Aperture & Focal Length comparisons

digital&film · Oct 17, 2014

I'm coming from APS-C digital and 35mm Film.

What I'm curious about is (with Mamiya RZ67) the focal length is roughly equivalent to "half", ie, 110mm ~ 55mm "normal lens" in 35mm.

Does this also hold true with Lens Aperture as well.. ie, F2.8 ~ F1.4 in 35mm?

:confused:

Karl A · Oct 18, 2014

Hi there, the f number is affected by the focal length of any lens. The reason is, it is actually a ratio which is calculated: focal length / aperture diameter. So the longer the lens, the higher the f number at a given aperture diameter.

I hope that helps, and I will let the real experts here chime in with any further mathematical analysis.

bdial · Oct 18, 2014

Generally the "normal" focal length is approximately equal to the diagonal of the format, so for 6x7 that is about 92 mm and 43 mm for 35. That the 6x7 normal length is around double is really coincidence, the next common film size is 4x5 inches with a diagonal around 160 mm, though often what's used for "normal" for 4x5 is a little shorter than that, 130 mm + or - is common.

The f number of a lens is the focal length divided by the diameter of the opening. Once you start getting into focal lengths over 100 mm, the diameter needed to get to an f/2.8 gets you into very big pieces of glass. Making fast lenses is a lot more practical with smaller lenses like 50's on 35mm cameras.
Often the larger cameras are not hand-held, so a fast lens isn't quite as useful as it might be on a 35, not to mention that the expense of a sharp 160 mm f 1.4 would be astronomical and the lens would be enormous.

snapguy · Oct 18, 2014

confused?

If you are a trifle confused by it all, join a very large club of us who am confoosed. And, you can ask about Circles of Confusion and really get balled up. Good luck.

MattKing · Oct 18, 2014

f/2.8 on a 90mm lens transmits the same amount of light as f/2.8 on a 50mm lens, or a 35mm lens, or a 28mm lens, or ....

If the correct exposure for a scene is f/2.8 at 1/125 second using a 50mm lens for your 35mm camera, than it will be the same for a 110mm lens for your RZ67.

However, as others have said above, in order to achieve that f/2.8 in the world of 110mm lenses, you need a physically bigger aperture and bigger pieces of glass.

This is because f/stops are ratios, and they take into account the focal length.

Depth of field will be affected with the change in formats.

shutterfinger · Oct 18, 2014

The aperture on any lens is the focal length divided by the diameter of the aperture opening as viewed through the entrance pupil of the lens.
A 110mm lens with an aperture opening of 110mm as viewed through the front element of the lens is an f1 lens regardless of format.

A f4 lens on an APS camera will allow as much light to the image plane as an f4 lens on 6x6, 6x7, 6x9, 4x5 or any other format will. This makes exposures consistent.
f4 on APS will have a larger depth of focus than it will on 6x7 which will be larger than 4x5. The larger the format the shallower the depth of focus at any aperture, its one of those things you learn to deal with and use to your advantage when going to a larger format.
http://www.largeformatphotography.info/ has two pdf files under Taking The Picture called Introduction to Depth of Field (a should read) and Depth of Field in Depth (optional read).
There are several on line DOF calculators that you can plug focal lengths and f stops into to see the differences of DOF in different formats for any given focused distance.

Slixtiesix · Oct 18, 2014

I don´t know for 6x7, but as a rule of thumb for 6x6, divide the focal length of the MF lens into half, add 10% and substract two aperture stops. So a 100/4 lens in 6x6 is like a 55/2 in 35mm.

Nuff · Oct 18, 2014

I think you are asking about the whole equivalence question that is popular on the digital forums if I'm not mistaken. I wouldn't worry about it too much.

If you want to know DOF for particular lens, there are calculators around that you can use to get the answer you are after.

Peltigera · Oct 18, 2014

"Does this also hold true with Lens Aperture as well.. ie, F2.8 ~ F1.4 in 35mm?"

No. The focal length and aperture are physical properties of the lens and not affected by what you attach it to. It is the angle of view that changes when a particular lens is moved from 6x7 to 35mm to APS-C. Everything apart from angle of view stays the same.

gone · Oct 18, 2014

That's quite a move, from 110 to 6x7. Whatever the focal lengths turn out to be, you're really going to enjoy those big negs. Now to line up someone to carry that thing! I suggest some serious reading of The Adventures of Huck Finn.

ME Super · Oct 18, 2014

A normal focal length lens for a given format has a focal length roughly equivalent to the diagonal of the format in question. For 35mm, this works out to about 43mm. 50mm is pretty close, so it is considered to be a normal length lens. For 6x6, normal works out to be around 80mm. 8x10 works out to have a normal focal length of around 325mm (12.8 inches). Depth of field depends on the diameter of the aperture, not the f/stop, so longer lenses have shallower depth of field for a given f/stop because the aperture is bigger. For example, at 50mm, f/4 has a diameter (as seen from the front lens element) of 12.5mm. At a focal length of 100mm, the aperture diameter at f/4 is now 25mm, resulting in depth of field being shallower with the 100mm lens than with the 50mm lens.

ic-racer · Oct 18, 2014

digital&film said:
I'm coming from APS-C digital and 35mm Film.

What I'm curious about is (with Mamiya RZ67) the focal length is roughly equivalent to "half", ie, 110mm ~ 55mm "normal lens" in 35mm.

Does this also hold true with Lens Aperture as well.. ie, F2.8 ~ F1.4 in 35mm?

:confused:

Yes, as the focal length doubles, the aperture is one-half (assuming roughly the same size opening in the lens). So two stops less light. In practical term, if you want a fast lens on a medium format camera the lens is going to have to be pretty big. The corollary of that is the fact that an f0.9 lens for 8mm movie camera is possible.

digital&film · Oct 18, 2014

Slixtiesix said:
I don´t know for 6x7, but as a rule of thumb for 6x6, divide the focal length of the MF lens into half, add 10% and substract two aperture stops. So a 100/4 lens in 6x6 is like a 55/2 in 35mm.

That's what I was wondering about.. so will the depth of field at f4 or f8.0 in 6x7 be more like f2 or f4 in 35mm??? :confused:

Gerald C Koch · Oct 18, 2014

"In optics, the f-number (sometimes called focal ratio, f-ratio, f-stop, or relative aperture) of an optical system is the ratio of the lens's focal length to the [physical or actual] diameter of the entrance pupil. It is a dimensionless number that is a quantitative measure of lens speed, and an important concept in photography. The number is commonly notated using a hooked f, i.e. f/N, where N is the f-number"

John Koehrer · Oct 18, 2014

^^^^^
Shouldn't that be: fl/D=f ?
IE: 100/50=2

John Koehrer · Oct 18, 2014

momus said:
That's quite a move, from 110 to 6x7. Whatever the focal lengths turn out to be, you're really going to enjoy those big negs. Now to line up someone to carry that thing! I suggest some serious reading of The Adventures of Huck Finn.

It's focal length, not film size.

RalphLambrecht · Oct 18, 2014

snapguy said:
If you are a trifle confused by it all, join a very large club of us who am confoosed. And, you can ask about Circles of Confusion and really get balled up. Good luck.

What is a circle of confusion? a bunch of photographer sitting around discussing depth of field?

RalphLambrecht · Oct 18, 2014

digital&film said:
I'm coming from APS-C digital and 35mm Film.

What I'm curious about is (with Mamiya RZ67) the focal length is roughly equivalent to "half", ie, 110mm ~ 55mm "normal lens" in 35mm.

Does this also hold true with Lens Aperture as well.. ie, F2.8 ~ F1.4 in 35mm?

:confused:

I always take film diagonal as normal;half that as wide angle and double the diagonal as portrait lens.works in any format. i have no suggestion on aperture other than; buy the fastest lens you can afford and stop it down to f/8-11 for max quality.

Gerald C Koch · Oct 18, 2014

John Koehrer said:
^^^^^
Shouldn't that be: fl/D=f ?
IE: 100/50=2

That is what the definition says.

John Koehrer · Oct 19, 2014

Wow, unnecessarily complex definition isn't it?
English can be so confusing.

cramej · Oct 20, 2014

digital&film said:
That's what I was wondering about.. so will the depth of field at f4 or f8.0 in 6x7 be more like f2 or f4 in 35mm??? :confused:

No. For example, if you have a 50mm lens on 35mm and a 50mm lens on MF, they will have similar DOF but different Angle of View so it looks like it is a totally different lens. F4 on a 50mm lens is F4 no matter which camera you have it attached to and DOF is calculated from aperture size and focus distance, not camera format. You cannot compare lenses of different focal lengths and try to equate apertures so 110mm lens ≠ 55mm lens.

Peltigera · Oct 20, 2014

The best advice, really, is to try the lens and see how you can make it work for you. You cannot make MF work like 35mm nor like digital - they are different in too many ways.

ic-racer · Oct 20, 2014

Depth of field is a human perception, not any physical property of a lens. The math to approximate it [DOF] requires many variables to obtain a result, making comparisons between this, that and the other thing, somewhat imperfect.

Ken Nadvornick · Oct 20, 2014

Here's a rough chart showing the equivalent normal focal lengths for 40 different film formats, based on the normal for the 35mm format. Displayed are horizontal, vertical, and diagonal orientations. Diagonal orientation is the dimension most people usually refer to in a discussion of equivalency.

The chart is rough because it currently relies on nominal measurements, not actual image sizes. Meaning, the 6x6 value was calculated from a nominal 60x60mm, and not the actual 56x56mm. I need to find a source of actual measurements for those formats I cannot directly measure myself before fine-tuning the output. But as it is it's close.

The chart was generated using the diagonal measurement for a 24x36mm frame as copied/pasted directly from the Windows calculator.

Ken

ic-racer · Oct 20, 2014

Ken Nadvornick said:
I need to find a source of actual measurements for those formats I cannot directly measure myself before fine-tuning the output.

Maybe just leave it at nominal measurements unless you can measure the film images yourself. Not all cameras of the same format have the film gate size. I made my table based on actual measurements of negatives exposed by my specific camera bodies. Also, with large format, one has to determine for if the exposed area produced by the film insertion slots are going to be part of the diagonal measurement or not.

Aperture & Focal Length comparisons

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