Yet Another Cycling Forum

Random Musings => Gallery => Phototalk => Topic started by: Tim Hall on January 03, 2021, 08:38:29 pm

Title: DSLR under exposing
Post by: Tim Hall 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?
Title: Re: DSLR under exposing
Post by: Lightning Phil on January 03, 2021, 08:44:27 pm
Was this in snowy and / or bright scenes?
Title: Re: DSLR under exposing
Post by: Tim Hall 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.
Title: Re: DSLR under exposing
Post by: Tim Hall on January 03, 2021, 08:54:45 pm
First example
(https://www.dropbox.com/s/moy28rlygtzq965/IMG_8012.JPG?raw=1)
1/200 f6.3 according to the Exif data
Second, taken on the same squeeze of the shutter
(https://www.dropbox.com/s/qj652jh6ztgobba/IMG_8013.JPG?raw=1)
1/200 f6.3 according to the Exif data
Title: Re: DSLR under exposing
Post by: Feanor 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.
Title: Re: DSLR under exposing
Post by: Lightning Phil on January 03, 2021, 09:02:35 pm
Both images have the same ISO assuming your DSLR is set to auto ISO?
Title: Re: DSLR under exposing
Post by: perpetual dan on January 03, 2021, 09:08:52 pm
Can you recreate the effect with the camera at room temperature?
Title: Re: DSLR under exposing
Post by: Tim Hall 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.
Title: Re: DSLR under exposing
Post by: Tim Hall 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
Title: Re: DSLR under exposing
Post by: Ham 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?
Title: Re: DSLR under exposing
Post by: Paul H 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.
Title: Re: DSLR under exposing
Post by: grams 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.
Title: Re: DSLR under exposing
Post by: Tim Hall on January 03, 2021, 10:25:59 pm
Ham - no. Good call though.
Paul, grams - well spotted. 
Title: Re: DSLR under exposing
Post by: Jaded on January 03, 2021, 11:26:49 pm
the branches at the top left are dead.

So, Ash dieback then?
Title: Re: DSLR under exposing
Post by: Tim Hall 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.