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04-09-2015, 09:09 PM   #1
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645D/Z with D-FA Lens: Constant aperture in exposure braket sequence

Hi All,

I'm new to the forum.

For the last year and a half I have been shooting 645D, and Z for last six months. I have ongoing work which requires shooting bracketed sequences of artworks containing light sources of varying intensities. These are shot from a solid tripod, using a cable release, AF disabled, constant aperture, while auto-bracketing the shutter speed. All is well when using A, FA, & 67 lenses, as I can set the aperture manually. But, when using the DFA 55 I can not figure out how to get the camera to shoot the sequence with a constant aperture. I get a proper bracket in terms of exposure values, but the shutter speed and aperture are varied according to the camera's logic. The only way I can work around this, is to bracket by hand, which can introduce camera movement.

Has anyone else encountered this, and more importantly, have you figured out a solution?

Thanks,

Philipp

04-09-2015, 09:34 PM   #2
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Use Av exposure mode. The camera will then vary just the shutter speed.
04-09-2015, 11:23 PM   #3
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Hi Ole,
Thanks for your reply. I tried that, but as the lighting changes, the Av responds of course. I'm need to have a known set of shutter speeds at a constant aperture, regardless of varying light levels. I guess I'm going to have to stick with the older lenses, maybe find a 55mm A or adapt a 67 55mm.
04-10-2015, 02:51 AM   #4
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QuoteOriginally posted by Philippp Quote
Hi Ole,
Thanks for your reply. I tried that, but as the lighting changes, the Av responds of course. I'm need to have a known set of shutter speeds at a constant aperture, regardless of varying light levels. I guess I'm going to have to stick with the older lenses, maybe find a 55mm A or adapt a 67 55mm.
Then use M mode and dial in a fixed shutter speed and aperture There is no functional difference between A, FA, and D FA lenses in terms of shooting modes, except that the two former lens series have an aperture ring, which is for the most part redundant on digital bodies. So, whatever you're doing with your old lenses should in fact work with the new ones as well.


Adam
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04-10-2015, 08:12 AM   #5
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You can't shoot AEB sequences in M mode on DFA lenses, Adam, for the purposes that most people are trying to use such sequences. The camera will not hold the aperture constant. In A and FA lenses, you can physically dial in a manual aperture setting on the lens ring, then the lens is holding the aperture constant. On DFA lenses lacking an aperture ring, the camera's AEB mode potentially resets both aperture and shutter speed on every frame of the sequence. (The same is true on the A and FA lenses if you set the aperture ring to "A".) Pentax's chosen interpretation of "auto-exposure bracketing" is that since both aperture and shutter speed contribute to the exposure value, the camera will optionally change both of them during AEB. No other camera manufacturer I've used takes that interpretation, but there it is.

The only way to hold constant aperture is with Av mode, but as the OP stated this is still an auto-metered mode and the camera will change the base exposure as the light changes. That's undesirable if you're doing something like a rotated HDR pano where you want to meter the light, set a base exposure for the entire series, and then rotate the camera through a series of positions shooting a bracketed sequence in each. Resulting in the same series of shutter speed values with a constant aperture. This is something I do all the time, and it's a pain in the butt with DFA lenses. There's no way to fully manually lock in the aperture for a base exposure while allowing the shutter speed alone to vary across the AEB sequence.

So that's one important functional difference between using DFA lenses, and all the older A, FA, 67, etc. lenses...
04-10-2015, 08:25 AM   #6
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QuoteOriginally posted by Royce Howland Quote
You can't shoot AEB sequences in M mode on DFA lenses, Adam, for the purposes that most people are trying to use such sequences. The camera will not hold the aperture constant. In A and FA lenses, you can physically dial in a manual aperture setting on the lens ring, then the lens is holding the aperture constant. On DFA lenses lacking an aperture ring, the camera's AEB mode potentially resets both aperture and shutter speed on every frame of the sequence. (The same is true on the A and FA lenses if you set the aperture ring to "A".) Pentax's chosen interpretation of "auto-exposure bracketing" is that since both aperture and shutter speed contribute to the exposure value, the camera will optionally change both of them during AEB. No other camera manufacturer I've used takes that interpretation, but there it is.

The only way to hold constant aperture is with Av mode, but as the OP stated this is still an auto-metered mode and the camera will change the base exposure as the light changes. That's undesirable if you're doing something like a rotated HDR pano where you want to meter the light, set a base exposure for the entire series, and then rotate the camera through a series of positions shooting a bracketed sequence in each. Resulting in the same series of shutter speed values with a constant aperture. This is something I do all the time, and it's a pain in the butt with DFA lenses. There's no way to fully manually lock in the aperture for a base exposure while allowing the shutter speed alone to vary across the AEB sequence.

So that's one important functional difference between using DFA lenses, and all the older A, FA, 67, etc. lenses...
Thanks for your reply Royce. Good to know I'm not the only one who finds Pentax's AEB design to be at odds with most other manufacturer's designs. I guess I'll start a letter writing campaign to see if this flaw could be addressed in a firmware update.
04-10-2015, 09:03 AM   #7
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you guys lost me. Philippp, are you saying that you want to maintain constant aperture but reset exposure(via manual shutter adjust or green button) for each set of bracketed shots to accommodate changing light values? Can you give or show an example of what you wish to do?
04-10-2015, 06:21 PM   #8
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Can't you tether that camera? Could you just use manual mode and set the appropriate settings via tether? Then you don't risk camera movement.

04-11-2015, 04:15 AM   #9
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Why not just set the values manually for each shot? I can't see there being any scene that the Z's DR can't capture in two or three exposures.
If you aren't using your custom modes, just set up different exposures on each one and flip through them for each shot.

I tend to bracket shots using a spot meter to determine the total dynamic range of the scene and use that to figure out how many shots I need from the brightest point to the darkest.
04-11-2015, 04:17 PM   #10
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Hi MIke SF,

Sorry if this is confusing. I need both a constant aperture, and a known set of shutter speeds for this to work. The two panos I've uploaded show no seams, because all image capture parameters per bracketed set of exposures are the same. This wouldn't be the case if the camera were responding to changing light levels in AV mode, depending on which part of the light work it was pointed at.

Uncle Vanya, your solution is the only workable one for controlling a DFA lens in this situation. Reviews state that Ricoh's tethering software works well. Thanks! I will order the software from B&H through this website.

Kolor-Pikker, for me the whole point of using AEB is is to ensure pixel-perfect alignment of a bracketed set of exposures. The DR of the 645z is great, but this work sometimes requires 4 - 5 exposures to retain highlight and shadow detail.

Why Pentax chose to design its AEB function this way is a mystery to me. Maintaining the same optical qualities (sharpness, DOF, bokeh, light fall-off) in a bracketed set is pretty important, especially in HDR panos as Royce mentioned.

Anyway, thank you all for your thoughtful input.

Philipp
04-11-2015, 07:16 PM   #11
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Could locking the exposure help (AE-L button)?
04-11-2015, 09:47 PM   #12
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QuoteOriginally posted by Philippp Quote
Hi MIke SF,

Sorry if this is confusing. I need both a constant aperture, and a known set of shutter speeds for this to work. The two panos I've uploaded show no seams, because all image capture parameters per bracketed set of exposures are the same. This wouldn't be the case if the camera were responding to changing light levels in AV mode, depending on which part of the light work it was pointed at.

Uncle Vanya, your solution is the only workable one for controlling a DFA lens in this situation. Reviews state that Ricoh's tethering software works well. Thanks! I will order the software from B&H through this website.

Kolor-Pikker, for me the whole point of using AEB is is to ensure pixel-perfect alignment of a bracketed set of exposures. The DR of the 645z is great, but this work sometimes requires 4 - 5 exposures to retain highlight and shadow detail.

Why Pentax chose to design its AEB function this way is a mystery to me. Maintaining the same optical qualities (sharpness, DOF, bokeh, light fall-off) in a bracketed set is pretty important, especially in HDR panos as Royce mentioned.

Anyway, thank you all for your thoughtful input.

Philipp
Philipp,
I am following you now and was initially puzzled because I shoot many panos and do not approach my exposures this way, not because I do not believe in the theory of maintaining constant DOF, etc, but because I do not find a tremendous problem with the alternative. Can you show an example of the kind of pano work you are doing so I can better understand the issue?
In my pano work, I use M and lock the exposure to mitigate brightest highlights of the entire pano. Then use stitching software to equalize overall blending of exposures.

Last edited by mikeSF; 04-11-2015 at 11:24 PM.
04-12-2015, 08:18 AM   #13
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QuoteOriginally posted by Philippp Quote
Reviews state that Ricoh's tethering software works well. Thanks! I will order the software from B&H through this website.
Note that as far as I know - I could be wrong with this - you can't use the software with mirror lockup. That may not be a problem for you. Though maybe you could set up the shot, drop out of tethered mode, shoot using a remote control and then re-enter tethered mode to set up the next shot.

Also, looking quickly through the manual, it doesn't seem that the exposure nor histogram is available from the tethering device. You'd need to read the exposure level on the camera (but no histogram there either).

So as an alternative, the Flucard connected via Wi-Fi to a mobile phone or tablet may also solve your problem. It has the same weaknesses as the Image Transmitter software in this use case, but costs less.
04-12-2015, 12:41 PM   #14
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Hi MikeSF,
Thanks for your reply. I use PTGUI for all my stitching, and it does a great job blending even when the exposure varies from one frame to the next. I know because I've inadvertently bumped my dial to Av, instead of leaving it on M. I tried uploading a couple of images yesterday, but I can't find them on the Forum, so here is a link to some samples of what I do:
CONSTANT_APERTURE_AEB_THREAD_PTX_FORUM_2015_04_12
Varying exposures in a pano sequence can work if your subject is forgiving. But these fluorescent works are often installed in white box galleries. Smooth white walls leave no room for blending errors, and they are hard to fix.

Hi The Ether
Thanks for the heads-up regarding the limitations of both software, and flu-card. All exposure parameters and review of histogram would be done before entering tethered mode, so as long as I can control aperture and shutter speed from laptop or tablet, I think it will work.

Philipp
04-12-2015, 01:16 PM   #15
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Philippp, just out of interest, for the white cube shots did you use a special head or rail for your work? For the outdoor scenes I presume that you simply rotated the camera around a fixed spot.
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