[Class A]

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This is a 14 minut exposure with the lens cap on, in a medium light room.

Now you can

a) reverse engineer the optical paths from this, and
b) sell it as art to some rich guy.

[MoiVous]

I can remember the same thing with my old Canon AT1 and T70 film SLR manuals as well. BTW, those cameras metered at full aperture (though stop down was available) and the text in both manuals warned you about light from the viewfinder.

Fortunately, the old manual cameras required you to meter BEFORE operating the shutter, and metering was through the viewfinder or your hand held light meter (yep - had one of those too ).

In the modern dSLR world, with all the automation available, it has an effect BUT remember, it will vary depending on ambient light. So a darkenned or shaded area behind the viewfinder will have much less effect if your shooting a brighter object.

It shouldn't have any major effect on autofocus however, as the mirror has a semi-transparent region which passes light into the base of the camera. There should be little problem from the viewfinder unless there is significant light from that side and your subject is low contrast anyway (see the SAFOX description up to version V at Pentax Auto Focus Systems - and of course RiceHighs critique is in his blog).

So the moral of the story - carry your viewfinder cap for those moments when you take your eye away from the viewfinder.

[jeffkrol]

Here is a pdf of the parts list for a Pentax D.
You can extrapolate the light path to the meter from these diagrams.
Pretty sure the meter pcb is 0-j100.

www.qualiteitems.com/images/pentaxdparts.pdf

[MoiVous]

That parts diagram shows the Photo Sensor block (O-J100) to be at the front top of the pentamirror, and the AF block (part O-M100) to be at the bottom of the camera - thanks for that Jeff!

[LaRee]

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.

I've changed the eyecup on my k10d to this one:
2 Eye Cups for Olympus OM-1 OM-2 OM-4 NEW Eyecup Cup - eBay (item 320300721544 end time Oct-17-08 07:09:28 PDT)
Nice that I got two in case I lose one. There are others available too, one from jinfinance on ebay for 7.95 free shipping.
After using a better cupped type eyepiece I can't go back to using the stock one. It always seems to let in too much light compared to the cupped one I use.
btw, when I do long exposures at night, I just put my hand over the eyepiece. Fiddling with that small piece after I get the shot lined up is just not an option for me!

[Jonson PL]

I’ve also noticed that I get different reading, looking through the finder; compared to looking at the top LCD. I just thought it was something funny with my camera. Thanks for bringing focus to it. I knew of it, but always thought it was some tech thing that people spent to much time thinking about.

I hope Pentax in a K2 D, or future upper models, will provide a nob to turn and put down blinds. Those viewfinder caps are easily lost, and not an ideal solution. Else next time, I’ll just use some cloth.


Another thing I wish for, is a hard neutral setting for diopter. So that it cannot get out of whack and setting.

I remember reading something about it in the K10D manual, and that's where I first knew about such an issue. Never really paid much attention to it though. I've dug it up, on page 80 of the K10D manual:

Exposure may be affected if light enters the viewfinder. Attach the provided ME viewfinder cap or use the AE lock function (p.162). (Ignore the light entering the viewfinder when the exposure mode is set to M)

Thanks

(I know I'm not using the AE lock function enough)

[jeffkrol]

That parts diagram shows the Photo Sensor block (O-J100) to be at the front top of the pentamirror, and the AF block (part O-M100) to be at the bottom of the camera - thanks for that Jeff!

Yes, I finally noticed that. It is in the same position as your eye is w/ a lens to project the image onto the small sensor block.

[Lloydy]

with nothing more than a pair of scissors, my trusty Swiss Army penknife and my wife's credit card I made this in a couple of minutes.





The K10 rubber eyepiece has a groove running around it that folds back easily, and takes a bit of credit card cut to 37mm x 21mm with the corners radiused very easily, and it's easier to use than the proper thing.

I'd also forgotton about blanking the viewfinder when using a tripod, and I'm an ex Canon man who used the curtain lever all the time on my old A1.

[MoiVous]

Lloydy - that is a neat solution! And a great use for those dead credit cards too!

[Douglas_of_Sweden]

Lloydy - that is a neat solution! And a great use for those dead credit cards too!

It works fine with a shewing gum as well

[Lloydy]

It works fine with a shewing gum as well

Yeah, but it's disgusting!

[falconeye]

the viewfinder open to light has on exposure, with sometimes as much as a 3 stop difference working off a tripod!

Gooshin, forgive me, but does this mean you aren't hiding below a large black cloth when you shoot? What a photographer are you? This was about the FIRST thing learnt in the history of photography

[Gooneybird]

Well, this has certainly been an interesting thread... As one of those longtime users, it was still news to me (but makes perfect sense once explained). Guess I'll get in the habit of using the AE-L button on the tripod!

Jim

This is a very enlightening subject for me, even having used my K10D for over 15 months, I never suspected this as the reason some of my AV shots were messed up. Having been using SLR cameras since the mid 1950s, where I alway used an external exposure meter and chose my settings from it (there was no auto anything then) I never even thought about this subject. In fact, with some of my telephoto shots, I now take a couple of shots to bracket, then select my Manual settings and never worry about this little potential sneaky screwup. But I'm glad to learn why some non-telephoto shots were effected - it worried me that something was going wrong.

Thanks for the explanation.

Olin

[Douglas_of_Sweden]

Yeah, but it's disgusting!

Only if you forget to remove it before looking in the viewfinder again!