Lens's focal length for half-frame camera

[alex_libinson]

There are several cameras giving half-frame for the "normal" 35 mm

film - that is 18x24 instead of 24x36 , e.g. Olympus-Pen. If its lens

is marked as "28mm" - does it mean its angular view corresponds to

55mm for the common 35 mm film camera, or the 28mm is the equivalent

focal length (so it is 14 mm really)?

Thanks in advance.

[ken_schroeder]

Alex, the 28mm on the half frame should be roughly equivalent to the 55 on the full frame camera. I haven't done the math, but I suspect it is a little wider than 55. The extra few millimeters of the 55 allow the lens to clear the mirror of a single lens reflex camera. I used an Olympus Pen years ago. It's a very nice little camera.

[ilkka_nissila]

28 mm on a 1.5x digital camera is the equivalent of 42 mm on the film camera, 35 mm is 52.5 mm on a full-frame.

[richard_oleson]

The lens focal lengths indicated on Olympus Pen and all other film cameras represent the actual focal length of the lens; the idea of "35mm equivalent focal length" was invented for digital cameras to give people a familiar frame of reference when dealing with odd digital formats.

 

The diagonal of the 18x24mm half frame is 30mm; the diagonal of the 24x36mm full frame is 43mm; so the "normal" focal length for a half frame would be about 30/43 x 50 or 35mm. The "35mm equivalent" of the 28mm on the Pen would be 43/30 x 28 or 40mm.

 

rick :)=

[david_h._hartman]

The distance from the lens to the subject determines the perspective. The focal length determines the magnification. The format, in this case 18x24 determines the angle of view.

 

Most people compare the diagonal distance when comparing formats. I normally compare the long side of the format. In this case I would compare by division 36/24=1.5. By this method a 28mm lens is equivalent to a 42mm lens (wide-normal) using 28*1.5=42. Using the diagonals 43.27/30.0=1.44 by this method 28*1.44=40.38 or 40.4mm (still wide-normal). Note that in comparing half frame to full frame NO magnification is occurring. It�s better to think of this as cropping.

 

These comparisons are kind of like speaking a new language by translating everything in and out of one�s native tongue. It�s easier to just speak the new language, e.g. an 80mm is a normal lens on 6x6, 150mm and 180mm are typical portrait lenses. A 50mm or 60mm are typical wide angles for 6x6.

 

Hope this helps,

[nic_olas]

Trying to understand this 'crop' or equivalence. So for examples, an 18mm Nikon lens on the Pen F film would be?

A 50mm Nikon Lens would be?

Thanks! Maths isn't my strength 

[nic_olas]

34 minutes ago, nic_olas said:

Trying to understand this 'crop' or equivalence. So for examples, an 18mm Nikon lens on the Pen F film would be?

A 50mm Nikon Lens would be?

Thanks! Maths isn't my strength 

NVM I believe I know the equivalence, 

18 x 1.5 = 27mm 

55 x 1.5 = 82.5

Is this roughly correct?

[michael_goldfarb]

For a quick guide, check this table of Pen F half-frame lenses, which includes the full-frame 35mm neg equivalents for their focal lengths.

Olympus Pen F - Wikipedia

[mike_halliwell]

It would seem x1.4 is more accurate.

So, 50mm is 70mm EQ.

[Dieter Schaefer]

4 minutes ago, mike_halliwell said:

It would seem x1.4 is more accurate.

Full frame is double the area of half frame - ergo the crop factor is SQRT(2) - which is close enough to 1.4. APS and half frame film formats are not the same (the "classic" APS-C film  is 25.1 × 16.7 mm vs 18 x 24 mm for half frame); the crop factor, however, is only marginally larger at 1.43. For digital, there's different APS-C formats: Canon chose 22.3×14.9 mm (1.6 crop factor), Nikon and others chose 23.5×15.6 mm (1.5 crop factor).

[ilkka_nissila]

1 hour ago, Dieter Schaefer said:

Full frame is double the area of half frame - ergo the crop factor is SQRT(2) - which is close enough to 1.4. APS and half frame film formats are not the same (the "classic" APS-C film  is 25.1 × 16.7 mm vs 18 x 24 mm for half frame); the crop factor, however, is only marginally larger at 1.43. For digital, there's different APS-C formats: Canon chose 22.3×14.9 mm (1.6 crop factor), Nikon and others chose 23.5×15.6 mm (1.5 crop factor).

Strictly speaking there is no APS-C film; most APS film cameras would let the user choose from three sizes of images on the negative (on the same roll) : APS-H 30.2 × 16.7 mm, APS-C 25.1 × 16.7 mm, and APS-P 30.2 × 9.5 mm. All the digital "APS-C" sensors are smaller than actual APS-C film frames. I wonder how did people start to refer to APS formats as sizes for digital sensors (when they're not the same size). Nikon always called theirs DX, making the distinction that this is a different format from film. Somehow I always though the "DX" borrows the "X" from "IX" which is the method of information encoding used by APS film (but I am just guessing). Digital cameras use EXIF tags to transmit this information.

[mike_halliwell]

From the table linked above comes this old favourite lens...

E.Zuiko Auto-T

100mm F3.5

140mm

17°

4-5

1-6

3.5–22

1.5m

43mm

250g

Where 100mm (35mm film format lens) is EQ to 140mm on the Olympus Pen F, so ~ x1.4.

Edited September 11, 2023 by mike_halliwell

[mike_halliwell]

Late EDIT, I mis-labelled the 100mm f3.5 as a 35mm film format lens, it's not!) Sorry... 😢

[Dieter Schaefer]

1 hour ago, ilkka_nissila said:

Strictly speaking there is no APS-C film

1 hour ago, ilkka_nissila said:

APS-H 30.2 × 16.7 mm, APS-C 25.1 × 16.7 mm, and APS-P 30.2 × 9.5 mm

Well, it's right there in the middle😏

APS_H film has even less to do with Canon's (and Leica's) digital APS-H format (which has a 1.3 crop factor). Nikon's DX is also a bit variable - some of the lower-tier bodies have crop factors of 1.53, 1.54, and 1.55 - when DX actually is 1.52.

1 hour ago, ilkka_nissila said:

I wonder how did people start to refer to APS formats as sizes for digital sensors (when they're not the same size)

Close enough, I suppose. Someone started it and it stuck.

[mike_halliwell]

I know some people don't like the Equivalence references, and I could understand that in the strict days of film, where there were very few different formats.

If you used, say 5 x 4 inch film you knew what kinda FOV you get from a 240mm lens, a 'standard lens' for MF was ~80mm and if you used 35mm film, that a v.wide angle was <24mm.... etc etc.

Now there are so many sensor sizes, especially in camera-phones, you NEED an equivalent 'cos who the heck knows what FOV a 4.2mm lens is going to give you?

Some phones now have 3 cameras, each with a different sensor size & different focal length, so to compare phones, esp. at the 'tele' end, is near impossible.

[Matthew Currie]

A small note.  I'm not sure how it was done everywhere, but I long ago had a Canon APS film camera.  I had assumed that the three print sizes were different sizes on the film itself, since I thought the whole point of APS was this new flexibility through film encoding and all.  I was surprised to find that, just as with some 35 mm. cameras, the different print sizes were just crop instructions to the processor.  Wide prints were  cropped horizontally, narrow ones cropped vertically with wider spaces between them.  No more or fewer pictures would go on the roll, whatever sizes you chose. The camera itself was very nicely made, a solid little all metal body.  Unfortunately the very compact body did not make up for the smaller negatives and the expensive film and processing, and it appears that very few of the touted features of APS were used by most processors anyway.

[mike_halliwell]

4 hours ago, Matthew Currie said:

the different print sizes were just crop instructions to the processor.

ie, a complete con!

I never figured out how they got away with it then..

Some even had masks to cover up some of the film in 'panoramic' mode...🤣

 

[ilkka_nissila]

5 hours ago, Matthew Currie said:

A small note.  I'm not sure how it was done everywhere, but I long ago had a Canon APS film camera.  I had assumed that the three print sizes were different sizes on the film itself, since I thought the whole point of APS was this new flexibility through film encoding and all.  I was surprised to find that, just as with some 35 mm. cameras, the different print sizes were just crop instructions to the processor.  Wide prints were  cropped horizontally, narrow ones cropped vertically with wider spaces between them.  No more or fewer pictures would go on the roll, whatever sizes you chose. The camera itself was very nicely made, a solid little all metal body.  Unfortunately the very compact body did not make up for the smaller negatives and the expensive film and processing, and it appears that very few of the touted features of APS were used by most processors anyway.

I see, so did I get it correctly: all APS film was exposed with APS-H sized images, and the encoding in the film indicated which part of each frame was to be included in the prints (implemented probably with digital cropping). So there is no such thing as an APS-C film negative if the cameras always exposed the APS-H area.

 

Interestingly the APS-H frame has an aspect ratio that is more panoramic than the typical widescreen 16:9, so they were moving towards that already.

Edited September 12, 2023 by ilkka_nissila