[jt_cph_dk]

Hi all, I need help with math ...

What tele lens would turn my AE 67 Finder Meter into a 1° spot meter? Who can do the math?

Pentax 67II (Medium Format) Film Size: 56 X 70 mm.

Frame: 100%=70 mm.
Spot: 15% =10.5 mm.

w/ AE 67 Finder Meter:
Visible horizontal view: 90%=63 mm.

Normal lens (105 mm.): 37°
Super-Tele lens (400 mm.): 10°
Super-Tele lens (800 mm.): 5°

Example:
Pentax 645N: 1° with 344 mm. lens (Film size: 45 X 60 mm./Spot dia. 6 mm.)
Angle with a normal lens: (apx) 80 mm./04.58°
(Source: Comparison of on-camera Spot Meters)

---------- Post added 29th Jan 2017 at 11:12 AM ----------

With a 1° spot meter: What 1° am I seeing (certainly not 1° of say a 400 or 800 mm. tele lens)? 1° of a normal lens (37°)?


Note 2. Feb. 2017: I found out, that Asahi already did the math and made an illustration.

Page 27 in the Digital Spot Meter Manual (http://www.cameramanuals.org/pentax_pdf/pentax_digital_spotmeter.pdf) is interestingly pointing exactly to the question of this thread:



---------- Post added 1st Feb 2017 at 01:14 PM ----------

Added comparison to AE 67 15° spot:



... make your own conclusions.

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Last edited by jt_cph_dk; 02-01-2017 at 09:12 PM.

[jt_cph_dk]

I understand, that I would always be seeing 15° in the spot, through whatever lens I mount on the camera.

[Silent Street]

I don't think it is sensible (or achievable!) to whittle down the in-camera spot to a perceived (rather than actual) degree of coverage.
Invest in a proper hand-held multispot/incident meter as this will give you a large range of opportunities to fine tune the exposure — much, much more than an in-camera meter can provide for.

[jt_cph_dk]

Well, I just did invest in a Minolta IV meter (in part because Im getting into flashwork with the 6X7 system) and an S-M-C Takumar 6X7 4/400. I could get the 5° spot (Minolta) viewfinder at some point and I probably will. The next year the ‘new‘ super-tele and my RC 2X mounted on 67II with the AE meter or the waist level finder will be my main photographic equipment—Im not suggesting using the tele for a spot meter and then changing the lens to shoot with a 55, 75 or 105mm (or whatever)—rather, Im asking: Where is the limit for a 1° spot meter? I don´t see how it would work at 100-200M/infinity range? At some point (for the 645 at 344mm) a tele lens gives a smaller spot, than a ‘normal‘ 1° spot meter does.

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Last edited by jt_cph_dk; 01-29-2017 at 05:04 PM.

[jt_cph_dk]

Here is a photo taken with my 55mm 67 lens—I can see how a 1° spot could have been useful—and then I pasted a 400mm test shot at 80M (K7, grey area is about 6x7 800mm format) into it, that I took the other day—would a 1° spot meter be of any use, better than the AE prism 15° spot in that small area?

[Digitalis]

you can get handheld spot-meters, I use a pentax modified zone VI meter - unfortunately these meters still command hefty prices due to the modifications. But if you can find one in good shape, it is a worthy investment if you shoot a lot of film.

[jt_cph_dk]

The math would be: horizontal field of view=37° (normal 105mm)/100 (1° spot)<horizontal field of view=5°(tele 800mm)/6 2/3 (15° spot)? Correct?

Let me answer that myself: No, it is not correct, but it does begin to describe the question.
Oh, Im terrible at math, have always been.

We know that a hand held 1° spot meter (normal) horizontal field of view is 37°
When we divide that by 100=0.37°

We know that at some point (distance) the diameter of this spot becomes smaller than the 15° TTL spot as viewed through a telelens of X mm.

... I don´t know how to do it?

What I can do is: I can draw it, illustrate it by applying the horizontal fields of views of a 400mm 6X7 through to 1000° 6X7 (10° - 4°) and then measure when the spot is larger than the 1° spot.

Or I can skip all that and make a guess based on the 645=344mm ... ad 1/3=500mm 6X7
Or ask some one with a hand held 1° spot meter experience: When shooting with a super-tele does it make sense to use such a meter?

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Last edited by jt_cph_dk; 01-31-2017 at 02:18 AM.

[Digitalis]

When shooting with a super-tele does it make sense to use such a meter?

If the FOV of the lens is bigger than the meter FOV then yes, absolutely. However, getting a 1 degree spot meter is cheaper than getting a lens with a 1 degree FOV on a 67 camera. And will likely produce metering values that are far more precise than the CWA metering the prism provides. Also the metering cells in a spot meter can be much more sensitive, and able to meter over a wider exposure range than the prism light meter can.

a tele lens gives a smaller spot, than a ‘normal‘ 1° spot meter does.

I think you're being a bit pedantic over your exposure parameters. The need for meter coverage relative to the lens FOV isn't entirely needed. Unless you are shooting subjects that are backlit or against shadow, you can get accurate ambient light exposure values using a simple meter equipped with a lumisphere.

[jt_cph_dk]

Thank´s :-) I have had a bit of fun making this sketch/illustration/answer of the question ...



---------- Post added 31st Jan 2017 at 02:29 PM ----------

shooting subjects that are backlit or against shadow

I will be shooting backlit images. Im going to be shooting an eagles nest in the woods, top of a tree and at about 200M distance (restricted area), this winter/spring. That´s why I just bought the 400mm 6X7 to combine with my RC 2X for a 800mm (aka 5° tele). This is the nest (aps-c 400mm):

[IMG][/IMG]

[Digitalis]

From the look of all the lichen growing on those trees, you won't be wanting for a 18% grey exposure target.

[jt_cph_dk]

From the look of all the lichen growing on those trees, you won't be wanting for a 18% grey exposure target.

Yeah, elaborate on that, please :-) zone V ... on that day it was overcast—Im planning on going on days with more light and earlier in the day, around noon, when the scene is lit from this side (south) of things. Im not at all finished planning for this shot. That day: ISO100, f16, 1/5sec ... I need 5-6 f-stops of light. I added a little bit of light and contrast to this in edit, not much. The lens combo was Super-Takumar 6X7 4/200+SMC 67 RC 2X, my ‘new‘ Takumar S-M-C 4/400 will yield better results

---------- Post added 31st Jan 2017 at 04:08 PM ----------

Here is the RAW with and without edit:

[UncleVanya]

I'm confused. I thought a 1degree spot meter was an absolute measurement covering 1 degree. The FOV of the spot meter is 1 degree - where did 37 degrees enter the picture?

Put another way. If I use a lensatic compass or a more accurate version and mark two points in the distance 1 degree apart the diameter of the area the meter covers should roughly be the same - right?

Thus a lens with a fov of X * 0.15 = 1 will give the answer to the fov needed of the lens to match the original question. (X = 1/.015 = 6.66 degrees) which is a very large telephoto on 6x7.

[RGlasel]

From the look of all the lichen growing on those trees, you won't be wanting for a 18% grey exposure target.

Ba da bing, ba da boom! That one comment made my day.

[Silent Street]

Or ask some one with a hand held 1° spot meter experience: When shooting with a super-tele does it make sense to use such a meter?

Yes. Chapter and verse could be written and discussed about the subject of spot meters, in-camera or hand-held.

In answer to your question, it is no less applicable with a super tele than it is for an ultrawide lens.

Of those trees; if a spot meter had been used (one with mean-weighted averaging over the range of spots), there would be no need at all for that horried fiddling in Fauxtoshoppe; there is no gradation of tone; it is extremely flat and dull.

Spot meters are not actually the tool of choice for all backlit subjects in a universal sense (for close-in subjects, an incident meter is better). Further, many spot meters e.g. Sekonic, interpret middle-grey as 16% (spot/reflected) and 12.6% (incident/invercone). What is the reading with the Minolta meter? So there is a bit more skill involved rather than interpreting the meter's results as gospel; a starting point, yes, but not gospel.

I don't think that Takumar is anything special. On the other hand, the 300 EID tele matched to a 67 is lip-smackingly good. Kind of an understatement given how well it stacks up against the old Taks. You might be able to hire this for your task.

[Digitalis]

Ba da bing, ba da boom! That one comment made my day.

i'm glad someone got that - you work with what you're given, and people who know what to look for will never go home short handed.

Spot meters are not actually the tool of choice for all backlit subjects in a universal sense (for close-in subjects, an incident meter is better). Further, many spot meters e.g. Sekonic, interpret middle-grey as 16% (spot/reflected) and 12.6% (incident/invercone). What is the reading with the Minolta meter? So there is a bit more skill involved rather than interpreting the meter's results as gospel; a starting point, yes, but not gospel.

Not Gospel, but this is also dependent on the quality of said spotmeter. The Pentax modified Zone VI meter has improved baffles in it to reduce veiling flare and enhanced light filtration to eliminate UV and IR wavelengths from impairing its accuracy. Minolta spot meters are pretty accurate in my experience. Nearly all Sekonic, meters have a propensity to be 1/8th of a stop over middle grey - I suspect this is because of inadequate UV filtration.

It is a shame Pentax didn't incorporate OTF metering in the Pentax 67, there is a surprising amount of empty space around the mirror box in that chassis.