math question: test shooting SLR for Super 8

[grainy]

Hi folks - I'm hoping to take my SLR out and use it to "previsualize" some night shots that I'd shoot with the super 8 later on. My hope was that I can set the SLR to more/less replicate the Super 8.
The way I see it there are three elements to consider:
1 - The difference between the shutter speeds (30/60 on the SLR vs. about ~40 shooting at 18fps),
2 - the ISO, I have two notches settings 100 and 200 on the SLR as opposed to 160 rating for the TriX.
3. The cameras' lens speeds. The SLR has a stunningly slow 3.5 zoom lens, while my Sankyo Super 8 has a blazing fast 1.2 lens.

I'm lost as to how to reconcile these all. I know it's not an exact science, and I have no idea whether the shutter shape of the Sankyo also affects this...
Any advice/help would be much appreciated!
G

[carllooper]

Use a light meter instead of the SLR.

OR

If the following gives the required response to a given scene:

ISO = 100
fstop = f/5.6

then I think the following will also give the same response (apart from difference in DOF):

ISO = 200
fstop = f/8.0

If the above example is correct then the following is mathematically correct for the above example:

ISO = 160
fstop = f/5.6 + 0.678 stops

since Log10(160)/Log10(2) - Log10(100)/Log10(2) = 0.678


And the difference in lenses (3.5 and 1.2) is given by:

Log10(3.5^2)/Log10(2) - Log10(1.2^2)/Log10(2)
= 3.08 stops


Carl

[grainy]

Hi Carl!
I've totally lost you on the math [this is why I ask
Using your equasions, if I shoot wide open with my f1.2 lens using 160 iso TriX, what's the equivelent setting on the f 3.5 SLR? I can modify shutter speed of the SLR to match...
G

[super8man]

Here are the basics you need to consider in your question:

And for the record, your question is: I want to use my 35mm camera to "preview" what a scene would like like in super 8. How do I do that?

First off, you are not really using a 35mm, you are using a digital SLR. The sensor size is NOT 24mmx36mm.

Your sensor size is unknown based on what you wrote - check out here:

http://www.cambridgeincolour.com/tutori ... r-size.htm

and be sure to read this section:

As sensor size increases, the depth of field will decrease for a given aperture (when filling the frame with a subject of the same size and distance). This is because larger sensors require one to get closer to their subject, or to use a longer focal length in order to fill the frame with that subject. This means that one has to use progressively smaller aperture sizes in order to maintain the same depth of field on larger sensors. The following calculator predicts the required aperture and focal length in order to achieve the same depth of field (while maintaining perspective).

You are using a larger sensor than that of super 8. Given than, you will need to have a longer focal length on your digital SLR compared to that on your Super 8 camera.

Let us know what SLR you are using and then we can do some math...

[grainy]

First off, you are not really using a 35mm, you are using a digital SLR. The sensor size is NOT 24mmx36mm...Let us know what SLR you are using and then we can do some math...

Hello super8man -- actually I am indeed talking about a regular ol' 35mm SLR that shoots regular old film.
I don't own a DSLR. I supposed it'd be handy but every year the state of the art is obsolete, whereas... [bla bla bla insert usual pro-film rant here] ;)

Do you need more stats on my SLR?
thanks much!
G

[super8man]

Hey, thanks - the reason I thought you were is that you said you had only 2 ASAs available on your 35mm...in fact, you can run any ASA since that is set by the film. If you can't run different ASA, I recommend you get a new camera! (any old Pentax K1000 will meter any ASA)

I don't have a depth of field scale handy for your Sankyo with a f1.2 lens but the DOF depends on your "zoom" or focal length on that camera. (Edit - found one!)

Super 8 film measures 4.01mm x 5.79mm resulting in a diagonal of 7.04mm according to Pythagorus.

Your SLR has a frame size of 24mm x 36mm resulting in a diagonal of 43.27mm.

Comparitively, to observe a similar scene in both format, you must multiply your super 8 camera zoom lens setting by 6 to get a similar view on a 35mm camera. (Edit - check my note at the end, I think the correct conversion factor is 4X, not 6X).

Now assuming you use "9mm" as your focal length on the Sankyo, this is "roughly" 54mm on a SLR.

The question then comes up of the f-stop and the problem of what focus setting your lens will be on.

On your Sankyo super 8, f1.2 on a 9mm lens yields a DOF of 6.32 feet through 24.5 at 10 foot on your focus. If you focus at 30 feet, your depth of field is 10.87 feet through infinity.

A similar zoom setting in your 35mm SLR camera is 63mm (9mm x 6).

To get a depth of field similar to 6.3 feet to 24 feet on a 63mm SLR lens, you need to have an f-stop of f25.

Notice I have not brought up ASA or film speed...basically, you have to figure out which things you have control over and which ones you don't.

So, on your Sankyo, you are shooting wide open at 18fps (which is 1/30th of a second for a 220-degree shutter)...assuming you set the camera to 10-feet on the focus and the lens is set at 9mm, everything from 6.32 feet to 24.5 feet will be in focus, regardless of what ASA film you use.

Same goes on your SLR: you set the focus to 10 feet, set your f-stop to f25 and your zoom lens to 63mm - you have now matched the DOF field.

PS - I may have screwed up on the ratio of super 8 to 35mm (the 6X factor) - perhaps someone can help me out? It may only need to be 4X which then makes the zoom lens setting at 36mm which then makes the f-stop on your SLR a much more reasonable f8.

[grainy]

woah, that's some serious math. It is blowing my non-math mind.

I forgot about DOFocus -- key in night shots (but also a bit of a wildcard because artificial light pops so much)

I gotta look at my SLR again (it's not in front of me), but I'm pretty sure the ISO is only settable by increments like 100, 200, 400. So I'd have to compensate for the 160 ISO of TriX somehow. I think....
How does that fall into the equasion?
G

[super8man]

Use 200ASA as "close enough" and use the 4X factor...pretty sure my 6X is wrong...but need to think about it more as to how the conversion works...I never really compare since I simply shoot what I like as I see it though the viewfinder of any camera I use.

[carllooper]

Hi Carl!
I've totally lost you on the math [this is why I ask
Using your equasions, if I shoot wide open with my f1.2 lens using 160 iso TriX, what's the equivelent setting on the f 3.5 SLR? I can modify shutter speed of the SLR to match...
G

At f1.2 the S8 lens is admitting 3 stops more light than the SLR at f3.5.

Log10(3.5^2)/Log10(2) - Log10(1.2^2)/Log10(2)
= 3.08 stops

To compensate for that you would EITHER need to use 1280 ASA film on the SLR:

Compensation / Required Film
0 stop 160 ASA
1 stop 320 ASA
2 stop 640 ASA
3 stop 1280 ASA

OR adjust the shutter by 3 stops (doubling the exposure time by two for every stop of compensation)

For example, if the S8 shutter were 1/48 sec (180 degree shutter @ 24fps) then

Compensation / Required Shutter
0 stop 1/48 sec
1 stop 1/24 sec
2 stop 1/12 sec
3 stop 1/6 sec

So for shutter compensation the answer is:

1. Use 160 ASA film (or nearest, eg. 200 ASA film)
2. Use a shutter of 1/6 sec (based on 180 degree shutter @ 24fps)

Note that setting the ASA/ISO on the camera is simply to ensure the internal light meter is calibrated to the film you are using. If using an external light meter, that setting is irrelevant.

[grainy]

cool, carl, thanks!
So if I shoot Tmax 400 with the SLR, that buys me an extra stop -- but conversely, I'm shooting 18fps rather than 24fps with my Sankyo, so that sorta balances it out?
So to err on the safe side, I should probably shoot using Tmax 400 at shutter speed 1/8 (I don't have a 1/6 shutter option).
Does that sound right?
Thanks for your help!!
G

[carllooper]

cool, carl, thanks!
So if I shoot Tmax 400 with the SLR, that buys me an extra stop -- but conversely, I'm shooting 18fps rather than 24fps with my Sankyo, so that sorta balances it out?
So to err on the safe side, I should probably shoot using Tmax 400 at shutter speed 1/8 (I don't have a 1/6 shutter option).
Does that sound right?
Thanks for your help!!
G

If using 400 film then:

The difference in stops betwen 400 and 160 is given by (using log10 since most calculators mean that by log):

log(400)/log(2) - log(160)/log(2) ~= 1.322 stops

So we only need to compensate for 3 - 1.322 ~= 1.678 stops ~= 1.7 stops

Do you know the shutter angle? Lets assume it's 180 degrees (we can always rerun the computation for a new shutter angle) then the exposure interval compensation for 18fps would be somewhere between 1/18 sec and 1/9 sec, but closer to 1/9sec:

0 1/36 sec
1 1/18 sec
2 1/9 sec

But we can work it out more exactly:

log(1/36) / log(2) = -5.169925001

We now add 1.7 stops

-5.16992501 + 1.7 = -3.469925001

And convert back to exposure interval:

2^-3.469925001 = 0.090250266 secs ~= 1/11 secs

Answer is: 1/11 secs

Or nearest SLR setting.

Carl

ps. As per super8man's insight the DOF will be incorrect. To match the DOF, shoot with a wider lens and crop the resulting image back to the dimensions of S8 relative to 35mm. And since the shutter time is different, to compensate, have your subject move in slow motion

[grainy]

If using 400 film then...
Answer is: 1/11 secs
Or nearest SLR setting.
Carl
ps. As per super8man's insight the DOF will be incorrect. To match the DOF, shoot with a wider lens and crop the resulting image back to the dimensions of S8 relative to 35mm. And since the shutter time is different, to compensate, have your subject move in slow motion

ha ha! Excellent! So I can load up some Tmax 400 and set at ~1/11 shutter speed and account for the slow motion.
Thanks much carllooper and super8man! Your math skills are <i>tres formidable</i>!

[super8man]

To be clear, the picture from the SLR will never be the same as the super 8 since super 8 requires each frame to move and your brain fills in the blanks. I daresay no matter what you do the pictures will be incredibly different. And let's not even think about how you plan on looking at your super 8 footage and being able to compare it to a still frame.

Also, your camera most definitely is a XL camera meaning it has a 220 degree shutter and therefore has a shutter speed of only 1/30 of a second. Only very old super 8 cameras with slow lenses (f1.8 or worse) are going to have a 1/40th or so shutter speed.

Best of luck.

[grainy]

Also, your camera most definitely is a XL camera meaning it has a 220 degree shutter and therefore has a shutter speed of only 1/30 of a second. Only very old super 8 cameras with slow lenses (f1.8 or worse) are going to have a 1/40th or so shutter speed.

Hi super8man - yes, it's a sankyo XL 420 sound.
So with a 1/30 shutter speed for the super 8, that puts me at an even slower speed on the SLR. Currently I'm at "1/11", which on my camera I rounded down to 1/8. But knowing this am I really looking at more like a 1/4?

G

[super8man]

Only for obtaining the same depth of field...you still need to shoot at 1/30 to capture what the motion would look like in a still frame.

Basically, you can't do what you are trying to do unless you change your needs.