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04-14-2020, 08:37 AM   #1
J. Lee
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K70 images under exposed when using ND or polarizer.

I own a Pentax K70 and a Pentax 16-85 f5.5-5.6 lens for more than a year. Recently, I found out when using a B+W brand ND filter or a Nikon brand polarizer with the camera set to Av mode, the result images were always under exposed a lot. Somehow the shutter speed would not go slower. Without any filters, the camera/lens worked fine when using small aperture or shooting under a low light scene; the shutter speed would automatically go slower to produce a perfect exposed image.
I've been using Canon, Nikon and Panasonic cameras / filters combos for a long time; and I've never run into such problem.
Any answer, comment?
Thank you

04-14-2020, 08:44 AM   #2
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does this also occur without the ND filter?
04-14-2020, 08:58 AM   #3
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QuoteOriginally posted by J. Lee Quote
I own a Pentax K70 and a Pentax 16-85 f5.5-5.6 lens for more than a year. Recently, I found out when using a B+W brand ND filter or a Nikon brand polarizer with the camera set to Av mode, the result images were always under exposed a lot. Somehow the shutter speed would not go slower. Without any filters, the camera/lens worked fine when using small aperture or shooting under a low light scene; the shutter speed would automatically go slower to produce a perfect exposed image.
I've been using Canon, Nikon and Panasonic cameras / filters combos for a long time; and I've never run into such problem.
Any answer, comment?
Thank you
Hola. Normalmente no uso los modos automáticos para disparar y mucho menos cuando uso un filtro ND. Al disparar en AV, la cámara calcula la velocidad de disparo, por lo que al oscurecer la imagen con el filtro la cámara aumenta el tiempo de exposición, por lo que te aconsejo que utilices la opción (M) para poder tu decidir el tiempo de exposición que necesites para realizar la fotografía idónea. Espero haber comprendido tu consulta y que mi respuesta te ayude.
Saludos desde España

Translation (added by moderator):

Hi. Normally I don't use the automatic shooting modes, much less when I use an ND filter. When shooting in AV, the camera calculates the shooting speed, so when darkening the image with the filter the camera increases the exposure time, so I advise you to use option (M) to be able to decide the shooting time. exposure you need to take the ideal photograph. I hope I understood your query and that my answer helps you.
Greetings from Spain

Last edited by BigMackCam; 04-14-2020 at 10:15 AM. Reason: Translation added per forum rules
04-14-2020, 10:43 AM   #4
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Are you covering the viewfinder when using the ND filter? I assume very long exposure times?

04-14-2020, 10:54 AM   #5
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QuoteOriginally posted by First Poster Quote
I found out when using a B+W brand ND filter or a Nikon brand polarizer with the camera set to Av mode, the result images were always under exposed a lot. Somehow the shutter speed would not go slower.
Are you saying that the camera is metering the same whether you hold an ND filter in front of the lens or not?
04-14-2020, 01:10 PM   #6
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What I would advise you to do is use the camera in Manual mode and make sure you cover the rear viewfinder of the camera with either a black piece of cloth (I use the ends of very small baby socks) or the viewfinder cover. When you meter, you may want to focus on an area of the scene that is moderate in exposure. Like, when I do river shots, I may focus on a grey river rock that is sort of at the middle of being exposed as opposed to the rest of the scene. Being in auto mode with an ND filter may not be as predictable as being in Manual mode.

As you said, you have been shooting long exposures prior to this, so you may just want to test in Manual mode to see if the output distinguishes itself from the results that you get in an auto mode.

I actually have been shooting long exposures with my Pentax 16-85 and Pentax K-3 II successfully for awhile, and have never had any distinct function problems in relation to ND filters being used. I use Manual or Bulb mode whenever I shoot long exposures with ND filters.

If you are shooting exposures that require more than the 30 seconds allowed by Manual mode, you may want to resort to Bulb Mode, which requires that you time the exposure yourself (with a watch for example) and also end the shot with a wired, infrared, or radio transmitter remote to avoid camera shake. There are also timers that are built for those type exposures.

Last edited by C_Jones; 04-14-2020 at 01:31 PM.
04-14-2020, 01:20 PM - 1 Like   #7
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QuoteOriginally posted by First Poster Quote
Somehow the shutter speed would not go slower.
Can you provide an example?

Here are a few things to consider:
  • There are limits to exposure times. On your camera, the shutter will not remain open longer than 30 seconds for other than timed B mode.
  • There are limits to the meter sensitivity. On your camera, the range is EV100 0 - 22. For reference, the lower limit of EV100 0 with an f/4 maximum aperture lens would be attained at low end of normal interior (think romantic) lighting
  • If the indicated exposure is outside available settings or if the light to the meter is outside its range, the rear LCD display and viewfinder display will blink.
  • When using a darker filter, the limits of meter sensitivity or maximum exposure time are fairly easily attained. When either happens, the shutter will still release, but the exposure may be inappropriate.
Problems such as in the list above are fairly common with ND filters, but unusual for most polarizer filters where a filter factor of about 1 is the norm.

Also, as noted above, when working with one's eye away from the viewfinder, it is important to either use the slip on cover that came with the camera or shade the finder with a finger or some other means. Bright light entering from the finder can bias the meter by several stops.


Steve
04-16-2020, 03:22 PM   #8
J. Lee
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K70 images under exposed when using ND or polarizer

Hi guys,
Thank you folks on pointing out to cover the view finder for longer time exposure. Apparently the K70 is much more susceptible than other cameras especially when the shutter speed calls for more than
1 second. Blame it on my own negligence. As I said, I do not have much under expose problems with other dslrs without covering the view finder when using filters. Usually I only have less than 1 EV stop expose difference. Thanks again.

04-16-2020, 03:42 PM   #9
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If, like me, you use the eyepiece magnifier, a piece of folded black card, held in place by the hot shoe cover (or equivalent - I use a spirit level instead of the original cover) will cover the eyepiece and exclude light, and will slip easily into your filter wallet when not in use, or can just be flipped up out of the way.
04-16-2020, 05:04 PM   #10
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QuoteOriginally posted by First Poster Quote
Thank you folks on pointing out to cover the view finder for longer time exposure. Apparently the K70 is much more susceptible than other cameras especially when the shutter speed calls for more than
I dunno...there is nothing particularly unusual about the K-70's viewfinder as compared to other auto-exposure SLR* cameras. The issue of meter bias due to errant light through the viewfinder of SLRs is as old as exposure automation on those cameras, meaning over 50 years ago. Some cameras have been equipped with a mechanical blind and others simply shipped with an eyepiece cap. Meter bias from not having the viewfinder blocked is usually not a big problem. It becomes more likely when light through the lens is dim relative to that striking the eyepiece glass such as with a strong filter, stop-down metering and/or with bright light from behind. An expected long exposure time is not a consideration for metering, but might lead to loss of contrast and/or veiling flare due to light leaking around the flipped up mirror during a long exposure. Potential for this latter issue exists for all SLRs regardless of metering or exposure system.

As I noted above, there may be a component of this problem that is caused simply by shooting with your filters in excessively dim light and/or narrow aperture. Your shutter will not stay open more than 30 seconds in Av mode and the meter is not linear when light to the cells is lower than the sensitivity of those cells.

Again...an example might be helpful with note indicating aperture, shutter speed, and ISO for the underexposed shot as well as the strength of the ND filter.

There is also the outside chance of a general issue with the shutter or aperture control, but that would happen regardless of whether a filter were attached.



Steve

* As might be obvious, mirrorless cameras, even those with EVF are immune to this sort of issue.

Last edited by stevebrot; 04-16-2020 at 05:13 PM.
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