I think my marumi ring flash is faulty

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George
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I have had my Marumi DRF14C Ring Flash for Canon for a few months now. I don't use it regularly but I always feel it is lacking in power and I have to drop the aperture and up the iso.

I wan't to prove this by experiment or understand if I just have it wrong.

So my understanding from the guide number is that

GN = distance x f-number at iso 100

So with my canon 100mm macro f2.8 at 1:1 (0.31m) I should have no problems at ISO100 and f22 as the max distance at this f/stop is:-

distance = GN / f-number = 0.64m

So at 0.31m the TTL should be kicking in and reducing power

What do you suggest as an experiment to check my theory that there is something wrong with the flash

camera is 450D
 
I think there is something a little bit odd about the amount of light reaching the sensor when you shoot at true macro distances and you lose a lot of light that is not accounted for by the chosen aperture value. In fact I have a vague memory of having read that at 1:1 reproduction you actually get about 2 stops less light than you were expecting. This will halve your GN, and hence your shooting range, when shooting at true macro.

So, your flash may indeed be outputting the claimed power, but the macro lens is losing you 2 stops above and beyond your chosen aperture. If you want to remove the FUD factor then try shooting at a distance of a couple of meters and see if the maths stacks up better. If you can set the flash to manual and fire at full power then at 2m (from the flash head) you should get proper illumination when shooting at 100 ISO and f/7.1...... in theory.

In fact, here's an even easier test, which I've just tried. Ignore flash for the minute. Take your camera and macro lens and aim it from 1-2m away from a plain surface such as a wall or door and focus (approximately) onto the wall/door. It doesn't have to be exact focus at all; just make sure you are well beyond the macro shooting range. Set your exposure mode to manual and dial in an exposure to give you a reading of +1 on the meter. Now, continue metering with a half press on the shutter button and, without moving or changing any other settings, simply manually turn the focus ring until you get to minimum focus distance. Watch the meter stay steady for a while, until you approach macro shooting range and you will see the meter dropping rapidly before your eyes.

Your flash may well be perfectly fine. It's your lens that's losing you the light.

To explain why that might be happening, consider the use of extension tubes to turn a regular lens into a "macro" lens. What you are doing is adding to the focal length of the lens, but your aperture is not becoming physically larger at the same time. If you double the focal length but do nothing to expand the aperture diameter then your f/ratio doubles too. e.g. what would have been f/2.8 becomes f/5.6. What should have been f/11 becomes f/22. What should have been f/22 becomes f/45. A macro lens basically has the extension tubes built in. When you hit the macro range the tubes come into play. So, if you think you're at f/22 you're possibly nearer to f/45, depending on the exact focus distance. With a GN of 14m and an aperture of f/45 your range at 100 ISO should be 14/45 = 0.31m.
 
Last edited:
Tim is 100% right
 
tdodd said:
So, if you think you're at f/22 you're possibly nearer to f/45, depending on the exact focus distance. With a GN of 14m and an aperture of f/45 your range at 100 ISO should be 14/45 = 0.31m.
Click to expand...

If that is the case it should still expose properly especially if I up to ISO 200.

I will check out what happens on the actual lens and light meter as you suggested and also try exposure outside of the macro range. I'll post some shots this evening to put it into perspective. It is only a feeling at the moment but when I am outside at 0.31m, f16 and iso 200 and still getting underexposed shots I feel something is fishy!

Thanks for the help
 
ok. Sorry for the image spamming that's coming next

first up 1:1 approx .31m

f22


f18


f14


f11


f9


f7.1
 
Tim is right. The quoted f/number is only correct for infinity focus and at 1:1 it is reduced by exactly two stops (it's an inverse square law effect). Some lenses can reduce this slightly, because they reduce focal length with their internal focusing mechanism and so the effective f/number is actually slightly lower than stated to start with, at very close range (if that makes sense!).

I'm guessing that this is the main cause of the under-exposure, and E-TTL is unable to compensate because the flash is already maxed out. An effective f/45 at ISO100 is asking a lot of any ring-flash, and the little Marumi is not the most powerful.

As a comment, the combination of ISO100 (for maximum image quality?) is at odds with f/22 which will seriously degrade sharpness with diffraction. Which in turn suggests that more 'normal' macro settings of maybe f/11-16 at ISO200-400 would not only solve the flash power problem, but deliver better image quality. Sure you'll lose some depth of field, but nobody said macro was easy - it's a question of choosing the best set of compromises, using the kit/budget you have to work with.
 
You are correct. I forgot to add that I repeated the experiment Tim described and indeed lose 2 stops going from infinity to 1:1.

So I guess we are saying the flash is working ok and it is my expectation of its performance that was wrong.
 
I think that with the tests you have performed you need to be cautious about interpreting results when using ETTL. At 31cm and 45cm I'm not seeing much difference between results shot at f/22 (f/45 actual) or f/11 (f/22 actual). That suggests to me that the flash is certainly outputting all the power asked of it up to those distances. The question is, how much further could you go without seeing the illumination fall off?

In your test results the leaf seems pretty equally lit throughout a large aperture range, suggesting the flash has not been asked to deliver more power than it can supply, even at f/22 (f/45).

With the peg board shots I think the white board has fooled the flash into quenching early, settling for something nearer to middle grey than proper white. Again this brightness remains steady across a broad range of apertures, so again the flash is not running out of juice as you stop down. The question is, can you get the whites looking properly white at that distance? Can you switch to manual flash and force it to fire at full power, and then see how far you can go? What I don't understand about this series is why the board is getting brighter at much larger apertures. ETTL should be knocking the power down to match the rest of the results.

The shots at 1.5m do show the flash running out of puff at f/22, even though the maths imply that the illumination should be sufficient. I wonder whether this is a consequence of the flash shaping/focusing the light for optimum results at macro distances, and basically not following inverse square law (no reason why it should since it is not a bare bulb point light source) at distances well beyond the macro range.
 
Cheers Tim. The problem with the marumi is that there is no manual mode so I can't compare with and without ETTL. I did try increasing flash compensation to +2 but there was no noticeable difference.

I'll see if I can have anither go at 1:1 and post some above f22

Richard, I will try the lower apertures as well as I hadn't considered the diffraction. I was just going for max quality iso 100 while using a high aperture as a lot of the time I am hand holding which makes 1:1 focusing somewhat difficult.

Above all, thanks for the advice / help
 
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