How to meter for manual bounce flash?

Question

I use an eBay flash in manual mode. Mostly indoors, so I bounce the flash off the ceiling. What is a good approach to read the camera metering and make adjustments to flash intensity?

My flash cannot communicate with my camera. No (E)TTL.

Asked by publicRavi

Answer

I have four manual flashes. I love them. I just make a test shot and look at the histogram on the back of my camera: adjust to suit.

Works great.

Check out the Strobist, he loves manual flashes (http://strobist.blogspot.com/)

Answered by Pat Farrell

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What is the relationship between Guide Number and flash power level?

Question

From this post, I understand that the flash guide number (GN) is given by the following formula:

Guide Number = Shooting Distance × f-number ÷ ISO factor

I'm assuming that the GN calculated via the equation above will be based on a flash that is firing at full power. Am I correct to assume that if I set the flash to fire at half power, that the guide number is effectively halved? i.e. the equation will now be:

Guide Number = Shooting Distance × f-number ÷ ISO factor * Power Level

where power level can be the following 1, 1/2, 1/4, 1/8, ... 1/64, etc?

Answer

The guide number is inversely proportional to the power squared. This is due to the way that light intensity diminishes with distance, at twice the distance light is spread over four times the area, so each bit of that area receives 1/4 of the light.

So the actual formula needs to take into account the square root of the power level:

Guide Number = Shooting Distance * f-number * (Power Level)^0.5 ÷ ISO factor

The guide number has the same inverse square relationship to the sensitivity, as detailed by the ISO factor, defined as follows:

ISO factor = (ISO / 100) ^ 0.5

Substituting and bringing sensitivity onto the top of the fraction gives a formula which you can simply plug numbers into:

Guide Number =  Shooting Distance * f-number * (Power Level)^0.5 * (ISO/100)^-0.5

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What's the relationship between flash power expressed as a fraction and as EV?

Question

I'm looking around for a second flash to play around with for off-camera stuff. Some flash specs I'm reading say that I get an adjustment range of plus or minus 2 EV or similar, while others say I get ½, ¹⁄₄, ¹⁄₁₆, and so on as manual settings.

What's the relationship between these different measurements? How do I convert from one to the other?

Answer

+-EV is used to measure compensation of automatically determined flash power (using either TTL or light sensor on flash). Fractions are used in manual mode and refer to how much of maximum power the flash is using.

Since these measures are used in different modes and there is no easy way to determine exactly how much power your flash uses in auto mode, you can't directly convert from compensation value to a manual setting. First, you'll have to determine flash power for correct exposure.

When you have determined manual flash power that gives you correct exposure, you have the equivalent of 0 EV. Now, if you double the power, that is +1EV; similarly, if you halve the power, it gives you -1EV. The amount of EVs gained by such compensation can be calculated as

log2( current setting / correct exposure setting )

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Manual photography cheat sheet — where can I find one, or what should be on it?

Question

I have been trying to shoot more in manual modes (both camera and flash). At this point, I understand all the concepts, but all the adjustments slow me down.

I'd like to have a "cheat sheet" with all the basic calculations/tables on it that I can refer to in order to speed up the process, in a format that I can easily keep in my camera bag.

Does anyone know of a good sheet, or alternatively, if I create it what information should I include?

At the moment I'd like to have at least:

• an f-stop list (just full stops)
• basic guide number information

Answer

Since there are 3 important variables here: aperture, shutter speed and ISO, I would Google for Exposure Triangle Cheat Sheet for example. Here are a few:

• Manual Mode Cheat Sheet (Muddyboots Photography Blog)
• Exposure Chart Cheat Sheet (Flickr)
• My Exposure Triangle Cheat Sheet (glark.org)
• Aperture, ISO, and Shutter Speed Explained (not a sheet, but you can compact it)

I see you updated your question. With respect to guide numbers here is one:

• Flash unit Guide Number calculations (for film)

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When to use the different modes of National PE-201C flash?

Question

I have a National PE-201C flash, which has the following guide chart:

I kind of understand the chart: if I'm standing 3.5 meters away from the subject and have ISO 100 film, I should use an aperture of ƒ/5.6. However, I'm unsure when to use the different modes of the flash and how to they actually affect the exposure. It has two auto modes: green and orange and a mode labeled M (presumably manual).

Can the situations, when to use which, be interpreted from the chart above? I've noticed that in the chart the green auto's line start from 1.2 m and the orange's from 1.8; does it have a meaning or is it decoration? How the ƒ-number can be the same with the overlapping distances — how do the auto modes actually differ? When should I use the M mode and how powerful is that?

The flash being 20+ years old, I probably shouldn't rely on the auto-modes as the light meter might have had happily retired. But, in theory, when to use which?

Answer

Pentax AF280T is a flash from similar era with similar green and red auto modes. According to its manual, these are "low output" and "high output" automatic modes.

For medium distances, you may use either; for edge cases (closer than 1.8 m or further than 5m on the PE-201C), you have to tell the flash if you need the light thrown carefully or quickly (the distances covered by green and red line should hint that).

So, when you take photos in green mode with ISO 100, set your aperture on 4 and the flash will adjust its power according to light reflecting back on its light sensor. For red mode, set your aperture on 2.8 with ISO 100.

Manual mode very likely just pops at full power each time (easy to verify if you actually have the flash). Out of distance, aperture and ISO, select two as you like and use the chart to select third value to match.

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Can I combine my Canon camera with my Nikon flash?

Question

I have a Canon 60D camera and a SB-600 flash for Nikon. Can I use this flash wirelessly with my camera? I had Nikon camera before which I replaced with Canon and I haven't purchased a flash yet, so this is my only option so far. It would be cool if it worked. I don't care for automatic features (like TTL, etc), I can use the flash in full manual mode, I just need the camera to fire it when taking the photo.

Answer

Yes, but only with extra gear like radio triggers or an external optical slave. The built-in slave can only operate in Nikon's CLS mode.

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Triggering Canon Speedlite manually

Question

I was planning to take a fake multiple-exposure picture: use a dark room, long exposure time, and manually trigger a flash for each exposure. But can I trigger my Canon Speedlite 430EX II manually without any extra gear?

This would be trivial if I had two camera bodies, but no such luck.

Of course there is the test button in the Speedlite unit. But exactly what does the test button do? How much flash power do I get this way; can it be adjusted by using the manual mode? Is it just a tiny burst of light so that I can see that the unit is alive, or can I use the test button to get consistent exposures?

Answer

Your test button should firing at whatever power you select manually. I can't speak to that specific model, but it has on a few different flash brands I've tried.

Its pretty easy to tell if it does.

• Just set it to high, take a picture and press the test button.

• Set it to low, take a picture and press the test button. Is there a big difference? Probably so.

You should get consistent exposures with it.

If for some crazy it doesn't, you can always electrically tape a couple of wires to the bottom pin and plate and just short the connection together with a momentary push button ( or just touching the wires together)- that's what the camera does.

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