ISO accuracy

I reciently purchased a K20D to enter the realm of digital photography. Started shooting film with a K1000 in 1977. In 1986 I bought a 645. I love film so this digital is a shock. Here is my problem. For years I have been doing a night Portrait. A full moon with snow on the ground and a clear sky makes for some awsome photo's. I tried to do this last night with the K20D. I tried all of the auto functions to no avail. even tried manual settings using my vivitar light meter. My meter was telling me 5.6@2sec ISO400. This manual setting did not produce even an outline of my subject. Unfortunately, I deleted all of the images before I found this site so I cannot post one. Is this something I just have to live with in digital or do I possibly have something wrong with the camera. In the studio this morning I went to my log book and tried some light setups and settings that have been working for years on film and could not produce an acceptable image. Are digital ISO and Film ISO the same?

I am using Pentax SMC 18-55 DA

Digital use a little bit different standard then film but the difference is around 1/3th of a stop to maybe 2/3 at most but reading your post it sounds it's 2 to even 3 stops too dark.
I've no idea what's going wrong.

What lens are you using?

DA 18-55

Is this the first time you've used your new camera?

Tried cleaning the contacts on the lens and camera body?

Be good if you could take and upload some images.

To all intents and purposes you can assume ISO is the same on film and digital.

The moonlit scenario is not straightforward, so I would bracket to ensure the correct exposure.

If you can't get a correct exposure in daylight then something is wrong. If you are trying to trigger studio lights with the built in flash then this will be the cause of your problems.

How frustrating. Something's not right if the automatic settings like Av didn't produce anything close to a correctly exposed shot by moonlight.

There are all sorts of issues here. Please excuse me if some of the following sounds like teaching grandma to suck eggs. You clearly have plenty of experience with film.

Firstly, clearly, we need to make sure the camera is working at all. Are you getting acceptable images when just using it in unchallenging conditions, e.g., just taking snapshots?

Secondly, to expand on John's comment. The pop-up flash emits a "pre-flash" to determine exposure. If you are using the flash to trigger the studio lights this pre-flash will fire your strobes prematurely. The K20 has settings about controlling flash in wireless mode which may help although most people prefer using a radio or IR triggers (I am not a studio person).

Lastly, moonlit scenes suffer from nature of a digital sensor's response curve. With film, bright areas of the image can retain detail even when quite over-exposed (obviously this varies with different types of film). Digital, however, has a straight reponse curve so that when highlights reach maximum brightness they become blown and contain no detail at all. The solution is to 'shoot to the right' which means checking that the histogram of light values (available during playback by pressing the INFO button) does not go over the right hand edge of the graph. This can result in what seem like dark, under-exposed images but by adjusting the levels in post processing (I use the fill light slider in lightroom) you can bring out the details in the dark areas. In extreme cases you might have to blend images taken at different exposures or use some form of HDR (high dynamic range) processing.

Hope this helps. K.

Thank you to all of you that helped with this problem. I believe I have resolved my issue. After reading the book over and over again for about the 50th time I think I found 2 reasons that caused my problem. First of all auto iso was set to span 100-200. That explained the lack of function in the auto modes. I was not aware you could set the range of the auto ISO. As to the manual usage. (Read exposure with a meter and set camera accordingly.) The problem was #18 in the custom menu was turned off. I turned it on. Set the amount to strong. guess what? It works! I didnt think my vivitar 45 could be wrong. At least I didnt try to fix it with a hammer LOL.

Thanks again to all of you.

Womble, thanks for all that information. However, before I go through all that trouble I think I'll bring one of the film cameras out of the collection and buy a roll of film.

Thanks,
Steven