newbie question about aperture

bogolisk · Expire: 2011-11-18

Newbie question:

My understanding is that the relative aperture (f/1.4, f/2.8, etc.) is equal (or is that proportional) to the focal-length/physical-aperture.

So for a given focal length, e.g. 50mm, at a specific f/X, the physical aperture should be the same regardless whether the lens is a FF, m4/3 or C-mount, right?

If so why would some lens vignette on FF and some not?

Thank you

visualopsins · Expire: 2025-04-11

That is the entrance aperture. There is also an exit aperture for the image circle projected by the lens onto film/sensor.

bogolisk · Expire: 2011-11-18

visualopsins · Expire: 2025-04-11

RioRico · Posted: Thu Aug 12, 2010 5:04 pm Post subject:

Entrance or exit apertures, the size of front and rear elements, all those are irrelevant here.

The aperture is the diameter of the iris opening. An f-stop is a fraction, the ratio between the iris diameter and the focal length, and is used because it is a standard for lenses of vastly different designs. The f-stop of the lens is independent of the size of a frame onto which it projects an image. A 50mm lens with an iris opening of 10mm is f/5, no matter whether it feeds a 6x6 or FF or APS-C or 110 frame.

But lenses of the same focal length and maximum aperture are built to project different size image circles, and at different registers (working distance from lens to frame), depending on the camera for which they are intended. For instance, a FF Leica LTM lens covers a 36x24mm frame (diagonal: 43mm) at a register of ~29mm. And a MF Pentax-67 lens covers a 77x56mm frame (diagonal: 90mm) at a register of ~85mm. An LTM lens *could* be mounted on a P67. But it won't focus to infinity, because the focal points (registers) don't match; and its image circle won't cover the P67 frame. Similarly, Pentax DA lenses (for APS-C cams) have the same register as FA lenses (for FF cams); but an APS-C frame is half the size of FF, and its image circle just can't reach FF corners.

That is vignetting -- the image circle doesn't cover the frame. CCTV lenses cover a small frame at a short register; even with a dSLR of the same register, that CCTV image circle won't cover the larger frame. The diameter of front and rear elements is irrelevant re: vignetting; only their focal characteristics matter. No matter the aperture, no matter the size of the exit glass, the rear elements are designed to project an image circle greater than the frame's diagonal, focused at the register distance.

bogolisk · Expire: 2011-11-18

Sorry, I'm a newbie and slow...

Let's pick 2 lenses: an FF one, Rokkor 50mm; an 4/3 one, ZD 50mm.

Both mounted on a m4/3 cam, e-p1 with proper adapter to match the register distance.

Will they have the same angle-of-view? I.e. will they take equivalent image ?(i.e. same geometry)

The image projected by the Rokkor is for FF thus bigger and cropped down by the 4/3 sensor. However, the image projected by the ZD is indeed for the 4/3 sensor and should be smaller. But if the final images are equivalent then, what did the cropping? the 4/3 ZD lens?

Thank you

RioRico · Posted: Thu Aug 12, 2010 6:22 pm Post subject:

I'll try some counter-examples.

Let's take that Rokkor 50, and perfect adaptors. We mount in on a Canon FF cam and SNAP! We then mount it on a Canon APS cam and SNAP! We then mount it on your m4/3 cam and SNAP! The lens projects the same image circle on each frame, in each format. But the FF frame diagonal (and thus the minimum diameter of that image circle) is ~43mm, the APS diagonal is ~30mm, and the m4/3 diagonal is ~22mm. The smaller sensors see less of the Rokkor's image circle; they have narrower FOV's. The sensor (frame) does the cropping. The geometry of the image doesn't change; perspective, the relationships of objects in the image, doesn't change.

Cut a picture from a magazine. Draw a 36x24mm rectangle on it. That's a FF frame. Inside that, draw a 24x18mm rectangle, half the FF size. That's an APS frame. Inside that, draw a 18x12mm rectangle, half the APS sice. That's a m4/3 frame. (Not exactly, but close enough.) The picture has not changed. But each smaller sensor sees a smaller slice of it, with less FOV.

Now I take a Zeiss 100mm lens I salvaged from an old 6x9 folder, and I put it on bellows, mount it on my APS-C Pentax K20D, and SNAP! Its image circle covers a 101mm diagonal frame. I replace it with a Meyer Trioplan 100mm lens, and SNAP! Its image circle covers a 43mm FF frame. I replace it with a Pentax DA 100mm lens, and SNAP! It's image circle covers a 30mm APS-C frame. Now I look at the pictures, each shot with the same aperture at the same subject at the same distance. THEY ARE ALL THE SAME!! Perspective (geometry) results from lens-to-subject distance, not focal length.

bogolisk · Expire: 2011-11-18

I see. If I understand what're you saying, for the same focal length, the image projected by the m4/3 or APS-C lens is only part of the image projected by the FF lens.

Thank you.

visualopsins · Expire: 2025-04-11

sichko · Joined: 20 Jun 2008 Posts: 2475 Location: South West UK

Seele · Joined: 17 Apr 2009 Posts: 742 Location: Sydney Australia

Let me give this a try.

A lens does not produce a flat image, but a three-dimensional, upside-down, miniaturised image of what is before it. Say, a subject closer to the lens will have its correspondent image further away from the lens, and vice versa. The action of focussing is to move the film plane so that it hits the image of the subject you want to focus on.

This explains why some lens/camera combination cannot reach infinity, as the film plane cannot be held close enough to the back of the lens, where the images of subjects at infinity reside.

The determining factor of the degree of miniaturisation is the focal length. Say, with a 50mm lens, the image of a certain subject measures 5mm; with a 100mm lens it will be 10mm, and with a 25mm lens it will be 2.5mm. That is inalterable. Same goes for the f/number, i.e. relative aperture which is dependent on the focal length.

So, if you use a certain format size, say 24mm X 36mm, a 50mm can be considered as a "standard lens". Switch to a 25mm lens, everything will become correspondingly smaller, so the same format area will encompass more of the scene, thus a "wide-angle lens". Switch to a 100mm lens, everything will become correspondingly bigger, this same format area will encompass less of the scene, thus "long-focus lens".

This is the "angle of view" which is dependent on the format size. Say, if you use a larger format size, say, 6cm X 9cm (nominal, actually a bit smaller), a 100mm lens could very well give about the same angle of view, as compared to a 50mm lens on the 24mm X 36mm format.

What is important is this: nowadays with the advent of cameras which can take lenses never intended to be matched with them, another factor, "angle of coverage" comes into play. Angle of coverage is an inherent attribute of lens design.

Look at the previous example again. On the 6cm X 9cm format, a 100mm lens would give an angle of view within its format area, similar to that of a 50mm lens on the 24mm X 36mm format. But if you take a 100mm lens from, say, a 35mm SLR and put it to this 6cm X 9cm camera, you get serious vignetting: you get something like looking through a tube. As the lens was originally designed to cover the 24mm X 36mm format, the designer saw no benefit at all to make an angle of coverage wide enough to encompass an area which will accommodate a 6cm X 9cm format area.

To summerize:

1. Actual image size is dependent on the focal length only.

2. Lenses of the same focal length will have the same angle of view on the same format size.

3. Lenses of the same focal length will have different angles of view depending on the format size: the larger the format, the more it takes in, so the greater the angle of view.

4. Angle of view is different from angle of coverage. A lens needs to have sufficient angle of coverage for the format size first.

Hope this makes sense.