Are 35mm legacy lenses 'slower' on m4/3?

One of the attractions of the m4/3 format is the possibility of using a wide range of older lenses originally designed for 35mm film cameras. This forum contains many examples of the impressive results obtained using such lenses, and users often refer to advantages of using 'fast' old lenses which have an effective focal length of double the 35mm equivalent because of the 'crop factor' of the smaller m4/3 sensor. My question is this: if most of the light gathered by legacy lenses is wasted because the sensor only occupies a fraction of the image circle, is the lens not 'slower' than the f-number suggests? In other words, the actual focal length of the lens is the same, but the effective diameter is smaller, so the effective f number is larger. IF my analysis is correct, the legacy lenses are not quite such good bargains. I can't get my head around this issue, and would appreciate any thoughts from the experts on the forum. Cheers,
Mike.

Given the 2x crop factor, compared to 35mm full frame, your lens will be for all practical purposes 2 stops slower.

but the story does not tell everything: given the crop factor, you have no soft corner issues. If you do, you can easily post process. Also, the focus is much easier than on anything else mentioning the word "film", so that full speed becomes an option. The option.

I have a f1.5/50 rangefinder lens. I hear that it was commonly used between f4 and f8. on the G1, it is always wide open.

The f-number deals with light per unit of area, so the exposure does not change with the sensor size -- you still get the same exposure per square centimeter.

As an analogy, if you use the lens on a full-frame sensor and later crop in the computer that does not change the original exposure.

Gato

--
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It's quite simple. Don't let 'equivalent' anything confuse you.

The light passing through the lens focusses onto some sq.mm of sensor. That remains the same regardless of whether the sensor is 24x35mm or 10x12 mm or whatever.

That's why f numbers are so useful - they represent an effective identity of the light, whatever the lens.

As I said, just don't think in terms of equivalent fls.

Hope it helps

Mike
--
Mike Davis
Photographing the public for over 50 years
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G1 FZ50

MYKC said:

... and users often refer to advantages of using 'fast' old lenses which have an effective focal length of double the 35mm equivalent because of the 'crop factor' of the smaller m4/3 sensor. My question is this: if most of the light gathered by legacy lenses is wasted because the sensor only occupies a fraction of the image circle, is the lens not 'slower' than the f-number suggests?

Click to expand...

MYKC said:

In other words, the actual focal length of the lens is the same, but the effective diameter is smaller, so the effective f number is larger.

Click to expand...

The lens did not change physically. You are just using a smaller part of the image circle - why would it be slower? You are still getting the same light per unit area striking the sensor.

--
Trevor

For legacy lenses:

• the effective focal length is 2x (so, 50mm = 100mm effective)
• the effective depth of field is 2x (so, 2.8 become 5.6)
• the exposure is the same (so, an FF and m43 will meter the same)

Also, it is an advantage to use FF lenses on m43 sensor. The best part of the lens is the centre part of it.. which is all that you are using when you mount an FF lens on an m43 body. This is why many pro cine lenses are hugely oversized.

MYKC said:

One of the attractions of the m4/3 format is the possibility of using a wide range of older lenses originally designed for 35mm film cameras. This forum contains many examples of the impressive results obtained using such lenses, and users often refer to advantages of using 'fast' old lenses which have an effective focal length of double the 35mm equivalent because of the 'crop factor' of the smaller m4/3 sensor. My question is this: if most of the light gathered by legacy lenses is wasted because the sensor only occupies a fraction of the image circle, is the lens not 'slower' than the f-number suggests? In other words, the actual focal length of the lens is the same, but the effective diameter is smaller, so the effective f number is larger. IF my analysis is correct, the legacy lenses are not quite such good bargains. I can't get my head around this issue, and would appreciate any thoughts from the experts on the forum. Cheers,
Mike.

Click to expand...

An f/1.4 lens is still an f/1.4 lens.

--
Charles
My family images are at http://www.stakeman.smugmug.com
Be sure of your subject.
Never, force the shot.

julieng said:

Given the 2x crop factor, compared to 35mm full frame, your lens will be for all practical purposes 2 stops slower.

Click to expand...

I don't think so. It should be the exact same speed and that's, in fact, what I see with my f1.4 50mm Minolta Rokkor lens on my G1 as well as my f.4 25mm PanLeica 4/3rds lens on my G1. In identical light conditions, for equivalent apertures, the G1 will set the exact same shutter speed. The Rokkor lens will have twice the "zoom" as the PanLeica but, otherwise, they are both equally fast.

Imagine the following scenario. In the darkroom you unroll a roll of 35mm film and on each frame you tape a border around the film, leaving the exposed portion of film only 1/2 the size as a normal 35mm frame. Then you load your roll of film into your 35mm camera.

You now have a 35mm camera loaded with a roll of 4/3rds sized film. Your lenses still expose the same center portion of the 35mm film frame as the equivalent-sized 4/3rds sensor but the rest of the film frame is simply masked off. But the un-masked portion sees the exact same amount of light from the lens as it would were the rest of the frame not masked off.

You develop your film and other than a white border around every frame, the negatives are identical to what you would have had, from an exposure point of view, had you not masked a portion of the film off.

You want to make some nice 4/3" prints of your pictures. You notice that you have to back the enlarger off a little more than you're used to to expose all of the 4/3" film sheets but other than that there is no difference.

CWilt said:

An f/1.4 lens is still an f/1.4 lens.

Click to expand...

Those who believe that anything other than changing the lens itself changes lens speed are ignorant of lens physics. The light cone coming out of the back of the lens transmits the same number of photons per second no matter what it is attached to. How each camera's sensor behaves with that light is a completely different matter.

Correct, but not everyone wants or needs to know that much to understand it. Thats why I said the simple answer was no. ;-)

--
Charles
My family images are at http://www.stakeman.smugmug.com
Be sure of your subject.
Never, force the shot.

MYKC said:

In other words, the actual focal length of the lens is the same, but the effective diameter is smaller, so the effective f number is larger.

Click to expand...

The effective f number, as far as light transmission goes, remains the same. The answer to your question is, No.

When I shoot my Pentax lenses on my G1, at f4 on the Pentax lens the exposure time is exactly the same as f4 on my 14-45 oem lens.

The depth of field is greater, though, than on the 1.5x Pentax cameras.

--
Cheers

Trevor G

http://www.computerwyse.com/photo.html

Jogger said:

For legacy lenses:

• the effective focal length is 2x (so, 50mm = 100mm effective)
• the effective depth of field is 2x (so, 2.8 become 5.6)

Click to expand...

A little clarification on the wording 'effective focal length'.

This holds true if you want to capture the same scene. Because of the crop factor, you will have to move further away from the subject, which results in larger dof.

Taking a photo from the same distance (same lens, but with two cameras: FF camera and m4/3), you will record the exact same dof. Only difference is that the m4/3 camera captures a smaller portion of the subject.

Damien

Jogger said:

• the exposure is the same (so, an FF and m43 will meter the same)

Also, it is an advantage to use FF lenses on m43 sensor. The best part of the lens is the centre part of it.. which is all that you are using when you mount an FF lens on an m43 body. This is why many pro cine lenses are hugely oversized.

MYKC said:

One of the attractions of the m4/3 format is the possibility of using a wide range of older lenses originally designed for 35mm film cameras. This forum contains many examples of the impressive results obtained using such lenses, and users often refer to advantages of using 'fast' old lenses which have an effective focal length of double the 35mm equivalent because of the 'crop factor' of the smaller m4/3 sensor. My question is this: if most of the light gathered by legacy lenses is wasted because the sensor only occupies a fraction of the image circle, is the lens not 'slower' than the f-number suggests? In other words, the actual focal length of the lens is the same, but the effective diameter is smaller, so the effective f number is larger. IF my analysis is correct, the legacy lenses are not quite such good bargains. I can't get my head around this issue, and would appreciate any thoughts from the experts on the forum. Cheers,
Mike.

Click to expand...

Click to expand...

--
http://www.flickr.com/photos/bilgy_no1

Those who state that using a 50mm f/2 lens on a 4/3 camera is like using a 100mm f/4 on full frame are right and wrong at the same time.

• Right from a field of view and DOF perspective, because of sensor size
• Wrong from a light gathering and speed perspective, as an apertue of f/2 will physically always remain f/2, be it on full frame or 4/3.

So a FF lense will not be slower on 4/3, but will have different FOV and DOF.

You can say that using a 50mm f/2 on 4/3 is like :

• using a 100mm f/4 on FF for FOV and DOF
• using a 50mm f/2 on FF for speed.

Hope it helps!

--
Cheers,

Frederic
http://azurphoto.com/blog/

I wil take your words for it that it is not the same difference on DoF as it is on shutter speed...

Pixnat2 said:

You can say that using a 50mm f/2 on 4/3 is like :

• using a 100mm f/4 on FF for FOV and DOF
• using a 50mm f/2 on FF for speed.

Click to expand...

To be more exact:
You can say that using a 50mm f/2 on 4/3 is like :

• using a 100mm f/2 for FOV
• using a 100mm f/2 for shutter speed and aperture and ISO
• using a 100mm f/4 for DOF

--
Vaughan
http://www.flickriver.com/photos/jvwpc/

Bilgy_no1 said:

Taking a photo from the same distance (same lens, but with two cameras: FF camera and m4/3), you will record the exact same dof. Only difference is that the m4/3 camera captures a smaller portion of the subject.

Click to expand...

Sure. But, nobody takes photos like that. They compose/frame according to what they see in the viewfinder... in which case, thinking of it as 2x DOF makes more sense.

MYKC said:

One of the attractions of the m4/3 format is the possibility of using a wide range of older lenses originally designed for 35mm film cameras. This forum contains many examples of the impressive results obtained using such lenses, and users often refer to advantages of using 'fast' old lenses which have an effective focal length of double the 35mm equivalent because of the 'crop factor' of the smaller m4/3 sensor. My question is this: if most of the light gathered by legacy lenses is wasted because the sensor only occupies a fraction of the image circle, is the lens not 'slower' than the f-number suggests? In other words, the actual focal length of the lens is the same, but the effective diameter is smaller, so the effective f number is larger. IF my analysis is correct, the legacy lenses are not quite such good bargains. I can't get my head around this issue, and would appreciate any thoughts from the experts on the forum. Cheers,
Mike.

Click to expand...

As others have pointed out - lenses for either 35mm film or m4/3 will transmit roughly the same amount of light at the same aperture. I say roughly because the f-number is just a ratio of the aperture diameter and focal length. Lenses with inefficient lens coatings or lots of elements can transmit slightly less light at the same aperture settings than those with better lens-multi-coating or less lens elements.

The main issue of using lenses for larger formats is that they project a larger image circle. In addition, lenses for film paid less attention to internal reflections. Digital sensors are far more reflective than film and so more light gets reflected back from the sensor and into the lens. This can cause some flare in some lighting, which lowers the contrast. Specific 4/3 lenses normally have a baffle at the rear to stop to much of the image circle being project into the camera to prevent this.

Thanks to everyone for the clarifications, my reasoning was clearly incorrect. I'm off to eBay now to get some fast FD glass. Cheers, Mike

MYKC said:

My question is this: if most of the light gathered by legacy lenses is wasted because the sensor only occupies a fraction of the image circle, is the lens not 'slower' than the f-number suggests?

Click to expand...

No. The lens transmits about 4x more light than needed to cover the sensor, but at the same time, the sensor has 1/4x the area as a 35mm film frame, so it all balances out. What the f-stop number is, roughly, telling you is illumination per unit of area on the sensor. With the same lens, each square millimeter of the sensor will receive the same illumination level, regardless of the sensor size.

Bilgy_no1 said:

Jogger said:

For legacy lenses:

• the effective focal length is 2x (so, 50mm = 100mm effective)
• the effective depth of field is 2x (so, 2.8 become 5.6)

Click to expand...

A little clarification on the wording 'effective focal length'.

Click to expand...

There's a fundamental problem with using phrases like "equivalent focal length" in these discussions; it's technically inaccurate as well as confusing.

We could be more precise if we used a phrase such as "equivalent angle of view", implying that a 50mm focal length lens on u4/3 has an "equivalent angle of view" of a 100mm focal length lens on 135 format.

So, instead of discussing focal lengths in millimeters, we should be talking angular degrees of field of view. I recommend using the angle subtended by the diagonal of the film format (i.e. the widest angle of view subtended by the format's frame), that way we can practically ignore differences in aspect ratio in such discussions.

As a peripheral comment, it amuses me to observe that, in many equipment-focused discussion forums such as DPReview, imprecise use of terminology is commonly applied to these concepts. As St. Ansel once quipped, something about "sharp images of fuzzy concepts".

Joe