K5 - Exposure bracketing for HDR

I'm experimenting with exposure bracking for HDR. However, I notice that bracketing on the K-5 is implemented by varying the aperture as well as the exposure time. I really don't want the aperture to be varied as it will change the depth of field and the lens artifacts, making alignment of the images for HDR problematic, to say the least.

In tried a night shot with a five exposure bracket in manual mode with a total range of +3.3EV to -3.3EV. The centre exposure was 6sec @ f/11. The +3.3EV was 20sec @ f/6.3 and the -3.3EV was 2sec @ f/20 (the maths adds up correctly to a factor of 10 either side).

An aperture range of f/6.3 to f/20 seems thoroughly nasty to me. Is there any way of ensuring that exposure bracketing is implemented by varying the exposure time only? Obviously I want the bracketing to be automated so that I don't disturb the camera on the tripod.

In fact, changing the aperture for bracketing seems horrible even if you're not interested in HDR. I suppose they'll be someone who doesn't want the exposure time to be varied, but varying both means none of us are happy!

Are you trying in camera HDR or separate exposure bracket sequence?

If the latter, the camera will do what you tell it based on the exposure mode setting you're using. To keep the aperture constant you need to be using the Av setting which keeps the aperture at what you set it and adjusts the shutter speed accordingly.

John

If you are using Manual mode, what is the problem with keeping the aperture fixed and just altering the shutter speed?
I can see that you don't want to disturb the camera itself in any way, but with a good tripod and everything locked up tight, how much do you expect it to move as you adjust one wheel?
Alternatively, did you try putting the body into Av mode, and then using the EV compensation? This should keep the aperture fixed, and give you the auto-bracketing that you want.
Geoff

Unless you're going for the sickly Acid trip overdone HDR then the K-5 has plenty enough dynamic range to take one image and compress the dynamics to 'HDR' taste.

Then you don't need to worry about aligning stuff.

JAK, sorry if I didn't make it clear. I was using exposure bracketing (not the in-built HDR). Thank you, your suggestion of using Av mode works. I had wondered if the exposure compensation behaviour varied with mode. Apologies for being too lazy to experiment before my original posting. A note in the manual would have helped me, but perhaps it's obvious to more experienced DSLR users. In case it helps anyone else my findings for exposure bracketing are:

P: Varies exposure time and aperture (depending on the program line)
Sv: Varies exposure time and aperture
Tv: Varies aperture
Av: Varies exposure time
TAv: Varies exposure time and aperture
M: Varies exposure time and aperture

In other words, Av and some P program lines are the only modes that keep the aperture constant when exposure bracketing.

Normally if I want to reduce exposure manually my first preference would be to reduce ISO in order to cut down noise, therefore I'm surprised that none of the modes do this, not even TAv mode.

DoctorJeff, I think even with a sturdy tripod you might get more movement than you'd want from rotating the dial. It must be preferable to use remote release and not touch the camera between exposures. Think how concerned some people get over mirror flop (incidentally, why can't the K-5 maintain mirror-up when exposure bracketing?). A 35mm lens has a horizontal field of view of about 38 degrees. Divide by 4928 and one pixel equates to an angle of less than half a minute. According to my sums that's less than 1/50th mm lateral shift between the front of the lens and the back of the body - that doesn't sound a lot.

Smeggypants, I think you are probably correct that the 14-bit output should normally be enough for HDR enhancement within the 'tasteful' range, but I'm new to DSLR and am still trying out what works best for the results I'm after. Some HDR purists (HDR and 'purist' don't really sit comfortably together) would say you can't get real HDR from a single image, but I'm not sure there's a valid distinction between an 8-bit image tone mapped from a 10-bit sensor bracketed two stops each way and an 8-bit image tone mapped from a single image from a 14-bit sensor. I think one argument for merging bracketed exposures is to reduce noise at the lower bit level, but I'm not even sure how valid that is if aperture and ISO are constant between exposures.

Then some people swear by processing two RAW files from a single image, one for highlights and one for shadows, then blend them in Photoshop CS5. I don't understand why you'd do that - surely if you post-process in 16-bits you've got all the information you need for tone mapping in a single file? I tried both methods and achieved much the same results, as I'd expect.

Like I said, I'm still experimenting!

in short: Bracketing while in Av mode will fix the aperture value and change exposure time. Bracketing in Tv mode will fix the exposure time and change aperture value.

Well it can. Both the 'delayed action' and 'remote control' Drive Modes for Exposure Bracketing pop the mirror up first. The delayed action setting pops the mirror up, waits for the time the delayed action is set to (2 or 12 seconds) and fires the exposures. The remote setting, used with a remote, needs two presses; the first to up the mirror and the second to fire the exposures, much the same as the dedicated mirror up settings.

John

Thanks flat4. You've put it so much better than me! Logically I'd have expected TAv mode to fix the exposure time and aperture value, and vary the ISO.

Thanks JAK. Excellent information again. I've just tried the 'delayed action' mode and that seems to work. I notice that the mirror still flaps between shots but I guess the delay is applied after each mirror flap. Does the mirror flap between exposures in order to do another meter reading and/or to check focus?

Hi all.
Glad i found this topic as it has answered my question to bracketing in manual, as i was a little puzzled out shooting the other day.

Thanks all.
Rob

Do you know I hadn't even realised it did do that! It does it on both delayed action and mirror up. Seems a bit daft, I'd say it is a programming oversight. The camera's 'take a shot' sub-routine goes back too far in the sequence. It needs to memorise the original exposure and keep the flipping mirror flipped up 'til it's all done! It could even do it quicker than it does now if it did.

John

People have recovered upto 5 stops from an erroneous underexposure on the K-5. i.e turning a black picture into something useable. So in terms of using multiple exposures to reduce noise in the blacks then I would think that only in extreme cases would you need to take a take a second exposure at 4 or 5EV lower than the original exposure.

There's certainly no need to take multiple exposures 1EV apart as some people do. Completelwaste of time.

I've found a workaround. Put the camera in Liveview mode before taking the sequence which of course causes the mirror to be flipped up throughout. The picture can still be framed in the viewfinder first if lighting conditions make viewing the screen impossible.

John

Thanks JAK. I had tried exposure bracketing with Live View but found that the mirror flipped between shots. However, I now realise that was because I'd enabled Phase Difference AF. Even with contrast AF enabled there is still some mechanical action going on between shots and I can feel a bit of vibration when hand held. Is that the aperture being stopped down? Perhaps it is lens specific.

I think I'll stick to your earlier suggestion of using the delayed action mode. Thanks again for all your useful help.

Probably that and the shutter mechanism. Maybe using SDM lenses might be less violent? I'll try it later.

John

I thought I'd add an update to this thread, in case someone else has a similar query regarding exposure bracketing.

I've found that when exposure bracketing in P mode the K-5 attempts to maintain constant the manual parameter settings you've applied. Eg, if you manually set ISO and aperture then the bracketing will be achieved by varying exposure time only, which is probably what you want for HDR merging.

I find this logical and all rather obvious, once you've realised it!

I've still not found a way of fixing exposure time and aperture, thereby bracketing on ISO alone.