Forgot Password
Pentax Camera Forums Home
 

Reply
Show Printable Version Search this Thread
02-01-2012, 05:22 AM   #1
Junior Member




Join Date: Jan 2011
Location: Cambridge
Photos: Albums
Posts: 31
newbie question - flash exposure compensation

Hi,

When using a Pentax-A manual lens with the on camera flash, I get some over exposed shots in some situation.

I then try to "chimp" to get the correct exposure. So tired to use the flash -2 ev.
Turn out that -2 ev is still not enough, still over exposed.

Then I start on the "normal" exposure compensation (i.e. not the flash ev).

I am using Av mode. When I pop the flash, shutter speed is always set to 1/180 (due to my setting that force it to always go to max flash sync speed? not aware of setting anything like this, I have only enable use of aperture ring in cus fun.).

When I have -2 ev, shutter still at 1/180. But the photo is less exposed (darker). So the "normal" ev control not the shutter speed but flash output?

If this is the case, when using Av mode with flash, the flash ev is defunc ? I can use the normal ev setting to get more than +/- 2.

A bit confused here.

This only occur when I use pentax-A lenses. With the AF kit lens, exposure and flash exposure compensation behave like I knew it. i.e flash ev control flash output power and ev control shutter speed. And I normally use this to get a balance between fore-ground subject vs back ground exposure.

Why with manual lens (pentax-A lenses) this is different?

BTW, if I don't pop the on camera flash, the exposure compensation control the shutter speed as per normal when I use these pentax-A lenses.

-kk

02-01-2012, 05:50 AM   #2
Senior Member




Join Date: Oct 2007
Location: Tallinn
Posts: 265
I know that with M lenses the built in flash always fires on maximum.
Don't know about the A lenses (because my only A lens is a fisheye and I haven't tried with it), but I wouldn't be surprised if it would also be the same for them.
02-01-2012, 07:17 AM   #3
Pentaxian
maxfield_photo's Avatar

Join Date: Mar 2011
Location: Cincinnati, OH
Posts: 1,216
I don't have a K-5 or an A series lens to test, but on the K20d in Av mode the camera will try to lock the shutter speed at a mid range value like 1/60th or 1/90th depending on ambient light, aperture and ISO at the time you pop the flash up. If you the sweep around it will adjust the shutter speed as necessary to take in particularly bright or dark objects. If you have auto ISO set, or hit the green button, it will try to predict the correct ISO and keep the flash power fairly consistent. If you have the ISO locked, it will adjust the flash output to compensate. Also, if you're using an A lens in something other than the 'A' setting you need to do stop-down metering as you would with a manual lens, even if you have the camera set to allow it (I think so anyway, it couldn't hurt).

Here's a couple things you can try:
  • Set the camera to Av, set the lens to A, allow the ISO to float, and try different flash compensation settings.
  • Same as above, but lock the ISO.
  • Try TAv mode.
  • Find the correct ambient exposure in Tv mode, make a note of the settings, then switch to Manual mode. Input those same settings, but then subtract a stop of light. Either close down the aperture by a stop, or decrease the ISO. Take a shot. Now try different flash compensation settings.
02-01-2012, 07:54 AM   #4
New Member




Join Date: Jan 2012
Posts: 18
If you use an M-lens, or an A-lens with the aperture ring not set in the A-mode, the built-in flash of your camera will always give a burst at full power.
The reason for this is that to determine correct flash exposure, the K5 gives a pre-flash, with the aperture wide open. It has to know what aperture wide-open is, and what aperture is going to be used when shooting the real photo.

Now, for an A-lens with the aperture ring locked in the A-setting, I'm not sure if P-TTL flashing should work, but I believe it is mentioned in the manual somewhere. Try searching for P-TTL and manual lenses in there.

02-01-2012, 11:15 AM   #5
Senior Member




Join Date: Apr 2011
Location: Lake Saint Louis
Posts: 126
Interesting thread. I just tried this.

ISO set to 80, A series lens (f2.8, 28mm) set to A, camera set to AV, on-camera flash on. The shutter set itself to 1/90 of a second by the camera.

I focused on a book case about 3 feet away and fired at f2.8, 3.5, 4, 5.6, 6.3, 8, 11, 16. The exposure from 2.8 to 5.6 was the same according to the histogram and the display. At f6.3 and up it started showing a darker image at each f-stop. I assume this happened because with the ISO of 80 the flash was no longer powerful enough to properly expose the image.

I then set the ISO to 800 and repeated the test and found my exposures were the same up to f16, at f22 they started getting darker according to the display and histogram. So, I guess the TTL function does work for A lenses if the flash is powerful enough for the ISO and lens opening. I did not have to use any flash compensation and the histogram showed proper exposures.
02-01-2012, 11:18 AM   #6
Veteran Member
sterretje's Avatar

Join Date: Dec 2008
Location: Roodepoort, South Africa
Photos: Gallery | Albums
Posts: 3,534
QuoteOriginally posted by kkx Quote
When I pop the flash, shutter speed is always set to 1/180 (due to my setting that force it to always go to max flash sync speed? not aware of setting anything like this, I have only enable use of aperture ring in cus fun.).
Page 182 of the manual

QuoteQuote:
Using the Flash in Av (Aperture Priority) Mode

• You can set the desired aperture value to take a flash photograph when
you want to change the depth of field or shoot a subject farther away.
• The shutter speed automatically changes with the ambient brightness.
• The shutter speed shifts automatically anywhere from 1/180 sec. to a
slow shutter speed (p.70) that reduces camera shake. The slowest
shutter speed depends on the focal length of the lens in use.
The shutter speed is locked at 1/180 sec. when a lens other than a DA,
DA L, D FA, FA J, FA or F lens is used.
QuoteOriginally posted by kkx Quote
When I have -2 ev, shutter still at 1/180. But the photo is less exposed (darker). So the "normal" ev control not the shutter speed but flash output?
No. When you start underexposing the 'normal' way, you will close the aperture. As a result, less flashlight will reach the sensor. So yes, the image will get darker.
02-01-2012, 11:23 AM   #7
Veteran Member
sterretje's Avatar

Join Date: Dec 2008
Location: Roodepoort, South Africa
Photos: Gallery | Albums
Posts: 3,534
QuoteOriginally posted by toooldtocare Quote
ISO set to 80, A series lens (f2.8, 28mm) set to A, camera set to AV, on-camera flash on. The shutter set itself to 1/90 of a second by the camera.
Hmmmm, according to the manual (see above post), it should not use 1/90. Maybe because you use the extended ISO range?
02-01-2012, 12:34 PM   #8
Senior Member




Join Date: Apr 2011
Location: Lake Saint Louis
Posts: 126
QuoteOriginally posted by sterretje Quote
Hmmmm, according to the manual (see above post), it should not use 1/90. Maybe because you use the extended ISO range?
Nope, goes to 90 even with extended ISO turned off. Don't know why. Tried a 50mm - f1.4 and get the same? As soon as I pop the flash up it changes to 90. If I go to X, which I assume is for use with an external flash, which is what I use most of the time, it does go to 180. But in AV, 90.


Last edited by toooldtocare; 02-01-2012 at 12:44 PM. Reason: addition of info
02-01-2012, 04:20 PM   #9
kkx
Junior Member




Join Date: Jan 2011
Location: Cambridge
Photos: Albums
Posts: 31
Original Poster
Actually, I am not using a proper pentax-A lens.

I short the a-pin on the camera mount and use my M42 and Pentax-M lens and try to get some pentax-A funtionality, such as Av mode and p-ttl flash.

QuoteOriginally posted by vloris Quote
The reason for this is that to determine correct flash exposure, the K5 gives a pre-flash, with the aperture wide open. It has to know what aperture wide-open is, and what aperture is going to be used when shooting the real photo.
I know I miss something important, the pre-flash.

I meter with my lens wide open, then AE-L, and stop down my lens to take the shots. The problem is that the pre-flash will measure exposure when my lens stopped down, which will be wrong.

To correct this, I either:
- set aperture on camera to wide open (i.e exif will be wrong) and stop down the lens.
or
- do what I did before, and use exposure compensation to "correct" the exposure.

QuoteOriginally posted by sterretje Quote
Page 182 of the manual

No. When you start underexposing the 'normal' way, you will close the aperture. As a result, less flashlight will reach the sensor. So yes, the image will get darker.
Thanks, I should have reread the manual again carefully.

But I can't see how the camera can close my aperture when I am in Av mode and using manual lens which the aperture is only controllable from the lens aperture ring.

Maybe I confused you with my poor explanation of my situation. Sorry.

QuoteOriginally posted by toooldtocare Quote
Interesting thread. I just tried this.

ISO set to 80, A series lens (f2.8, 28mm) set to A, camera set to AV, on-camera flash on. The shutter set itself to 1/90 of a second by the camera.

I focused on a book case about 3 feet away and fired at f2.8, 3.5, 4, 5.6, 6.3, 8, 11, 16. The exposure from 2.8 to 5.6 was the same according to the histogram and the display. At f6.3 and up it started showing a darker image at each f-stop. I assume this happened because with the ISO of 80 the flash was no longer powerful enough to properly expose the image.

I then set the ISO to 800 and repeated the test and found my exposures were the same up to f16, at f22 they started getting darker according to the display and histogram. So, I guess the TTL function does work for A lenses if the flash is powerful enough for the ISO and lens opening. I did not have to use any flash compensation and the histogram showed proper exposures.
I am interested to know why your shutter is at 1/90, instead of 1/180.

I also find that the exposure is 1ev darker from F8. I am not sure the limited power of the flash cause this. It could be, but it seems unlikely for my case. Still trying to find out why.

-kk
02-01-2012, 05:16 PM   #10
Senior Member




Join Date: Apr 2011
Location: Lake Saint Louis
Posts: 126
kk,

I just did another test. Instead of using my lens on set A, I set it on 2.8 via aperture ring, the camera set at AV, and shot the same bookcase. This would be the same as using your screw mount lens. The flash did not do a pre-flash, just fired full power and of course was way over exposed. I then stopped it down a few f-stops and did the same and got the same result, even at f22 there was no difference. The lens did not stop down, stayed wide open to 2.8 verified by the info in the display. Also, the shutter speed was auto set to 180, where it is 90 with the lens set to A. I think this is what you are also getting. When the aperture ring is not in A the display on top displays F--- which I think means it has no idea. From this I can assume that the camera does not know where the lens is set like it does when it is in the A setting, the same for a M lens.

If I set the camera to M than the flash fires at full power and the lens does stop down. If I knew the guide number of the flash I could calculate the correct f-stop like we used to do before all this automatic stuff. Also in M, the shutter speed top setting is 180, but you can drop it down to slower shutter speeds.

Because of this I would recommend getting an off-camera flash with an automatic setting. I use a Sunpak 544 for weddings. I preset the flash to the same ISO as the camera, I also set the flash a f-stop that covers my needed range, and the camera to the same f-stop. The flash records the light bouncing back and turns off when needed, which is something apparently the K5 cannot do.

Last edited by toooldtocare; 02-01-2012 at 05:25 PM.
02-01-2012, 10:22 PM   #11
Veteran Member
sterretje's Avatar

Join Date: Dec 2008
Location: Roodepoort, South Africa
Photos: Gallery | Albums
Posts: 3,534
Just working towards some explanations

QuoteOriginally posted by kkx Quote
I know I miss something important, the pre-flash.
Do you mean that you don't get the pre-flash? If so, it explains the situation. Your lens is, in some way, still considered to be a lens that does not support A. You can check if you get a pre-flash using the 2 second delay. You will get a flash before the mirror goes up and two seconds later you will get a flash during the exposure.

QuoteOriginally posted by kkx Quote
I short the a-pin on the camera mount and use my M42 and Pentax-M lens and try to get some pentax-A funtionality, such as Av mode and p-ttl flash.
I've read somewhere (I think it was here on the forum) that the camera uses distance information provided by the lens to 'calculate' the flash power with pTTL. And your lenses don't provide that information. But I might be wrong; if I'm right however it might explain it.

QuoteOriginally posted by kkx Quote
But I can't see how the camera can close my aperture when I am in Av mode and using manual lens which the aperture is only controllable from the lens aperture ring.
The camera can't; but you did not mention that in your opening post (). Fact stays that if you close the aperture (either via the camera with a proper A lens or using the aperture ring), less flashlight will reach the sensor; and that was the point: you don't affect flash power using 'normal eV adjustment'.
02-01-2012, 10:42 PM   #12
Veteran Member
sterretje's Avatar

Join Date: Dec 2008
Location: Roodepoort, South Africa
Photos: Gallery | Albums
Posts: 3,534
A note on shutter speed. With a 'proper' lens, the camera decides based on focal length as well as ambient light which shutter speed it will use (the algorithm might vary a bit per model). I posted this a while ago in https://www.pentaxforums.com/forums/pentax-dslr-discussion/170163-k-x-flash-s...-question.html

QuoteQuote:
On a side note:
In Av mode, the camera sets a shutter speed (in my experience) for flash that is partially based on the focal length used. In a dark scene, the K100D in combination with DA55-300 uses roughly 1/FL (1/60 @55mm, 1/125 @ 100mm, 1/180s @ 150mm). In a brighter scene the same combo uses 1/180s regardless of focal length, possibly attempting 'fill-in flash'.

K5 with FA31Ltd or DFA100WR is more conservative and uses 1 / (2 * FL) in a dark scene; not tested in a bright scene.
Reply

Bookmarks
  • Submit Thread to Facebook Facebook
  • Submit Thread to Twitter Twitter
  • Submit Thread to Digg Digg
Tags - Make this thread easier to find by adding keywords to it!
camera, dslr, exposure, flash, k-5, k-5 ii, k-5 iis, k5, lens, lenses, pentax k-5, pentax-a, shutter
Thread Tools Search this Thread
Search this Thread:

Advanced Search


Similar Threads
Thread Thread Starter Forum Replies Last Post
Camera exposure compensation vs flash compensation raider Photographic Technique 4 09-29-2011 05:02 PM
Newbie Q about flash exposure compensation Kitty Pentax DSLR Discussion 1 01-15-2011 05:50 AM
Flash vs. camera exposure compensation jms698 Photographic Technique 2 08-22-2009 04:27 PM
Flash Exposure Compensation Question roger.hayslip Flashes, Lighting, and Studio 4 02-28-2008 08:25 AM
540 FGZ + K10D flash exposure compensation WMBP Pentax DSLR Discussion 12 12-06-2007 05:26 PM



All times are GMT -7. The time now is 12:02 AM. | See also: NikonForums.com, part of our network of photo forums!
  • Red (Default)
  • Green
  • Gray
  • Dark
  • Dark Yellow
  • Dark Blue
  • Old Red
  • Old Green
  • Old Gray
  • Dial-Up Style
Hello! It's great to see you back on the forum! Have you considered joining the community?
register
Creating a FREE ACCOUNT takes under a minute, removes ads, and lets you post! [Dismiss]
Top