Metering with mf / stop down lenses on Canon DSLR's

bobcominitaly · Joined: 22 May 2009 Posts: 71 Location: Terontola (I)

In an earlier topic on this forum, concerning the use of af confirm adapters, @Orio wrote:

Olivier · Expire: 2015-08-06

Orio · Expire: 2012-12-04

Bob, you need to focus your mind Wink

on how the EOS cameras work.
EOS cameras are built to be used with EF lenses.
EF lenses always stay wide open until you press the shutter, and EOS cameras always meter the light at wide open. They have full automation, so when you stop the lens down (using camera's wheels), you do not actually close the iris. The iris stays open. Follow me so far?
OK, so what happens is that when you tell your EOS camera that your lens is stopped down at f/8, your EOS camera meters the light at wide open (it reads the wide open value from the lens' chip), and then CALCULATES the actual light for a f/8 aperture NOT based on the ACTUAL light that would pass through an f/8 aperture, but based on a mathematical table that it has built inside, which has corresponding values for all the potential apertures. And it applies that value which is theoretical, not real.
Still with me?
Ok, so when you use your MF lens with an adapter, the adapter tells the EOS camera what is the wide open value for your lens. Which might or might not be true for your lens, because your lens' wide open, and the preset aperture of the chip, might or might not coincide. But that has not much importance for what comes after.
What comes after, is that you set your MF lens to,say, f/8.
This closes the PHYSICAL iris on your lens.
But what does the camera know about that? NOTHING!
Because your adapter's chip is not telling the camera that the iris is stopped down to f/8. THE CAMERA STILL THINKS THAT YOUR LENS IS WIDE OPEN (at the preset value of your adapter's chip).
And this is the reason why, the farther you move stopping down from that preset value, the more the lens over-exposes. BECAUSE THE CAMERA THINKS THAT THE LIGHT IT GETS FROM YOUR F/8 STOPPED DOWN LENS, IS THE LIGHT THAT THE LENS RECEIVES WIDE OPEN. Thus, logically, the camera makes a compensation, based on the built-in table. And this generates the error.
Which happens also in the opposite direction: if you have a chip that says f/2.8 as preset aperture, and your lens is actually f/1.4 wide open, when you shoot wide open, the camera will apply a compensation by under-exposing.

All the above, is the reason why an intermediate aperture, such as f/3.5, is the best possible compromise for a non-programmable adapter's chip.
Such intermediate value will minimize the shift in the exposure in both directions, since there is only 2,5 stops distance from a f/1.4 wide open aperture and 2,5 stops distance from a f/8 aperture, which is usually the most stopped down aperture that people uses in order to avoid the negative effect of diffraction. Sometimes you will need to use the lens fully stopped down, but usually those occasions are, like, 5% or less of the actual lens use.

Hope this can help!!

Olivier · Expire: 2015-08-06

Ok Orio.
I followed you very clearly.

I found my adapter is set on F2 and 50mm.
So, I have to underexpose each time I close my lenses, or use slower lenses than F2.
What I didn't catch was that when using a 3.5 lens, I have to underexpose 1/3 when wide open, instead of setting the meter to 0. Now, I will be more cautious.

Orio · Expire: 2012-12-04

poncho_morales · Expire: 2013-02-21

I agree with orio that usually -1/3 compensation per full stop works fine, and usually end up underexposing by 1 stop when going from 5.6 to 8 if my chipped adapter is f2. In some cases though, i have found better results by just using unchipped adapters. so the camera will do the metering with the actual amount of light while stopped down manually in the lens...

Olivier · Expire: 2015-08-06

I agree too with -1/3 compensation per full stop. It works also for me.
Generally, with my f2 chipped adapter, it gives :
f2.8=0 or -1/3
f4=-1/3 or -2/3
f5.6=-2/3 or -1
f8=-1 or -4/3
f11=-4/3 or -5/3
f16=-5/3 or -2
f22=-2

I think I'll try to mask the chip with scotch tape.
And I will also have a look to full manual shooting.

Thanks.

naplam · Expire: 2013-11-30

I think the chip theory is a bit flimsy, I have no chipped adapters and still have to compensate sometimes with small apertures. The chip might just make the problem worse.

bobcominitaly · Joined: 22 May 2009 Posts: 71 Location: Terontola (I)

@Oliver, you probably saw the same post as I did, and I can't find it back either- It was a russian description with pictures of an adapter with a piece of black cardboard mounted on the inside. In the cardboard was a rectangular cutout to serve as a sensorsized mask. You can figure the dimensions: assuming the adapter just can contain a mask for a 24 x 36 mm sensor, you should project a cutout of 0,63 x the dimensions of this mask in the middle of your adapter.

Don't know if it will work, though: according to @Orio's reasoning, probably not. I still think that having both both af confirm and correct metering would be ideal, but I can live with one of the two. I also find that 1/3 of a stop per stop usually is adequate compensation, but most of the time I work in full manual mode, using the camera as a light meter: focus and metering wide open, remembering exposure time, and stopping down by counting clicks. For every click (or two clicks, depending on the lens) I just double the exposure time.

Maybe I'll give this adapter a try: it's not only programmable for aperture, but also for focus correction, and it allows focus trapping on a Caqnon - would turn my 40d into a Pentax.

Olivier · Expire: 2015-08-06

Yes, that's it, and I found it :
http://forum.mflenses.com/someone-could-explain-accessory-for-crop-bodies-t17827,highlight,metering+cropsize+mask.html

and here is the link to the russian site, translate in english by google :
http://translate.google.com/translate?js=n&prev=_t&hl=en&ie=UTF-8&u=http%3A%2F%2Fwww.penta-club.ru%2Fforum%2Findex.php%3Fshowtopic%3D41859%26st%3D0&sl=ru&tl=en&history_state0=

I saw the chip you're talking about.
I think i'll keep on working with my -1/3 stops and see what i'll do when i'll receive my 40D metering glass.

Cheers

greg · Expire: 2012-12-03

I have found that the use of f5.6 with an adjustable aperture value adapter (the focal length value is for reference only as an exif value and doesn't mean anything to the camera) and shooting RAW that I have more than enough leeway to adjust the RAW settings. This with a Canon 300D and XTi

hinnerker · Expire: 2015-08-09

In a german forum we discuss this a long period.

If you take an adapter 2.0 indicating and a lens which is for example with a f-stop <2.8, the results are often in AV mode this way:

!! Different ISO settings on both cams !!

Porst 1.2/55mm Integralmeasure on EOS 5D (AF Chip 2.0)

1.2= 1/500
2.0= 1/250
2.8= 1/180
4.0= 1/90
5.6= 1/45
8.0= 1/15
11= 1/8

same Porst 1.2 on a Canon 400D (AF chip 2.0)

1.2 = 1/1250
2.0 = 1/1250
2.8 = 1/800
4.0 = 1/400
5.6 = 1/125
8.0 = 1/20
11 = 1/8

Nikkor-S.C 1.2/55 an der EOS 40D, AF chip (f2.0), mittenbetont:

f1.2 - 1/800
f1.4 - 1/640
f2.0 - 1/500
f2.8 - 1/250
f4.0 - 1/80
f5.6 - 1/20
f8.0 - 1/8

there are two conclusions.

First of all, the smaller Crop sensor Cams like the 400D cannot handle apertures smaller than 2.8.

The images in a test row are going to be overexposured.

The second conclusion is, there are differences in using a crop cam or a semipro cam like the 5D.

A lot of speculations are running around like the building of the aperture and so on.

Its difficult to find a correct explanation for this phenomenon.

I solve the problem in that way:

Measurement with open Aperture, each aperture stop counting and stretch the time how its correct in physical terms.

1.4 = 1/1000
2.0 = 1/500
2.8 = 1/250

etc...

just the way you did it with an old exposure meter ring.

This works fine for me an the results are consistent.

Orio already did describe the correct electrical coupling with EF Lenses.
And he wrote the reason, why the cam cant give the correct results.
There is no coupling sensor and MF Lens who indicates or set the flag is given for a specified aperture stopp from the user. So the measurement unit always think, its a new open aperture value.. and goes wrong because there is no compensation.

The programming chip of some asiatic sellers only serve the Aperture Value you want to the exifs, but doesnt play any rule for the behavior of the Measurment.

cheers
Hinnerker

pbking · Joined: 27 Dec 2008 Posts: 43

I have no such problems here (5D, C/Y 50/1.7 and Vario-Sonnar 28-85) with using spot metering and then chimping the histogram afterwards, gives an accurate representation of what was captured.

Re: the screen thing. Of course it will compensate meter-wise as Canon SLRs meter through the focus-screen. The -S screens are of higher-precision and can appear 'darker' hence this adjustment in the camera, so it can factor in the screen to its exposure metering algorithms

L