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Analyzing the results of the guide number test

From: Lighting for Photographers: Flash Exposure Fundamentals

Video: Analyzing the results of the guide number test

So we just finished shooting our guide number test. I took a series of portraits of Gabby bracketing the aperture, and now we're going to see how to use those images to establish what are guide number is. So let's get straight into that. So here are the images that we just took for the guide number test, and you can see that the bracketing worked. We went from f/22 all the way down to 5.6. Clearly, the first five or six of these are underexposed, and the last three or four are overexposed. So just by eyeballing it, I can tell.

Analyzing the results of the guide number test

So we just finished shooting our guide number test. I took a series of portraits of Gabby bracketing the aperture, and now we're going to see how to use those images to establish what are guide number is. So let's get straight into that. So here are the images that we just took for the guide number test, and you can see that the bracketing worked. We went from f/22 all the way down to 5.6. Clearly, the first five or six of these are underexposed, and the last three or four are overexposed. So just by eyeballing it, I can tell.

I'm going to open up this image, the one taken at f/11. I'm going to open up the one at 10, at 9, and at f/8. So let's just take a look at these guys, and we're going to do that in the Raw processor. So what I'm looking for is to see which exposure holds detail in the white shirt and gives me values of roughly 245 in the brightest spot, because then I can work with that exposure to really give me a good tight exposure.

Here you can see I'll be looking at the RGB values up here. I am going to scroll the cursor over the white shirt. I'm looking to get close to 245 in the highlights. I'm nowhere near anywhere in the white shirt, and even in the white banding on the gray card it's low. The gray card itself in the gray area looks pretty close to what medium gray would call for, but what I'm really interested in again is getting the highest values of white without blowing the detail.

Here I'm looking at the image that was taken at f/10, scrolling through the highlights and white shirt, still not getting to 245. Just take a look at the image at 9. Here I'm getting close 242. I hit getting in here. Let's see let's go back here. So you can see the gray value is getting pretty high, but this is the image that I'm liking so far. It hit a hotspot here at 245 at one point. Now I'm going into the exposure taken at f/8, and I'm getting really close to blowing the highlights.

You can see that by looking at the histogram where I pushed all the way over to the right-hand side. The blacks themselves are not deep enough. Where I'm getting black over here is from the curtains. The exposure that I'm going to choose based on holding detail on the white close to medium gray is the exposure taken at f/9. So what I'm going to do to find my guide number is multiply the aperture times the flash to subject distance.

So if you recall, we shot at 10 feet. The exposure that I like is at f/9. So my guide number is 90. This is the image that the manufacturer's guide number would have yielded. The manual calls I think for a guide number of 136 or 138. Here we are clearly underexposed at 10 feet at f/14. So this is the reason that we suggested that you do your guide number test, because if you were to rely on the manufacturer, you'd be a full stop and a third underexposed whenever you're shooting manual flash.

Now we've taken a look at the results of the guide number test. We saw the best exposure occurred at the aperture value of f/9. I multiplied that aperture value by my flash to subject distance of 10 feet to give me a guide number of 90 for my flash on my camera at ISO 100. The manufacturer's guide number for the same model is right around 140, which is 1 & 1/3 stops brighter. Therefore, if I were using the published numbers, I'd be underexposing my flash images. That's the reason I encourage you to do this test for yourself.

Next, I'll get into more detail about the guide number and how and why it works in manual flash photography.

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