TTL Flash for Mirrorless

As far as I know the Z series cameras are compatible to existing Nikon flashes and can do TTL with them. I wonder how TTL flash is done with the Z. With film SLR the TTL flash sensor measures the light reflected off the film and shut off the flash when there is sufficient exposure. With DSLR it uses a constant power preflash, the light sensor in the viewfinder then determine how much flash power is need for the actual exposure.

Since there is no dedicated light sensor on the Z, how would it do it?

As far as I know the Z series cameras are compatible to existing Nikon flashes and can do TTL with them. I wonder how TTL flash is done with the Z. With film SLR the TTL flash sensor measures the light reflected off the film and shut off the flash when there is sufficient exposure. With DSLR it uses a constant power preflash, the light sensor in the viewfinder then determine how much flash power is need for the actual exposure.

Since there is no dedicated light sensor on the Z, how would it do it?

Possibly they measure the total lightness/darkness of the signals coming from the pixels of the sensor.

I suspect the Nikon Z will work in the same way as a Sony A7xxx. Light from the pre-flash is measured by the sensor itself, cleared, then exposed with the full, regulated flash for keeps. There is a noticeable lag compared to non-flash operation.

Nikon DSLRs, like their film predecessors, use flash sensors in the floor of the mirror box. Instead of reflecting from the film, a incident and pre-flash light is reflected from special paint on the closed shutter. The sensor itself is too shiny to measure exposure by reflected light. A Leica M-9 (perhaps others) reflects from the closed shutter in the same way for measuring both flash and non-flash exposure.

Nikon Z system mirrorless cameras are iTTL/CLS compatible, just like all Nikon DSLRs from the D2 family (2004) and D70 family (also 2004) on. They use a pre-flash to evaluate the exposure.

The Nikon DSLRs use a pre-flash before the mirror flips up and measure the flash exposure with the regular meter inside the prism. In mirrorless Z cameras, obviously there is no mirror. I would imagine it just uses the image sensor to measure pre-flash exposure. But we need to verify that with Nikon.

Nikon DSLRs prior to 2004 use D-TTL flash, namely the D1 family and D100, although the D2 family is both D-TTL and i-TTL compatible. In D-TTL, Nikon paints the outward-facing side of the shutter curtain gray. The pre-flash occurs after the mirror flips up but before the shutter opens. Sensors inside the mirror box measure the pre-flash reflected off the gray shutter curtain. Nikon hasn’t used that technology again in over a decade.

"Nikon DSLRs, like their film predecessors, use flash sensors in the floor of the mirror box."

- No. Nikon's DSLRs use the sensors in the prism housing for metering flash, same as for ambient light. The mirror box only houses the AF sensor(s), and the shutter blinds have no metering pattern on them. As can be plainly seen by simple visual inspection of same.

Otherwise there would be no incompatibility between film and DSLR TTL flash, and no need to introduce the i-TTL pre-flash system.

I'm unsure whether Nikon's i-TTL flash protocol uses a fixed time delay between pre-flash and main flash, or whether the main flash is triggered by the shutter action itself. I suspect the latter. Either way, the measured delay between pre and main flash is on the order of several tens of milliseconds. (So much for the touted 'instant' response of a DSLR versus a MILC!)

Whatever. The basic sequence of TTL flash operation in Nikon's Z cameras must follow the same protocol as in their DSLRs. Namely: Pre-flash, followed by pre-flash metering analysis, followed by transmission of setup data to any remote off-camera flash, followed by pre-trigger signal to off-camera flash, followed finally by main flash at computed power.

All the above takes up time and wastes flash power.

I suspect that such backward compatibility also prevents full use of the electronic shutter with flash, and limits the X-synch speed to a stately and dated 1/200th second.

- No. Nikon's DSLRs use the sensors in the prism housing for metering flash, same as for ambient light.

(Except, as Shun said, for the early ones with D-TTL.)

Either way, the measured delay between pre and main flash is on the order of several tens of milliseconds. (So much for the touted 'instant' response of a DSLR versus a MILC!)

Did someone claim that? The shutter lag on Nikon dSLRs has tended to be of the order of 40ms anyway, and that might even have been with mirror lock up. DSLRs do tend to turn on faster though.

I was surprised how slow flash actually is, having believed it to be kind of instant. Shooting a dragonfly with flash and still getting blur is how I learnt that the full power flash duration is still several ms.

All the above takes up time and wastes flash power.

I'm not sure how bright the preflash is (though it's clearly visible with second curtain flash). In theory it needn't be that much power. I'm not sure it's more wasteful than the degree of reflectivity off film. You can still manually set the flash, of course - I did this with the dragonflies specifically because I wanted to avoid the iTTL delay.

I suspect that such backward compatibility also prevents full use of the electronic shutter with flash, and limits the X-synch speed to a stately and dated 1/200th second.

Electronic shutter readout isn't all that fast, so unless you want a ridiculously slow sync speed and to have the flash at a different offset from ambient exposure for different parts of the frame, I can see flash with a full electronic shutter being limited. (I assume for a fully electronic shutter the sensor normally resets and reads lines in a rolling pattern similar to the way aperture blades follow each other above the sync speed. I'm also aiming that resetting a scan line is faster than reading it, so EFCS sets up the sensor at the speed the shutter blade moves.)

Still, the Z series have mechanical shutters, and I don't think the 1/200s limit is anything to do with mirrorless (except possibly that the shutters were redesigned to be thin).

(Except, as Shun said, for the early ones with D-TTL.)

 

 

 

Did someone claim that? The shutter lag on Nikon dSLRs has tended to be of the order of 40ms anyway, and that might even have been with mirror lock up. DSLRs do tend to turn on faster though.

 

I was surprised how slow flash actually is, having believed it to be kind of instant. Shooting a dragonfly with flash and still getting blur is how I learnt that the full power flash duration is still several ms.

 

 

 

I'm not sure how bright the preflash is (though it's clearly visible with second curtain flash). In theory it needn't be that much power. I'm not sure it's more wasteful than the degree of reflectivity off film. You can still manually set the flash, of course - I did this with the dragonflies specifically because I wanted to avoid the iTTL delay.

 

 

 

Electronic shutter readout isn't all that fast, so unless you want a ridiculously slow sync speed and to have the flash at a different offset from ambient exposure for different parts of the frame, I can see flash with a full electronic shutter being limited. (I assume for a fully electronic shutter the sensor normally resets and reads lines in a rolling pattern similar to the way aperture blades follow each other above the sync speed. I'm also aiming that resetting a scan line is faster than reading it, so EFCS sets up the sensor at the speed the shutter blade moves.)

 

Still, the Z series have mechanical shutters, and I don't think the 1/200s limit is anything to do with mirrorless (except possibly that the shutters were redesigned to be thin).

- Well, Nikon managed to get 1/500th X-synch using an electronic 'shutter' in one of their early DX DSLRs. Has technology gone backwards?

Surely the electronic 'shutter' gating time has nothing to do with the readout time? Or maybe the changeover from CCD to CMOS technology was a backward step in this respect?

Even leaving the electronic shutter aspect aside; I see no reason why the DSLR X-synch speed of 1/320th shouldn't have been retained on a MILC. Unless it has more to do with RF triggering delays than the actual shutter.

However, my reply to Bebu's question still stands. The operation of TTL flash in the MILCs must, of necessity, broadly follow the same protocol as that of i-TTL. Otherwise there'd be no backward compatibility. The major difference being that pre-flash metering is taken directly from the sensor, not from the viewfinder prism.

FWIW. Here's a 'scope trace of a typical speedlight output at full power.

X axis is time in milliseconds.

Well, Nikon managed to get 1/500th X-synch using an electronic 'shutter' in one of their early DX DSLRs. Has technology gone backwards?

If you are happy with that level image quality, yes, these things can be implemented. But most people are not happy with D70 image quality any more, and quality got precedence over flash sync in the eyes of the majority of the market. The fast sync was possible using CCD (though some artifacts such as blooming was reported) but CMOS allowed better high ISO image quality, so it took over.

D70 Sensor/Electronic Shutter problems...: Nikon Pro DX SLR (D500, D300, D200, D100) Talk Forum: Digital Photography Review

CMOS global shutter cameras do exist for e.g. super 35mm Canon C700 GS for $30000.

also I found this:

Full frame 48MP sensor with global shutter and 8K video hits market from CMOSIS

Pretty expensive for just a monochrome sensor.

Yes, I was under the impression that the CMOS global shutter cameras like the D1 dedicated some die area per pixel to the stored value and this limited the sensor area available for image capture - along with other transfer issues. I may have a confused memory though.

Personally I don't care for high flash sync but I think mirrorless cameras should be able to have global shutter in the Z's price range.

I'm not sure how easy it generally is to achieve. BlackMagic tried to make the micro cinema camera have a global shutter, but had problems. I'd hope the stacked sensor tech Sony has started using might be a step in the right direction.

Personally I don't care for high flash sync but I think mirrorless cameras should be able to have global shutter in the Z's price range.

- But a lot of people do care. That's why leaf shutter MF lenses were (and still are) very popular.

I find it totally ridiculous that I can use a cheap old digital bridge camera at speeds up to 1/2000th with perfect X-synch, while the £3000+ Z7 is stuck with a 1/200th X-synch speed that was bettered 30 years ago by any film Nikon.

And don't mention power-losing HSS as a suitable solution. A guide number of around 20 is no substitute for 1200 watt-seconds or more of studio strobe.

If Nikon were serious about real professional use, then a leaf shutter lens would be on the cards. I'm sure modern materials technology could easily produce a leaf shutter with a top speed of at least 1/1000th s. In fact it might now be possible to make a Z to F adapter containing such a shutter.

- But a lot of people do care. That's why leaf shutter MF lenses were (and still are) very popular.

 

I find it totally ridiculous that I can use a cheap old digital bridge camera at speeds up to 1/2000th with perfect X-synch, while the £3000+ Z7 is stuck with a 1/200th X-synch speed that was bettered 30 years ago by any film Nikon.

 

And don't mention power-losing HSS as a suitable solution. A guide number of around 20 is no substitute for 1200 watt-seconds or more of studio strobe.

 

If Nikon were serious about real professional use, then a leaf shutter lens would be on the cards. I'm sure modern materials technology could easily produce a leaf shutter with a top speed of at least 1/1000th s. In fact it might now be possible to make a Z to F adapter containing such a shutter.

I was hoping for the Z that can sync at top speed of 1/8000 without HSS. To me HSS is rather useless. I found when I do fill flash in daylight I need much higher flash power than when I shoot in low light condition and the flash is the main light source. When shooting in bright sunlight and using the flash to fill the shadow the flash has to light the shadow almost to the same level as sunlight perhaps 2/3 or 1 stop less. In low light I can crank the ISO up and I wouldn't need as much flash power.

Sony can only use flash with a mechanical shutter, although the front shutter can be electronic. There are two very quick but discernible operations of the shutter for TTL flash. The first probably terminates the pre-flash measurement and the second triggers the exposure. Since the same sensor is used for both exposure and taking in both the A7 and Z, I suspect both will operate in a similar manner. If you use a manual flash, you must still use a mechanical shutter (electronic front shutter allowed), but there is only a single shutter operation. Both the flash shoe and auxiliary flash ports (A9 and A7Riii) are disabled above 1/250.

A Nikon D2X had no shutter speed limit with manual flash. In fact, above 1/250 second, only the electronic shutter was effective. You had to use the separate flash port on the body, or a dumb cable in the flash shoe, for this to work. The shutter could be as short as 1/16000, faster than most flash units. You could see the light taper off as you approached the flash duration speed.

There is a workaround for the crippled X-synch speed, and that's to use flash in FP mode.

The elongated pulse mode of HSS uses FP synch, where the flash fires just as the first shutter blind begins its travel. The catch being that there's no manual switch to FP mode, and it can only be forced by fitting an HSS capable speedlight to the camera's hotshoe, and setting the camera shutter faster than whatever the X-synch speed is.

However, once such a speedlight has been fitted, there's absolutely nothing to stop you 'piggy backing' a flash with some decent power onto it. In fact you can even point the stupid speedlight away from the subject and use a proper flash, or flashes, as your sole light.

The only proviso is that the flash duration must be long enough to cover the FP shutter's entire travel, and this rules out thyristor or IGBT control of the flash 'power'. It also rules out some short duration studio units, but the majority of flashes are useable at full output.

As can be seen from the 'scope trace I posted above, a typical flash duration gives some useful light over a 3 or 4 millisecond period. This is long enough to cover the travel time of a modern FP shutter.

OK, the light isn't completely even over the frame, but it's not as bad as you might think from looking at the intensity curve. And if the flash is being used as outdoor fill, then the tail off becomes very unnoticeable.

So all that's needed is to fit any HSS capable speedlight to the camera hotshoe, point it at the sky and forget it's there, while you wire your proper flash to the P-C socket. A cumbersome arrangement I'll admit, but it does work.

Of course if Nikon just allowed FP-synch mode to be enabled with a menu option, then such a subterfuge wouldn't be necessary. It would also show that they actually care about making equipment for professional use, more than they care about shifting overpriced speedlights. Sadly, that seems not to be the case.

FP operation is not continuous, but pulsed at a high rate (240 fps?). That was sufficient for older cameras, like the Leica M2 and M3, which had a traverse time on the order of 1/50 second. Shutters which traverse at 4 or 5 times that speed are likely to produce inconsistent or banded results with an FP flash mode.

You can get by with 1/200 second if you reduce the level of direct sunlight with a tent or scrim, and use a flash with enough power to balance with existing light. You can add ND filters to bring the aperture into a more useable range. You can also use an Hasselblad X1D, which has a lens shutter speed of 1/2000.

A Nikon D2X had no shutter speed limit with manual flash. In fact, above 1/250 second, only the electronic shutter was effective. You had to use the separate flash port on the body, or a dumb cable in the flash shoe, for this to work. The shutter could be as short as 1/16000, faster than most flash units. You could see the light taper off as you approached the flash duration speed.

You may have the Nikon D70/D70S in mind. The D2X has a CMOS sensor and no electronic shutter. The D70 is the one that has an electronic shutter.

There are leaf shutter cameras on the market (e.g., Fuji X100F, Leica S, Hasselblad X1D) if you need this feature.

Elinchrom offer Hi-Sync (which is their implementation of flash syncronization at high shutter speeds) provided that a suitable flash head is used. I have one of those heads for a 400 Ws battery based system. They say it minimizes the gradient by optimizing the timing. I haven't used it in such a situation where the remaining gradient would be noticeable. They also offer a 500Ws system with HSS (which should be similar to Nikon's FP sync but with a more powerful head).

I am sure Elinchrom's competitors offer something similar.

There are leaf shutter cameras on the market (e.g., Fuji X100F, Leica S, Hasselblad X1D) if you need this feature.

And the Coolpix A, I feel honour-bound to mention. :-) With several of these the available sync speed does depend on the aperture, though, since the blades are dual-purposed.

I am not following your thought. With leaf shutters, every shutter speed is a sync speed. There should be no unevenness of the light across the frame due to the shutter speed selected.

However, fast shutter speeds may not be available at wide apertures because the blades don't open fast enough. This doesn't have to do with sync speed per se. The effectiveness of flash with long flash duration may be reduced at wide apertures but if a short duration flash is used, this effect should be reduced.

I didn't know that leaf shutter lenses could have one set of blades, I thought they always had two sets, one for aperture and the other for shutter.

A leaf shutter will vignette at higher speeds unless it is located exactly at one of the nodes. Because it opens from the center outward, and vice versa, the center is exposed longer than the edges.

If it's at the node, presumably this is often desirable, because it gives closer to gaussian bokeh (rather than a constant illumination)? Minolta, I believe, had a "vary the aperture during the exposure" mode. I've been trying to get around to setting up a Photoshop action to do some multi-shot blending for aperture bracketing and report it back to the forum - I've just been too tired to do the maths.