flash Techniques for Digital Wedding and Portrait Photography," and I would like confirm I understand the information on page 50, the grid of test shots of various ISO, apertures, and shutter speeds. Let us label the pictures

1 2 3
4 5 6
7 8 9

First, I want to understand which pictures we would expect to be the same. If we double the ISO number (so comparing rows 2 and 3), both the flash and ambient are increased by a stop. Therefore, to get the same exposure at the same shutter speed, we should close the aperture one-stop. This suggests that the pairs (5, 7) and (6, 8) should be identical. At the same ISO (so comparing rows 1 and 3), we expect that a 2x increase in shutter speed will also require opening the aperture one stop to get equivalent exposure. This suggests that (1,8) and (2,9) should be the same. If we half the ISO and double the shutter speed, we have made the ambient 4 times darker. Thus we need to open up two stops comparing rows 1 and 2. This suggests that (1,6) should be the only similar shot for those two rows. (I notice that there is some confirmation in that we've established 1, 6, and 8 all match in multiple ways.)

My second question is flash power. Neil is using TTL in this illustration, but since I am currently a manual only user, I would like to know how to adjust the flash appropriately. This probably most easily thought of within a row. Take for example pair (1,2). The aperture closes one stop, but the exposure of the subject remains the same. Does this imply that the flash doubled its output? I could always chimp my way to the solution, but it would be better to be able to anticipate the results of closing down a stop and increasing my flash power by a step on the output guide.

Thanks for reading this lengthy post.

Comments

  • Neil vNNeil vN Administrator
    Hi there Mark ..

    The essence of this is explained in this post:
    http://neilvn.com/tangents/2009/11/10/when-aperture-does-not-control-flash-exposure/

    The main idea that I wanted to explain on that page in the book (and in that blog post), is that with TTL flash, the flash exposure will remain consistent. The ambient exposure will change though.

    This is because your camera will control your TTL flash exposure, as you change your aperture and ISO.

    This is how TTL flash is completely different than manual flash.
    You will have to unhook your mind from the idea that aperture controls TTL flash exposure. It doesn't. The camera will control the flash to give more or less light, to (hopefully) give consistent flash exposure.

    Go over the images again, and see if it makes sense now. :)
  • Thank you, Neil, for the speedy reply.

    Here I thought I had read your entire blog, but you sent me to a new page.

    I *think* I understand your points about TTL, and I hope they are well taken. At the moment, I am operating on an inexpensive manual flash. From my perspective TTL is just a short hand for my manually upping my flash power as I drop ISO or closing aperture. The same result could be achieved with manual flash by riding the controls and chimping. Feel free to correct me if I am missing the point of TTL. I hope to join the ranks of the TTL users someday, but for now, budget contrains me to a manual flash (for reference, I'm _way_ off the reservation, shooting on a Panasonic G1 with a a Yongnuo 450-II).

    That said, I think you are selling yourself short with respect to the amount of information on page 50 of your book. Let us say I walked into a room and blasted off a test shot at a given flash power, ISO, shutter speed, and aperture (let's assume TTL flash for simplicity). I look at the LCD and decide I want more/less DOF or more/less ambient. How do I get there from my test shot? Which controls do I change? I think that information is in these test shots, and my previous post attempts to provide a set of rules to do so. Just writing it out was helpful to me, so perhaps we need discuss this no further.

    Perhaps I can close by framing my previous paragraph as a question. If you were at max flash power and decided you needed 1 more stop of DOF, what settings would you change? I suspect the answer is that you would double ISO and close down the aperture 1 stop. The flash exposure would not change, and the doubled ISO would keep the ambient exposure the same with the smaller aperture.

    I hope my response clarifies rather than making matters more muddy.
  • Neil vNNeil vN Administrator
    Yup, you're right in how manual flash and TTL would relate ... that with TTL flash the power is adjusted as you change your settings.
    markmfredrickson said: I look at the LCD and decide I want more/less DOF or more/less ambient. How do I get there from my test shot? Which controls do I change?
    The changes you need to make for more or less DoF, remain the same as ever with ambient light ... and your TTL flash will follow in terms of exposure.
    markmfredrickson said: If you were at max flash power and decided you needed 1 more stop of DOF, what settings would you change? I suspect the answer is that you would double ISO and close down the aperture 1 stop.
    Exactly that! You'd change your ISO one stop up, and you could change your aperture by a stop .. giving you that one stop more depth-of-field.
  • Neil vNNeil vN Administrator
    markmfredrickson said: I hope my response clarifies rather than making matters more muddy.
    I think you're *there* ... you understand it all.
    It's just that these things / decisions need to become second nature now.

  • Thanks Neil! All the credit goes to you for your clear teaching.
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