I am planning to take photographs that have a shallow depth of field (50mm f/1.8 on a crop sensor body) and that are also lighted by an external off-camera flash. I am comparing two external flashes. One has guide number 43 meters (ISO 100), and manual power 1 - 1/128 in 1/3 stops. The other has guide number 44 meters (ISO 100), and manual power 1 - 1/32 in full stops. There's a third theoretical choice as well with guide number 44, but it only supports 1, 1/2, 1/8 and 1/32, so no full stops there. Because the third seems to be worse than the second and at the same time more expensive, I have ruled it out.

I understand the flash is so fast that the camera's fastest shutter speed 1/4000 does not limit the exposure. The minimum ISO is 100. So, the flash will essentially be like a shutter. I don't want the flash to be so powerful that the photograph consists of only the foreground with the background being so dark it's essentially invisible. So, the power of the flash needs to be limited.

Do I need the ability to adjust the flash power down to 1/128 or is 1/32 enough? There's a 2.3x difference in the price of these external flashes.

Oh, and one more important piece of information: the background light level can be anything. Daylight, street lights during the night, I want everything to be possible. Of course, during daylight, I need more power than during night. (EDIT: or do I? shorter exposure could take care of that)

  • 2
    \$\begingroup\$ "... the flash is so fast that the camera's fastest shutter speed 1/4000 does not limit the exposure." The camera's flash sync speed will limit the available exposure lengths ("shutter speeds"). When your camera take a photo at 1/4000 second, it takes about 1/200 second for the slit between the two shutter curtains to transit the sensor from top to bottom. Please see this question and its answers \$\endgroup\$
    – Michael C
    Mar 3, 2019 at 23:54

6 Answers 6


For off-camera reproducible fill-in work, you don't really need all the frills of modern digital-TTL-capable flashes. Have you considered using something like a Metz Mecablitz 40MZ-3 or its cousins? They are comparatively affordable preowned, offer reduction in power up to 1/256 in steps of 1/3EV, zoom from 20mm (with built-in diffusor plate) to 105mm, and their "nominal" guide number of 40 is actually at focal length 50mm, matching the power of flashes typically advertised with guide number 54 (namely at 105mm focal length) these days. Also their elongated shape keeps the tilt/swivel reflector comparatively close to the hot shoe, making them more similar to studio flashes when mounting them in light stands and/or behind umbrellas.

  • \$\begingroup\$ Buying a used flash is like buying a used light bulb. You never know how many more flashes it has before the bulb blows. \$\endgroup\$
    – Michael C
    Mar 4, 2019 at 1:03
  • \$\begingroup\$ Given that a such flashes garage-sell at $5-$15, you can do the same buying strategy as with lightbulbs - you buy them in bulk. Obviously, when carry weight/bulk is a concern, that isn't an option - but for critical use scenarios (field job, limited carry, pro grade reliability expected) a fully dedicated TTL unit is like the better choice anyway due to less potential for hassle and user error. \$\endgroup\$ Mar 4, 2019 at 9:32

Those GN are basically the same. The MAX reach of the flash is the GN / aperture f-stop. Let's make the math easy, we'll use GN 44m with an aperture of f/2.0 (you have a little further going to f/1.8).

The GN is not regulated. It can be at various ISO (It is usually a rating at ISO 100).

It can be different at different lens focal lengths. Some flashes are rated at 50mm focal length. Others use the rating of their max zoom - let's say 105mm.

For instance a flash may have a GN of 44m at 70mm. It might perform at:
GN 52m with 105mm focal length (if it zooms that far) GN 44m with 75mm lens
GN 37m with 50mm lens
GN 32m with 35mm lens

So let's assume you have a flash with GN44 that covers a 75mm equiv lens (50mm on APS-C) to:

44m at f/1
22m at f/2
11m at f/4

But what about the range when power is not at 100%?

One quarter the power is half the distance. For instance:

GN 44m reaches to 22m at f/2 with full power.
1/2 power = GN 32 and reach is 16m
1/4 power = GN 22 and reach is 11m
1/8 power = GN 16 and reach is 8m
1/16 power = GN 11 and reach is 5.6m
1/32 power = GN 7.8 and reach is 4m
1/64 power = GN 5.6m and reach is 2.8m
1/128 power = GN 4 and reach is 2m 1/256 power = GN 2.8 and reach is 1.4m

Many flashes can use even lower power in automatic TTL mode than their lowest manual power setting. But you seem to want full manual control for consistent results.

So you can see you need lower power levels if you're using close subject distances.

Lastly, to not have an overexposed subject lit by flash and a dark background not lit as much you would use slow sync. Basically, you would calculate the shutter duration required to properly expose the background with no flash. If the camera is set to this or 1/3 stop less your flash will just kill shadows. If you set the camera to 3 stops less than the ambient light, then your exposure will be effectively just light from the flash.

Balancing the proportion of ambient light to flash light in the exposure is basically done by adjusting the iso/aperture/shutter speed from -3 stops to the -1/3 stop for the ambient light. At -3 stops exposure ambient is dark and you only light up however strong the flash is set to.

Using our GN 44m example, at 1/16 power and f/2:

Subjects much closer than 5.5m will be overexposed
Subjects near 5.5m will be properly exposed by the flash
Subjects much beyond 5.5m won't be lit enough by the flash

Subjects (background) beyond 10m will be mostly lit only by your ambient settings controlled by exposure time ("shutter speed").

Lastly, flash sync (aside from HSS - fast overlapping strobing - or a leaf shutter). Your APS-C camera will have a flash sync speed. Let's say 1/160 second. Faster shutter speed than this won't work. Slower shutter time than this brings in that ambient light. Meaning, you can't use your 1/4000 sec highest shutter speed with regular flash. Nice thought, but let's just say you can't go faster than your flash sync speed.

Like you understand, the flash duration can be thought of as short but the way the fast shutter speeds are achieved is that one curtain closely follows the other one like a gap or slit. For instance, at a 1/800 shutter speed setting the first curtain starts to open. Once it gets halfway open, the second curtain begins to close behind it and is now "chasing" the first curtain across the sensor. When the 1st curtain gets to fully open, the 2nd curtain is already at half closed. If at this moment the flash fires (let's say for your flash 1/128 power is 1/20,000 sec) you have half a picture that got flash, other half got none. The 2nd curtain continues and when it's closed the exposure is over.

At 1/2000 sec, the 2nd curtain started closing when the first was 10% open. Thus 1st curtain is 50% open and 2nd curtain is 40% closed. This 10% exposed gap moves down until 1st curtain is fully open and 2nd curtain is 90% closed. Flash fires. Only 10% of picture saw flash.

At max flash sync speed of 1/160 (or whatever your camera has, this depends on curtain speed), you have first curtain opens fully - it is open then flash fires - then 2nd curtain starts closing (1/160 second after 1st curtain started opening).

In one millisecond (1/1000 sec) increments:
0ms - all closed
1ms - 1st curtain is 1/3 open
2ms - 1st curtain is 2/3 open
3ms - 1st curtain is fully open
3.01ms - flash fires
4ms - flash is either finished or finishing at a much reduced brightness
5ms - flash is either finished or finishing as shutter remains open
6ms - 2nd curtain starts closing
7ms - 2nd curtain is 1/3 closed
8ms - 2nd curtain is 2/3 closed
9ms - 2nd curtain closed exposure finished

The 1/160 sec (6.25ms) exposure took almost 1/100th sec (10ms) to fully occur. The 2nd curtain followed the first curtain by a delay of 1/160th sec (6.25ms).


Some calculation, please downvote if incorrect:

Subject distance will be at least 1.7 meters, probably longer than that.

Guide number is simply the F-number multiplied by subject distance. So, required minimum guide number is 1.8 * 1.7 = 3.06, probably larger than that but this is the absolute minimum.

According to this calculator: http://dpanswers.com/content/genrc_flash_calc.php

...the less expensive flash requires at F/1.8 and 1/32 power a 4.3 meter distance. Of course I could move the flash to be 4.3 meters away with the camera closer, but space might not be enough for that.

For those who want to do the calculations: 44 / sqrt(32) = 7.778, larger than 3.06, so the less expensive flash won't do.

However, 44 / sqrt(128) = 3.889, larger than 3.06, so even the more expensive flash needs to be a bit further away than 1.7 meters (it needs to be 2.16 meters away, to be exact).

The background will be taken care of by selecting the correct exposure time: more during the night, less during daytime.

  • 1
    \$\begingroup\$ ...and it seems Canon has done the decision for me: the 2000D has a broken hot-shoe, compatible only with Canon EX flashes: photo.stackexchange.com/questions/103688/… ...so I have to buy the expensive Canon flash with 1/128 power level anyway. \$\endgroup\$
    – juhist
    Mar 3, 2019 at 11:40
  • \$\begingroup\$ Not necessarily. At least one E-TTL compatible radio trigger will allow using third party flashes with the 2000D. Please see: Canon EOS 4000D Hot Shoe for Sync (with Studio Flashes). Most radio trigger transmitters also have PC connectors to fire manual external flashes. \$\endgroup\$
    – Michael C
    Mar 4, 2019 at 1:00
  • \$\begingroup\$ Adding modifiers (soft box, shoot through umbrella, etc.) between the flash and subject will also reduce the amount of power that actually reaches the subject, so you may not need as much reduced power as you would with a bare flash. \$\endgroup\$
    – Michael C
    Mar 4, 2019 at 1:09

I understand the flash is so fast that the camera's fastest shutter speed 1/4000 does not limit the exposure.

That's unlikely unless you are talking about the weakest settings of the flash. High-powered studio flashes will reach that kind of speed but then you are not talking about guide numbers of 40.

In my experience, for fill-in flashes it tends to be more reliable to use "automatic" exposure on the flash (not manual, but not TTL either) and then turn the fill-in contribution to a fraction by overriding ISO or aperture. For direct flash from in-room distances, this allows you to quash quite more than the typically available fractions of manual power would and when you change from direct light to bounce, you don't need to start playing with flash power from scratch since the flash will adjust to a good degree.

Of course, the more modern and communicative your setup is, the more cumbersome it may be to override the decisions it would take for making the flash the default sole light source.

Another way to reduce the direct light amount at equal nominal power is to choose wider zoom settings than the framing would require. That distributes the light more, usually increasing the softening and weaker amount of indirect light.

It's also worth noting that these days guide numbers tend to be inflated since flash manufacturers have turned to advertising the guide numbers for the longest zoom setting (often 105mm) instead of 50mm (for a period of time the standard for specifying zoom-capable flashes) or even 35mm (typical coverage of non-zooming flashes). Particularly for off-camera flashes, the guide numbers are misleading since the narrow angles for which they are specified are only relevant for spot highlighting where the guide number tends to be "more than enough" anyway.

  • \$\begingroup\$ To anyone who doesn't pretty much already know enough about flash to know the answer to the question, this answer is filled with so much flash specific jargon as to be be meaningless. \$\endgroup\$
    – Michael C
    Mar 4, 2019 at 1:07

I understand the flash is so fast that the camera's fastest shutter speed 1/4000 does not limit the exposure.

That's not true. At speeds higher than your usual sync speed (1/250s) you will have to set your camera and your flash to High Speed Sync (HSS). During HSS the flash is intentionally very slow, making the flash actually a permanent light source for a short period of time.

This will (a) heavily reduce the flash output and (b) make it depend on the shutter speed like the availble light. Faster shutter speed, more output reduction. During daylight you probably won't be able to overpower the sun at normal distances.


My two cents. Some topics have already been addressed.

1 - 1/32 in full stops

In my opinion, this is too limited...

1, 1/2, 1/4, 1/8, 1/16, 1/32

I almost never use full power (and rarely 1/2), so my batteries last longer, the flash recharges faster. This would limit me to 4 powers. But in general, I do stay in the range of 1/4 - 1/32.

In some cases, you could need even less power, for some subtle effect. But probably that will come later when you get some more flashes to play around.

1/3 stops

The fine tuning can be achieved by moving your flash closer or further away. Not a big deal for your first flash.

There's a 2.3x difference in the price of these external flashes.

Hum. The limitations on the specs you mention make me feel you are looking for the amazon basics flash. Just keep in mind you need the remote trigger, I would totally recommend using a radio trigger instead of cable.

I can not recommend or de-recommend the basic models, they are a real option. Just keep in mind that when you buy some other flashes more "capable" for example managing "groups" our first purchase will have a different radio signal.

For a little bit more you can have one with radio receiver included, (you still need the emitter) 1/128, 1/3 stops, and group receiver ready.

So I would think about those things, not only on the limitations you mention.

camera's fastest shutter speed 1/4000 does not limit the exposure

To understand the sync speed... Take a look at this: https://youtu.be/CmjeCchGRQo?t=182

In that case, you need an HSS flash

Of course, during daylight, I need more power than during the night. Do I?

Not really. The difference would be the relationship between background ambient light and the surroundings.

In practice, you potentially change your ISO during the day, let's say to 100 and during the night to let's say 800, so yes. You need more power during the day, but for that change in usage, not for the ambient light directly.

The variables of the flash are power, distance, and modifiers. The ambient can be controlled and separated, as you say, by shutter speed.

In some cases, you can add another variable, a filter on the camera, to overcome some scenarios like using a wide number, let's say f1.8 during the day.


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