F4.0 > f2.8?

*sinus007* · 02-25-2009, 06:01 PM

Hi,
I was testing my new Metz 48 and came across an interesting thing: pictures taken @F4.0 brighter than @F2.8. How it can be?
Here they are:
1. w/DA40, ISO 100, F2.8

2. w/DA40, ISO 100, F4.0

I tried another lens - same thing
Thank you very much,

Lowell Goudge · 02-25-2009, 06:10 PM

it could be a couple of things

first, although your lens is F2.8 have you ever checked the linearity of exposure on your camera? maybe it is f2.9 or something.

Second, are you certain the flash is absolutely correct in exposure, the light output may not be the same flash to flash depending on the charge of the capacitor, how accurate is the flash?

third, you giove apature but not shutter speed, as you stop down, is the camera compensating by increasing natural light exposure?

remember, p-ttl calculates time, as opposed to actually measuring light through the lens when stopped down.

but, having said all these great ideas, if you measure the grey wall between the couch and painting, the grey scale value is 162 for one and 163 for the other, implying exposure is within about 1/40th of a stop

nanok · 02-25-2009, 06:47 PM

i think lowell is spot on: hard to tell with such small pictures, but look at the light from the lamp on the wall, i think i can see that the contrast is higher on the wall, so perhaps the ambient light had more work done at f2.8 than at f4.0 (would make sense, if the shutter speed remained constant and the flash varied power to keep the exposure within limits). if anything, this proves the flash system worked perfectly

*sinus007* · 02-25-2009, 07:01 PM

Hi Lowell,
Sorry, I forgot to mention the exposure on both shots is 1/60s
Also, I tried to bring the #1 to #2 by changing ev in LR - the difference is about 0.5.
I guess you're right about P-TTL. I'll experiment a bit more.
Thanks for the reply,

**Damn Brit** · 02-25-2009, 07:13 PM

Moved to accessories forum

mattdm · 02-25-2009, 08:48 PM

Originally Posted by sinus007

Hi Lowell,
Sorry, I forgot to mention the exposure on both shots is 1/60s

So, here's an experiment: put the camera on M, and turn off the flash. Take a shot at ISO 100 and ¹⁄₆₀th of a second at f/2.8 and again f/4, ignoring the part where the camera is telling you the image is horribly underexposed.

Take a look at the results. That's the portion of the exposure that's not contributed by the flash. (This is another way of saying: it doesn't really matter what you have the shutter speed set to, unless it's slow enough to let in a meaningful amount of ambient light.)

Ben_Edict · 02-26-2009, 03:48 AM

Originally Posted by sinus007

Also, I tried to bring the #1 to #2 by changing ev in LR - the difference is about 0.5.

How are we supposed to make sensible comments on two images, if you tried to ameliorise the differences under discussion?

Ben

Lowell Goudge · 02-26-2009, 06:14 AM

ben, he actually did a good job too, i can't find anywhere, on any object, a difference in greyscale of more than 1 between the 2 photos.

Unless I totally messes up in copying the itty bitty photos into my editor, I would have to say they are as identical as possible.

*sinus007* · 02-26-2009, 07:13 AM

Hi,
Thanks all for the replies.

Originally Posted by mattdm

So, here's an experiment: put the camera on M, and turn off the flash. Take a shot at ISO 100 and ¹⁄₆₀th of a second at f/2.8 and again f/4, ignoring the part where the camera is telling you the image is horribly underexposed.

Take a look at the results. That's the portion of the exposure that's not contributed by the flash. (This is another way of saying: it doesn't really matter what you have the shutter speed set to, unless it's slow enough to let in a meaningful amount of ambient light.)

I'm not sure I understand - can you explain please.

Originally Posted by Ben_Edict

How are we supposed to make sensible comments on two images, if you tried to ameliorise the differences under discussion?

Ben

The posted pictures are not pp'd. I just converted them to jpg in LR and resized. I "oops'd" with the size - here's the link to Picasa web album: Picasa Web Albums - Anatoly - flash test
where you can clearly see the difference.
My question is why the flash didn't provide linear lighting based on 1stop difference in aper. and the same exposure?
Thank you very much,