D90 viewfinder cap: really necessary?

When I bought my first DSLR I was taking some pictures of roses, and I had the camera on a tripod. I had taken a few pictures and I had a thought that I should take a few with the self-timer; my tripod is rather old and not the best and I had concerns that my pressing the shutter release might cause some blur. I set the timer and pressed the shutter release and backed away from the camera. The camera was pointed down at a rose and the eyepiece was pointed up at the sky. It took me a long time to figure out why the pictures taken using the timer had a different exposure than the ones taken with me looking through the viewfinder.

Many years ago I was watching a photographer taking pictures of my Daughter’s girl’s softball team and he had his camera on a tripod and he was using a corded-remote. When he had the girls as he wanted them, he would back away from the camera and take the shot while talking to the girls to get then to smile. What really caught my eye was that he hung his cap over the back of the camera; it wasn’t until years later that it dawned on me why he did that.

--
Brooks
http://bmiddleton.smugmug.com/

I guess I was wrong to think I could ignore the cap!

I ran a simple test of my own, just to verify the effect. I put the D90 on a tripod and pointed it at a wastebasket in my office, but angled it down so the viewfinder was pointing back directly at one of my overhead lights. It was in auto-no-flash and auto iso. Here are the results of 4 shots:

cap on ---------- 1/13 - f3.5 - iso800
cupped hand - 1/13 - f3.5 - iso800
no cover ------- 1/25 - f5 ---- iso400
flashlite --------- 1/30 - f5.6 - iso320

I doubt the effect no longer! Notice that a simple overhead bulb falling on the viewfinder results in about a 3 ev underexposure! (The flashlite was pointed at the viewfinder to take the effect to the extreme. In fact, I sort of had to angle it around various ways before the camera would even take the picture.) Imagine a tripod-mounted picture, outside, pointing down at something, and with the sun at your back!

I'm gratified at least that a cupped hand (just away from the camera, not touching it) is enough to remove the problem.

I read in another thread, where someone was asking about why the mirror came down just before you took a shot in liveview, that the mirror comes down in order to meter the exposure. So, I repeated this experiment in liveview:

cap on ---------- 1/13 --- f3.5 - iso800
cupped hand - 1/13 --- f3.5 - iso800
no cover ------- 1/15 --- f4 ---- iso800
flashlite --------- 1/100 - f10 --- iso200

So, its clear, this effect applies even if you are shooting stills in liveview.

(The inconsistency between the two flashlite shots above is understandable, in each case I was moving the flashlite around until it would take a shot. I'm not sure why the two "no cover" shots were inconsistent. Maybe there is some effect from the "quick look" aspect of the liveview metering.)

I agree with one of the posters above that using this cap is royal pain in the , but at least I am going to be keeping it in my camera bag and trying hard not to lose it now. I guess my formula will be: left thumb or cupped hand if present behind the camera, little cap otherwise.

This factor is sort of a spoiler. From now on, when someone reports reports an underexposed image or some other sort of strange inconsistency with the D90, we are going to have ask about whether they covered the viewfinder.

I can't fault Nikon too much for the D90 not having everything the D300 has, but I do fault Nikon for having this fairly serious effect on getting good shots relegated to one little obscure and rather out-of-context note at the bottom of one page of the manual

Thanks everybody.

--Milt--
--
My site: http://milton.barber.name

The effect on the exposure metering can easily be proven with a flashlight.

There is also a very small amount of light leaking onto the sensor during the exposure. I did an experiment with a 15' exposure and lens cap (not viewfinder cap) on. Comparing an exposure taken with the viewfinder covered with an exposure pointing a flashlight into the viewfinder shows a barely recognizable shimmer on the upper part of the picture with flashlight. I doubt that it will disturb in usual situations. For long time exposures with ND filters in broad daylight, especially with sun in the back it could be recommended to put the viewfinder cap on.

Matt

Its not the viewfinder light reaching the picture that is the problem, its the viewfinder light affecting the exposure reading when you are on auto exposure. Did you see my data? A change of 3 ev just from indoor lights is a lot!!

--Milt--
--
My site: http://milton.barber.name