How to measure light output of different flash setups (e.g., in lux)?

[MacroBen]

I've been messing around with different macro flash setups, including designing various diffusers. As I test them out, it does seem anecdotally that some designs eat up more of the flash's light than others. I'm looking for a way to quantify this to better compare and refine my designs.

So far I've just tried out smartphone light meter apps, but those have come up short. The best one was an iOS app called "Light Meter" that did a great job of roughly measuring ambient light in the room, flashlights, etc. But none of my camera flashes registered. I suspect it's because the sampling rate of the app just didn't light up with the super-quick flash of a camera flash.

Would a regular inexpensive light meter, like are all over Amazon, work for this? I don't want to spend money on this kind of device just to find out it has a similar sampling rate problem as the apps.

What I'm NOT looking for is a traditional photography light meter, that measures the ambient light and tells you what camera settings to use. And I know I can manually adjust the power of my flash with different setups. I still want to know how different diffusers affect output in some objective way.

I don't need the measurements to be totally accurate either, which is why I first looked at smartphone apps. I just need something better than my subjective sense of brightness.

I appreciate any help you all can provide. Thanks in advance!

 

[tugwilson]

I'm pretty sure that no cheap meter that measures light output in Lux or Foot Candles will meter a flash. A light meter that has a mode that meters flash (Like the Sekonic L-308) will measure flash output in EV when can be converted to Lux.

 

[Gato Amarillo]

If all you need is to judge the relative light output of different modifiers you can get a pretty good estimate without a meter, provided you know how to set up manual exposure using the flash power settings.

Set up a standard repeatable target - a gray card would be ideal, but almost anything with middle tones can work. Set up so you can repeat the same working distance, ISO and f-number with different modifiers. Shoot with your each of your modifiers, keeping the setup as close as possible to the same, to find the power level that gives correct exposure with each.

Compare the power settings to judge the difference between the modifiers.

Otherwise the only choice I see is to go with a flash meter and compare the readings for each modifier.

Gato

 

[MacroBen]

I'm pretty sure that no cheap meter that measures light output in Lux or Foot Candles will meter a flash. A light meter that has a mode that meters flash (Like the Sekonic L-308) will measure flash output in EV when can be converted to Lux.

Okay that's helpful! So the big expensive photography light meters have a mode for measuring this? Do you happen to know what that mode is usually called?

I guess next I'll look into how I can get access to one of those without spending hundreds of dollars. Thank you for the help!

 

[bclaff]

You could adapt the technique I used when I wrote this article

SB-800 - Power versus Shutter Speed (D70 and D200)

"Shots were taken of a highly reflective white target and the linear data in the center of the image was analyzed."

You would need to shoot NEF and use something like RawDigger for the analysis.

 

[Travis Burnside]

This is literally what a photographic light meter with flash capability is made for. It will provide EV values from the subject position when the flash fires. Unfortunately they're not quite cheap.

As others have suggested, you can make a repeatable setup with a white or gray card, and fixed camera and light power settings. You can then note the resulting exposure differences between different modifiers. This doesn't cost anything but time, but it won't give you easy EV measurements like a meter will.

Just some food for thought...

 

[tugwilson]

I'm pretty sure that no cheap meter that measures light output in Lux or Foot Candles will meter a flash. A light meter that has a mode that meters flash (Like the Sekonic L-308) will measure flash output in EV when can be converted to Lux.

Okay that's helpful! So the big expensive photography light meters have a mode for measuring this? Do you happen to know what that mode is usually called?

I guess next I'll look into how I can get access to one of those without spending hundreds of dollars. Thank you for the help!

The L-308 isn't very big and costs ~£180 in the UK. It's widely available second hand. I'm not sure what the mode's called.

 

[fotowbert]

I'm pretty sure that no cheap meter that measures light output in Lux or Foot Candles will meter a flash. A light meter that has a mode that meters flash (Like the Sekonic L-308) will measure flash output in EV when can be converted to Lux.

Okay that's helpful! So the big expensive photography light meters have a mode for measuring this? Do you happen to know what that mode is usually called?

I guess next I'll look into how I can get access to one of those without spending hundreds of dollars. Thank you for the help!

The Sekonic L-308 and the like are typically also called "flash meters" because they can measure flash output in addition to ambient light when you put the meter into one of its "flash" modes. They generally have a flash mode for wired flash triggering via PC cord and a cordless flash mode where the meter will wait for 90 seconds while you trigger the flash manually.

If you want to only be measuring the flash output I recommend you kill all ambient light while taking the measurement. More advanced meters like my Sekonic L-358 reports a % Flash value which is the proportion of the total exposure produced by the flash. I find it easier to just take measurements in the dark rather than calculating the flash content from the % Flash value. Also the % Flash value is usually reported in 10 % increments.

Download the manual for the Sekonic L-308 for details.

• John

 

[Ellis Vener]

Any good quality basic flash meter will do this. Sekonic is the current leader if you are looking to buy new.

You might want to look on the used market for a Minolta Flashmeter IV or V. A quick glance at eBay tells me that mint-quality ones sell for around $100.00 or less.

I also like the new Datacolor LightColor Meter. As the name indicates it not only meters the intensity but also the color of the light, and it does this for both constant and electronic flash. https://www.datacolor.com/spyder/products/lightcolor-meter

Another aspect of the Datacolor LCM that you might like for your macro setups is that it is physically smaller than a standard flash meter.

 

[MacroBen]

Thank you, everyone, for all the fantastic responses! While I've shot macro (mostly wildlife, in the field) for decades, I'm relatively new to flash photography and am kind of happy I have so much more to learn!

I plan to look into seeing if I can find a good sub-$100 flash meter. In the meantime, I might try to get a setup going with a gray card and see if I can do some comparative measurements that way for cheap.

 

[John Vickers]

You could adapt the technique I used when I wrote this article

SB-800 - Power versus Shutter Speed (D70 and D200)

"Shots were taken of a highly reflective white target and the linear data in the center of the image was analyzed."

You would need to shoot NEF and use something like RawDigger for the analysis.

Your link seems to be broken. This works:

SB-800 - Power versus Shutter Speed (D70 and D200)

(The non-working link is: http://http/www.PhotonsToPhotos.net...Power_versus_Shutter_Speed_(D70_and_D200).htm ).