Attention math nerds!

I need your help.

I recently had the idea to start carrying around an empty 35mm slide to hold up and help me "see" photo opportunities. My question is as follows:

How far would I need to hold a 24mm x 36mm cutout from my eye in order to accurately frame the field of view of a 35mm lens on a full frame sensor?

You could easily just take a picture, look at it and figure that out yourself with the slide...

Director's viewfinder

It's obviously more expensive than a paper cutout, but you get focal length readout from 18-200mm or so, and it won't look as dumb

That said; why would you want to do this? It's not a photo opportunity if the camera isn't actually in front of your eye.

CameraMan wrote in post #15416071
You could easily just take a picture, look at it and figure that out yourself with the slide...

Obviously I've considered that but it's much too imprecise for my needs.

Here is an example of what I'm trying to reverse engineer. Notice the tape measure on the right hand side. It tells you the focal length you'd need to reproduce the view in the cutout based on its distance from your eye.

I'd just buy one of those things but they are unavailable

Kolor-Pikker wrote in post #15416099
Director's viewfinder

It's obviously more expensive than a paper cutout, but you get focal length readout from 18-200mm or so, and it won't look as dumb

That said; why would you want to do this? It's not a photo opportunity if the camera isn't actually in front of your eye.

You're saying photo opportunities don't exist unless there is a camera in front of your eye? I think you've been reading too many quantum mechanics books, friend.

I'm sure there is an answer to my question.

Kolor-Pikker wrote in post #15416099
Director's viewfinder

It's obviously more expensive than a paper cutout, but you get focal length readout from 18-200mm or so, and it won't look as dumb

That said; why would you want to do this? It's not a photo opportunity if the camera isn't actually in front of your eye.

Ever sit down and plan out a series of photos that are required to produce your desired vision for an art exhibit or something? If you are just in planning stages, then you may not want to bother lugging a pile of heavy expensive equipment around with you just to see if something is going to look exactly the way you want it to for framing.

Plus, the small cardboard cut outs easily fit in a wallet, and go everywhere with you. If you pass something that is going to be there the next day, you can scout the location with composition in mind. If you can frame it up in a way you like with the simple tool, then you know it is worth your time to come back later with the actual gear that you don't want to have to lug everywhere with you.

Luckless wrote in post #15416460
Ever sit down and plan out a series of photos that are required to produce your desired vision for an art exhibit or something? If you are just in planning stages, then you may not want to bother lugging a pile of heavy expensive equipment around with you just to see if something is going to look exactly the way you want it to for framing.

Plus, the small cardboard cut outs easily fit in a wallet, and go everywhere with you. If you pass something that is going to be there the next day, you can scout the location with composition in mind. If you can frame it up in a way you like with the simple tool, then you know it is worth your time to come back later with the actual gear that you don't want to have to lug everywhere with you.

Thanks, I was too lazy to type all that myself LOL

I have been poking around with google trying to find an article on this that I had read awhile ago that laid things out in nice detail. Sadly I can't seem to find it.

However, another site I did find suggested a simple method of just eyeballing things. Cut out a card of a size you are happy with in the correct aspect ratio for your camera. Place the camera on a tripod with the lens in question on, and aim it at something such that you have two distinct markings just touching the left and right of the frame. Place your card at the sensor plane of your camera (Canon bodies have a mark somewhere around the top, finally something useful for it to do!) and move your head back and forth till the framing in your card matches that of your camera. Measure this to a point on your face that you can consistently line up properly, and you're done. 99+% accurate and calibrated.

Other sources suggest that it is sensor size hole + actual focal length, and then scale them together if you change one. But I don't really see how this can work accurately due to variations in human faces and measuring points. (Measuring out from my cheek is very different from a friend who has far flatter face, which means something like comparing a 28/30mm to 35mm is going to be 'interesting' without some method of calibration)

Luckless wrote in post #15416460
Ever sit down and plan out a series of photos that are required to produce your desired vision for an art exhibit or something? If you are just in planning stages, then you may not want to bother lugging a pile of heavy expensive equipment around with you just to see if something is going to look exactly the way you want it to for framing.

"Opportunity" means a chance or circumstance for something to potentially happen; planning is mutually exclusive to probability and it should have been worded as such.

No one goes just looking for potential locations to shoot unless they already have an idea of what it is they're looking for, assuming a plan of some sort, this obviously limits subject matter to locations, unless you intend to hire models or something.

I don't get it... either you think you're fancy and feel the need to scout a location and frame it up in your head with no future reference, or you're a hobbyist lacking the presumptuousness to lug a 2-3lb camera around and maybe you'll even find a photo "opportunity"... If a dSLR is too cumbersome for you, get an iPhone and set it to a field of view that closely matches a 35mm lens on FF as you say and use that, you'll even have GPS tagging to boot.

You don't need any templates or "aids" to carry around - you already have them with you.
Just make a pistol with the thumb and forefinger of both hands and invert them to make a frame.
Oldest trick in the book.
It's almost identical to 3x2 framing.

philwillmedia wrote in post #15417734
You don't need any templates or "aids" to carry around - you already have them with you.
Just make a pistol with the thumb and forefinger of both hands and invert them to make a frame.
Oldest trick in the book.
It's almost identical to 3x2 framing.

Stand in a small room, is your widest lens wide enough to actually capture what you are considering? Can you be 100% sure if it is worth bothering to come back to that location to capture the image you are trying to get?

I really don't see what is so hard about understanding how such a simple tool could be useful to someone who is interested in using it.

I don't know about the rest of you, but I've often been out (With my gear even) and have spotted a location that looked like it could be interesting to shoot from. (Last one was an old half fallen tree over a river) I left my gear with a friend, and climbed the tree without my expensive camera. Why? To prove whether or not I could safely climb it. (It was out over a river, if I was going for a sudden swim then I didn't want my camera in my hands) If I was confident in the safety of the location, I could then easily return with my gear. However, it wasn't exactly a total cake walk to get up there in the first place, so having an accurate framing tool that would have allowed me to make decisions while I was already up there would have saved me a second trip up. (Turned out that my 28mm, the widest I had with me, was in fact not wide enough to capture both sides of the river where I wanted to, it was close, but not close enough. I nearly dropped an expensive camera in a river trying to get a shot that was physically impossible for me to take with the gear I had at hand.)

I'll take a crack at the OP's question. (But Luckless already posted the right answer.) The explanation below is only valid for fairly distant objects (distance from camera to object much larger than the lens focal length). Probably other simplifying assumptions were made, but it ought to be about right.

The angle of view observed by a sensor of dimension a with a lens of focal length f is
theta=2 arctan(a/(2f))

The angle of view through an aperture of dimension s held at distance r from the eye is
phi=2 arctan(s/(2r))

So, for a paper cutout the same size as your camera's sensor (s=a), to observe the same angle of view (theta=phi) as would be observed by your camera with a lens of focal length f, you must hold your paper cutout a distance r=f away from your eye.

Hope that helps.

Kolor-Pikker wrote in post #15417575
"Opportunity" means a chance or circumstance for something to potentially happen; planning is mutually exclusive to probability and it should have been worded as such.

No one goes just looking for potential locations to shoot unless they already have an idea of what it is they're looking for, assuming a plan of some sort, this obviously limits subject matter to locations, unless you intend to hire models or something.

I don't get it... either you think you're fancy and feel the need to scout a location and frame it up in your head with no future reference, or you're a hobbyist lacking the presumptuousness to lug a 2-3lb camera around and maybe you'll even find a photo "opportunity"... If a dSLR is too cumbersome for you, get an iPhone and set it to a field of view that closely matches a 35mm lens on FF as you say and use that, you'll even have GPS tagging to boot.

Thank you for defining the word "opportunity" for us, you nailed it. Unfortunately you've erred in imposing, perhaps unconsciously, a time limit for "something to potentially happen." Photo opportunities are everywhere, and planning is most certainly not mutually exclusive to spotting them. Philippe Petit saw the opportunity to walk on a wire between the Twin Towers before they even existed.

IMAGE LINK: http://www.flickr.com/​photos/noahfence/80623​01095/

I just tried an experiment using an Olyupus OM-4 with 50mm lens, and a 135 slide holder...I had to hold the slide holder about 1.75" (twofinger widths) from my eyeball to approximate the FOV of the 50mm lens on the camera. And at 1.75" from my eyeball, the edges of the slide holder were so out of focus to prevent any more that a GUESStimate of the FOV! I find it far better to guestimate (with greater accuracy) by spreading the thumb and small finger out and holding that out at arm length and using twice that hand distance as 'normal lens' (50mm on FF)...better than viewing blurry slide holder so close to my eyeball.

Wilt wrote in post #15421577
I just tried an experiment using an Olyupus OM-4 with 50mm lens, and a 135 slide holder...I had to hold the slide holder about 1.75" (twofinger widths) from my eyeball to approximate the FOV of the 50mm lens on the camera. And at 1.75" from my eyeball, the edges of the slide holder were so out of focus to prevent any more that a GUESStimate of the FOV! I find it far better to guestimate (with greater accuracy) by spreading the thumb and small finger out and holding that out at arm length and using twice that hand distance as 'normal lens' (50mm on FF)...better than viewing blurry slide holder so close to my eyeball.

This is a fair point. You can scale the aperture size and distance from the eye together to deal with the focus problem. For a 50mm lens, an aperture of 36 cm (long dimension) held at 50 cm from the eye gives the correct field of view for a full-frame sensor.

Of course, your paper cutout isn't really "wallet size" anymore, unless you care to fold it up.