[akhawar]

Page 65 of AF-540's manual (http://www.pentax.jp/english/support/man-pdf/acc_st_af540fgz.pdf) shows that HSS is not available in Manual Flash photography? Why can't I set the flash output to 1/32 and use it in HSS mode? Why would that be the case? Why is Pentax limiting this?

I see one explanation. flash output power really becomes dependent on the shutter speed as faster the shutter speed, the less flash output will be used for exposure. But that could just be a simple note in the manual saying that 1/32 or 1/4 don't correspond to the same values in other standard sync modes.

I also notice that if the camera (checked on both K-20d & K-5) mode is in M, the body doesn't allow you to set a faster shutter speed than 1/180. if the attached 540 is in HSS mode. Again, I fail to understand why there are "Automatic" limitations like this when in Manual mode? Isn't Manual the mode for ultimate freedom?

Nothing that frustrates me but I am curious on this design limitation...

[enoeske]

You have to start with how the shutter and its curtains work.

When you start an exposure, the mirror flips up out of the way, the first curtain falls from the top exposing the whole sensor. For a split second, the whole sensor is exposed to the light from the lens. The second curtains falls, blocking the sensor and finally the mirror flips back down. The fastest the shutter mechanism can do this is 1/180 of a second.

In order to achieve an exposure faster than 1/180, the curtains cheat a little. The second curtain will start falling before the entire sensor is exposed. This creates a moving "slit" that exposes the whole sensor, but not all at the same time. Here is where the problem lies.

Speedlights flash very fast. In fact, when at low power (1/16, 1/32 ...) they can hit more than 1/10,000 of a second. Much faster than the shutter on the camera can achieve. Thus, changing the shutter speed has almost no effect on the flash exposure. Even if you could set your shutter to 1/8000, the flash is still faster than that and so the shutter won't do anything to lower the exposure from the flash. To adjust the exposure, you need to simply block some of the light using the aperture.

Anyway, in the case where the shutterspeed is slower than 1/180, flash photography is easy because the whole sensor is exposed. The curtains open and expose the sensor. The flash goes off. The curtains close. Nice and neat.

But when you try to use a shutterspeed faster than 1/180, you run into this moving "slit". When the flash goes off, only the part of the sensor that is currently exposed will be lit up by the flash. The rest of the exposure won't see the flash (remember, the flash is so fast). This is sometimes called banding because you will usually get a big black band in the photo. Pentax doesn't let you run into this situation and just shuts off the flash when you are above 1/180.

HSS can let you get around this shutter limitation. Instead of one flash, it strobes very quickly several times during the exposure. This allows the whole sensor to get some light from the flash as the curtain slit is moving down. The problem with this, is that now the shutter is indeed blocking some of the light in the same way that it blocks bright sunlight. So the faster your shutter in HSS mode, the more light the shutter is blocking.

[akhawar]

Thanks for your reply. However, everything except the last para is about standard flash operation. I understand all that. I also understand that for HSS, the flash is pulsing. The pulsing starts with the first curtain and keeps on going until the second curtain falls...

With all this in the background, my question is on why there is this limit on HSS availability with Manual exposure control on camera?

And also, why HSS is not available with Manual flash output?

[enoeske]

You can use HSS in when in manual mode on the camera. You can't use HSS when the speedlight is in manual mode. In manual mode, the flash does not talk to the camera body, so it won't know how to sync up with the moving curtain slit. It simply pops the user entered power value when the hotshoe tells it the sensor is exposed.

[akhawar]

Yes, your explanation makes sense for the flash in Manual mode... I kind of feel, that is a fair limitation of the fact that the Flash is a dumb device in Manual flash control.

However, as you are expecting the HSS to work in M camera mode (on the body), I expect that too. But please look at the link that I provided to the User Manual of the flash. The table shows that HSS is not usable when camera is in Manual mode. That to me is very strange behavior. What would limit that? As I also mentioned in my initial post, the body doesn't let you set a shutter speed that is faster than 1/180 in this case. It just feels so odd... that I am in M mode but I can't set the shutter speed as I want to.

[D4rknezz]

Banding be damned, I want to be able to set my shutter speed at 1/250, and then crop the result or whatever.
I know.

I think its a limitation made so that point and shooters, not strobist, would not try and set their cameras to get impossible results, and blame it on the camera.
Its like ...putting a speed delimiter on a corvette that clearly says it can reach 160MPH (or whatever corvette can reach). Most people wont ever try to go above 100MPH, but when someone does, the delimiter stops them from going above...say, 120MPH.

I think its an executive decision, rather than technical, because clearly the camera is capable of higher shutter speed. It is just ordered not to, when a flash is put on it.

[akhawar]

We know there will not be any banding issue with High-speed sync mode on the flash and higher shutter speeds on the body. It works in other modes. But HSS doesn't work with M (Manual) mode on camera body..

[L33tGreg]

Yes, your explanation makes sense for the flash in Manual mode... I kind of feel, that is a fair limitation of the fact that the Flash is a dumb device in Manual flash control.

However, as you are expecting the HSS to work in M camera mode (on the body), I expect that too. But please look at the link that I provided to the User Manual of the flash. The table shows that HSS is not usable when camera is in Manual mode. That to me is very strange behavior. What would limit that? As I also mentioned in my initial post, the body doesn't let you set a shutter speed that is faster than 1/180 in this case. It just feels so odd... that I am in M mode but I can't set the shutter speed as I want to.

Limitations in the AF-540 I think. The Metz 58 (I believe) can do HSS with the camera in M.

Greg

[maxfield_photo]

Limitations in the AF-540 I think. The Metz 58 (I believe) can do HSS with the camera in M.

This is 100% correct. The Metz flashes will also let you use manual flash in any camera mode with the possible exception of green mode (I've never tried).

[SOldBear]

I also notice that if the camera (checked on both K-20d & K-5) mode is in M, the body doesn't allow you to set a faster shutter speed than 1/180. if the attached 540 is in HSS mode.

That is strange. My K-7 and K10D allow me to set speed higher than 1/180, with the AF540FGZ in HSS mode. The manual says HSS is possible in Tv, Av, and M modes.

This is a test photo with my K-7, shutter speed 1/400s: