What happens to effective focal length

[raid]

I am a newbie to micro 4/3 cameras, so I have a few questions on how lenses from a 35mm camera or from a half frame camera will do on a 4/3 EP-2 camera with a half size sensor.

I know that the focal length is doubled with a 35mm lens. A 50mm lens is effectively a 100mm lens.
edit: [what is covered]

question 1: What about lenses for the half frame cameras? If I use a 38mm lens from the PEN FT, with it still be effectively a 38mm lens or will it be a 76mm lens? Similarly, what if I use a lens made for the Zeiss Tenax, which is a square format?

question 2: Someone here said that when using legacy lenses with max aperture 2.0 on the EP-2 wil result in DOF corresponding to 4.0. Is this correct? Will it also be applicable to lenses for the PEN FT?


question 3: If we pick any two legacy lenses that work well with a film camera, is it true that we may see "new" differences when used on a digital camera? Why?


question 4: When using a fish-eye lens on a 4/3 camera like the EP-2, will distortion be much smlaler due to using the center half of the lens?

 

[ronnies]

This is where it's good to remember that the focal length of a lens doesn't change with the sensor size, just the field of view is different. A 38mm lens will give the same view on an EP-2 whether the lens was designed for full frame 35mm, half frame 35mm or even medium format.

Ronnie

 

[Roger Hicks]

This is where it's good to remember that the focal length of a lens doesn't change with the sensor size, just the field of view is different. A 38mm lens will give the same view on an EP-2 whether the lens was designed for full frame 35mm, half frame 35mm or even medium format.

Ronnie

Dear Ronnie,

Exactly. It still doubles, unless you're looking at 'equivalent' focal lengths, which are already buggered up. A 38mm lens off a Pen F gives the same FOV on 4/3 as a 38mm lens off a 16mm movie camera, 24x24mm Tenax (or Robot), 24x36mm 'full frame', 44x66mm (Alpa), 56x56 (Hasselblad)...

Cheers,

R.

 

[willie_901]

Focal length can not double. The focal length just is. Your 38 mm lens will always be a 38 mm lens.

When the sensor size is halved, the angle of view is halved. Think about this. If you had a really high MP 36 X 24 mm sensor, the m4/3 would just be a crop out of the image from that hypothetical sensor.

The reduction in sensor size does affect the DOF. All things being equal, the DOF is doubled. A f 2 lens still transmits the same amount of light, but the DOF will be similar to a f 4 lens.

All your lenses produce a circular image. So the half-size or square format they were intended for is irrelevant. What is relevant is do they cover a m 4/3 sensor with adequate IQ.

The lenses may perform much differently than they do with film. With film the angle that the light enters the sensor (in this case a film-dye granule) does not have much impact on the efficiency of the chemical change caused by the light. A digital sensor photo-site requires a micro-lens to direct the light onto the sensor surface. Now the angle of the light has a great effect on the photon measuring process. This is why Leica and Fujifilm use different sensor micro-lenses for different parts of the sensor.

Using legacy lenses on m 4/3 cameras is attractive because the performance depends greatly how the light exits the rear element and how it interacts with the micro-lenses. The variety of lens characteristics and micro-lens configurations is diverse. People enjoy spending lots of time seeing how certain lenses work with certain sensors.

In my limited experience I got the best results using the lenses designed for my camera (LUMIX G1). One reason I abandoned the m 4/3 system is I did not enjoy playing with old lenses to see how they compared to each other. Others do enjoy this and the flexibility of the m 4/3 system is a strong attraction.

 

[berlincontemporary]

I know that the focal length is doubled with a 35mm lens. A 50mm lens is effectively a 100mm lens.

Noooooooooo! Its still a 50mm lens. Imagine a film shot with the borders cropped off. Crop factor is not that hard to understand...

question 2: Someone here said that when using legacy lenses with max apertute 2.0 on the EP-2 wil result in DOF corresponding to 4.0. Is this correct?

To achieve a similar "look" of a 50/1.4 on full frame you would need a 25/0.5 or something on MFT. What I call "look" is angle of view and corresponding depth of field. It is not "focal length" which does not change.

question 4: When using a fish-eye lens on a 4/3 camera like the EP-2, will distortion be much smlaler due to using the center half of the lens?

Imagine a full frame fisheye picture with the borders cropped off. Its pretty simple.

 

[berlincontemporary]

Here you go. Focal length does not change with the size of sensor you put behind a lens.

 

[ferider]

Regarding qu. 3, Raid, I believe yes, bokeh will be different, but have no data. A test to proove this would be nice ... Hint. 🙂

 

[Roger Hicks]

Focal length can not double. The focal length just is. Your 38 mm lens will always be a 38 mm lens.

When the sensor size is halved, the angle of view is halved. Think about this. If you had a really high MP 36 X 24 mm sensor, the m4/3 would just be a crop out of the image from that hypothetical sensor.

The reduction in sensor size does affect the DOF. All things being equal, the DOF is doubled. A f 2 lens still transmits the same amount of light, but the DOF will be similar to a f 4 lens.

All your lenses produce a circular image. So the half-size or square format they were intended for is irrelevant. What is relevant is do they cover a m 4/3 sensor with adequate IQ.

The lenses may perform much differently than they do with film. With film the angle that the light enters the sensor (in this case a film-dye granule) does not have much impact on the efficiency of the chemical change caused by the light. A digital sensor photo-site requires a micro-lens to direct the light onto the sensor surface. Now the angle of the light has a great effect on the photon measuring process. This is why Leica and Fujifilm use different sensor micro-lenses for different parts of the sensor.

Using legacy lenses on m 4/3 cameras is attractive because the performance depends greatly how the light exits the rear element and how it interacts with the micro-lenses. The variety of lens characteristics and micro-lens configurations is diverse. People enjoy spending lots of time seeing how certain lenses work with certain sensors.

In my limited experience I got the best results using the lenses designed for my camera (LUMIX G1). One reason I abandoned the m 4/3 system is I did not enjoy playing with old lenses to see how they compared to each other. Others do enjoy this and the flexibility of the m 4/3 system is a strong attraction.

Of course you are absolutely right. Sloppy terminology on my part. The angle of coverage halves because the equivalent focal length (compared with 35mm) doubles.

For the rest you are absolutely right too. Purpose-designed lenses should always give the technically 'best' results. The only reasons to use 'legacy' lenses are (1) because you have them and (2) because you might like them.

Cheers,

R.

 

[Thardy]

Regarding qu. 3, Raid, I believe yes, bokeh will be different, but have no data. A test to proove this would be nice ... Hint. 🙂

The oof area (bokeh) looks like the bokeh for that focal length.

 

[ferider]

The oof area (bokeh) looks like the bokeh for that focal length.

Nope.

First of all for the same focus distance and lens, same print size and viewing distance, etc., the DOF is more shallow on cropped vs. full frame format. Second, the "bokeh" will look different on digital vs. film, since film is thicker.

Knowing Raid, before long, he will test this 🙂

 

[raid]

Oh well. I am seeing lots of useful feedback. Thank you all. I asked what many would ask when using digital with cropping for the first time. I should have clarified that what I meant with "effective focal length" was actually what "you see in the image in terms of coverage". Sloppy use of words.

 

[PentHassyKon]

Here you go. Focal length does not change with the size of sensor you put behind a lens.

Focal length can not double. The focal length just is. Your 38 mm lens will always be a 38 mm lens.

When the sensor size is halved, the angle of view is halved. Think about this. If you had a really high MP 36 X 24 mm sensor, the m4/3 would just be a crop out of the image from that hypothetical sensor.

The reduction in sensor size does affect the DOF. All things being equal, the DOF is doubled. A f 2 lens still transmits the same amount of light, but the DOF will be similar to a f 4 lens.
....snipped....

I'm not sure I understand the statement in BOLD ITALIC given the above visualization of a "cropped" APS-C image.
I'm inclined to think that the DOF for a 50mm f1.4 lens is the same whether it is full frame or cropped - no?

 

[raid]

DOF seems to be a tricky issue here. I had no clue about such a thing before I saw a remark being made at RFF a few days ago.

 

[greyelm]

If I can add my 2 pennies worth, I have tred to simplify all of this to myself. Firstly the lens does not know on which camera it is fitted so its optical characteristics such as bokeh and DOF should remain the same. Secondly (as already stated) the captured image will be a crop of that seen by a full frame sensor. With 35mm film cameras we got used to the characteristics of the various focal lengths, all we have to do is do this with other camera formats.

 

[tlitody]

Another thing is that when using tele lenses on FF you get the visual effect that background is brought forward. I can tell just looking at images that it was made with a tele lens. But a 50mm lens on a small format sensor which is equivalent to a 100mm lens on FF sensor will not give the same look as 100mm lens on FF. So the only equvalence is angle of view and not focal length but we use focal length terminology when we should be using AOV terminology.

 

[berlincontemporary]

First of all for the same focus distance and lens, same print size and viewing distance, etc., the DOF is more shallow on cropped vs. full frame format.

Only half correct. The part in red is false.
For the same focus distance, DOF is always the same. It doesnt matter if you crop a part of the projected image or not.

 

[dave lackey]

What the hell is a legacy lens anyway? 😛

I guess it won't fit the D2H, D2X or either of my Leica cameras?

 

[raid]

This is getting more interesting with each new posting.
Eventually, we will agree on the same thing. Right?

 

[berlincontemporary]

I'm inclined to think that the DOF for a 50mm f1.4 lens is the same whether it is full frame or cropped - no?

Yes it is.

With a smaller sensor you have to lengthen the distance to the subject compared to a larger sensor, given a lens of identical focal length is used. Thus the focus distance changes and depth of field becomes greater.

This is all pretty simple once you start to USE different cameras and see for yourself. There was a moment when I had three Canon digital bodies (Full frame, 1.3x, 1.6x) and made comparison pictures with the same 50/1.4 lens. When I did not change the camera position the smaller sensors simply produced crops of the full frame image. When I changed position to fit the same scene into the smaller frame, focus distance changed and depth of field became greater.

 

[greyelm]

Yes it is.

With a smaller sensor you have to lengthen the distance to the subject compared to a larger sensor, given a lens of identical focal length is used. Thus the focus distance changes and depth of field becomes greater.

This is all pretty simple once you start to USE different cameras and see for yourself. There was a moment when I had three Canon digital bodies (Full frame, 1.3x, 1.6x) and made comparison pictures with the same 50/1.4 lens. When I did not change the camera position the smaller sensors simply produced crops of the full frame image. When I changed position to fit the same scene into the smaller frame, focus distance changed and depth of field became greater.

Well put, a good summary.