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10-14-2008, 01:55 PM   #16
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Absolutely it can make a difference. This is the reason the Spotmatic ESII came with the great innovation of having a shutter speed dial setting which when selected closed a pair of blinds over the viewfinder. It's wonderful for tripod shooting for precisely that feature alone. Set up your shot, turn the dial to close the blinds, -click-, turn the dial to open them again.

10-14-2008, 02:01 PM   #17
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Gooshin,

Thanks for pointing this out. I'm not surprised that I've never noticed it, but I am surprised that I've never heard this phenomenon mentioned before. I just did a few more tests here and I guess I DO see it happening. Took a couple of quick and dirty shots illustrating the idea:

Picasa Web Albums - William - 20081014 mete...

In these two shots I observed a roughly half-stop difference in exposure - and the exposure was better when the viewfinder was covered by my hand. I would note that the aperture's here were fairly wide and you seem to be saying that a smaller aperture might make a difference to this phenomenon.

I am curious now about why or how this happens. I guess that through-the-lens metering involves a sensor inside the camera. The sensor is (I assume) positioned so that it's mainly receiving light coming in through the lens - but I guess there's an open corridor between the lens and the viewfinder, and so you get some "backwash" of light from the finder. Does my explanation make sense and sound correct?

Thanks for this thread.

Will
10-14-2008, 02:47 PM   #18
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When on a tripod, I always use the AE-L before pulling away from the viewfinder to avoid that problem. That little button is there for something!!!
10-14-2008, 03:17 PM   #19
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QuoteOriginally posted by Gooshin Quote
at anything blow F4.0 i would think that the viewfinder hole is already bigger than the diaphragm, at f8 you would definetly get more light... and since M42's on a modern pentax are the only lenses that would showcase this, perhaps this is why its such a rare complain from people.
Actually, you get stop-down metering on any non-"A" lens when using the green button or DOF preview. So you got me curious, and I just did a little testing with my M100/2.8. Sure enough, if I stopped down to f/8 or greater and pointed my camera directly at the ground with the viewfinder pointed up toward an overhead light fixture, I saw a 1 EV or more difference. That is, holding the DOF preview level and reading the meter from the top LCD, I was the reading change by 1 EV or more when I put my finger over the viewfinder. Similarly, hitting the green button yielded shutter speeds that differed by a full stop or more depending on whether I had my finger over the viewfinder or not. It was harder to reproduce when pointing the camera such that the amount of light at the viewfinder was not obviously greater than the amount at the lens, and it was virtually impossible to reproduce with the aperture wide open no matter how I pointed the camera. But I was definitely able to reproduce it with an "M" (not M42) lens.

10-14-2008, 03:46 PM   #20
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I had this exact same experience when I was at the zoo about a month ago. I was trying to take a self portrait with the 12 second timer, set the camera up, checked focus, then pressed the shutter, walked over and stood where I wanted. Went back to check it out, and the photo was a sea of blackness, lol, tried again, same result. It hit me all of a sudden, the sun was low, and shining directly in the viewfinder. I took my ( clean ) hanky, draped the camera strap over it, and put it over the eyepiece, took the shot again, and wha la !! perfect exposure, lol
10-14-2008, 03:47 PM   #21
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Light entering the prism from behind is a historic issue. The detectors were usually located inside the prism housing on either side of the eyepiece. Light would enter through the eyepiece and bounce back off the focus screen (and other places) into the detectors and bias the meter setting.

As Lowell noted the Ricoh XR2s had a viewfinder shutter. This feature or some other device was present in many other cameras from multiple manufacturers. My Ricoh XR7 came with a slip-on device that slid onto the camera strap. The strap fabric blocked the viewfinder when it was slipped onto the eyepiece. (Very hokey...The XR7 was the successor to the XR2s. You would think that the "improved" version would have kept this elegant and useful feature.)

Steve
10-14-2008, 03:57 PM   #22
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damn, things that slip your mind. shooting landscapes all weekend and the exposures were all over the place (many about 1 stop over / under) got really annoying. drove home (couple hours) trying to think through everything that I was doing that may have caused it. totally forgot about this
10-14-2008, 04:12 PM   #23
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QuoteOriginally posted by Gooshin Quote
getting to know the camera better i realized the other day how BIG of an impact having the viewfinder open to light has on exposure, with sometimes as much as a 3 stop difference working off a tripod!

just by blocking it with my thumb i see the readings change drasticly.

my next purchase item will be a cupped eye-piece to eliminate light streaks during regular use.
The exposure sensor is almost in a straight line into the camera from the eyepiece. It's on one of the faces of the prism.....


10-14-2008, 05:11 PM   #24
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QuoteOriginally posted by Gooshin Quote
getting to know the camera better i realized the other day how BIG of an impact having the viewfinder open to light has on exposure, with sometimes as much as a 3 stop difference working off a tripod!

just by blocking it with my thumb i see the readings change drasticly.

my next purchase item will be a cupped eye-piece to eliminate light streaks during regular use.
My Nikon F2s came with an eyepiece blind specifically to combat this problem, and to keep stray light from bleeding past the mirror during long exposures.
What happens is that the metering is done off the focusing screen, so stray light coming in the eyepiece can affect metering.

I'm of the opinion that any upper end camera should come with a closable eyepiece blind just for this reason.
Sadly, it seems they would rather listen to their accountants than the people who use the equipment.

As an aside, if you leave a K1000 sitting in a place where sunlight can hit the eyepiece for a few days, it will kill the battery, as the meter is activated by light hitting the photocell.
10-14-2008, 05:24 PM   #25
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Just got home from work.

It turns out, without my face pressed against the eypiece to block ambient light, I was able to get a variation of one to two stops.

This was most noticable with brighter light to my side.

I am sure now that shooting with glasses on, can cause a problem with exposure.

I wounder if this is a contributing factor to the "K10D underexposes" myth/fact/truth/rumor.

Gooshin, Thanks for bringing this topic up.

Eric.
10-14-2008, 06:32 PM   #26
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QuoteOriginally posted by jeffkrol Quote
The exposure sensor is almost in a straight line into the camera from the eyepiece. It's on one of the faces of the prism.....
Jeff,

Where in your diagram is the "exposure sensor"? Is it the translucent screen? The "film"?

Will
10-14-2008, 07:16 PM   #27
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QuoteOriginally posted by KungPOW Quote

I wounder if this is a contributing factor to the "K10D underexposes" myth/fact/truth/rumor.

.
I doubt it. When one of my K10D bodies came back from Pentax (autofocus problem), it consistently underexposed by about 2/3 stop, regardless of the light, exposure mode, etc. Before it went to Pentax it was pretty much on the money, exposure-wise. So I set exposure comp to +.7 and that pretty much eliminated that problem.

I shoot a lot of scenics with the camera on a tripod. If the sun happens to be shining on the viewfinder, I simply shade it with my hand. Otherwise I don't bother. However, I am almost always standing right behind the camera though not with my face against the viewfinder. No exposure problems.

Richard
10-14-2008, 07:17 PM   #28
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QuoteOriginally posted by flyer Quote
When on a tripod, I always use the AE-L before pulling away from the viewfinder to avoid that problem. That little button is there for something!!!
Great idea! It's amazing the things you forget about.

Richard
10-14-2008, 08:32 PM   #29
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QuoteOriginally posted by WMBP Quote
Jeff,

Where in your diagram is the "exposure sensor"? Is it the translucent screen? The "film"?

Will
either where marked or the next flat spot on the prism above it. I have a repair manual of a DS somewhere showing it better then this"generic" picture.
10-15-2008, 03:03 AM   #30
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RE: what a difference blocking the viewfinder makes!

Humour me.

Put camera on tripod, in S.AF mode.
Put eye to viewfinder.
Focus on a subject that doesnt have much contrast, but still focuses fine.
Take eye/body away from viewfinder.
Attempt focus again.
Doesnt lock focus?
Why is that? Anyone have an explanation?
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