[BruceBanner]

So... I own a Cactus RF60x and I am also interested in perhaps picking up a Metz M400 flash, more for those 'running and gunning' situations than more serious studio stuff.

How can I get an idea of what the M400 will be like, in terms of power output? I mean it's a smaller more compact flash (which is great for the purposes I want), however that should mean it's quite a bit weaker than the RF60x I'm imagining... but in real terms by how much? Like the Metz M400 does HSS, but is it pretty bad at it? Are the recycle times going to be frustratingly slow because it needs to be at maximum power even at 1/500 when shooting wide open on some lenses outdoors etc...

I'm wondering if there is something I can do with my V6ii and RF60x, perhaps replicate or nerf/cap the power output that flash has and then pretend it's a M400, take it outside and do some tests to help decipher whether the HSS feature on it (the M400) is worth anything at all! Because there is always the smaller and even lighter Pentax AF 201 FG which is also appealing (however cannot do HSS), but if the M400 performs so poorly with HSS (test with a modifier on as well) then I might forget the smaller compact flashes for any of that kind of work and treat it purely as an indoor flash only (and save myself some bulk and cash at the same time... by choosing not to get the M400 etc).

TIA!

Bruce

[inkista]

Typically, this is where you look at the guide numbers in the specs.

The M400's speced guide number is 40m (at iso 100 and 105mm zoom).

The RF60x's speced guide number is 56m (at iso 100 and 105mm zoom).

guide number = f-number x distance

Does that help?

[beholder3]

How can I get an idea of what the M400 will be like, in terms of power output?

The Gold standard is to compare GN at ISO 100 at the same focal length (!) and the latter should be typical, like 50mm.

In the last 10 years flash makers have tried to make idiot users by advertising only the light output at the narrowest possible reflector.

The M400 has a GN of 28 (metric) at ISO 100 and 50mm. So at F4 you can brighten up a target in 7 m distance.


The RF60X has a GN of 45 (metric) at ISO 100 and 50mm. So at F4 you can brighten up a target in 11 m distance.
Thats the same GN as the Pentax AF540 I btw.

[Lord Lucan]

The Gold standard is to compare GN at ISO 100 at the same focal length (!) and the latter should be typical, like 50mm.

Good point, although strictly speaking it should be at the angle of spread for a standard lens, 50mm in the case of full frame.

In the last 10 years flash makers have tried to make idiot users by advertising only the light output at the narrowest possible reflector..
... The M400 has a GN of 28 ...The RF60X has a GN of 45 ... Thats the same GN as the Pentax AF540

So they are also laughing at us with the naming of their units. There was a time when the number in the name was a clue to its GN, like the Pentax AF280T had a GN of 28 (280 decimeters perhaps )

[BruceBanner]

Typically, this is where you look at the guide numbers in the specs.

The M400's speced guide number is 40m (at iso 100 and 105mm zoom).

The RF60x's speced guide number is 56m (at iso 100 and 105mm zoom).

guide number = f-number x distance

Does that help?

Not really... as I'm still kinda new to the whole flash thing.

The Gold standard is to compare GN at ISO 100 at the same focal length (!) and the latter should be typical, like 50mm.

In the last 10 years flash makers have tried to make idiot users by advertising only the light output at the narrowest possible reflector.

The M400 has a GN of 28 (metric) at ISO 100 and 50mm. So at F4 you can brighten up a target in 7 m distance.


The RF60X has a GN of 45 (metric) at ISO 100 and 50mm. So at F4 you can brighten up a target in 11 m distance.
Thats the same GN as the Pentax AF540 I btw.

I think this makes a little more sense.

So, if in HSS mode (bright day), with the M400, if I was shooting wide open at f1.8 with the FA50 and ISO 100, I could have the target at around 15.5m away and still be lit? (28 divided by 1.8 = 15.55). How do I calculate for for an FA77 then? Or... if using a diffuser such as a Maggel or Magsphere, which will soften the light but also reduce it's reach... how can I factor stuff like that into the equation?

Can I somehow nerf my Cactus RF60x to having a guide number of 28 instead of 45 to see what kind of real life results might be feasible (of course the recycle time would be longer on the M400)?

[bschriver11]

I'm going to say no, the guide numbers are for at or below max sync speed. HSS fires at a reduced light output depending on your shutter speed, the faster the shutter speed the less the distance will be. Try downloading a manual for the unit your looking at as they usually have such information in the back.

[West Penn]

So, if in HSS mode (bright day), with the M400, if I was shooting wide open at f1.8 with the FA50 and ISO 100, I could have the target at around 15.5m away and still be lit? (28 divided by 1.8 = 15.55). How do I calculate for for an FA77 then? Or... if using a diffuser such as a Maggel or Magsphere, which will soften the light but also reduce it's reach... how can I factor stuff like that into the equation?

Can I somehow nerf my Cactus RF60x to having a guide number of 28 instead of 45 to see what kind of real life results might be feasible (of course the recycle time would be longer on the M400)?

No. You would need to be a sync speed or below at f1.8 and the flash at 15.5m as bschriver11 stated. When in HSS mode, you lose one stop every time you double you shutter speed above your sync speed. So, if you round the distance in your calculation to 16m for convenience (it's easier to work out the calculations in your head if you follow aperture settings, e.g., 4, 5.6. 8, 11, etc.) then at 1/400 sec you'd need to move the light to 11m, at 1/800 sec you'd need to move the light to 8m, at 1/1600 you'd need to mov the light to 5.6m, etc, These number would be for the bare flash.

EDIT: the guide number for the M400 was given as 28 at 105mm, so the numbers above are for a lens of that focal length, not 50mm. The guide number would be lower for a 50mm lens. Again, all of this should be in the users manual.

When you add the defuser, you'd have to calculate in how much light is lost. Maybe the user manuals gives that information. If not, try one stop of light lost when adding the defuser and see how that works for you.

If you use the RF60x at 1/4 power, it will have a guide number of 28. There should be a table of guide numbers in your user manual (or simply use the table from another flash that has the same guide number at the same focal length as the RF60x).

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Last edited by West Penn; 10-14-2018 at 08:00 AM.

[UncleVanya]

HSS isn't directly related to GN. Different flashes have different HSS output even with the same base guide number.

Understanding Flash Guide Numbers, HSS GN Calculator

[beholder3]

So, if in HSS mode (bright day), with the M400, if I was shooting wide open at f1.8 with the FA50 and ISO 100, I could have the target at around 15.5m away and still be lit? (28 divided by 1.8 = 15.55).

HSS is no base for calculation as it just means your shutter is faster than 1/200 on the K-1. HSS power absoliuetely depends on the exact shutter speed. The faster the less power.
An Pentax AF540 in non-HSS has the abovementioned GN 45. If you go to 1/1000 sec HSS it drops to 11.7 (only 1/4 of the power).

You do need exact parameters from you desired shooting and need to compare it with exact figures from the manufacturer which you need to goggle or ask their support about. Homework for you to do.

How do I calculate for for an FA77 then?

You check the power tables for the used product for either 70mm or 80mm.

Or... if using a diffuser such as a Maggel or Magsphere, which will soften the light but also reduce it's reach... how can I factor stuff like that into the equation?

No data for such stuff anywhere. You need to try it out to find out how many stops of light they suck up.

Can I somehow nerf my Cactus RF60x to having a guide number of 28 instead of 45 to see what kind of real life results might be feasible (of course the recycle time would be longer on the M400)?

That is simple. You just stop down your lens in the test 45/28=x1,6 times more than you would do in the real scenario. In practise that means by about one stop. So if you actually want to do a portrait at F1.4 you just need to run your test at F2 to simulate the M400.

[AstroDave]

Do you have access to an oscilloscope? (Or, a friend with one?) For a few bucks worth of parts, you can build a light detecting circuit similar to the one I used here: Pentax AF540FGZ flash

These devices are remarkably linear (i.e. the voltage output is directly proportional to the power of the light shining on the detector). With something like that, you could directly measure the relative light output of your various flashes. (Does require that you actually have them in hand!!)

Or, you could send them to me, and I'd be glad to measure them (postage to/from Oz may be prohibitive, though!!)

[derekkite]

The Metz documentation is quite good and gives tables of power. The guide number of 48 on the Metz 48 will have a guide number of 21 in high speed sync mode, or less than half the power.

There are a few things to know about flashes and light. The intensity of light decreases by the square of the distance. The further away the less light. For a light of a given intensity, if you focus the light in one direction, the intensity at distance is greater than if it wasn't focused. Flashes have focus settings based on the field of view that is illuminated; if you have a wide angle 20mm lens the flash would have to illuminate close to 180 degrees, vs a longer lens with a narrower field of view, so the flash settings at 50mm, 105mm refer to the field of view. Obviously a given lamp focused to iluminate for a 105mm lens will be expose brighter than if that same lamp is focused to illuminate for a 50mm lens.

That is what the guide numbers are referring to, the illumination at a distance with a specific field of view.

Flashes control the level of light by time. Cameras control shutter speed by opening and closing two curtains over the sensor. The K3 at more than 1/180 shutter speed doesn't expose the whole sensor at one time; a slit crosses the face of the sensor. So typically you set your shutter speed to 1/180, which means that the whole sensor is exposed, and the flash then illuminates the whole sensor. At full power the flash is on for the full duration of the exposure. 1/8 power it is on for 1/4000 - 1/6000 of a second, depending on the flash model. To freeze movement you limit the power of the flash.

High speed sync is a different animal. The slit between the front and rear curtain as it goes across the sensor at high shutter speeds gets illuminated by a series of flash emissions, making the sensor at all times illuminated by the flash. It pulses very quickly. The available power of the flash is substantially less than 1/1 power.

High speed sync is typically used to illuminate shadows. If you are taking a portrait outside on a sunny day, you can fill the shadows, and have all the shutter speed adjustment that you normally would have without flash.

[stevebrot]

So, if in HSS mode (bright day), with the M400, if I was shooting wide open at f1.8 with the FA50 and ISO 100, I could have the target at around 15.5m away and still be lit? (28 divided by 1.8 = 15.55). How do I calculate for for an FA77 then? Or... if using a diffuser such as a Maggel or Magsphere, which will soften the light but also reduce it's reach... how can I factor stuff like that into the equation?

A few comments related to your question and to flash use in general:

• Shutter speed is usually incidental to what happens with flash
• HSS is a special case where guide number will decrease as shutter speeds increase. Think of if as continuous lighting. That is one reason why HSS requires some form of TTL flash metering on most cameras. Note that effective guide numbers are usually lower, even best case than with normal sync.
• Guide number aperture calculation is the same regardless of lens focal length
• Guide numbers do change when the angle of coverage is changed. Wider --> lower guide number. The flash manual should include a table.
• The ability to decrease flash output by powers of two is very powerful. If guide number indicates f/2.8 and you want to shoot at f/2.0, simply decrease output to 1/2 power.
• Flash modification using reflectors or diffusers will reduce the effective guide number. You can do your own measurements or fall back to some form of TTL metering or on-flash metering.

Steve

[mcgregni]

Yes Steve, it's probably not so helpful to be talking about specific lenses here..... It's the flash zoom setting (angle of coverage) that matters. You can use any flash zoom setting with any lens, they are not tied together. For most portraits there is no need to match flash zoom and lens focal length, it is more efficient to set a longer flash zoom setting than lens focal length, as the subject is often in the center areas of the frame.

[West Penn]

Sure you can use any flash zoom setting with any lens, but if you want to cover the entire frame with light from the flash, you'll not get it when using a 28mm lens with a flash that has the zoom set to 105mm. Best to look at the manual and it's guide number chart, and then experiment some.

[mcgregni]

Yes, but with the 105mm flash setting we could light up something a lot further away, something that might look very small through that 28mm lens.

I suppose I was trying to avoid us giving the impression to the OP that GN was somehow related to the lens on the camera, as it isn't. GN is affected by changes to flash zoom settings, as the longer settings throw light' further along a narrower pathway. Guide number can be calculated even without any lens.