Author Topic: DSLR under exposing  (Read 895 times)

Tim Hall

  • Victoria is my queen
DSLR under exposing
« on: January 03, 2021, 08:38:29 pm »
Out for a walk today I shot a series of phtographic masterpieces on my Pentax K-S2 fitted with a Tamron 18mm-250mm zoom. When I got back I found some where very under exposed, generally/always the first in a bunch of shots. The Exif info shows that the under exposed and correctly exposed shots have the same/very close shutter speed & aperture (I was shooting mostly on Aperture Priority.)

My hypothesis is the aperture mechanism was a bit sluggish, possibly due to the low temperature (around 3C), so took some time open up to the required stop on pressing the shutter.

Does that sound reasonable? What can I do to stop it happening again whwn out and about in the cold?
There are two ways you can get exercise out of a bicycle: you can
"overhaul" it, or you can ride it.  (Jerome K Jerome)

Re: DSLR under exposing
« Reply #1 on: January 03, 2021, 08:44:27 pm »
Was this in snowy and / or bright scenes?

Tim Hall

  • Victoria is my queen
Re: DSLR under exposing
« Reply #2 on: January 03, 2021, 08:48:51 pm »
Was this in snowy and / or bright scenes?
No, wintry over cast. Hang on, I'll put an  example up.
There are two ways you can get exercise out of a bicycle: you can
"overhaul" it, or you can ride it.  (Jerome K Jerome)

Tim Hall

  • Victoria is my queen
Re: DSLR under exposing
« Reply #3 on: January 03, 2021, 08:54:45 pm »
First example

1/200 f6.3 according to the Exif data
Second, taken on the same squeeze of the shutter

1/200 f6.3 according to the Exif data
There are two ways you can get exercise out of a bicycle: you can
"overhaul" it, or you can ride it.  (Jerome K Jerome)

Feanor

  • It's mostly downhill from here.
Re: DSLR under exposing
« Reply #4 on: January 03, 2021, 09:01:17 pm »
Out for a walk today I shot a series of phtographic masterpieces on my Pentax K-S2 fitted with a Tamron 18mm-250mm zoom. When I got back I found some where very under exposed, generally/always the first in a bunch of shots. The Exif info shows that the under exposed and correctly exposed shots have the same/very close shutter speed & aperture (I was shooting mostly on Aperture Priority.)

My hypothesis is the aperture mechanism was a bit sluggish, possibly due to the low temperature (around 3C), so took some time open up to the required stop on pressing the shutter.

Does that sound reasonable? What can I do to stop it happening again when out and about in the cold?

But isn't the aperture 'normally open', and only stops down during the exposure?
That's why there's a Depth-of-field preview button.

So a sluggish aperture mechanism would tend to over-expose?

Or perhaps I'm out-of-date.

Re: DSLR under exposing
« Reply #5 on: January 03, 2021, 09:02:35 pm »
Both images have the same ISO assuming your DSLR is set to auto ISO?

Re: DSLR under exposing
« Reply #6 on: January 03, 2021, 09:08:52 pm »
Can you recreate the effect with the camera at room temperature?

Tim Hall

  • Victoria is my queen
Re: DSLR under exposing
« Reply #7 on: January 03, 2021, 09:18:03 pm »
Feanor - good point. I think you're right (off to experiment and check)
Phil - yes same ISO
Dan - experiment coming up

I had the Aperture block replaced under warranty when I bought the camera 2nd hand from a dealer. It might be that issue again - I've just found some instructions on how to check it.
There are two ways you can get exercise out of a bicycle: you can
"overhaul" it, or you can ride it.  (Jerome K Jerome)

Tim Hall

  • Victoria is my queen
Re: DSLR under exposing
« Reply #8 on: January 03, 2021, 09:55:40 pm »
Experiment indoors at 20C.
1/8s f8 badly under exposed
next shot 1/10s f8 correctly exposed
next shot 1/10s f8 correctly exposed
next shot 1/10s f8 correctly exposed

Change aperture
2/5s f14 correctly exposed
2/5s f14 correctly exposed
2/5s f14 correctly exposed

Change aperture
1/40s f3.5 correctly exposed
1/40s f3.5 correctly exposed

Change back to f8, still correctly exposed

So, an intermittent fault/first time it's used after an undetermined time fault.
Next is to try a different lens and this lens on another body
There are two ways you can get exercise out of a bicycle: you can
"overhaul" it, or you can ride it.  (Jerome K Jerome)

Re: DSLR under exposing
« Reply #9 on: January 03, 2021, 09:59:34 pm »
Just on the offchance, you haven't switched into spot or centre weighted metering mode by mistake, have you?

Re: DSLR under exposing
« Reply #10 on: January 03, 2021, 10:12:54 pm »
Despite what the Exif says, the DoF in the two images you've posted indicates that they were not shot at the same aperture.  So yes, looks like a mechanical issue.

Re: DSLR under exposing
« Reply #11 on: January 03, 2021, 10:18:21 pm »
Despite what the Exif says, the DoF in the two images you've posted indicates that they were not shot at the same aperture.  So yes, looks like a mechanical issue.

I was going to say exactly this - the branches at the top left are a dead giveaway.

Tim Hall

  • Victoria is my queen
Re: DSLR under exposing
« Reply #12 on: January 03, 2021, 10:25:59 pm »
Ham - no. Good call though.
Paul, grams - well spotted. 
There are two ways you can get exercise out of a bicycle: you can
"overhaul" it, or you can ride it.  (Jerome K Jerome)

Jaded

  • The Codfather
  • Formerly known as Jaded
Re: DSLR under exposing
« Reply #13 on: January 03, 2021, 11:26:49 pm »
the branches at the top left are dead.

So, Ash dieback then?
If you don't like your democracy, vote against it.

Tim Hall

  • Victoria is my queen
Re: DSLR under exposing
« Reply #14 on: January 04, 2021, 05:50:55 pm »
Trying the lens on a different body - normal exposure

Trying a differenrt lens on this body - first shot is underexposed.

In depth web based reasearch A brief google points to the solenoid on the aperture block misbehaving.  There's a couple of tweaks I'll try and cross my fingers, otherwise it looks like a replacement will be needed.
There are two ways you can get exercise out of a bicycle: you can
"overhaul" it, or you can ride it.  (Jerome K Jerome)