EOS 1n RS light loss due to the mirror - input from users requested

Hi,

I got my EOS 1N RS the other day and had time to play with it a bit today but didn't get to shoot a roll of film through it (yet). The manual says there's a 2/3 of a stop of light loss due to the mirror. Has anyone verified this? I just compared my N RS to two EOS 3 bodies while spot metering different surfaces using the same lens and have found that mine actually has more like a 1+1/3rd of a stop of light loss - 2/3rds of a stop more than advertised. And yes, I've triple checked f-stops and ISO settings as well as compensations and metering modes. I'm somewhat puzzled by this and am wondering whether the camera could be faulty (mirror looks clean) or whether this is normal for this body? I know that only shooting some film through it can give me definite answers but it would be great to hear about the experiences of other guys using the camera so I know whether I should send it in for checking just to be safe. Thanks!

I have a pair of the earlier RT bodies. I've never measured the light loss, I can't say it's ever bothered me any. The best way to know if there's a problem would be to shoot a roll of transparency film. I also have the 1N-RS, though in five years I've never shot with it, it's too big and heavy.

You are using a spot meter to check the film plane luminance with the back open? You need some diffusing surface at the film plane, otherwise you are pointing the spot meter at an aerial image. Perhaps that is the problem. Your best bet is to use the same roll of film in both cameras and shoot some zone I frames with the same lens using zenon flash illumination (to eliminate the shutter) and compare the density.

@(there was a url link here which no longer exists): I've used a RT over 10 years ago and that triggered my want for a camera with a pellicle mirror. Weight isn't an issue for me, my EOS3 /w BP-E2 aren't much lighter and I use them as my main bodies. I like a good hefty camera in my hands, but that's personal taste. I'm also not bothered by the light loss itself but I find the discrepancy between what's advertised and what my camera shows a bit... well, odd, makes my scratch my head.

@(there was a url link here which no longer exists): No, I tested them with the backs closed, same lens, same everything. First I used evaluative metering and noticed the discrepancy, then I confirmed it with spot metering because at first I thought it was the different evaluative metering systems which can show different results on different camera generations. I've just loaded up a roll of HP5+ and will shoot some test images. I know that stock by heart so I should notice any differences in exposure - I will shoot some Zone I stuff /w flash as well, thanks for the hint!

I have a couple of old Canon Pellix's I still use and can tell you that the pellicle mirrors age differently. One is relatively clear and looks new, the other has yellowed ( a bit). The yellowed one does produce a somewhat dimmer view through the viewfinder that I'm sure has an impact on the meter. I'm not sure what material Canon used for the pellicle mirror on the RS 1N, but it may be aging "differently," and affecting your meter.

I agree that the best way to test is to shoot a roll of transparency film.

Jim B.

Hi moviemaniac,

I agree with Jim B. - the differences that the meter tells you only tells you the meters are off.

To find the 2/3 stop light loss (or is it more due to age) you should shoot some film that is sensitive to exposure variations - transparency film.

You want to find out how much light hits the film.

It is a bit weird to think of losing 2/3 stop through a Pellicle in available light. A friend showed me her brother's which had the 50mm f/1.2 lens. It makes you pause when you realize that you are only getting f/1.8 out of it. But I won't dwell on that, you still get the bokeh.

Sorry I had it wrong. I thought it automatically electronically compensated on the lens aperture (like a Bolex Rex) so you can use a hand held meter and flash with no extra compensation, thus making it difficult to find out how much light actually passes through without a film test. But I re-read the manual and I am wrong. Ether way you did get a curious result.

Okay, guys, I did some more testing today and I encountered something odd: My initial comparison of the meters was done in the evening under flourescent lighting in my study (and outside while the sun had already set). Now I just went out into the midday sun and the difference between the EOS3 and the 1N RS is actually about 2/3rds of a stop pretty consistently whatever I meter it on. Thus the meter of the 1N RS seems to react differently unter low(er) light situations. That's fine with me, because when I'm shooting low-light wide open I'm going to reach for the EOS 3 anyway (or a 1V if I can one day get my hands onto a not horribly overpriced and used-to-death body over here... ).
Oh, and the test roll of HP5+ turned out great. I did some zone system metering on several frames and they turned out spot on to what I metered. I will shoot some Velvia with it when the leaves start to fall though, that will be the ultimate test.

moviemaniac said:

Okay, guys, I did some more testing today and I encountered something odd: My initial comparison of the meters was done in the evening under flourescent lighting in my study (and outside while the sun had already set). Now I just went out into the midday sun and the difference between the EOS3 and the 1N RS is actually about 2/3rds of a stop pretty consistently whatever I meter it on. Thus the meter of the 1N RS seems to react differently unter low(er) light situations. That's fine with me, because when I'm shooting low-light wide open I'm going to reach for the EOS 3 anyway (or a 1V if I can one day get my hands onto a not horribly overpriced and used-to-death body over here... ).
Oh, and the test roll of HP5+ turned out great. I did some zone system metering on several frames and they turned out spot on to what I metered. I will shoot some Velvia with it when the leaves start to fall though, that will be the ultimate test.

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The difference in readings you experienced under fluorescent lighting might be due to the nature of your light source or the difference in the way that the meters respond to continuous light sources (the sun) vs. a source that by its nature is somewhat sinusoidal (the flourescent lights).

I'd suggest a similar test under low lvel evening sky illumination.

I have this Minolta flashmeter III and the Booster II with film plane attachment. If you have this you could measure at the film plane with the same lens, mounted on a non pellice camera and then the RS it will show you the amount of light loss due to the mirror.
I believe the dicrepancy that you saw is more of the meter between the 2 cameras are not the same. I think Canon would calibrate the meter acounting for the amount of light loss. Besides the amount of light that the meter see is the amount of light reflected by the mirror and the amount of light that reaches the film is the amount of light that goes thru the mirror. These are not the same and if I am not wrong the amount reflected is significantly less than the amount that goes thru so that with the RS one would rather suffer more of a dark viewfinder than light loss to the actual exposure.

If 2/3 stop less light is going thru the mirror then it's about 64 percent is reaching the film. The remaining 36 percent refected into the viewfinder and that is about 1.4 stop less light than a mirror that reflects 100% of the light.

It should not be of concern how much light loss, in published fact 0.6 stop.

A mirror such as in the Canon EOS1n loses close to (but not precisely) 0.3 stop — you lose much more putting an f3.5 lens on the front! More still, adding a polariser. Where is the issue? The pellicle mirror provides known (but not exactly earth-shattering) benefits of course and they are provided with a trade-off given as a very small amount of light loss. Changing focusing screens can also institute light loss.

Negative film is not the stuff to use when checking meter accuracy. Using negative film allows you a huge amount of latitude, especially HP5+. Two-thirds stop of anything on that will not have any affect at all. Run a roll of transparency film though the camera to sharpen up on what the meter is doing.

A word on meter referencing: the spot meter in the EOS 3 uses a later generation algorithm than that in the EOS 1, 1N, 1N RS bodies, and has altered weighting on account of the 21 zone meter as opposed to the 16 zones double-function spot meter of the 1N series. I would not consider it a reliable parallel reference. If the camera does continue to throw metering out with obvious derangement (again, use transparency film because it is sensitive to +/– 0.3 stop variations, and a variation of +/– 0.6 is indisputably obvious), then a bench test is warranted. Before you do that, check custom functions to ensure the meter is reading in 0.5 vs 0.3 step.

Over long and hard use, the embedded metering eye beneath the mirror can cop a bit of dust. It is very, very sensitive to scratching — look, but don't touch!

Thanks for that technical insight!
The camera is in pristine, rarely used condition (i.e. a few odd rolls for the family album), everything's squeaky clean, including the metering eye. I've already ordered a few fresh rolls of Velvia - it's about time I shot some slides again so I'll use them to really test the meter thoroughly.

Atta boy!
Velvia will tell all, bluntly and brutally, but a lot of what happens depends on shooting in the right light. Use the different on-board meters, including partial additional to evaluative and partial spot. Bright sun and shadows will cause most meters to tremble and fit. Velvia in diffuse light is a beautiful thing to look at, but if the meter buggers up a simple, uncomplicated scene, then you will know, like being hit by lightning, something is wrong. But I hope not. None of the 1N-series cameras are softies to squirm at drama. I do hope it all works out for such a great camera.