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Camera's meter reading different to light meter


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I've just been comparing the auto-exposure readings from my Canon 518 Auto Zoom to those taken with a Sekonic L-308B light meter set to 100 ASA and 18fps. Camera was loaded with Ektachrome 100D, so no filter engaged, and set to 18fps. 

In all the lighting conditions in which I tested and compared results, the camera's internal meter (set to auto) showed an aperture two full stops less than the external meter. So: if external meter read f/2, camera would say f/4.

I understand that reversal stock has very little latitude for over/under exposure, so these results seem to suggest that using an external meter with this camera is not a good idea, as the film will be highly over-exposed.

I wondered if anyone more technically savvy than me might be able to suggest a reason for this huge difference?

I have to say that when I have exposed reversal stock in this camera on auto-exposure the results have been pretty spot on, but the difference between the two readings is very puzzling indeed!

I wonder what Martin Baumgarten might have to say..? 

All replies welcome - thanks for reading.

 

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It can depend on how you take your readings. There's also some light loss due to the beam-splitter in the viewfinder,  maybe 1/2 to 2/3, but that should give a larger aperture reading, not smaller. Maybe the meter needle isn't reading accurately.

If you take reflected readings off a grey card you may get a closer reading, but in the circumstances if you're getting good results I'd be inclined to stick to the camera meter.

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Thanks Mark -

Yes, I've read about the light loss from the beam-splitter but was puzzled that the reading was smaller. You seem to be suggesting that the camera's meter may be showing a misleading f-stop while nevertheless providing the correct exposure? I never thought of that - thanks.

Anyone else have any other ideas..?

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2 hours ago, Stephen Gordon said:

the camera's meter may be showing a misleading f-stop while nevertheless providing the correct exposure

You could check that with a reading off a grey card (or even a sheet of white paper- you're only checking, after all). Just make sure you get in fairly close so the reading is only influended by the card.

I don't know about the 518, but some zooms have a smaller effective stop at the long end, so stay at wide angle.

Edited by Mark Dunn
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I've shot about 6 rolls of the new Ektachrome and can tell you that this stock will handle overexposure just fine. It's rated at 100 ASA but seems to meter better at 80 ASA. Other's are starting to use this approach from my readings online. My rule has been to open up 1 full stop on cloudy days, or in shaded areas, and a 1/2 stop on bright sunny days. My results have been excellent! 

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Possibly worth having your Sekonic checked and calibrated? Two stops over is a large discrepancy if the internally metered footage looks OK, even with some exposure latitude. As mentioned, normally an external meter reading would be under-exposing because of the light lost to lens, viewfinder and internal meter.

I tend not to trust old camera internal meters, they can easily go out of calibration after so many years, or the battery voltage varies, or the optics reflecting light to them may have yellowed, etc, but the proof is in the pudding. For a better sense of it, you could shoot a bracket test with a stop over and under the internal reading (note the aperture values) and then see what actually looks best. 

I guess it's possible that the viewfinder f stop scale you're seeing is out compared to the actual aperture f stop. Again, if you shoot a bracket test using the Sekonic and see what looks best you can try to work out an offset value.

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Thanks Mark - I will follow your advice.

Shane - your reply is appreciated, and I am reassured by your experience with the stock!

Dom - thanks very much: the Sekonic L-308B meter provides readings which exactly match my recently-serviced Sekonic L-398, so I'm happy to rule that line of enquiry out. The camera meter batteries are new, and the correct voltage (2 x 1.35v) but I'm interested in your theory that the calibration has simply drifted after so many years, and your other point about the optics is also good. You also seem to agree with Mark's original idea.

I'm very grateful to you all for your time. I've just retrieved another 518 AZ I own which was at work - I will try comparing readings from the two cameras and see if I can garner any useful additional data!

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Thanks Mark - I will follow your advice.

Shane - your reply is appreciated, and I am reassured by your experience with the stock!

Dom - thanks very much: the Sekonic L-308B meter provides readings which exactly match my recently-serviced Sekonic L-398, so I'm happy to rule that line of enquiry out. The camera meter batteries are new, and the correct voltage (2 x 1.35v) but I'm interested in your theory that the calibration has simply drifted after so many years, and your other point about the optics is also good. You also seem to agree with Mark's original idea.

I'm very grateful to you all for your time. I've just retrieved another 518 AZ I own which was at work - I will try comparing readings from the two cameras and see if I can garner any useful additional data!

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I woke up last night realising I may have made a rookie error: my Canon 518AZ is taking a reflected reading, but I was taking incident readings with my external meter! Oops.

Will do some more tests this weekend to establish whether or not this accounts for the large discrepancies, which in that case will not actually be discrepancies at all.

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1 hour ago, Stephen Gordon said:

I woke up last night realising I may have made a rookie error: my Canon 518AZ is taking a reflected reading, but I was taking incident readings with my external meter! Oops.

Will do some more tests this weekend to establish whether or not this accounts for the large discrepancies, which in that case will not actually be discrepancies at all.

Aargh. That's the exact difference there should be between incident and reflected readings. Kindly admonish yourself.😀

Incident readings will usually be more accurate as you probably know. You still need to allow for the light loss in the finder.

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Thanks Mark  - I've sent myself to the foot of our stairs.

I didn't know, though, that that this was the exact difference to expect, so thanks for making that point clear and continuing my education!

I've always found your posts among the most enlightening on this forum - your replies here have been no exception to that rule.

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Yes, as you probably know, a reflected reading off a white card will underexpose by about 2 stops, conversely for black, because the meter assumes you are reading off a grey card of 18% reflectance. So the diffuser dome you clip on to take an incident reading absorbs 2 stops, to allow for that difference. See "zone system"- but don't get too bogged down with it!

You're fine with a reflected reading with scenes of average brightness, but anything else will put you out. So you're right to want to try to use an incident reading which doesn't depend on the scene, but on the light.

No problem, HTH. But if you want enlightenment (pun?) try David Mullen. I talk the talk, but he walks the walk, every day.

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You know what Mark? I didn't know that. And the manual I found online for my L-308 didn't explain that properly, if at all. I clearly have a lot to learn about reflected readings, but your starters-for-ten are super-useful - thanks so much!

As for the venerable Mr Mullen, I spent a good part of lockdown working my way through every page of 'Ask David Mullen anything' on REDUSER.net (I feel sure you will know it well yourself). An astonishing resource. Doesn't hurt that I've loved every piece of work of his that I've been able to acquire...

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  • 5 months later...

I used Kodak Ektachrome 7294 cartridge on my Agfa Movexoom 10.  There was a good 2 stop reading difference between my "spotmeter" and the camera meter. 

 

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1. Super 8-cameras like the Agfa Movexoom 10 have a beam splitter. Shares of the light passing the lenses always get deflected to the viewfinder and to the exposure meter by this beam splitter. These shares are therefore not available for the exposure of the film. The internal exposure system of the camera is adjusted accordingly. An external handheld lightmeter does not take this light loss into account, so you get a difference.

 

2. The Agfa Movexoom 10 reads the film speed automatically, but it can read and adjust only 2 values: ISO 40/17° and ISO 160/23°. Ektachrome 7294 is ISO 100/21°, so the exposure system works on a wrong basis: ISO 160/23° instead of ISO 100/21°. You get an underexposure of 2/3 f-stop.

Solution: The Movexoom 10 offers backlight compensation. It is the +-Position of the switch-on-off button, to be used instead of the A-Position. The backlight compensation works with + 2/3 f-stop. Using the backlight compensation you have  an precise compensation for the wrong ISO-setting.

Backlight compensation can nevertheless be made by the manual aperture setting und aperture fixing button.

Hope this helps. Best regards

Harald

 

 

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5 hours ago, Harald Vonhoegen said:

1. Super 8-cameras like the Agfa Movexoom 10 have a beam splitter. Shares of the light passing the lenses always get deflected to the viewfinder and to the exposure meter by this beam splitter. These shares are therefore not available for the exposure of the film. The internal exposure system of the camera is adjusted accordingly. An external handheld lightmeter does not take this light loss into account, so you get a difference.

 

2. The Agfa Movexoom 10 reads the film speed automatically, but it can read and adjust only 2 values: ISO 40/17° and ISO 160/23°. Ektachrome 7294 is ISO 100/21°, so the exposure system works on a wrong basis: ISO 160/23° instead of ISO 100/21°. You get an underexposure of 2/3 f-stop.

Solution: The Movexoom 10 offers backlight compensation. It is the +-Position of the switch-on-off button, to be used instead of the A-Position. The backlight compensation works with + 2/3 f-stop. Using the backlight compensation you have  an precise compensation for the wrong ISO-setting.

Backlight compensation can nevertheless be made by the manual aperture setting und aperture fixing button.

Hope this helps. Best regards

Harald

 

 

Harald,

 

the difference was not 2/3rd of a stop but good 2 stops over. anyways I use manual exposure settings.

good advice regarding the use of + button for filming in backlight or beaches etc. 

Most of Super 8 cameras are useful only if manual exposure is possible otherwise they are no good. most of them can't read modern stock's speed.

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  • 2 weeks later...

The difference between a hand held Exposure Meter and the Super 8mm camera's built in meter is significant.   Some cameras, usually lower end models or those intended for very low light filming use an exterior meter window on the camera and are not TTL (thru the lens metering).  The early KODAK XL cameras had a great design, 230 degree shutter opening, non reflex and external meter window to avoid robbing any light going to the film.  So they are still unbeat in low light filming [of course, 99.9% of these are no longer running due to the neoprene drive gear on the motor shaft having crumbled to dust, but that's another story].

   Most Super 8mm camera's instruction manuals will point out this exposure variation when filming in Manual Exposure Mode.   Depending on the size of the zoom lens etc, and beam splitter system used, the exposure variation is any where from 1-Stop to 2-Stops.  Even so, it's best to conduct a test to synch  your hand held exposure meter to using with whatever Super 8mm camera you have.  Do NOT be fooled by the aperture display in the viewfinder.  This is the F-Stop, which is determined by the lens opening size and affects exposure and depth of field, BUT....it is NOT the same in determining exposure due to the light loss from the  built in light meter which has some light deflected to it, as well as the beam splitter deflecting light to the viewfinder.   Some later Super 8mm cameras used a wedged cut type prism so that there would be less light loss or none at all, and some others used a tiny optic fiber cable or tiny prism just to bring light to the viewfinder, again, not any light loss.  This was mostly common in later XL cameras, and it depends on the model.   I know my SANKYO XL620 Supertronic camera, despite some of its oddities, can film in very low light due to hardly any light loss, and is accurate to a hand held light meter within 1/3 stop or less from my own tests.   This is one reason I chose this camera for Anamorphic filming since there is some additional light loss caused by the huge A-lens....totaled...since much less than most Super 8mm conventional prism design cameras.

   The caveat here is that with some cameras such as the BEAULIEU interchangeable lens cameras, exposure can be measure accurately with a hand held light meter without any extra adjustment, except for favoring some over or under exposure by the filmmaker.  They use a mirror guilotine shutter design deflecting 100% of the incoming light from the lens to the viewfinder and the film.    Some Super 8mm cameras rob more light and others less, and to do a full review would be exhaustive.   It is best to just conduct your own test, factoring in any information the instruction manual provides, and try some brackets shots, slating them so you will know what setting you are using and/or what deviation from your hand held light meter.

  As always, don't waste an entire cartridge of Super 8mm film on such tests....each shot can be very short, only a couple seconds or so is needed per exposure bracket.  Then use the rest of the film for whatever you would like.    I of course, have the luxury of shooting a few feet or less for such tests, removing the exposed film to process, and reload the film cartridge for continued use later.  I realize this is beyond the scope of most here....so just be careful and do write detailed notes for yourself to avoid exposure mistakes and wasted film later.  Super 8mm use has gotten so incredibly expensive in recent times, sad, but I'm glad new film stock is still available.   To minimize costs, I am as careful as I can be, and also process my own films as I have done since I was a young teenager long ago at 14.

  I would've added my comments sooner, but others quite knowledgeable have shared their information, which is terrific.  There was a recent posting and seeing the new activity, I thought I would add some information of my own.  I hope it helps.

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